Autosomal Recessive Osteopetrosis 3

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Classifications

ISDS Skeletal Nosology

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Mappings

NCIT

NCIT:C118438 Osteopetrosis with Renal Tubular Acidosis

NCIT names the same clinical entity by its historical descriptive name.

NCIT

NCIT:C118438 Osteopetrosis with Renal Tubular Acidosis

NCIT names the same clinical entity by its historical descriptive name.

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Inheritance

1

Autosomal Recessive HP:0000007

Biallelic CA2 variants may be homozygous or compound heterozygous. Clinical expression varies within and between families; the available clinical and enzyme series do not establish population-wide penetrance. Asymptomatic obligate heterozygotes had approximately half-normal erythrocyte CA II in the founding biochemical study.

Autosomal recessive inheritance Expressivity: VARIABLE

Show evidence (3 references)

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"The etiology is bi-allelic loss-of-function mutations of CA2 that encodes CA II."

States the biallelic loss-of-function etiology directly.

PMID:6405388 SUPPORT Human Clinical

"Levels of CA II in erythrocyte hemolysates from asymptomatic obligate heterozygotes are about half of normal."

Gene-dosage behaviour of the enzyme in asymptomatic carriers, consistent with recessive inheritance and the basis for biochemical carrier testing.

PMID:9453381 SUPPORT Human Clinical

"highlight the inter-/intra-familial variability of the disease in this part of the world despite the overwhelming prevalence of the same"

Supports VARIABLE expressivity: clinical severity differs within and between families carrying the same founder allele.

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Discussions and Knowledge Gaps

5

By what route does loss of cytosolic carbonic anhydrase II produce parenchymal calcium deposition in basal ganglia and thalami, and why does the calcium burden not track neurodevelopmental outcome?

KNOWLEDGE GAP OPEN ca2_cerebral_calcification_mechanism

Attached to

pathophysiology#Intracranial Parenchymal Calcium Deposition phenotypes#Cerebral calcification phenotypes#Intellectual disability

Intracranial calcification and cognitive impairment are observed, but their causal relationship and their route from CA II deficiency remain unresolved. Calcification can be absent during childhood. In a ten-patient series, calcium score did not significantly associate with age, developmental quotient or optic canal diameter; this small negative study does not prove that calcium burden can never affect neurological outcome. The early transplant series suggested delayed calcification while developmental impairment persisted.

Show evidence (3 references)

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"Although this enzymopathy reveals how CA II is important for the skeleton and kidney tubule, the pathogenesis of the mental subnormality and cerebral calcification is less well understood."

The explicit statement that this arm is the unexplained one, contrasted with the two arms that are explained.

PMID:39667299 SUPPORT Human Clinical

"Correlation analysis revealed no significant associations between intracranial parenchymal calcium score and age, DQ score, or optic canal diameter."

The negative correlation that blocks the obvious inference from calcium burden to cognitive outcome.

PMID:22120147 SUPPORT Human Clinical

"Variability of brain calcification, cognitive disturbance and optic nerve involvement may imply additional genetic or epigenetic influences affecting the course of the disease."

Genotype-identical patients diverge on exactly the features in question, which is what makes this a mechanism gap rather than a sampling problem.

Why does the Car-2 null mouse reproduce the renal and growth arms of carbonic anhydrase II deficiency but not the osteopetrosis, and what model is therefore valid for studying the human bone arm?

HUMAN MODEL MISMATCH OPEN ca2_mouse_null_lacks_osteopetrosis

Attached to

pathophysiology#Osteoclast Resorption Lacuna Acidification Failure pathophysiology#Impaired Osteoclastic Bone Resorption animal_models#Mouse

Car2-null mice reproduce growth restriction and renal tubular acidosis but did not develop osteopetrosis even at old ages. The reason for this species difference is unresolved. It limits translation of skeletal endpoints from this mouse, while leaving the model useful for renal physiology and for investigating compensation. The negative skeletal result should not be generalized to every cellular resorption assay.

Show evidence (2 references)

PMID:3126501 SUPPORT Model Organism

"However, the prominent osteopetrosis found in humans with CA II deficiency could not be detected even in very old homozygous null mice."

The negative skeletal result in a confirmed protein null, which is the mismatch itself.

PMID:3126501 SUPPORT Model Organism

"Like humans with the same inherited enzyme defect, animals homozygous for the new null allele are runted and have renal tubular acidosis."

The same animal reproduces the renal and growth arms, so the failure is specific to bone rather than a failed model overall.

Is the short stature of carbonic anhydrase II deficiency caused by chronic metabolic acidosis, by the skeletal disease, or by growth hormone deficiency?

KNOWLEDGE GAP OPEN ca2_growth_failure_attribution

Attached to

phenotypes#Short stature pathophysiology#Systemic Hyperchloremic Metabolic Acidosis

Short stature is well documented clinically, but contributions from acidosis, skeletal disease and endocrine dysfunction remain unresolved. In CA II-deficient mice, low GH secretion was male-specific, and randomized GH administration increased linear growth despite persistent acidosis. This demonstrates a modifiable mouse growth phenotype, not an established human GH-deficiency mechanism or clinical treatment effect.

Show evidence (4 references)

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"Several mouse models of CA II deficiency have shown growth hormone deficiency, yet currently there is no standard pharmacologic therapy for patients."

Introduces the growth-hormone candidate, and in the same sentence records that no therapy is standardised.

PMID:3126501 SUPPORT Model Organism

"Like humans with the same inherited enzyme defect, animals homozygous for the new null allele are runted and have renal tubular acidosis."

Growth restriction and acidosis co-occur in the null mouse, which is the observation behind the acidosis hypothesis - but it is co-occurrence, not a demonstrated mechanism.

PMID:9688969 SUPPORT Model Organism

"Randomized administration of rGH from approximately 5 to approximately 12 wk to CAD mice improved food efficiency and increased serum IGF-I levels, final length, and weight compared with placebo without affecting blood pH."

Mouse intervention improves growth without correcting acidosis; it is not human treatment evidence.

+ 1 more reference

If CA II is the only soluble carbonic anhydrase isoenzyme in the kidney, why do CA II-deficient patients still respond to intravenous acetazolamide with a rise in urinary pH and bicarbonate excretion?

OPEN QUESTION OPEN ca2_residual_renal_carbonic_anhydrase

Attached to

pathophysiology#Impaired Proximal Tubular Bicarbonate Reclamation

Patients with undetectable erythrocyte CA II nevertheless show a normal renal response to acetazolamide. The original authors set out three explanations - that the deficiency is not expressed in kidney, that another acetazolamide-sensitive soluble isoenzyme is expressed there in these patients, or that the response reflects the membrane-bound luminal carbonic anhydrase, a different gene product unaffected by the CA2 mutation - and favoured the third. This is recorded as an open question rather than settled because the entry's renal node depends on it: if luminal membrane-bound activity is intact, the human renal defect is a partial loss of tubular carbonic anhydrase function, not a total one, and the residual activity is a potential target. It also bears on the interpretation of acetazolamide challenge as a diagnostic test, which would be uninformative here.

Show evidence (2 references)

PMID:3932950 SUPPORT Human Clinical

"Following acetazolamide infusion, the CA II-deficient patients exhibited a prompt rise in urinary pH and HCO3- excretion similar to the response seen in control subjects."

The observation that creates the question.

PMID:3932950 SUPPORT Human Clinical

"These findings indicate that CA II-deficient patients, who lack detectable CA II in their erythrocytes, still expressed an acetazolamide-inhibitable CA activity in their kidneys."

The authors' inference that residual acetazolamide-sensitive activity persists in the CA II-deficient kidney.

Does the combination of osteopetrosis and renal tubular acidosis always mean CA II deficiency, given that coinherited TCIRG1 and ATP6V1B1 defects can reproduce it?

INTERPRETATION OPEN ca2_osteopetrosis_rta_phenocopy

Attached to

phenotypes#Osteopetrosis phenotypes#Renal tubular acidosis genetic#CA2

The clinical triad is a very strong pointer to CA2 and is often treated as diagnostic, but it is not pathognomonic, and the documented exception is instructive rather than merely pedantic. One kindred with osteopetrosis and distal renal tubular acidosis had normal CA II levels and no CA2 defect; instead two separate recessive disorders were segregating, a TCIRG1 deletion producing the osteopetrosis and an ATP6V1B1 mutation producing the distal RTA. Both genes encode subunits of the same proton pump, in tissue-specific isoforms. The practical consequence is that the triad justifies CA2 testing and the enzyme assay, but a negative CA2 result in a patient with the full phenotype should prompt a search for coinherited pump subunit defects rather than a presumption of a cryptic CA2 variant.

Show evidence (1 reference)

PMID:12566520 SUPPORT Human Clinical

"In this kindred, two separate recessive disorders are penetrant, each affecting a different, tissue specific subunit of the vacuolar proton pump (H(+)-ATPase), providing a highly unusual, novel genetic explanation for the coexistence of osteopetrosis and dRTA."

The documented genetic phenocopy of the CA II deficiency phenotype.

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Pathophysiology

11

Biallelic CA2 Loss of Function

Mechanism confidence: Established

Biallelic splice, nonsense, frameshift or missense variants reduce CA II abundance or activity. Erythrocyte CA II was virtually absent in the founding biochemical series, with CA I preserved, but this does not establish complete loss of activity in every tissue for every allele. Recombinant His107Tyr retains greatly reduced activity. The exon 7 frameshift produces an inactive, unstable truncated protein that accumulates in insoluble aggregates; p.Trp123Ter is a truncating nonsense allele, not a full-length missense product.

erythrocyte CL:0000232

CA2 hgnc:1373

Genetic context CA2 hgnc:1373 variant_origin: GERMLINE functional_impact_category: LOSS_OF_FUNCTION

Both homozygous and compound-heterozygous genotypes occur; the index American siblings carried His107Tyr in trans with an intron 5 splice-acceptor variant.

carbonic anhydrase II (cytosolic CO2 hydratase) activity GO:0004089 ↓ DECREASED

Experimental models

Recombinant His107Tyr CA II OTHER Bacterially expressed mutant CA II retains detectable but greatly reduced catalytic activity.

CA2 exon 7 frameshift expression in COS cells CELL LINE The patient-associated exon 7 frameshift produces a truncated, inactive protein with aggregation and accelerated degradation in transfected COS cells.

Purified Gly145Arg and His94Tyr CA II proteins OTHER Site-directed mutant proteins were expressed and purified for spectroscopic stability and activity studies; both showed local structural changes and reduced native-state stability.

Downstream

Reduced CA II-Dependent Proton and Bicarbonate Generation Reduced CA II activity slows the enzyme-catalyzed hydration of CO2; it does not abolish all cellular production of protons or bicarbonate.

Intracranial Parenchymal Calcium Deposition Cerebral calcification is an observed component of CA2 deficiency, but the intermediate route from enzyme dysfunction to regional mineral deposition is unresolved.

Show evidence (6 references)

PMID:3925334 SUPPORT Human Clinical

"Virtual absence of the carbonic anhydrase II peak on high-performance liquid chromatography, of the esterase and carbon dioxide hydratase activities of carbonic anhydrase II, and of immunoprecipitable isozyme II was demonstrated on extracts of erythrocyte hemolysates from all patients studied."

Demonstrates absence of the enzyme by three independent methods across 18 patients from 11 unrelated families.

PMID:6405388 SUPPORT Human Clinical

"We report here biochemical and immunological evidence for the virtual absence of CA II in erythrocytes of patients affected with this condition, whereas CA I level is not reduced."

The founding observation, and establishes that the deficiency is isoenzyme-specific rather than a general carbonic anhydrase failure.

PMID:8128957 SUPPORT In Vitro

"The 27-kD truncated enzyme produced when the mutant cDNA was expressed in COS cells was enzymatically inactive, present mainly in insoluble aggregates, and detectable immunologically at only 5% the level of the 29-kD normal carbonic anhydrase II"

Expression study showing one route to loss of function - truncation with aggregation and accelerated degradation.

+ 3 more references

Reduced CA II-Dependent Proton and Bicarbonate Generation

Mechanism confidence: Established

Reduced CA II activity impairs rapid reversible CO2 hydration to bicarbonate and protons. This limits enzyme-supported substrate supply for osteoclast and renal transport. Other carbonic anhydrase activities can persist: CA II-deficient patients retained an acetazolamide-sensitive renal response. The observed brain phenotype is associated with CA2 deficiency, but its intermediate mechanism is unresolved.

one-carbon (CO2/bicarbonate) interconversion GO:0006730 ↓ DECREASED

cytosolic CO2 hydration to proton and bicarbonate GO:0004089 ↓ DECREASED

Downstream

Osteoclast Resorption Lacuna Acidification Failure Reduced rapid proton generation impairs acidification of the osteoclast resorption compartment.

Impaired Proximal Tubular Bicarbonate Reclamation Reduced enzyme activity impairs the proximal component of bicarbonate reclamation.

Impaired Distal Tubular Proton Secretion Reduced enzyme-supported proton generation impairs distal urinary acidification.

Show evidence (1 reference)

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"Carbonic anhydrase II deficiency (OMIM # 259730), initially called "osteopetrosis with renal tubular acidosis and cerebral calcification syndrome", reveals an important role for the enzyme carbonic anhydrase II (CA II) in osteoclast and renal tubule function."

Establishes that the one enzyme's loss is what couples the osteoclast and renal tubule phenotypes.

Osteoclast Resorption Lacuna Acidification Failure

Mechanism confidence: Established

CA II-dependent proton generation supports acidification of the sealed osteoclast resorption compartment. Deficiency impairs this process despite the presence of osteoclasts. The proximal lesion is in intracellular enzyme activity, distinct from variants directly affecting the ruffled-border transport machinery.

osteoclast CL:0000092

acidification of the sealed resorption lacuna GO:0045851 ↓ DECREASED proton secretion across the osteoclast ruffled border GO:1902600 ↓ DECREASED

bone tissue UBERON:0002481

Downstream

Impaired Osteoclastic Bone Resorption Hydroxyapatite dissolution is acid-dependent, so a lacuna that is not acidified does not release bone mineral.

Show evidence (2 references)

PMID:6405388 SUPPORT Human Clinical

"provide genetic evidence implicating CA II in osteoclast function and bone resorption"

Human genetic evidence that CA II is required for osteoclast function - the inference the naturally occurring null makes available.

PMID:6405388 SUPPORT INDIRECT Human Clinical

"explain previous observations that carbonic anhydrase inhibitors block the normal parathyroid hormone-induced release of calcium from bone"

Pharmacological corroboration: inhibiting carbonic anhydrase phenocopies the resorption block. Indirect because it argues from inhibitor data rather than from the patients' osteoclasts.

Impaired Osteoclastic Bone Resorption

Mechanism confidence: Established

Impaired acidification reduces osteoclast-mediated resorption of mineralized bone. The coupling of mineral dissolution and matrix removal explains defective remodeling without requiring an increase in the rate of bone formation.

osteoclast CL:0000092

osteoclastic bone resorption GO:0045453 ↓ DECREASED bone remodeling GO:0046849 ↓ DECREASED

Downstream

Accumulation of Unresorbed Sclerotic Bone with Preserved Osteoclast Number

Mechanism confidence: Established

Reduced resorption allows dense, poorly remodeled bone to accumulate despite the presence of osteoclasts. Bone density does not imply normal mechanical strength. Resolution of osteopetrosis after hematopoietic transplantation supports an osteoclast-lineage defect; it does not prove that every extra-skeletal outcome is unaffected.

bone element UBERON:0001474

Animal models

Car-2 null mouse (ENU-induced) Mouse fails to recapitulate The defining human phenotype is absent from the mouse null. Osteopetrosis could not be detected even in very old homozygotes.

Downstream

Cranial Foramen and Optic Canal Narrowing

Craniofacial Disproportion and Upper Airway Narrowing

Osteopetrosis

Recurrent fractures Dense bone in this disease is brittle rather than strong, and fracture is a presenting feature.

Show evidence (2 references)

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"Although CA II deficiency is an "osteoclast-rich" OPT, and therefore transplantation of healthy osteoclasts can improve the skeletal disease, the RTA and central nervous system difficulties persist."

Describes an osteoclast-rich disorder in which transplantation can improve bone disease despite persistent renal and central nervous system difficulties.

PMID:23877423 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"mutations in TCIRG1, CLCN7, OSTM1, SNX10 and PLEKHM1 lead to osteoclast-rich ARO (in which osteoclasts are abundant but have severely impaired resorptive function), whereas mutations in TNFSF11 and TNFRSF11A lead to osteoclast-poor ARO"

Defines the osteoclast-rich versus osteoclast-poor axis this node is placed on. Indirect because this review enumerates the other recessive loci and does not itself mention CA2.

Cranial Foramen and Optic Canal Narrowing

Mechanism confidence: Established

Dense skull-base bone can narrow cranial foramina. In the 23-patient cohort, optic nerve involvement affected 23 of 46 eyes and correlated with optic canal narrowing, supporting compression as an important mechanism. This association does not exclude an additional neural contribution. Internal auditory canal size was normal in that cohort.

bone element UBERON:0001474

Downstream

Optic atrophy Compression of the optic nerve within a narrowed bony canal.

Cranial nerve compression

Show evidence (2 references)

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"The disorder presents late in infancy or early in childhood with fracturing, developmental delay, weakness, short stature, and/or cranial nerve compression and palsy."

Lists cranial nerve compression and palsy among the presenting features.

PMID:22120147 SUPPORT Human Clinical

"All internal auditory canals were normal in size, and no patient had clinically significant hearing loss."

Bounds the node: foraminal narrowing in this cohort spared the internal auditory canals, so the claim is not that every cranial foramen narrows.

Craniofacial Disproportion and Upper Airway Narrowing

Mechanism confidence: Provisional

Skeletal overgrowth of the skull produces a large cranial vault and broad forehead, with midfacial and mandibular disproportion. The inference that this geometry narrows the upper airway enough to cause obstructive sleep apnea rests on a case report plus the general osteopetrosis literature, so the node is recorded as provisional.

Downstream

Obstructive sleep apnea

Show evidence (1 reference)

PMID:22120147 SUPPORT Human Clinical

"All patients had osteopetrosis, renal tubular acidosis, developmental delay, short stature and craniofacial disproportion with large cranial vault and broad forehead."

Craniofacial disproportion with large cranial vault and broad forehead was present in all 23 patients.

Impaired Proximal Tubular Bicarbonate Reclamation

Mechanism confidence: Established

CA II-dependent intracellular CO2 hydration supports proximal bicarbonate reclamation. Deficiency can produce a proximal component of renal tubular acidosis, but the relative proximal and distal contributions vary between patients. Acetazolamide challenge in two affected individuals still increased urinary pH and bicarbonate excretion, indicating residual renal carbonic anhydrase activity rather than complete loss of all isoenzymes.

epithelial cell of proximal tubule CL:0002306

tubular bicarbonate reclamation GO:0015701 ↓ DECREASED

kidney UBERON:0002113

Downstream

Systemic Hyperchloremic Metabolic Acidosis

Show evidence (2 references)

PMID:12566520 SUPPORT Human Clinical

"Most often, the RTA is of mixed proximal and distal type, but kindreds are reported in which either distal or proximal RTA predominates."

The direct statement that the renal defect is usually mixed, with kindred-level variation - the reason this entry does not assert a single RTA tier.

PMID:3932950 SUPPORT Human Clinical

"These findings indicate that CA II-deficient patients, who lack detectable CA II in their erythrocytes, still expressed an acetazolamide-inhibitable CA activity in their kidneys."

Qualifies the node: carbonic anhydrase activity is not wholly absent from the CA II-deficient kidney, so the renal defect is a partial rather than a total loss of tubular carbonic anhydrase function.

Impaired Distal Tubular Proton Secretion

Mechanism confidence: Established

CA II supports proton secretion by collecting-duct intercalated cells. Its deficiency can impair distal urinary acidification. Car2-null mice additionally show depletion of intercalated-cell markers and reduced bicarbonate-transporter expression; these model findings do not establish an identical cellular population defect in human kidneys.

collecting-duct alpha-intercalated cell CL:0005011

tubular proton secretion GO:1902600 ↓ DECREASED

kidney UBERON:0002113

Animal models

Car-2 null mouse (ENU-induced) Mouse partially recapitulates Collecting-duct cellular and transporter changes support disruption of distal acid-base transport in Car2-null mice.

Downstream

Systemic Hyperchloremic Metabolic Acidosis

Show evidence (2 references)

PMID:12566520 SUPPORT Human Clinical

"Most often, the RTA is of mixed proximal and distal type, but kindreds are reported in which either distal or proximal RTA predominates."

The direct statement that the renal defect is usually mixed, with kindred-level variation - the reason this entry does not assert a single RTA tier.

PMID:3932950 SUPPORT Human Clinical

"These findings indicate that CA II-deficient patients, who lack detectable CA II in their erythrocytes, still expressed an acetazolamide-inhibitable CA activity in their kidneys."

Qualifies the node: carbonic anhydrase activity is not wholly absent from the CA II-deficient kidney, so the renal defect is a partial rather than a total loss of tubular carbonic anhydrase function.

Systemic Hyperchloremic Metabolic Acidosis

Mechanism confidence: Established

Renal bicarbonate loss and/or impaired net acid excretion produce metabolic acidosis, commonly hyperchloremic, sometimes with hypokalemia. Oral alkali can improve the acid-base disturbance. The combination with osteopetrosis is a strong diagnostic clue but can also occur with coinherited TCIRG1 and ATP6V1B1 disease.

Animal models

Car-2 null mouse (ENU-induced) Mouse recapitulates Homozygous null mice develop renal tubular acidosis, as humans with the same enzyme defect do.

Downstream

Renal tubular acidosis

Hyperchloremic metabolic acidosis

Show evidence (1 reference)

PMID:35035649 SUPPORT Human Clinical

"The suspicion of carbonic anhydrase II deficiency was confirmed by arterial blood gases revealing a marked metabolic acidosis fulfilling the diagnostic triad."

A worked clinical instance of the systemic acidosis being the measurable expression of this node.

Intracranial Parenchymal Calcium Deposition

Mechanism confidence: Established

Intracranial mineral deposition is directly observed on imaging, commonly involving basal ganglia, thalami and the frontal grey-white junction. It can progress during childhood and can be absent at a particular assessment. The lesion is established; the causal intermediates connecting CA II deficiency to deposition and cognitive outcomes remain unresolved. A small clinical series found no significant association between calcium burden and developmental measures.

basal ganglion UBERON:0002420

Downstream

Cerebral calcification

Show evidence (2 references)

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"Cerebral calcification uniquely appears in early childhood."

Establishes that the calcification is an early-childhood feature specific to this osteopetrosis.

PMID:39667299 SUPPORT Human Clinical

"Intraparenchymal calcifications in CAII-D are common but unrelated to developmental delay and neuropsychiatric symptoms, suggesting complex pathophysiology."

Supports an established imaging lesion while the lack of association with developmental measures in this small series leaves its causal relationship to neurological impairment unresolved.

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Pathograph

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Referential integrity issues (1):

Source 'pathophysiology#Biallelic CA2 Loss of Function' (for edge to 'Erythrocyte carbonic anhydrase II') not found in named elements

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Phenotypes

29

Blood 1

Anemia HP:0001903

Show evidence (1 reference)

PMID:11938359 SUPPORT Human Clinical

"Five patients were blind secondary to optic nerve entrapment and 2 patients developed anemia and secondary erythropoesis due to bone marrow involvement."

Marrow-related anemia was reported in this clinically diagnosed cohort; CA II activity was measured in nine participants.

Ear 2

Hearing impairment HP:0000365

Hearing loss occurs in some reports but was not clinically significant in any patient of the 23-person founder-allele cohort. These observations show variable expression; they do not exclude hearing involvement from the disease.

Show evidence (1 reference)

PMID:11264157 SUPPORT Human Clinical

"Two children, one from each family, have undergone allogeneic bone marrow transplantation because of severe progressive visual and hearing loss."

Progressive hearing loss severe enough to be a transplantation indication in two related Irish families.

Conductive hearing impairment HP:0000405

Show evidence (1 reference)

PMID:37662627 SUPPORT Human Clinical

"she presented right optic nerve atrophy with visual impairment, strabismus, horizontal nystagmus, and conductive hearing loss."

The full report specifies conductive loss; the abstract’s broader deafness term is less precise.

Eye 3

Optic atrophy OCCASIONAL HP:0000648

Show evidence (2 references)

PMID:39667299 SUPPORT Human Clinical

"Optic nerve atrophy was observed in 22.2 %, while brain calcifications were present in 70.0 % of cases."

22.2% in the ten-patient pediatric series falls within OCCASIONAL; the abstract does not state the denominator with evaluable optic-nerve assessment.

PMID:22120147 SUPPORT Human Clinical

"Globes and retinae were normal, but optic nerve involvement was present in 23/46 eyes and was variable in severity, random in occurrence and statistically correlated with degree of optic canal narrowing."

Optic nerve involvement in 23/46 eyes is a broader outcome than optic atrophy and must not be pooled with the patient-level estimate. Canal correlation supports, but does not prove exclusivity of, compression.

Nystagmus FREQUENT HP:0000639

Show evidence (1 reference)

PMID:22120147 SUPPORT Human Clinical

"Saccadic abnormalities were present in two, while half of these patients had sensory or accommodative strabismus, and seven had congenital nystagmus."

Seven of 23 patients in the 2011 founder-allele cohort had congenital nystagmus (30.4%, FREQUENT). This is a cohort estimate, not an independently measured frequency across all CA2 genotypes.

Strabismus FREQUENT HP:0000486

Show evidence (1 reference)

PMID:22120147 SUPPORT Human Clinical

"Saccadic abnormalities were present in two, while half of these patients had sensory or accommodative strabismus, and seven had congenital nystagmus."

Half of the cohort corresponds to FREQUENT (30–79%); the allele-specific cohort does not establish a rate for every genotype.

Genitourinary 3

Renal tubular acidosis HP:0001947

Renal tubular acidosis may be proximal, distal or mixed. The Puerto Rican exon 7 frameshift proband had no detectable acidosis, whereas six later patients with the same allele did. A missing renal component does not exclude CA2 deficiency.

Show evidence (3 references)

PMID:12566520 SUPPORT Human Clinical

"Most often, the RTA is of mixed proximal and distal type, but kindreds are reported in which either distal or proximal RTA predominates."

The basis for binding the parent term rather than a proximal or distal child term.

PMID:8128957 SUPPORT Human Clinical

"Six subsequent Hispanic patients of Caribbean ancestry, all of whom had osteopetrosis and renal tubular acidosis but who varied widely in clinical severity, were found to be homozygous for the same mutation."

Supports the association while the same paper documents absence of detectable RTA in its index patient.

PMID:38655726 SUPPORT Human Clinical

"Further evaluation showed renal tubular acidosis type 2."

A case in which the tubular defect was proximal (type 2), illustrating the segment-level variation the parent-term binding accommodates.

Nephrocalcinosis HP:0000121

Show evidence (1 reference)

PMID:9453381 SUPPORT Human Clinical

"At the age of 14 months, ultrasound studies in the girl showed medullary nephrocalcinosis which has not been previously described in association with CA II, while cerebral CT scan revealed dilated ventricles."

The first report of nephrocalcinosis in this syndrome.

Nephrolithiasis HP:0000787

Show evidence (1 reference)

PMID:9453381 SUPPORT Human Clinical

"The patient from family 2, who had two previously reported affected siblings, developed bilateral recurrent renal stones and hypercalciuria but no nephrocalcinosis."

Documents stones and hypercalciuria with intrafamilial variability; it does not supply a disease-wide frequency.

Head and Neck 8

Amelogenesis imperfecta HP:0000705

Show evidence (1 reference)

PMID:37662627 SUPPORT Human Clinical

"The oral manifestations included anterior open bite, posterior crossbite, tooth eruption impairment, and hypoplastic amelogenesis imperfecta (AI)."

The single report of amelogenesis imperfecta in a CA2-confirmed patient.

Delayed eruption of teeth HP:0000684

Show evidence (1 reference)

PMID:37662627 SUPPORT Human Clinical

"The oral manifestations included anterior open bite, posterior crossbite, tooth eruption impairment, and hypoplastic amelogenesis imperfecta (AI)."

Records tooth eruption impairment in a CA2-confirmed patient.

Broad forehead HP:0000337

Show evidence (1 reference)

PMID:22120147 SUPPORT Human Clinical

"All patients had osteopetrosis, renal tubular acidosis, developmental delay, short stature and craniofacial disproportion with large cranial vault and broad forehead."

Craniofacial disproportion with broad forehead in 23/23 patients.

Microcephaly HP:0000252

Show evidence (1 reference)

PMID:37662627 SUPPORT Human Clinical

"She presented with osteopetrosis, microcephaly, cerebral calcifications, developmental delay, short stature, limb malformations, and recurrent fractures of the femur, tibia, and fibula"

Direct clinical description of the individual case.

Anterior open-bite malocclusion HP:0009102

Show evidence (1 reference)

PMID:37662627 SUPPORT Human Clinical

"The oral manifestations included anterior open bite, posterior crossbite, tooth eruption impairment, and hypoplastic amelogenesis imperfecta (AI)."

Direct oral findings in one affected individual.

Persistence of primary teeth HP:0006335

Show evidence (1 reference)

PMID:37662627 SUPPORT Human Clinical

"Later, at age 17, she presented with retained primary and impacted permanent teeth."

Direct longitudinal dental finding.

Multiple impacted teeth HP:0001571

Show evidence (1 reference)

PMID:37662627 SUPPORT Human Clinical

"Later, at age 17, she presented with retained primary and impacted permanent teeth."

The plural impacted permanent teeth supports this binding.

Dental crowding HP:0000678

Show evidence (1 reference)

PMID:37662627 SUPPORT Human Clinical

"She presented mixed dentition with generalized enamel hypoplasia, high-arched and ogival palate, anterior open bite, posterior crossbite, crowded teeth, ectopic eruption, and altered tooth shape."

Direct dental examination in the Brazilian case.

Metabolism 2

Hyperchloremic metabolic acidosis HP:0004918

Show evidence (1 reference)

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"In CA II deficiency, OPT is uniquely accompanied by renal tubular acidosis (RTA) of proximal, distal, or combined type featuring hyperchloremic metabolic acidosis, rarely with hypokalemia and paralysis."

Names hyperchloremic metabolic acidosis as the acid-base phenotype.

Hypokalemia HP:0002900

Show evidence (1 reference)

PMID:38328522 SUPPORT Human Clinical

"On the fifth day of admission, he was reported to have a low potassium level again (2.9 mmol/L)."

Documents recurrent low potassium in the reported patient without assigning every episode solely to CA2.

Musculoskeletal 3

Osteopetrosis HP:0011002

Show evidence (1 reference)

PMID:22120147 SUPPORT Human Clinical

"All patients had osteopetrosis, renal tubular acidosis, developmental delay, short stature and craniofacial disproportion with large cranial vault and broad forehead."

Osteopetrosis in 23/23 patients from 10 unrelated families.

Cerebral calcification FREQUENT HP:0002514

Show evidence (2 references)

PMID:39667299 SUPPORT Human Clinical

"Optic nerve atrophy was observed in 22.2 %, while brain calcifications were present in 70.0 % of cases."

Frequency of brain calcification in a consecutive pediatric series.

PMID:22120147 SUPPORT Human Clinical

"Brain calcification was generally progressive and followed a distinct distribution, involving predominantly basal ganglia and thalami and grey-white matter junction in frontal regions more than posterior regions."

Establishes the anatomical distribution recorded in the preferred_term.

Recurrent fractures HP:0002757

Show evidence (2 references)

PMID:15300855 SUPPORT Human Clinical

"Other features include developmental delay, short stature, cognitive defects, and a history of multiple fractures by adolescence."

Records multiple fractures by adolescence as a recognised feature.

PMID:1301935 SUPPORT Human Clinical

"The presence of mental retardation and relative infrequency of skeletal fractures distinguish the clinical course of the patients with the Arabic mutation from those of the American and Belgian patients with the His 107-->Tyr mutation."

Genotype-dependent fracture frequency - the basis for the qualification in the description.

Nervous System 6

Global developmental delay FREQUENT HP:0001263

Show evidence (1 reference)

PMID:39667299 SUPPORT Human Clinical

"For neurodevelopmental symptoms, 60.0 % of patients presented with global developmental delay, 20.0 % had intellectual disability, and the remaining 20.0 % had normal development."

Quantifies developmental delay and, importantly, records that 20% had normal development - so this is frequent, not obligate.

Intellectual disability HP:0001249

Show evidence (3 references)

PMID:22120147 SUPPORT Human Clinical

"Mental retardation was present in approximately two-thirds and varied from mild to severe."

Frequency and severity range in 23 patients from 10 families.

PMID:7959703 SUPPORT Human Clinical

"All our patients were homozygous for this mutation and were mentally retarded, a characteristic feature of the phenotype of patients with an Arabic background."

Ties cognitive involvement to the intron 2 founder genotype.

PMID:9580777 SUPPORT Human Clinical

"We describe an Italian boy with this disease whose mental retardation was relatively mild and whose renal tubular acidosis had only a distal component."

Documents the mild end of the cognitive spectrum alongside a distal-predominant renal phenotype.

Cranial nerve compression HP:0001293

The deep-research report offered HP:0031815 for this phenotype. That CURIE resolves to "Abnormal oral physiology" - the report's own Term Validation section flagged it as naming a different term - so HP:0001293 was looked up independently and bound instead.

Show evidence (1 reference)

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"The disorder presents late in infancy or early in childhood with fracturing, developmental delay, weakness, short stature, and/or cranial nerve compression and palsy."

Names cranial nerve compression and palsy as a presenting feature.

Obstructive sleep apnea HP:0002870

Show evidence (1 reference)

PMID:30109220 SUPPORT Human Clinical

"Herein we report a case of osteopetrosis with RTA associated with severe OSAS successfully treated with continuous positive airway pressure (CPAP)."

The single reported case linking this syndrome to severe OSA.

Hydrocephalus HP:0000238

Show evidence (1 reference)

PMID:38655726 SUPPORT Human Clinical

"presented at 8-month-old with hydrocephaly, brain shunt, and developmental delay."

Individual clinical observation; no general frequency or mechanism is inferred.

Seizure HP:0001250

Show evidence (1 reference)

PMID:35035649 SUPPORT Human Clinical

"During the hospitalization period, he developed a single seizure attack characterized as tonic seizure associated with up rolling of the eyes and cyanosis which was relieved without medications."

Reports the event and its clinical context rather than attributing it specifically to calcification.

Growth 1

Short stature HP:0004322

Show evidence (1 reference)

PMID:22120147 SUPPORT Human Clinical

"All patients had osteopetrosis, renal tubular acidosis, developmental delay, short stature and craniofacial disproportion with large cranial vault and broad forehead."

Short stature in 23/23 patients.

🧬

Genetic Associations

1

CA2

Gene: CA2 hgnc:1373 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (5 references)

PMID:15300855 SUPPORT Human Clinical

"With one exception, all patients with osteopetrosis and renal tubular acidosis examined have proven to have CA II deficiency. All CA II-deficient patients analyzed have been found to have mutations in the CA2 gene."

The bidirectional gene-disease specificity statement, and the reason this is curated as a single disease rather than a grouping.

PMID:6410391 SUPPORT In Vitro

"Segregation analysis of the molecular marker and chromosomes in cell hybrids indicated a clear correlation between the presence of chromosome 8 and the human CA II gene (CA2)."

Somatic cell hybrid mapping of CA2 to chromosome 8, the step that made molecular diagnosis possible.

PMID:15300855 SUPPORT Human Clinical

"These mutations were scattered over the genome from exon 2 to 7."

Establishes that pathogenic variants are distributed across the gene rather than clustering in one domain, which is why diagnostic sequencing covers all seven exons.

+ 2 more references

Variants (5)

c.232+1G>A (intron 2 founder splice-donor variant) Pathogenic

splice site

The recurrent intron 2 splice-donor allele is explicitly reported as NM_000067.3:c.232+1G>A in the Jordanian case and as c.232+1G>A in other primary clinical reports. Earlier series associated it with developmental impairment and fewer fractures, but the 2024 boy had age-appropriate development and recurrent low-trauma fractures. The Iranian transplant abstract instead reports c.232+1G>T; that discordant substitution should not be silently equated with c.232+1G>A.

Show evidence (4 references)

PMID:1301935 SUPPORT Human Clinical

"We have analyzed DNAs from members of six unrelated Arabic kindreds and found five to be homozygous and one heterozygous for a novel splice junction (donor site) mutation at the 5' end of intron 2."

Original description of the intron 2 splice-junction variant.

PMID:7959703 SUPPORT Human Clinical

"This mutation is found exclusively in patients with an Arabic background and thus may be confined to this ethnic group."

Describes the population distribution observed in that historical series, not proof that the allele is exclusive to an ancestry today.

PMID:35035649 SUPPORT Human Clinical

"Whole exome sequencing confirms the diagnosis of CA II deficiency with homozygous mutation (CA2 NM_000067.3:c.232+1G>A) inherited in an autosomal recessive manner."

The full clinical report supplies explicit transcript-level nomenclature for the recurrent splice allele.

+ 1 more reference

His107Tyr (p.H107Y) Pathogenic

missense

C-to-T transition substituting tyrosine for the invariant histidine at position 107, disrupting hydrogen bonding to Glu-117 and Tyr-194 and destabilising the protein. Unusually among CA2 alleles it leaves detectable residual activity, which has been proposed as the explanation for the milder phenotype and absent cognitive impairment in the families that carry it.

Show evidence (3 references)

PMID:1928091 SUPPORT Human Clinical

"The change is a C-to-T transition which results in the substitution of Tyr (TAT) for His (CAT) at position 107."

Original identification of the variant in a consanguineous Belgian family.

PMID:1542674 SUPPORT In Vitro

"We show that the mutant enzyme expressed in bacteria from the cDNA containing the His-107----Tyr mutation has detectable, though greatly reduced, activity."

Recombinant expression establishing that this allele is hypomorphic rather than null.

PMID:1542674 SUPPORT Human Clinical

"The three affected siblings from this family are compound heterozygotes, each having inherited two different mutations in the structural gene for carbonic anhydrase II."

Documents compound heterozygosity (His107Tyr with an intron 5 splice acceptor variant) in the index American family.

Exon 7 single-base deletion (frameshift) Pathogenic

frameshift

Single-base deletion in exon 7 producing a frameshift and premature termination, homozygous in seven unrelated Hispanic families of Caribbean ancestry. The truncated 27-kD product is inactive, aggregates, and is rapidly degraded.

Show evidence (1 reference)

PMID:8128957 SUPPORT Human Clinical

"She proved to be homozygous for a single-base deletion in the coding region of exon 7 that produces a frameshift that changes the next 12 amino acids before leading to chain termination"

Molecular description of the Caribbean Hispanic founder allele.

c.368G>A p.W123X (nonsense) Pathogenic

nonsense

Homozygous c.368G>A, p.Trp123Ter was reported in a Chinese proband. The study combined structural analysis with a transfected HEK293T expression model. The abstract does not separate all modeled zinc-binding predictions from experimental protein-expression findings, so the specific structural mechanism remains qualified.

Show evidence (2 references)

PMID:33555497 SUPPORT Human Clinical

"Sequencing and genetic analysis have revealed the homozygous nonsense mutation of CA2 gene (c.368G > A, p.W123X) in the exon 4 of chromosome 8 of the proband"

Identification of the variant in the proband and its absence in unaffected relatives.

PMID:33555497 SUPPORT INDIRECT Computational

"Properties and structure of mutated CA2 proteins changed after mutation, especially in change of protein modification and hindrance of zinc ions binding"

Used for the structural-analysis hypothesis about zinc binding, not as a direct zinc-binding assay or a quantitative protein-expression result. The connection to enzyme dysfunction is inferential.

c.753delG, p.Asn252Thrfs*14 Likely Pathogenic

frameshift

Homozygous exon 7 deletion reported in the Brazilian woman with generalized hypoplastic enamel and other CA II-deficiency findings. Her mother was heterozygous; the father was unavailable for testing. The report notes prior observation in two Mexican siblings.

Show evidence (1 reference)

PMID:37662627 SUPPORT Human Clinical

"Molecular analysis revealed a CA2 homozygous deletion (c.753delG, p.Asn252Thrfs*14) and confirmed the clinical diagnosis."

The molecularly characterized Brazilian dental case carried this homozygous frameshift; the full report classified it as likely pathogenic.

💊

Medical Actions

9

Oral alkali therapy

Action: Pharmacotherapy NCIT:C15986

Agent: sodium bicarbonate CHEBI:32139

Platform: Small molecule

Oral bicarbonate or citrate treats the acid-base disturbance. In the child reported with obstructive sleep apnea, sodium bicarbonate and potassium citrate improved blood acidosis and bicarbonate. Earlier treatment was proposed to explain milder outcomes in a separate cohort, but that comparison does not establish a disease-modifying neurological effect.

Mechanism Target:

MODULATES Systemic Hyperchloremic Metabolic Acidosis — Exogenous alkali can improve systemic acidosis without correcting the underlying genotype.

Show evidence (1 reference)

PMID:30109220 SUPPORT Human Clinical

"Oral therapy with sodium bicarbonate and potassium citrate was started with improvement of blood acidosis (Table 1) and blood HCO3-."

Direct clinical response supports correction of acidosis; it does not establish normalization of all disease manifestations.

Show evidence (2 references)

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"Several mouse models of CA II deficiency have shown growth hormone deficiency, yet currently there is no standard pharmacologic therapy for patients."

Establishes that no pharmacologic therapy is standardised for this disease - the reason alkali therapy is described here as the mainstay for one arm rather than as a protocol.

PMID:30109220 SUPPORT Human Clinical

"Oral therapy with sodium bicarbonate and potassium citrate was started with improvement of blood acidosis (Table 1) and blood HCO3-."

Direct clinical response supports correction of acidosis; it does not establish normalization of all disease manifestations.

Allogeneic hematopoietic stem cell transplantation

Action: allogeneic hematopoietic stem cell transplantation Ontology label: Hematopoietic Cell Transplantation NCIT:C15431

Platform: Cell therapy

Allogeneic transplantation supplies donor hematopoietic precursors and can resolve osteopetrosis. In two related Irish families, two transplanted children had histological and radiographic skeletal improvement with stabilized hearing and vision at two and three years. Renal tubular acidosis and developmental delay persisted; the older transplanted child subsequently developed cerebral calcification. The authors suggested delayed calcification, which is distinct from proving neurological correction or proving that transplantation has no cerebral effect. A later Iranian child had improving vision and complete donor chimerism at twelve months. These small reports do not establish routine use for every patient.

Mechanism Target:

RESTORES Osteoclast Resorption Lacuna Acidification Failure — Donor hematopoietic precursors can restore the osteoclast lineage; skeletal histology and imaging support correction of bone disease rather than a direct measurement of lacunar pH.

Show evidence (1 reference)

PMID:11264157 SUPPORT Human Clinical

"Allogeneic bone marrow stem cell replacement cures the osteoclast component of CAII deficiency and retards the development of cerebral calcification, but it appears to have little or no effect on the renal lesions."

States precisely which arm the transplant corrects and which it does not, which is what makes this an arm-specific mechanism link.

Show evidence (5 references)

PMID:11264157 SUPPORT Human Clinical

"Post-transplantation evaluation at 2 and 3 years demonstrates histologic and radiologic resolution of their osteopetrosis with stabilization of hearing and vision."

Documents resolution of the skeletal disease after transplantation.

PMID:38655726 SUPPORT Human Clinical

"He Engrafted on day +13, and 95% chimerism was achieved."

A recent worked case with matched related donor engraftment, confirming feasibility in this specific genotype.

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"Although CA II deficiency is an "osteoclast-rich" OPT, and therefore transplantation of healthy osteoclasts can improve the skeletal disease, the RTA and central nervous system difficulties persist."

The review summarizes skeletal improvement with persistent renal and central nervous system difficulties; it does not establish that every extra-skeletal outcome is unchanged.

+ 2 more references

Continuous positive airway pressure for obstructive sleep apnea

Action: continuous positive airway pressure Ontology label: Continuous Positive Airway Pressure NCIT:C124040

Platform: Device

CPAP for craniofacial obstructive sleep apnea. Reported in a single child in whom adherence built up gradually from a minimal starting pressure. Relevant because the airway obstruction in this disease is skeletal in origin, so adenotonsillectomy alone may not resolve it - in the reported family the twin had a severe respiratory complication after that surgery.

Target Phenotypes: Obstructive sleep apnea HP:0002870

Show evidence (1 reference)

PMID:30109220 SUPPORT Human Clinical

"Herein we report a case of osteopetrosis with RTA associated with severe OSAS successfully treated with continuous positive airway pressure (CPAP)."

The single report of CPAP use in this disease.

Ophthalmologic surveillance for compressive optic neuropathy

Action: serial ophthalmologic examination and optic canal assessment Ontology label: Eye Examination NCIT:C38060

Ophthalmologic assessment is relevant because optic nerve involvement can accompany canal narrowing, and progressive visual loss prompted transplantation in reported patients. The clinical series supports the indication but does not establish a surveillance interval.

Target Phenotypes: Optic atrophy HP:0000648

Show evidence (5 references)

PMID:22120147 SUPPORT Human Clinical

"Globes and retinae were normal, but optic nerve involvement was present in 23/46 eyes and was variable in severity, random in occurrence and statistically correlated with degree of optic canal narrowing"

The case for surveillance rather than symptom-driven referral: half of eyes affected, occurrence random so not predictable from the rest of the phenotype, and a measurable bony parameter that tracks it.

PMID:39667299 SUPPORT Human Clinical

"Optic nerve atrophy was observed in 22.2 %, while brain calcifications were present in 70.0 % of cases."

Frequency of optic nerve atrophy in a consecutive pediatric series, which is the prevalence that justifies routine rather than selective assessment.

PMID:11264157 SUPPORT Human Clinical

"Two children, one from each family, have undergone allogeneic bone marrow transplantation because of severe progressive visual and hearing loss."

Surveillance has a consequence here: detected progressive visual loss was the stated indication for escalating to transplantation in these families.

+ 2 more references

Dental and orthodontic care

Action: restorative and orthodontic dental procedures Ontology label: Dental Procedure NCIT:C38052

Dental evaluation and appropriate restorative or orthodontic care address eruption disturbances, malocclusion and enamel defects. The clinical evidence characterizes the indication; it does not establish a tested CA2-specific dental treatment protocol.

Target Phenotypes: Amelogenesis imperfecta HP:0000705 Delayed eruption of teeth HP:0000684

Show evidence (2 references)

PMID:37662627 SUPPORT Human Clinical

"The oral manifestations included anterior open bite, posterior crossbite, tooth eruption impairment, and hypoplastic amelogenesis imperfecta (AI)."

The specific oral findings this care addresses - two malocclusion patterns, impaired eruption, and an enamel defect.

PMID:37662627 SUPPORT Human Clinical

"This study suggests that AI can be another feature of CAII deficiency syndrome."

The authors' own hedged statement that amelogenesis imperfecta belongs to this syndrome. Quoted as hedged because it rests on one patient.

Genetic counseling, carrier testing and prenatal diagnosis

Action: Genetic Counseling NCIT:C15240

Counseling can explain recessive inheritance and support informed reproductive choices. Familial CA2 variant testing enables carrier and prenatal diagnosis. Historical erythrocyte enzyme studies also detected reduced activity in obligate carriers.

Show evidence (3 references)

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"Prenatal diagnosis requires mutational analysis of CA2."

Establishes that prenatal diagnosis is molecular.

PMID:15300855 SUPPORT Human Clinical

"In two opportunities for prenatal diagnosis, one from cultured amniocytes and one from chorionic villus biopsy, we demonstrated the general utility of the direct sequencing method for prenatal DNA diagnosis."

Documents prenatal diagnosis on both sample types.

PMID:9580777 SUPPORT Human Clinical

"The identification of the mutation in this family provided the opportunity for prenatal diagnosis in a subsequent pregnancy."

A worked instance of family-specific prenatal diagnosis.

Closed fracture reduction and cast immobilization

Action: closed fracture reduction and cast immobilization Ontology label: Therapeutic Procedure NCIT:C49236

Platform: Other

Closed reduction with an above-knee cast was used after medical stabilization in the 2024 child with bilateral tibial fractures. The report describes subsequent healing; it does not establish a comparative orthopedic protocol.

Target Phenotypes: Recurrent fractures HP:0002757

Show evidence (1 reference)

PMID:38328522 SUPPORT Human Clinical

"A closed reduction and above-knee cast was applied after stabilization."

Directly reported orthopedic treatment in this patient.

Potassium replacement for hypokalemia

Action: Pharmacotherapy NCIT:C15986

Agent: potassium chloride CHEBI:32588

Platform: Small molecule

Potassium chloride was given for recurrent hypokalemia during the 2024 admission. This is correction of an electrolyte complication, not evidence of modifying the underlying CA2 disorder.

Show evidence (1 reference)

PMID:38328522 SUPPORT Human Clinical

"He was started on potassium chloride (KCl) 30 mEq/L added to intravenous (IV) fluid."

Documents use in the individual report; no general efficacy estimate is inferred.

Vitamin D supplementation

Action: Pharmacotherapy NCIT:C15986

Agent: vitamin D3 CHEBI:28940

Platform: Small molecule

Vitamin D3 supplementation was included in the Jordanian child’s discharge treatment. The report does not establish a CA2-specific skeletal or neurological benefit.

Show evidence (1 reference)

PMID:35035649 SUPPORT Human Clinical

"The patient was discharged on sodium bicarbonate therapy, lactulose and vitamin D3 supplements and has been followed up regularly."

Documents use in the individual report; no general efficacy estimate is inferred.

🔬

Biochemical Markers

1

Erythrocyte carbonic anhydrase II (Virtually absent in affected individuals by HPLC peak, esterase activity, CO2 hydratase activity and immunoprecipitation; approximately half-normal in asymptomatic obligate heterozygotes; carbonic anhydrase I unaffected.)

Pathograph Readouts

Readout Of pathophysiology#Biallelic CA2 Loss of Function Present Absent

Absence of erythrocyte CA II establishes the enzymatic diagnosis; the half-normal carrier level makes the same assay a carrier test.

Show evidence (1 reference)

PMID:6405388 SUPPORT Human Clinical

"We report here biochemical and immunological evidence for the virtual absence of CA II in erythrocytes of patients affected with this condition, whereas CA I level is not reduced."

The measurement behind this readout, and the isoenzyme specificity that makes an absent result interpretable.

Show evidence (2 references)

PMID:3925334 SUPPORT Human Clinical

"Reduced levels of isozyme II were found in obligate heterozygotes."

Supports the carrier-detection use of the assay.

PMID:3925334 SUPPORT Human Clinical

"Virtual absence of the carbonic anhydrase II peak on high-performance liquid chromatography, of the esterase and carbon dioxide hydratase activities of carbonic anhydrase II, and of immunoprecipitable isozyme II was demonstrated on extracts of erythrocyte hemolysates from all patients studied."

Establishes absence of the enzyme by four concordant measurements in all patients studied.

🔬

Diagnosis

3

Clinical and radiographic recognition

Osteopetrosis, renal tubular acidosis and cerebral calcification form the classic clinical pattern. The complete triad is not required at assessment, and the pattern alone is not fully gene-specific. Skeletal imaging, acid-base studies and brain imaging characterize the presentation.

Show evidence (2 references)

PMID:30109220 SUPPORT Human Clinical

"The deficiency of carbonic anhydrase II causes a moderate form, presenting classically as a triad of osteopetrosis, renal tubular acidosis (RTA), and cerebral calcification."

States the classical triad.

PMID:8128957 SUPPORT Human Clinical

"The proband was a 2 1/2-year-old Hispanic girl of Puerto Rican ancestry who was unique clinically, in that she had no evidence of renal tubular acidosis, even though she did have osteopetrosis, developmental delay, and cerebral calcification."

The clinical triad is incomplete in this molecularly confirmed patient, so all three features are not required for recognition.

Molecular confirmation

Identify biallelic disease-causing CA2 variants and interpret them with the clinical findings. Published methods include sequencing all seven exons and whole-exome sequencing; splice and truncating alleles require variant-specific assessment.

Show evidence (2 references)

PMID:15300855 SUPPORT Human Clinical

"we amplified all seven exons by PCR from genomic DNA and directly sequenced the amplified products"

The molecular confirmation method, covering all seven exons.

PMID:1542674 SUPPORT Human Clinical

"The three affected siblings from this family are compound heterozygotes, each having inherited two different mutations in the structural gene for carbonic anhydrase II."

Molecular diagnosis includes compound heterozygosity, not only homozygous variants.

Erythrocyte CA II assay

Marked reduction of CA II protein and activity with preserved CA I supports the enzymatic diagnosis. Erythrocyte measurements do not establish enzyme activity in every tissue.

Show evidence (1 reference)

PMID:6405388 SUPPORT Human Clinical

"We report here biochemical and immunological evidence for the virtual absence of CA II in erythrocytes of patients affected with this condition, whereas CA I level is not reduced."

The measurement behind this readout, and the isoenzyme specificity that makes an absent result interpretable.

📈

Progression

3

Presentation in late infancy to early childhood

Age: late infancy to early childhood

Onset is insidious rather than acute. Diagnosis is usually prompted by fracture, failure to thrive, developmental concern, or an incidental radiograph.

Show evidence (1 reference)

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"The disorder presents late in infancy or early in childhood with fracturing, developmental delay, weakness, short stature, and/or cranial nerve compression and palsy."

Gives both the age window and the presenting features.

Progressive accrual of intracranial calcification through childhood

Age: childhood

Serial imaging over up to 10 years in a 23-patient cohort showed progressive, regionally patterned calcification. Absence of calcification in childhood does not exclude the diagnosis.

Show evidence (2 references)

PMID:22120147 SUPPORT Human Clinical

"Brain calcification was generally progressive and followed a distinct distribution, involving predominantly basal ganglia and thalami and grey-white matter junction in frontal regions more than posterior regions."

Longitudinal imaging establishing the progressive course and its distribution.

PMID:22120147 SUPPORT Human Clinical

"At least one child had no brain calcification at age 9 years, indicating that brain calcification may not always be present in carbonic anhydrase type II deficiency syndrome during childhood."

Bounds the claim: the calcification arm is age-dependent and not obligate in childhood, which matters for using imaging as a diagnostic criterion.

Skeletal improvement in adult life

Age: adulthood

The skeletal arm tends to stabilise or improve with age, which is why this is classed an intermediate rather than malignant osteopetrosis. The renal tubular acidosis persists.

Show evidence (1 reference)

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"The skeletal findings may improve by adult life, and CA II deficiency can be associated with a normal life-span."

States the favourable skeletal trajectory and lifespan directly.

📊

Prevalence

2

Worldwide, cumulative case reports

Cases In Literature Ultra Rare

More than 100 affected individuals reported as of 2023, concentrated in the Middle East and Mediterranean basin, which reflects consanguinity and the intron 2 founder allele rather than a true regional prevalence estimate. No population-based prevalence or incidence figure has been published.

Show evidence (1 reference)

PMID:36709914 SUPPORT INDIRECT REVIEW SYNTHESIS Other

"CA II deficiency now describes >100 affected individuals encountered predominantly from the Middle East and Mediterranean region."

Cumulative reported-case count and geographic concentration.

North African and Middle Eastern countries

Cases In Literature Ultra Rare

A 1992 tally of the then-39 reported cases found 72% from North African and Middle Eastern countries. This is a proportion among ascertained cases, not a population rate, and is reported here only to document the geographic skew.

Show evidence (1 reference)

PMID:1301935 SUPPORT Human Clinical

"Of the 39 reported cases of the carbonic anhydrase II deficiency syndrome, 72% were patients from North African and Middle Eastern countries, most, if not all, of whom were of Arabic descent."

Quantifies the geographic distribution of ascertained cases.

🧫

Experimental Models

3

Recombinant His107Tyr CA II OTHER

Bacterially expressed mutant CA II retains detectable but greatly reduced catalytic activity.

Publication

PMID:1542674

Pathograph links

Biallelic CA2 Loss of Function partially recapitulates observes: molecular Bacterially expressed mutant CA II retains detectable but greatly reduced catalytic activity.

Limitations: Residual enzyme activity in a heterologous system does not measure patient-tissue protein abundance or establish the cause of milder cognition.

Show evidence (1 reference)

PMID:1542674 SUPPORT In Vitro

"We show that the mutant enzyme expressed in bacteria from the cDNA containing the His-107----Tyr mutation has detectable, though greatly reduced, activity."

Direct biochemical activity assay of the recombinant allele.

Show evidence (1 reference)

PMID:1542674 SUPPORT In Vitro

"We show that the mutant enzyme expressed in bacteria from the cDNA containing the His-107----Tyr mutation has detectable, though greatly reduced, activity."

Direct biochemical activity assay of the recombinant allele.

CA2 exon 7 frameshift expression in COS cells CELL_LINE

The patient-associated exon 7 frameshift produces a truncated, inactive protein with aggregation and accelerated degradation in transfected COS cells.

Publication

PMID:8128957

Pathograph links

Biallelic CA2 Loss of Function partially recapitulates observes: molecular The patient-associated exon 7 frameshift produces a truncated, inactive protein with aggregation and accelerated degradation in transfected COS cells.

Limitations: Overexpression in COS cells measures allele-specific processing and activity, not human osteoclast resorption or renal transport.

Show evidence (1 reference)

PMID:8128957 SUPPORT In Vitro

"The 27-kD truncated enzyme produced when the mutant cDNA was expressed in COS cells was enzymatically inactive, present mainly in insoluble aggregates, and detectable immunologically at only 5% the level of the 29-kD normal carbonic anhydrase II expressed from the wild-type cDNA."

Direct expression and biochemical measurements in a heterologous cell line.

Show evidence (2 references)

PMID:8128957 SUPPORT In Vitro

"The 27-kD truncated enzyme produced when the mutant cDNA was expressed in COS cells was enzymatically inactive, present mainly in insoluble aggregates, and detectable immunologically at only 5% the level of the 29-kD normal carbonic anhydrase II expressed from the wild-type cDNA."

Direct expression and biochemical measurements in a heterologous cell line.

PMID:8128957 SUPPORT In Vitro

"Metabolic labeling revealed that this 27-kD mutant protein has an accelerated rate of degradation."

Pulse-labeling provides evidence of altered turnover in the cell-expression model.

Purified Gly145Arg and His94Tyr CA II proteins OTHER

Site-directed mutant proteins were expressed and purified for spectroscopic stability and activity studies; both showed local structural changes and reduced native-state stability.

Publication

PMID:31778702

Pathograph links

Biallelic CA2 Loss of Function partially recapitulates observes: molecular Site-directed mutant proteins were expressed and purified for spectroscopic stability and activity studies; both showed local structural changes and reduced native-state stability.

Limitations: Protein stability assays do not reproduce tissue-selective clinical disease and should not be treated as measurements in patient organs.

Show evidence (1 reference)

PMID:31778702 SUPPORT In Vitro

"Studies suggest that the mutant proteins undergo local structural perturbations and have compromised native state stability."

Purified-protein studies used circular dichroism, fluorescence, NMR and activity assays.

Show evidence (1 reference)

PMID:31778702 SUPPORT In Vitro

"Studies suggest that the mutant proteins undergo local structural perturbations and have compromised native state stability."

Purified-protein studies used circular dichroism, fluorescence, NMR and activity assays.

🐁

Animal Models

2

Car-2 null mouse (ENU-induced)

ENU-induced Car2-null mice lack detectable CA II, develop growth restriction and renal tubular acidosis, and did not show osteopetrosis even in aged homozygotes. This establishes a model of selected renal and growth findings with an important skeletal divergence.

Species

Mouse

Genotype

Car-2 null (ENU-induced, homozygous)

Background

(C57BL/6J x DBA/2J)F1-derived

Publication

PMID:3126501

Pathograph links

Systemic Hyperchloremic Metabolic Acidosis recapitulates fidelity: moderate observes: organism Homozygous null mice develop renal tubular acidosis, as humans with the same enzyme defect do.

Limitations: Renal tubular acidosis is reproduced, but the founding phenotype does not by itself resolve the proximal and distal contributions. Later kidney studies show collecting-duct cellular and transporter changes whose extent in human CA2 deficiency remains unestablished.

Readouts

Renal tubular acidosis in homozygous null mice

Direction: ALTERED

Interpretation: The acid-base phenotype of the model matches the human renal arm.

Show evidence (1 reference)

PMID:3126501 SUPPORT Model Organism

"Like humans with the same inherited enzyme defect, animals homozygous for the new null allele are runted and have renal tubular acidosis."

Reports the renal tubular acidosis in the null mouse.

Show evidence (1 reference)

PMID:3126501 SUPPORT Model Organism

"The CA II protein, which is normally expressed in most tissues, could not be detected by immunodiffusion analysis in any tissues of the CA II-deficient mice"

Confirms the model is a true protein null, so it is informative for the consequences of losing the enzyme.

Impaired Distal Tubular Proton Secretion partially recapitulates fidelity: moderate observes: cellular Collecting-duct cellular and transporter changes support disruption of distal acid-base transport in Car2-null mice.

Limitations: Intercalated-cell markers and transporter abundance are proxy readouts, not direct measurements of proton secretion. The same cellular depletion has not been established in human kidney.

Readouts

Collecting-duct intercalated cell depletion

Direction: DECREASED

Interpretation: Beyond loss of enzyme activity, the acid-secreting cell population itself is depleted and replaced by principal cells - a cell-population phenotype, not only an enzymatic one.

Show evidence (1 reference)

PMID:8594870 SUPPORT Model Organism

"These results suggest that intercalated cells are severely depleted and are replaced by principal cells in CAII-deficient mice."

Immunocytochemical quantification of intercalated-cell loss in the CAR2-null collecting duct.

Collecting-duct bicarbonate transporter expression

Direction: DECREASED

Interpretation: Pendrin, Slc26a7 and AE1 expression all fall, consistent with loss of the intercalated-cell phenotype rather than an isolated enzyme deficit.

Show evidence (1 reference)

PMID:18209476 SUPPORT Model Organism

"CAII deficiency results in a significant decrease in the gene and protein expression of bicarbonate transport proteins from Slc26 gene family - Slc26a4 (pendrin) and Slc26a7."

Quantifies the transporter expression changes in the null kidney.

Show evidence (2 references)

PMID:8594870 SUPPORT Model Organism

"These results suggest that intercalated cells are severely depleted and are replaced by principal cells in CAII-deficient mice."

Immunocytochemical quantification of intercalated-cell loss in the CAR2-null collecting duct.

+ 1 more reference

Accumulation of Unresorbed Sclerotic Bone with Preserved Osteoclast Number fails to recapitulate fidelity: low observes: organism The defining human phenotype is absent from the mouse null. Osteopetrosis could not be detected even in very old homozygotes.

Limitations: The model does not reproduce overt human osteopetrosis, limiting its use for that skeletal endpoint. This negative organism-level finding does not demonstrate normality of every osteoclast process; the basis of the species difference remains unresolved.

Readouts

Skeletal density in aged homozygous null mice

Direction: UNCHANGED

Interpretation: A genuine negative result: no osteopetrosis at any age examined.

Show evidence (1 reference)

PMID:3126501 SUPPORT Model Organism

"However, the prominent osteopetrosis found in humans with CA II deficiency could not be detected even in very old homozygous null mice."

Direct statement of the negative skeletal finding.

Show evidence (1 reference)

PMID:3126501 REFUTE Model Organism

"However, the prominent osteopetrosis found in humans with CA II deficiency could not be detected even in very old homozygous null mice."

The negative skeletal finding demonstrates non-recapitulation of overt osteopetrosis, not exclusion of all mechanistic value of the model.

Show evidence (1 reference)

PMID:2495727 SUPPORT Model Organism

"The CA II-deficient mice differed from controls in evidencing an age dependent medial calcification of small arteries in a number of organs."

The model does develop an age-dependent ectopic calcification, though arterial rather than cerebral. Recorded here as a characterised feature of the model, not as a model of the human brain phenotype - see the cerebral calcification discussion.

CA II-deficient mice in recombinant growth hormone studies

CA II-deficient acidotic mice were studied for growth-hormone/IGF physiology and randomized recombinant growth hormone response. Reduced GH secretion was observed in males, not females. Treatment increased growth despite persistent acidosis, and the follow-up time-course analysis showed a progressively increasing response. These experiments do not establish human GH deficiency or justify clinical efficacy claims.

Species

Mouse

Publication

PMID:9688969

Show evidence (3 references)

PMID:9688969 SUPPORT Model Organism

"Randomized administration of rGH from approximately 5 to approximately 12 wk to CAD mice improved food efficiency and increased serum IGF-I levels, final length, and weight compared with placebo without affecting blood pH."

Mouse intervention improves growth without correcting acidosis; it is not human treatment evidence.

PMID:9688969 SUPPORT Model Organism

"Thus growth failure in CAD mice is associated with a decrease in GH secretion in males but not in females."

Sex-specific finding limits extrapolation to a universal endocrine mechanism.

PMID:10975300 SUPPORT Model Organism

"Administration of GH increased standardized growth progressively up to 5-9 weeks."

Follow-up analysis emphasizes duration of exposure in the mouse experiment.

{ }

Source YAML

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name: Autosomal Recessive Osteopetrosis 3
creation_date: '2026-09-10T00:00:00Z'
category: Mendelian
description: Autosomal recessive osteopetrosis 3 (ARO3, OPTB3) is carbonic anhydrase II deficiency, caused by biallelic CA2 loss-of-function variants. Reduced CA II activity impairs rapid cytosolic generation of protons and bicarbonate, affecting osteoclast-mediated bone resorption and renal acid-base transport. Osteopetrosis, renal tubular acidosis and cerebral calcification form the classic clinical triad, but renal acidosis or brain calcification can be absent at assessment. The route to cerebral calcification and cognitive impairment remains incompletely understood. Clinical expression varies within and between families, including fractures, growth impairment, visual or hearing abnormalities and developmental difficulties. Skeletal findings may improve in adulthood. Hematopoietic transplantation can correct osteopetrosis in selected reported patients, while renal tubular acidosis and developmental impairment persisted in the early transplant series; delayed cerebral calcification was suggested, not a demonstrated universal neurological cure.
disease_term:
  preferred_term: autosomal recessive osteopetrosis 3
  term:
    id: MONDO:0009818
    label: autosomal recessive osteopetrosis 3
synonyms:
- Carbonic anhydrase II deficiency syndrome
- CA2 deficiency syndrome
- Osteopetrosis with renal tubular acidosis and cerebral calcification
- Marble brain disease
- Guibaud-Vainsel syndrome
- OPTB3
parents:
- Sclerosing Bone Dysplasias
notes: 'CA2 deficiency is a gene-defined disorder distinct from other osteopetroses. Coinherited TCIRG1 and ATP6V1B1 defects can produce osteopetrosis with renal tubular acidosis despite normal CA II, so that clinical combination alone does not establish a CA2 diagnosis. Genotype-associated tendencies are not deterministic: normal psychomotor development and recurrent low-trauma fractures have been reported with the intron 2 c.232+1G>A allele. General osteopetrosis treatment findings require gene-specific interpretation.'
mappings:
  ncit_mappings:
  - term:
      id: NCIT:C118438
      label: Osteopetrosis with Renal Tubular Acidosis
    mapping_predicate: skos:exactMatch
    mapping_source: NCIT
    mapping_justification: NCIT names the same clinical entity by its historical descriptive name.
classifications:
  isds_skeletal_category:
  - classification_value: osteopetrosis_and_related
    notes: ISDS 2023 Table 1, Group 24, lists NOS 24-0110 as osteopetrosis with renal tubular acidosis, CA2-related (AR; MIM 259730). The publisher table was directly inspected. The generated PMID cache captures the group-scope paragraph but its extracted table stops before Group 24; the adjacent cached quote therefore supports the group scope indirectly.
    evidence:
    - reference: PMID:36779427
      reference_title: 'Nosology of genetic skeletal disorders: 2023 revision.'
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: OTHER
      snippet: to highlight the fact that the osteopetroses represent disorders in the number or function of osteoclasts
      explanation: Establishes the scope of the ISDS osteopetrosis group as disorders of osteoclast number or function. CA2 deficiency is such a disorder, so the assignment follows by one inference step rather than from a list entry - the cached record does not name CA2.
inheritance:
- name: Autosomal Recessive
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  expressivity: VARIABLE
  description: Biallelic CA2 variants may be homozygous or compound heterozygous. Clinical expression varies within and between families; the available clinical and enzyme series do not establish population-wide penetrance. Asymptomatic obligate heterozygotes had approximately half-normal erythrocyte CA II in the founding biochemical study.
  evidence:
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: The etiology is bi-allelic loss-of-function mutations of CA2 that encodes CA II.
    explanation: States the biallelic loss-of-function etiology directly.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
  - reference: PMID:6405388
    reference_title: Carbonic anhydrase II deficiency identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Levels of CA II in erythrocyte hemolysates from asymptomatic obligate heterozygotes are about half of normal.
    explanation: Gene-dosage behaviour of the enzyme in asymptomatic carriers, consistent with recessive inheritance and the basis for biochemical carrier testing.
  - reference: PMID:9453381
    reference_title: Nephrocalcinosis and urolithiasis in carbonic anhydrase II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: highlight the inter-/intra-familial variability of the disease in this part of the world despite the overwhelming prevalence of the same
    explanation: 'Supports VARIABLE expressivity: clinical severity differs within and between families carrying the same founder allele.'
prevalence:
- population: Worldwide, cumulative case reports
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: More than 100 affected individuals reported as of 2023, concentrated in the Middle East and Mediterranean basin, which reflects consanguinity and the intron 2 founder allele rather than a true regional prevalence estimate. No population-based prevalence or incidence figure has been published.
  evidence:
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: CA II deficiency now describes >100 affected individuals encountered predominantly from the Middle East and Mediterranean region.
    explanation: Cumulative reported-case count and geographic concentration.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
- population: North African and Middle Eastern countries
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: A 1992 tally of the then-39 reported cases found 72% from North African and Middle Eastern countries. This is a proportion among ascertained cases, not a population rate, and is reported here only to document the geographic skew.
  evidence:
  - reference: PMID:1301935
    reference_title: A splice junction mutation in intron 2 of the carbonic anhydrase II gene of osteopetrosis patients from Arabic countries.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Of the 39 reported cases of the carbonic anhydrase II deficiency syndrome, 72% were patients from North African and Middle Eastern countries, most, if not all, of whom were of Arabic descent.
    explanation: Quantifies the geographic distribution of ascertained cases.
progression:
- phase: Presentation in late infancy to early childhood
  age_range: late infancy to early childhood
  notes: Onset is insidious rather than acute. Diagnosis is usually prompted by fracture, failure to thrive, developmental concern, or an incidental radiograph.
  evidence:
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: The disorder presents late in infancy or early in childhood with fracturing, developmental delay, weakness, short stature, and/or cranial nerve compression and palsy.
    explanation: Gives both the age window and the presenting features.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
- phase: Progressive accrual of intracranial calcification through childhood
  age_range: childhood
  notes: Serial imaging over up to 10 years in a 23-patient cohort showed progressive, regionally patterned calcification. Absence of calcification in childhood does not exclude the diagnosis.
  evidence:
  - reference: PMID:22120147
    reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Brain calcification was generally progressive and followed a distinct distribution, involving predominantly basal ganglia and thalami and grey-white matter junction in frontal regions more than posterior regions.
    explanation: Longitudinal imaging establishing the progressive course and its distribution.
  - reference: PMID:22120147
    reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: At least one child had no brain calcification at age 9 years, indicating that brain calcification may not always be present in carbonic anhydrase type II deficiency syndrome during childhood.
    explanation: 'Bounds the claim: the calcification arm is age-dependent and not obligate in childhood, which matters for using imaging as a diagnostic criterion.'
- phase: Skeletal improvement in adult life
  age_range: adulthood
  notes: The skeletal arm tends to stabilise or improve with age, which is why this is classed an intermediate rather than malignant osteopetrosis. The renal tubular acidosis persists.
  evidence:
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: The skeletal findings may improve by adult life, and CA II deficiency can be associated with a normal life-span.
    explanation: States the favourable skeletal trajectory and lifespan directly.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
pathophysiology:
- name: Biallelic CA2 Loss of Function
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  description: Biallelic splice, nonsense, frameshift or missense variants reduce CA II abundance or activity. Erythrocyte CA II was virtually absent in the founding biochemical series, with CA I preserved, but this does not establish complete loss of activity in every tissue for every allele. Recombinant His107Tyr retains greatly reduced activity. The exon 7 frameshift produces an inactive, unstable truncated protein that accumulates in insoluble aggregates; p.Trp123Ter is a truncating nonsense allele, not a full-length missense product.
  genetic_context:
    gene:
      preferred_term: CA2
      term:
        id: hgnc:1373
        label: CA2
    variant_origin: GERMLINE
    functional_impact_category: LOSS_OF_FUNCTION
    description: Both homozygous and compound-heterozygous genotypes occur; the index American siblings carried His107Tyr in trans with an intron 5 splice-acceptor variant.
  molecular_functions:
  - preferred_term: carbonic anhydrase II (cytosolic CO2 hydratase) activity
    modifier: DECREASED
    term:
      id: GO:0004089
      label: carbonate dehydratase activity
  cell_types:
  - preferred_term: erythrocyte
    term:
      id: CL:0000232
      label: erythrocyte
  chemical_entities:
  - preferred_term: zinc(2+) (catalytic metal of carbonic anhydrase II)
    term:
      id: CHEBI:29105
      label: zinc(2+)
  downstream:
  - target: Reduced CA II-Dependent Proton and Bicarbonate Generation
    causal_link_type: DIRECT
    description: Reduced CA II activity slows the enzyme-catalyzed hydration of CO2; it does not abolish all cellular production of protons or bicarbonate.
    evidence:
    - reference: PMID:6405388
      reference_title: Carbonic anhydrase II deficiency identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: we postulated a defect in carbonic anhydrase II (CA II), the only one of the three soluble isozymes of carbonic anhydrase that is known to be synthesized in kidney and brain
      explanation: The founding study identifies deficiency of this isoenzyme; it does not establish absence of every compensating activity in all tissues.
  - target: Intracranial Parenchymal Calcium Deposition
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: Cerebral calcification is an observed component of CA2 deficiency, but the intermediate route from enzyme dysfunction to regional mineral deposition is unresolved.
    evidence:
    - reference: PMID:36709914
      reference_title: Carbonic anhydrase II deficiency.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: Although this enzymopathy reveals how CA II is important for the skeleton and kidney tubule, the pathogenesis of the mental subnormality and cerebral calcification is less well understood.
      explanation: Supports the association while explicitly stating that the pathogenesis of the cerebral arm is not understood, which is why the edge is typed as indirect with unknown intermediates.
      quote_role: REVIEW_SYNTHESIS
      directness: INDIRECT
  evidence:
  - reference: PMID:3925334
    reference_title: Carbonic anhydrase II deficiency in 12 families with the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Virtual absence of the carbonic anhydrase II peak on high-performance liquid chromatography, of the esterase and carbon dioxide hydratase activities of carbonic anhydrase II, and of immunoprecipitable isozyme II was demonstrated on extracts of erythrocyte hemolysates from all patients studied.
    explanation: Demonstrates absence of the enzyme by three independent methods across 18 patients from 11 unrelated families.
  - reference: PMID:6405388
    reference_title: Carbonic anhydrase II deficiency identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: We report here biochemical and immunological evidence for the virtual absence of CA II in erythrocytes of patients affected with this condition, whereas CA I level is not reduced.
    explanation: The founding observation, and establishes that the deficiency is isoenzyme-specific rather than a general carbonic anhydrase failure.
  - reference: PMID:8128957
    reference_title: 'Carbonic anhydrase II deficiency: single-base deletion in exon 7 is the predominant mutation in Caribbean Hispanic patients.'
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: The 27-kD truncated enzyme produced when the mutant cDNA was expressed in COS cells was enzymatically inactive, present mainly in insoluble aggregates, and detectable immunologically at only 5% the level of the 29-kD normal carbonic anhydrase II
    explanation: Expression study showing one route to loss of function - truncation with aggregation and accelerated degradation.
  - reference: PMID:1928091
    reference_title: 'Carbonic anhydrase II deficiency syndrome in a Belgian family is caused by a point mutation at an invariant histidine residue (107 His----Tyr): complete structure of the normal human CA II gene.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: His-107 appears to have a stabilizing function in the structure of all CA molecules, and its substitution by Tyr apparently disrupts the critical hydrogen bonding of His-107 to two other similarly invariant residues, Glu-117 and Tyr-194, resulting in an unstable CA II molecule.
    explanation: A second route to loss of function - destabilisation of a full-length protein by substitution of an invariant residue.
  - reference: PMID:31778702
    reference_title: Effect of disease-linked mutations on the structure, function, stability and aggregation of human carbonic anhydrase II.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: Studies suggest that the mutant proteins undergo local structural perturbations and have compromised native state stability.
    explanation: Purified-protein biophysics confirming reduced native-state stability for two disease-linked missense substitutions.
  - reference: PMID:1542674
    reference_title: Molecular basis of human carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: We show that the mutant enzyme expressed in bacteria from the cDNA containing the His-107----Tyr mutation has detectable, though greatly reduced, activity.
    explanation: Direct residual-activity measurement prevents interpreting all CA2 alleles as complete catalytic nulls.
  genes:
  - preferred_term: CA2
    term:
      id: hgnc:1373
      label: CA2
- name: Reduced CA II-Dependent Proton and Bicarbonate Generation
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  description: 'Reduced CA II activity impairs rapid reversible CO2 hydration to bicarbonate and protons. This limits enzyme-supported substrate supply for osteoclast and renal transport. Other carbonic anhydrase activities can persist: CA II-deficient patients retained an acetazolamide-sensitive renal response. The observed brain phenotype is associated with CA2 deficiency, but its intermediate mechanism is unresolved.'
  molecular_functions:
  - preferred_term: cytosolic CO2 hydration to proton and bicarbonate
    modifier: DECREASED
    term:
      id: GO:0004089
      label: carbonate dehydratase activity
  biological_processes:
  - preferred_term: one-carbon (CO2/bicarbonate) interconversion
    modifier: DECREASED
    term:
      id: GO:0006730
      label: one-carbon metabolic process
  chemical_entities:
  - preferred_term: carbon dioxide
    term:
      id: CHEBI:16526
      label: carbon dioxide
  - preferred_term: bicarbonate
    term:
      id: CHEBI:17544
      label: hydrogencarbonate
  - preferred_term: proton
    term:
      id: CHEBI:24636
      label: proton
  downstream:
  - target: Osteoclast Resorption Lacuna Acidification Failure
    causal_link_type: DIRECT
    description: Reduced rapid proton generation impairs acidification of the osteoclast resorption compartment.
    evidence:
    - reference: PMID:36709914
      reference_title: Carbonic anhydrase II deficiency.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: CA II deficiency is the paradigm OPT featuring failure of osteoclasts to resorb bone due to inability to acidify their pericellular milieu.
      explanation: States the causal step from enzyme loss to failed pericellular acidification in the osteoclast.
      quote_role: REVIEW_SYNTHESIS
      directness: INDIRECT
  - target: Impaired Proximal Tubular Bicarbonate Reclamation
    causal_link_type: DIRECT
    description: Reduced enzyme activity impairs the proximal component of bicarbonate reclamation.
    evidence:
    - reference: PMID:30109220
      reference_title: 'Carbonic Anhydrase II Deficiency: A Rare Case of Severe Obstructive Sleep Apnea.'
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      directness: INDIRECT
      snippet: This defect causes a mixed form of RTA due to waste of bicarbonates proximally and an incapacity to acidify the urine distally.
      explanation: The case report discussion explicitly describes the proximal bicarbonate-loss and distal acidification components; this is mechanistic synthesis rather than an original segment-specific transport assay.
  - target: Impaired Distal Tubular Proton Secretion
    causal_link_type: DIRECT
    description: Reduced enzyme-supported proton generation impairs distal urinary acidification.
    evidence:
    - reference: PMID:30109220
      reference_title: 'Carbonic Anhydrase II Deficiency: A Rare Case of Severe Obstructive Sleep Apnea.'
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      directness: INDIRECT
      snippet: This defect causes a mixed form of RTA due to waste of bicarbonates proximally and an incapacity to acidify the urine distally.
      explanation: The case report discussion explicitly describes the proximal bicarbonate-loss and distal acidification components; this is mechanistic synthesis rather than an original segment-specific transport assay.
  evidence:
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: 'Carbonic anhydrase II deficiency (OMIM # 259730), initially called "osteopetrosis with renal tubular acidosis and cerebral calcification syndrome", reveals an important role for the enzyme carbonic anhydrase II (CA II) in osteoclast and renal tubule function.'
    explanation: Establishes that the one enzyme's loss is what couples the osteoclast and renal tubule phenotypes.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
- name: Osteoclast Resorption Lacuna Acidification Failure
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: CA II-dependent proton generation supports acidification of the sealed osteoclast resorption compartment. Deficiency impairs this process despite the presence of osteoclasts. The proximal lesion is in intracellular enzyme activity, distinct from variants directly affecting the ruffled-border transport machinery.
  cell_types:
  - preferred_term: osteoclast
    term:
      id: CL:0000092
      label: osteoclast
  biological_processes:
  - preferred_term: acidification of the sealed resorption lacuna
    modifier: DECREASED
    term:
      id: GO:0045851
      label: pH reduction
  - preferred_term: proton secretion across the osteoclast ruffled border
    modifier: DECREASED
    term:
      id: GO:1902600
      label: proton transmembrane transport
  locations:
  - preferred_term: bone tissue
    term:
      id: UBERON:0002481
      label: bone tissue
  downstream:
  - target: Impaired Osteoclastic Bone Resorption
    causal_link_type: DIRECT
    description: Hydroxyapatite dissolution is acid-dependent, so a lacuna that is not acidified does not release bone mineral.
    evidence:
    - reference: PMID:36709914
      reference_title: Carbonic anhydrase II deficiency.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: CA II deficiency is the paradigm OPT featuring failure of osteoclasts to resorb bone due to inability to acidify their pericellular milieu.
      explanation: 'The same sentence carries the next step as well: failed acidification is given as the reason resorption fails.'
      quote_role: REVIEW_SYNTHESIS
      directness: INDIRECT
  evidence:
  - reference: PMID:6405388
    reference_title: Carbonic anhydrase II deficiency identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: provide genetic evidence implicating CA II in osteoclast function and bone resorption
    explanation: Human genetic evidence that CA II is required for osteoclast function - the inference the naturally occurring null makes available.
  - reference: PMID:6405388
    reference_title: Carbonic anhydrase II deficiency identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: explain previous observations that carbonic anhydrase inhibitors block the normal parathyroid hormone-induced release of calcium from bone
    explanation: 'Pharmacological corroboration: inhibiting carbonic anhydrase phenocopies the resorption block. Indirect because it argues from inhibitor data rather than from the patients'' osteoclasts.'
- name: Impaired Osteoclastic Bone Resorption
  biological_scale: TISSUE
  mechanism_confidence: ESTABLISHED
  description: Impaired acidification reduces osteoclast-mediated resorption of mineralized bone. The coupling of mineral dissolution and matrix removal explains defective remodeling without requiring an increase in the rate of bone formation.
  cell_types:
  - preferred_term: osteoclast
    term:
      id: CL:0000092
      label: osteoclast
  biological_processes:
  - preferred_term: osteoclastic bone resorption
    modifier: DECREASED
    term:
      id: GO:0045453
      label: bone resorption
  - preferred_term: bone remodeling
    modifier: DECREASED
    term:
      id: GO:0046849
      label: bone remodeling
  downstream:
  - target: Accumulation of Unresorbed Sclerotic Bone with Preserved Osteoclast Number
    causal_link_type: DIRECT
- name: Accumulation of Unresorbed Sclerotic Bone with Preserved Osteoclast Number
  biological_scale: TISSUE
  mechanism_confidence: ESTABLISHED
  description: Reduced resorption allows dense, poorly remodeled bone to accumulate despite the presence of osteoclasts. Bone density does not imply normal mechanical strength. Resolution of osteopetrosis after hematopoietic transplantation supports an osteoclast-lineage defect; it does not prove that every extra-skeletal outcome is unaffected.
  locations:
  - preferred_term: bone element
    term:
      id: UBERON:0001474
      label: bone element
  downstream:
  - target: Cranial Foramen and Optic Canal Narrowing
    causal_link_type: DIRECT
  - target: Craniofacial Disproportion and Upper Airway Narrowing
    causal_link_type: DIRECT
  - target: Osteopetrosis
    causal_link_type: DIRECT
  - target: Recurrent fractures
    causal_link_type: DIRECT
    description: Dense bone in this disease is brittle rather than strong, and fracture is a presenting feature.
    evidence:
    - reference: PMID:15300855
      reference_title: 'Carbonic anhydrase II deficiency syndrome (osteopetrosis with renal tubular acidosis and brain calcification): novel mutations in CA2 identified by direct sequencing expand the opportunity for genotype-phenotype correlation.'
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Other features include developmental delay, short stature, cognitive defects, and a history of multiple fractures by adolescence.
      explanation: Records repeated fracture as a consequence of the skeletal disease.
  evidence:
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Although CA II deficiency is an "osteoclast-rich" OPT, and therefore transplantation of healthy osteoclasts can improve the skeletal disease, the RTA and central nervous system difficulties persist.
    explanation: Describes an osteoclast-rich disorder in which transplantation can improve bone disease despite persistent renal and central nervous system difficulties.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
  - reference: PMID:23877423
    reference_title: 'Osteopetrosis: genetics, treatment and new insights into osteoclast function.'
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: OTHER
    snippet: mutations in TCIRG1, CLCN7, OSTM1, SNX10 and PLEKHM1 lead to osteoclast-rich ARO (in which osteoclasts are abundant but have severely impaired resorptive function), whereas mutations in TNFSF11 and TNFRSF11A lead to osteoclast-poor ARO
    explanation: Defines the osteoclast-rich versus osteoclast-poor axis this node is placed on. Indirect because this review enumerates the other recessive loci and does not itself mention CA2.
    quote_role: REVIEW_SYNTHESIS
- name: Cranial Foramen and Optic Canal Narrowing
  biological_scale: TISSUE
  mechanism_confidence: ESTABLISHED
  description: Dense skull-base bone can narrow cranial foramina. In the 23-patient cohort, optic nerve involvement affected 23 of 46 eyes and correlated with optic canal narrowing, supporting compression as an important mechanism. This association does not exclude an additional neural contribution. Internal auditory canal size was normal in that cohort.
  locations:
  - preferred_term: bone element
    term:
      id: UBERON:0001474
      label: bone element
  downstream:
  - target: Optic atrophy
    causal_link_type: DIRECT
    description: Compression of the optic nerve within a narrowed bony canal.
    evidence:
    - reference: PMID:22120147
      reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Globes and retinae were normal, but optic nerve involvement was present in 23/46 eyes and was variable in severity, random in occurrence and statistically correlated with degree of optic canal narrowing.
      explanation: The cohort supports an association between canal narrowing and optic nerve involvement; it does not experimentally exclude all primary neural injury.
  - target: Cranial nerve compression
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: The disorder presents late in infancy or early in childhood with fracturing, developmental delay, weakness, short stature, and/or cranial nerve compression and palsy.
    explanation: Lists cranial nerve compression and palsy among the presenting features.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
  - reference: PMID:22120147
    reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: All internal auditory canals were normal in size, and no patient had clinically significant hearing loss.
    explanation: 'Bounds the node: foraminal narrowing in this cohort spared the internal auditory canals, so the claim is not that every cranial foramen narrows.'
- name: Craniofacial Disproportion and Upper Airway Narrowing
  biological_scale: TISSUE
  mechanism_confidence: PROVISIONAL
  description: Skeletal overgrowth of the skull produces a large cranial vault and broad forehead, with midfacial and mandibular disproportion. The inference that this geometry narrows the upper airway enough to cause obstructive sleep apnea rests on a case report plus the general osteopetrosis literature, so the node is recorded as provisional.
  downstream:
  - target: Obstructive sleep apnea
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:30109220
      reference_title: 'Carbonic Anhydrase II Deficiency: A Rare Case of Severe Obstructive Sleep Apnea.'
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: This condition leads to specific craniofacial dysmorphisms associated with upper airway obstruction that may result in obstructive sleep apnea.
      explanation: States the craniofacial-to-airway-obstruction step for CA II deficiency specifically.
  evidence:
  - reference: PMID:22120147
    reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: All patients had osteopetrosis, renal tubular acidosis, developmental delay, short stature and craniofacial disproportion with large cranial vault and broad forehead.
    explanation: Craniofacial disproportion with large cranial vault and broad forehead was present in all 23 patients.
- name: Impaired Proximal Tubular Bicarbonate Reclamation
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: CA II-dependent intracellular CO2 hydration supports proximal bicarbonate reclamation. Deficiency can produce a proximal component of renal tubular acidosis, but the relative proximal and distal contributions vary between patients. Acetazolamide challenge in two affected individuals still increased urinary pH and bicarbonate excretion, indicating residual renal carbonic anhydrase activity rather than complete loss of all isoenzymes.
  cell_types:
  - preferred_term: epithelial cell of proximal tubule
    term:
      id: CL:0002306
      label: epithelial cell of proximal tubule
  biological_processes:
  - preferred_term: tubular bicarbonate reclamation
    modifier: DECREASED
    term:
      id: GO:0015701
      label: bicarbonate transport
  locations:
  - preferred_term: kidney
    term:
      id: UBERON:0002113
      label: kidney
  downstream:
  - target: Systemic Hyperchloremic Metabolic Acidosis
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:36709914
      reference_title: Carbonic anhydrase II deficiency.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: In CA II deficiency, OPT is uniquely accompanied by renal tubular acidosis (RTA) of proximal, distal, or combined type featuring hyperchloremic metabolic acidosis, rarely with hypokalemia and paralysis.
      explanation: Names the tubular defect and the systemic acid-base consequence it produces, together with the range of tubular segments involved.
      quote_role: REVIEW_SYNTHESIS
      directness: INDIRECT
  evidence:
  - reference: PMID:12566520
    reference_title: 'A phenocopy of CAII deficiency: a novel genetic explanation for inherited infantile osteopetrosis with distal renal tubular acidosis.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Most often, the RTA is of mixed proximal and distal type, but kindreds are reported in which either distal or proximal RTA predominates.
    explanation: The direct statement that the renal defect is usually mixed, with kindred-level variation - the reason this entry does not assert a single RTA tier.
  - reference: PMID:3932950
    reference_title: Positive renal response to intravenous acetazolamide in patients with carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: These findings indicate that CA II-deficient patients, who lack detectable CA II in their erythrocytes, still expressed an acetazolamide-inhibitable CA activity in their kidneys.
    explanation: 'Qualifies the node: carbonic anhydrase activity is not wholly absent from the CA II-deficient kidney, so the renal defect is a partial rather than a total loss of tubular carbonic anhydrase function.'
- name: Impaired Distal Tubular Proton Secretion
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: CA II supports proton secretion by collecting-duct intercalated cells. Its deficiency can impair distal urinary acidification. Car2-null mice additionally show depletion of intercalated-cell markers and reduced bicarbonate-transporter expression; these model findings do not establish an identical cellular population defect in human kidneys.
  cell_types:
  - preferred_term: collecting-duct alpha-intercalated cell
    term:
      id: CL:0005011
      label: renal alpha-intercalated cell
  biological_processes:
  - preferred_term: tubular proton secretion
    modifier: DECREASED
    term:
      id: GO:1902600
      label: proton transmembrane transport
  locations:
  - preferred_term: kidney
    term:
      id: UBERON:0002113
      label: kidney
  downstream:
  - target: Systemic Hyperchloremic Metabolic Acidosis
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:36709914
      reference_title: Carbonic anhydrase II deficiency.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: In CA II deficiency, OPT is uniquely accompanied by renal tubular acidosis (RTA) of proximal, distal, or combined type featuring hyperchloremic metabolic acidosis, rarely with hypokalemia and paralysis.
      explanation: Names the tubular defect and the systemic acid-base consequence it produces, together with the range of tubular segments involved.
      quote_role: REVIEW_SYNTHESIS
      directness: INDIRECT
  evidence:
  - reference: PMID:12566520
    reference_title: 'A phenocopy of CAII deficiency: a novel genetic explanation for inherited infantile osteopetrosis with distal renal tubular acidosis.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Most often, the RTA is of mixed proximal and distal type, but kindreds are reported in which either distal or proximal RTA predominates.
    explanation: The direct statement that the renal defect is usually mixed, with kindred-level variation - the reason this entry does not assert a single RTA tier.
  - reference: PMID:3932950
    reference_title: Positive renal response to intravenous acetazolamide in patients with carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: These findings indicate that CA II-deficient patients, who lack detectable CA II in their erythrocytes, still expressed an acetazolamide-inhibitable CA activity in their kidneys.
    explanation: 'Qualifies the node: carbonic anhydrase activity is not wholly absent from the CA II-deficient kidney, so the renal defect is a partial rather than a total loss of tubular carbonic anhydrase function.'
- name: Systemic Hyperchloremic Metabolic Acidosis
  biological_scale: ORGANISM
  mechanism_confidence: ESTABLISHED
  description: Renal bicarbonate loss and/or impaired net acid excretion produce metabolic acidosis, commonly hyperchloremic, sometimes with hypokalemia. Oral alkali can improve the acid-base disturbance. The combination with osteopetrosis is a strong diagnostic clue but can also occur with coinherited TCIRG1 and ATP6V1B1 disease.
  downstream:
  - target: Renal tubular acidosis
    causal_link_type: DIRECT
  - target: Hyperchloremic metabolic acidosis
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:35035649
    reference_title: 'Blessing in disguise: when head trauma solves the riddle of carbonic anhydrase II deficiency.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The suspicion of carbonic anhydrase II deficiency was confirmed by arterial blood gases revealing a marked metabolic acidosis fulfilling the diagnostic triad.
    explanation: A worked clinical instance of the systemic acidosis being the measurable expression of this node.
- name: Intracranial Parenchymal Calcium Deposition
  biological_scale: TISSUE
  mechanism_confidence: ESTABLISHED
  description: Intracranial mineral deposition is directly observed on imaging, commonly involving basal ganglia, thalami and the frontal grey-white junction. It can progress during childhood and can be absent at a particular assessment. The lesion is established; the causal intermediates connecting CA II deficiency to deposition and cognitive outcomes remain unresolved. A small clinical series found no significant association between calcium burden and developmental measures.
  locations:
  - preferred_term: basal ganglion
    term:
      id: UBERON:0002420
      label: basal ganglion
  downstream:
  - target: Cerebral calcification
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Cerebral calcification uniquely appears in early childhood.
    explanation: Establishes that the calcification is an early-childhood feature specific to this osteopetrosis.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
  - reference: PMID:39667299
    reference_title: The correlation of intracranial parenchymal calcium score and the severity of neurological clinical presentation in carbonic anhydrase deficiency type 2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Intraparenchymal calcifications in CAII-D are common but unrelated to developmental delay and neuropsychiatric symptoms, suggesting complex pathophysiology.
    explanation: Supports an established imaging lesion while the lack of association with developmental measures in this small series leaves its causal relationship to neurological impairment unresolved.
phenotypes:
- category: Skeletal
  name: Osteopetrosis
  phenotype_term:
    preferred_term: Osteopetrosis
    term:
      id: HP:0011002
      label: Osteopetrosis
  description: Generalized increase in skeletal density due to impaired resorption. All 23 patients in the 2011 founder-allele cohort had osteopetrosis; this cohort observation is not a population-wide penetrance estimate.
  diagnostic: true
  evidence:
  - reference: PMID:22120147
    reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: All patients had osteopetrosis, renal tubular acidosis, developmental delay, short stature and craniofacial disproportion with large cranial vault and broad forehead.
    explanation: Osteopetrosis in 23/23 patients from 10 unrelated families.
- category: Renal
  name: Renal tubular acidosis
  phenotype_term:
    preferred_term: Renal tubular acidosis, most often of mixed proximal and distal type
    term:
      id: HP:0001947
      label: Renal tubular acidosis
  description: A proximal, distal or mixed tubular acidification defect can occur; different clinical series report different predominant components. Some molecularly confirmed patients lacked detectable acidosis at assessment.
  diagnostic: true
  notes: Renal tubular acidosis may be proximal, distal or mixed. The Puerto Rican exon 7 frameshift proband had no detectable acidosis, whereas six later patients with the same allele did. A missing renal component does not exclude CA2 deficiency.
  evidence:
  - reference: PMID:12566520
    reference_title: 'A phenocopy of CAII deficiency: a novel genetic explanation for inherited infantile osteopetrosis with distal renal tubular acidosis.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Most often, the RTA is of mixed proximal and distal type, but kindreds are reported in which either distal or proximal RTA predominates.
    explanation: The basis for binding the parent term rather than a proximal or distal child term.
  - reference: PMID:8128957
    reference_title: 'Carbonic anhydrase II deficiency: single-base deletion in exon 7 is the predominant mutation in Caribbean Hispanic patients.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Six subsequent Hispanic patients of Caribbean ancestry, all of whom had osteopetrosis and renal tubular acidosis but who varied widely in clinical severity, were found to be homozygous for the same mutation.
    explanation: Supports the association while the same paper documents absence of detectable RTA in its index patient.
  - reference: PMID:38655726
    reference_title: 'Allogenic hematopoietic stem cell transplantation in an Iranian patient with osteopetrosis caused by carbonic anhydrase II deficiency: A case report.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Further evaluation showed renal tubular acidosis type 2.
    explanation: A case in which the tubular defect was proximal (type 2), illustrating the segment-level variation the parent-term binding accommodates.
- category: Laboratory
  name: Hyperchloremic metabolic acidosis
  phenotype_term:
    preferred_term: Hyperchloremic metabolic acidosis
    term:
      id: HP:0004918
      label: Hyperchloremic metabolic acidosis
  description: 'The systemic, measurable expression of the tubular defect: a normal-anion-gap metabolic acidosis, occasionally with hypokalemia and consequent paralysis.'
  diagnostic: true
  evidence:
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: In CA II deficiency, OPT is uniquely accompanied by renal tubular acidosis (RTA) of proximal, distal, or combined type featuring hyperchloremic metabolic acidosis, rarely with hypokalemia and paralysis.
    explanation: Names hyperchloremic metabolic acidosis as the acid-base phenotype.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
- category: Neurologic
  name: Cerebral calcification
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Cerebral calcification, predominantly basal ganglia and thalami
    term:
      id: HP:0002514
      label: Cerebral calcification
  description: 'Intracranial parenchymal calcification, the feature behind the historical name "marble brain disease". Present in 70% of a modern 10-patient series and progressive on serial imaging, but age-dependent: a child without calcification at 9 years has been documented.'
  diagnostic: true
  evidence:
  - reference: PMID:39667299
    reference_title: The correlation of intracranial parenchymal calcium score and the severity of neurological clinical presentation in carbonic anhydrase deficiency type 2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Optic nerve atrophy was observed in 22.2 %, while brain calcifications were present in 70.0 % of cases.
    explanation: Frequency of brain calcification in a consecutive pediatric series.
  - reference: PMID:22120147
    reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Brain calcification was generally progressive and followed a distinct distribution, involving predominantly basal ganglia and thalami and grey-white matter junction in frontal regions more than posterior regions.
    explanation: Establishes the anatomical distribution recorded in the preferred_term.
- category: Neurologic
  name: Global developmental delay
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  description: Developmental delay is among the presenting features and was present in 60% of a consecutive pediatric series, with a median developmental quotient of 63.5.
  evidence:
  - reference: PMID:39667299
    reference_title: The correlation of intracranial parenchymal calcium score and the severity of neurological clinical presentation in carbonic anhydrase deficiency type 2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: For neurodevelopmental symptoms, 60.0 % of patients presented with global developmental delay, 20.0 % had intellectual disability, and the remaining 20.0 % had normal development.
    explanation: Quantifies developmental delay and, importantly, records that 20% had normal development - so this is frequent, not obligate.
- category: Neurologic
  name: Intellectual disability
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  description: Cognitive impairment varies. Approximately two-thirds of the 23-patient founder-allele cohort had intellectual disability, whereas the later ten-patient series separately reported intellectual disability in 20% and global developmental delay in 60%. These differently defined outcomes should not be pooled. Normal psychomotor development was reported in an 11-year-old with c.232+1G>A.
  evidence:
  - reference: PMID:22120147
    reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Mental retardation was present in approximately two-thirds and varied from mild to severe.
    explanation: Frequency and severity range in 23 patients from 10 families.
  - reference: PMID:7959703
    reference_title: A unique mutation underlying carbonic anhydrase II deficiency syndrome in patients of Arab descent.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: All our patients were homozygous for this mutation and were mentally retarded, a characteristic feature of the phenotype of patients with an Arabic background.
    explanation: Ties cognitive involvement to the intron 2 founder genotype.
  - reference: PMID:9580777
    reference_title: Clinical and molecular heterogeneity in carbonic anhydrase II deficiency and prenatal diagnosis in an Italian family.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: We describe an Italian boy with this disease whose mental retardation was relatively mild and whose renal tubular acidosis had only a distal component.
    explanation: Documents the mild end of the cognitive spectrum alongside a distal-predominant renal phenotype.
- category: Growth
  name: Short stature
  phenotype_term:
    preferred_term: Short stature
    term:
      id: HP:0004322
      label: Short stature
  description: Growth impairment is well documented, including all 23 patients of the founder-allele cohort. Chronic acidosis, skeletal disease and endocrine effects are possible contributors; male-specific low growth hormone in the mouse model does not establish human GH deficiency.
  evidence:
  - reference: PMID:22120147
    reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: All patients had osteopetrosis, renal tubular acidosis, developmental delay, short stature and craniofacial disproportion with large cranial vault and broad forehead.
    explanation: Short stature in 23/23 patients.
- category: Skeletal
  name: Recurrent fractures
  phenotype_term:
    preferred_term: Recurrent fractures
    term:
      id: HP:0002757
      label: Recurrent fractures
  description: Repeated fractures of dense but brittle bone, typically accumulating through childhood. Notably, fractures are reported as relatively infrequent in patients carrying the intron 2 founder allele compared with the His107Tyr families.
  evidence:
  - reference: PMID:15300855
    reference_title: 'Carbonic anhydrase II deficiency syndrome (osteopetrosis with renal tubular acidosis and brain calcification): novel mutations in CA2 identified by direct sequencing expand the opportunity for genotype-phenotype correlation.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Other features include developmental delay, short stature, cognitive defects, and a history of multiple fractures by adolescence.
    explanation: Records multiple fractures by adolescence as a recognised feature.
  - reference: PMID:1301935
    reference_title: A splice junction mutation in intron 2 of the carbonic anhydrase II gene of osteopetrosis patients from Arabic countries.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The presence of mental retardation and relative infrequency of skeletal fractures distinguish the clinical course of the patients with the Arabic mutation from those of the American and Belgian patients with the His 107-->Tyr mutation.
    explanation: Genotype-dependent fracture frequency - the basis for the qualification in the description.
- category: Ophthalmologic
  name: Optic atrophy
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Optic atrophy from optic canal narrowing
    term:
      id: HP:0000648
      label: Optic atrophy
  description: Optic nerve atrophy was reported in 22.2% of the later pediatric series. The earlier cohort reported the broader finding of optic nerve involvement in 23 of 46 eyes, which is a different outcome and denominator. Its association with canal narrowing supports a compressive contribution.
  evidence:
  - reference: PMID:39667299
    reference_title: The correlation of intracranial parenchymal calcium score and the severity of neurological clinical presentation in carbonic anhydrase deficiency type 2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Optic nerve atrophy was observed in 22.2 %, while brain calcifications were present in 70.0 % of cases.
    explanation: 22.2% in the ten-patient pediatric series falls within OCCASIONAL; the abstract does not state the denominator with evaluable optic-nerve assessment.
  - reference: PMID:22120147
    reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Globes and retinae were normal, but optic nerve involvement was present in 23/46 eyes and was variable in severity, random in occurrence and statistically correlated with degree of optic canal narrowing.
    explanation: Optic nerve involvement in 23/46 eyes is a broader outcome than optic atrophy and must not be pooled with the patient-level estimate. Canal correlation supports, but does not prove exclusivity of, compression.
- category: Neurologic
  name: Cranial nerve compression
  phenotype_term:
    preferred_term: Cranial nerve compression
    term:
      id: HP:0001293
      label: Cranial nerve compression
  description: Compression of cranial nerves in narrowed skull-base foramina, with palsy. The optic nerve is the best documented; the internal auditory canals were of normal calibre in the 23-patient cohort.
  notes: The deep-research report offered HP:0031815 for this phenotype. That CURIE resolves to "Abnormal oral physiology" - the report's own Term Validation section flagged it as naming a different term - so HP:0001293 was looked up independently and bound instead.
  evidence:
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: The disorder presents late in infancy or early in childhood with fracturing, developmental delay, weakness, short stature, and/or cranial nerve compression and palsy.
    explanation: Names cranial nerve compression and palsy as a presenting feature.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
- category: Ophthalmologic
  name: Nystagmus
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Nystagmus
    term:
      id: HP:0000639
      label: Nystagmus
  description: Seven of 23 patients in the 23-patient cohort had congenital nystagmus, most often in association with afferent visual disturbance.
  evidence:
  - reference: PMID:22120147
    reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Saccadic abnormalities were present in two, while half of these patients had sensory or accommodative strabismus, and seven had congenital nystagmus.
    explanation: Seven of 23 patients in the 2011 founder-allele cohort had congenital nystagmus (30.4%, FREQUENT). This is a cohort estimate, not an independently measured frequency across all CA2 genotypes.
- category: Dental
  name: Amelogenesis imperfecta
  phenotype_term:
    preferred_term: Hypoplastic amelogenesis imperfecta
    term:
      id: HP:0000705
      label: Amelogenesis imperfecta
  description: Generalized hypoplastic enamel affecting primary and permanent dentitions was documented in a Brazilian woman with homozygous CA2 c.753delG. The association is supported by a case report, without a disease-wide frequency estimate. A contribution from systemic acidosis was not excluded.
  evidence:
  - reference: PMID:37662627
    reference_title: Carbonic anhydrase II deficiency syndrome with amelogenesis imperfecta linked to a homozygous CA2 deletion.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The oral manifestations included anterior open bite, posterior crossbite, tooth eruption impairment, and hypoplastic amelogenesis imperfecta (AI).
    explanation: The single report of amelogenesis imperfecta in a CA2-confirmed patient.
- category: Dental
  name: Delayed eruption of teeth
  phenotype_term:
    preferred_term: Tooth eruption impairment
    term:
      id: HP:0000684
      label: Delayed eruption of teeth
  description: Impaired tooth eruption, expected in a sclerosing bone disease where unresorbed bone obstructs the eruption path, and documented alongside malocclusion in a CA2-confirmed patient.
  evidence:
  - reference: PMID:37662627
    reference_title: Carbonic anhydrase II deficiency syndrome with amelogenesis imperfecta linked to a homozygous CA2 deletion.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The oral manifestations included anterior open bite, posterior crossbite, tooth eruption impairment, and hypoplastic amelogenesis imperfecta (AI).
    explanation: Records tooth eruption impairment in a CA2-confirmed patient.
- category: Renal
  name: Nephrocalcinosis
  phenotype_term:
    preferred_term: Medullary nephrocalcinosis
    term:
      id: HP:0000121
      label: Nephrocalcinosis
  description: Reported in one Kuwaiti patient at 14 months and described there as not previously associated with CA II deficiency. Affected siblings in the same series did not have it.
  evidence:
  - reference: PMID:9453381
    reference_title: Nephrocalcinosis and urolithiasis in carbonic anhydrase II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: At the age of 14 months, ultrasound studies in the girl showed medullary nephrocalcinosis which has not been previously described in association with CA II, while cerebral CT scan revealed dilated ventricles.
    explanation: The first report of nephrocalcinosis in this syndrome.
- category: Renal
  name: Nephrolithiasis
  phenotype_term:
    preferred_term: Recurrent renal stones with hypercalciuria
    term:
      id: HP:0000787
      label: Nephrolithiasis
  description: Bilateral recurrent stones with hypercalciuria in one patient, whose affected siblings had neither stones nor nephrocalcinosis.
  evidence:
  - reference: PMID:9453381
    reference_title: Nephrocalcinosis and urolithiasis in carbonic anhydrase II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The patient from family 2, who had two previously reported affected siblings, developed bilateral recurrent renal stones and hypercalciuria but no nephrocalcinosis.
    explanation: Documents stones and hypercalciuria with intrafamilial variability; it does not supply a disease-wide frequency.
- category: Respiratory
  name: Obstructive sleep apnea
  phenotype_term:
    preferred_term: Obstructive sleep apnea
    term:
      id: HP:0002870
      label: Obstructive sleep apnea
  description: Severe obstructive sleep apnea attributable to craniofacial dysmorphism rather than to adenotonsillar hypertrophy alone, reported in one child and successfully managed with CPAP.
  evidence:
  - reference: PMID:30109220
    reference_title: 'Carbonic Anhydrase II Deficiency: A Rare Case of Severe Obstructive Sleep Apnea.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Herein we report a case of osteopetrosis with RTA associated with severe OSAS successfully treated with continuous positive airway pressure (CPAP).
    explanation: The single reported case linking this syndrome to severe OSA.
- category: Audiologic
  name: Hearing impairment
  phenotype_term:
    preferred_term: Hearing impairment
    term:
      id: HP:0000365
      label: Hearing impairment
  description: Progressive hearing loss was a transplantation indication in two children, without the hearing-loss type specified in that report. No patient in the 23-person founder-allele cohort had clinically significant hearing loss. The separately annotated Conductive hearing impairment records the specific subtype reported in the Brazilian woman.
  notes: Hearing loss occurs in some reports but was not clinically significant in any patient of the 23-person founder-allele cohort. These observations show variable expression; they do not exclude hearing involvement from the disease.
  evidence:
  - reference: PMID:11264157
    reference_title: Bone marrow transplantation corrects osteopetrosis in the carbonic anhydrase II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Two children, one from each family, have undergone allogeneic bone marrow transplantation because of severe progressive visual and hearing loss.
    explanation: Progressive hearing loss severe enough to be a transplantation indication in two related Irish families.
- category: Craniofacial
  name: Broad forehead
  phenotype_term:
    preferred_term: Broad forehead with large cranial vault
    term:
      id: HP:0000337
      label: Broad forehead
  description: Part of the craniofacial disproportion present in all 23 patients of the 23-patient cohort, and the visible expression of skull overgrowth.
  evidence:
  - reference: PMID:22120147
    reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: All patients had osteopetrosis, renal tubular acidosis, developmental delay, short stature and craniofacial disproportion with large cranial vault and broad forehead.
    explanation: Craniofacial disproportion with broad forehead in 23/23 patients.
- name: Strabismus
  category: Ophthalmologic
  phenotype_term:
    preferred_term: Strabismus
    term:
      id: HP:0000486
      label: Strabismus
  description: Sensory or accommodative strabismus affected approximately half of the 23-person founder-allele cohort. This is a cohort estimate.
  evidence:
  - reference: PMID:22120147
    reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Saccadic abnormalities were present in two, while half of these patients had sensory or accommodative strabismus, and seven had congenital nystagmus.
    explanation: Half of the cohort corresponds to FREQUENT (30–79%); the allele-specific cohort does not establish a rate for every genotype.
  frequency: FREQUENT
- name: Microcephaly
  category: Neurologic
  phenotype_term:
    preferred_term: Microcephaly
    term:
      id: HP:0000252
      label: Microcephaly
  description: Reported in the Brazilian woman with homozygous c.753delG; no disease-wide frequency or exclusive causal attribution is inferred.
  evidence:
  - reference: PMID:37662627
    reference_title: Carbonic anhydrase II deficiency syndrome with amelogenesis imperfecta linked to a homozygous CA2 deletion.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: She presented with osteopetrosis, microcephaly, cerebral calcifications, developmental delay, short stature, limb malformations, and recurrent fractures of the femur, tibia, and fibula
    explanation: Direct clinical description of the individual case.
- name: Hypokalemia
  category: Metabolic
  phenotype_term:
    preferred_term: Hypokalemia
    term:
      id: HP:0002900
      label: Hypokalemia
  description: Recurrent hypokalemia occurred during the 2024 boy’s admission with acidosis and fractures; the acute respiratory presentation also involved asthma treatment.
  evidence:
  - reference: PMID:38328522
    reference_title: 'Carbonic Anhydrase II Deficiency: Unusual Presentation of the Arabic Mutation. A Case Report.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: On the fifth day of admission, he was reported to have a low potassium level again (2.9 mmol/L).
    explanation: Documents recurrent low potassium in the reported patient without assigning every episode solely to CA2.
- name: Hydrocephalus
  category: Neurologic
  phenotype_term:
    preferred_term: Hydrocephalus
    term:
      id: HP:0000238
      label: Hydrocephalus
  description: Hydrocephaly requiring a shunt was reported in the Iranian child subsequently transplanted; the abstract does not resolve its mechanism.
  evidence:
  - reference: PMID:38655726
    reference_title: 'Allogenic hematopoietic stem cell transplantation in an Iranian patient with osteopetrosis caused by carbonic anhydrase II deficiency: A case report.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: presented at 8-month-old with hydrocephaly, brain shunt, and developmental delay.
    explanation: Individual clinical observation; no general frequency or mechanism is inferred.
- name: Seizure
  category: Neurologic
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  description: A single tonic seizure was reported during the Jordanian child’s admission after head injury. That observation does not establish a disease-wide epilepsy syndrome.
  evidence:
  - reference: PMID:35035649
    reference_title: 'Blessing in disguise: when head trauma solves the riddle of carbonic anhydrase II deficiency.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: During the hospitalization period, he developed a single seizure attack characterized as tonic seizure associated with up rolling of the eyes and cyanosis which was relieved without medications.
    explanation: Reports the event and its clinical context rather than attributing it specifically to calcification.
- name: Conductive hearing impairment
  category: Auditory
  phenotype_term:
    preferred_term: Conductive hearing impairment
    term:
      id: HP:0000405
      label: Conductive hearing impairment
  description: Conductive hearing loss was explicitly described in the Brazilian woman. This is a specific subtype of the broader Hearing impairment annotation; the two entries are not independent prevalence observations.
  evidence:
  - reference: PMID:37662627
    reference_title: Carbonic anhydrase II deficiency syndrome with amelogenesis imperfecta linked to a homozygous CA2 deletion.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: she presented right optic nerve atrophy with visual impairment, strabismus, horizontal nystagmus, and conductive hearing loss.
    explanation: The full report specifies conductive loss; the abstract’s broader deafness term is less precise.
- name: Anterior open-bite malocclusion
  category: Dental
  phenotype_term:
    preferred_term: Anterior open-bite malocclusion
    term:
      id: HP:0009102
      label: Anterior open-bite malocclusion
  description: Observed with posterior crossbite and eruption impairment in the Brazilian dental case.
  evidence:
  - reference: PMID:37662627
    reference_title: Carbonic anhydrase II deficiency syndrome with amelogenesis imperfecta linked to a homozygous CA2 deletion.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The oral manifestations included anterior open bite, posterior crossbite, tooth eruption impairment, and hypoplastic amelogenesis imperfecta (AI).
    explanation: Direct oral findings in one affected individual.
- name: Persistence of primary teeth
  category: Dental
  phenotype_term:
    preferred_term: Persistence of primary teeth
    term:
      id: HP:0006335
      label: Persistence of primary teeth
  description: Retained primary teeth were observed in the dental case by age 17.
  evidence:
  - reference: PMID:37662627
    reference_title: Carbonic anhydrase II deficiency syndrome with amelogenesis imperfecta linked to a homozygous CA2 deletion.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Later, at age 17, she presented with retained primary and impacted permanent teeth.
    explanation: Direct longitudinal dental finding.
- name: Multiple impacted teeth
  category: Dental
  phenotype_term:
    preferred_term: Multiple impacted teeth
    term:
      id: HP:0001571
      label: Multiple impacted teeth
  description: Impacted permanent teeth accompanied impaired eruption in the dental case.
  evidence:
  - reference: PMID:37662627
    reference_title: Carbonic anhydrase II deficiency syndrome with amelogenesis imperfecta linked to a homozygous CA2 deletion.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Later, at age 17, she presented with retained primary and impacted permanent teeth.
    explanation: The plural impacted permanent teeth supports this binding.
- name: Dental crowding
  category: Dental
  phenotype_term:
    preferred_term: Dental crowding
    term:
      id: HP:0000678
      label: Dental crowding
  description: Crowded teeth were documented on oral examination; the evidence does not quantify prevalence.
  evidence:
  - reference: PMID:37662627
    reference_title: Carbonic anhydrase II deficiency syndrome with amelogenesis imperfecta linked to a homozygous CA2 deletion.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: She presented mixed dentition with generalized enamel hypoplasia, high-arched and ogival palate, anterior open bite, posterior crossbite, crowded teeth, ectopic eruption, and altered tooth shape.
    explanation: Direct dental examination in the Brazilian case.
- name: Anemia
  category: Hematologic
  phenotype_term:
    preferred_term: Anemia
    term:
      id: HP:0001903
      label: Anemia
  description: Two children in the historical 35-patient clinical series developed anemia with marrow involvement. Not all participants had enzyme confirmation, so the observation is retained without a genotype-confirmed frequency.
  evidence:
  - reference: PMID:11938359
    reference_title: Long-term follow up of carbonic anhydrase II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Five patients were blind secondary to optic nerve entrapment and 2 patients developed anemia and secondary erythropoesis due to bone marrow involvement.
    explanation: Marrow-related anemia was reported in this clinically diagnosed cohort; CA II activity was measured in nine participants.
genetic:
- name: CA2
  gene_term:
    preferred_term: CA2
    term:
      id: hgnc:1373
      label: CA2
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  presence: Biallelic disease-associated CA2 variants
  notes: CA2 encodes cytosolic carbonic anhydrase II. Splice-site, nonsense, frameshift and missense variants have been reported. Homozygous and compound-heterozygous genotypes occur. A predicted truncation or splice disruption does not by itself demonstrate absent protein in every tissue; His107Tyr retains residual activity in recombinant assays. Coexistent osteopetrosis and renal tubular acidosis can also result from coinherited defects in other genes.
  variants:
  - name: c.232+1G>A (intron 2 founder splice-donor variant)
    type: splice site
    clinical_significance: PATHOGENIC
    description: The recurrent intron 2 splice-donor allele is explicitly reported as NM_000067.3:c.232+1G>A in the Jordanian case and as c.232+1G>A in other primary clinical reports. Earlier series associated it with developmental impairment and fewer fractures, but the 2024 boy had age-appropriate development and recurrent low-trauma fractures. The Iranian transplant abstract instead reports c.232+1G>T; that discordant substitution should not be silently equated with c.232+1G>A.
    evidence:
    - reference: PMID:1301935
      reference_title: A splice junction mutation in intron 2 of the carbonic anhydrase II gene of osteopetrosis patients from Arabic countries.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: We have analyzed DNAs from members of six unrelated Arabic kindreds and found five to be homozygous and one heterozygous for a novel splice junction (donor site) mutation at the 5' end of intron 2.
      explanation: Original description of the intron 2 splice-junction variant.
    - reference: PMID:7959703
      reference_title: A unique mutation underlying carbonic anhydrase II deficiency syndrome in patients of Arab descent.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: This mutation is found exclusively in patients with an Arabic background and thus may be confined to this ethnic group.
      explanation: Describes the population distribution observed in that historical series, not proof that the allele is exclusive to an ancestry today.
    - reference: PMID:35035649
      reference_title: 'Blessing in disguise: when head trauma solves the riddle of carbonic anhydrase II deficiency.'
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Whole exome sequencing confirms the diagnosis of CA II deficiency with homozygous mutation (CA2 NM_000067.3:c.232+1G>A) inherited in an autosomal recessive manner.
      explanation: The full clinical report supplies explicit transcript-level nomenclature for the recurrent splice allele.
    - reference: PMID:38328522
      reference_title: 'Carbonic Anhydrase II Deficiency: Unusual Presentation of the Arabic Mutation. A Case Report.'
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Psychomotor development is appropriate for chronological age.
      explanation: The reported 11-year-old with homozygous c.232+1G>A did not have developmental impairment at assessment; the allele does not determine intellectual disability.
  - name: His107Tyr (p.H107Y)
    type: missense
    clinical_significance: PATHOGENIC
    description: C-to-T transition substituting tyrosine for the invariant histidine at position 107, disrupting hydrogen bonding to Glu-117 and Tyr-194 and destabilising the protein. Unusually among CA2 alleles it leaves detectable residual activity, which has been proposed as the explanation for the milder phenotype and absent cognitive impairment in the families that carry it.
    evidence:
    - reference: PMID:1928091
      reference_title: 'Carbonic anhydrase II deficiency syndrome in a Belgian family is caused by a point mutation at an invariant histidine residue (107 His----Tyr): complete structure of the normal human CA II gene.'
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: The change is a C-to-T transition which results in the substitution of Tyr (TAT) for His (CAT) at position 107.
      explanation: Original identification of the variant in a consanguineous Belgian family.
    - reference: PMID:1542674
      reference_title: Molecular basis of human carbonic anhydrase II deficiency.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: We show that the mutant enzyme expressed in bacteria from the cDNA containing the His-107----Tyr mutation has detectable, though greatly reduced, activity.
      explanation: Recombinant expression establishing that this allele is hypomorphic rather than null.
    - reference: PMID:1542674
      reference_title: Molecular basis of human carbonic anhydrase II deficiency.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: The three affected siblings from this family are compound heterozygotes, each having inherited two different mutations in the structural gene for carbonic anhydrase II.
      explanation: Documents compound heterozygosity (His107Tyr with an intron 5 splice acceptor variant) in the index American family.
  - name: Exon 7 single-base deletion (frameshift)
    type: frameshift
    clinical_significance: PATHOGENIC
    description: Single-base deletion in exon 7 producing a frameshift and premature termination, homozygous in seven unrelated Hispanic families of Caribbean ancestry. The truncated 27-kD product is inactive, aggregates, and is rapidly degraded.
    evidence:
    - reference: PMID:8128957
      reference_title: 'Carbonic anhydrase II deficiency: single-base deletion in exon 7 is the predominant mutation in Caribbean Hispanic patients.'
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: She proved to be homozygous for a single-base deletion in the coding region of exon 7 that produces a frameshift that changes the next 12 amino acids before leading to chain termination
      explanation: Molecular description of the Caribbean Hispanic founder allele.
  - name: c.368G>A p.W123X (nonsense)
    type: nonsense
    clinical_significance: PATHOGENIC
    description: Homozygous c.368G>A, p.Trp123Ter was reported in a Chinese proband. The study combined structural analysis with a transfected HEK293T expression model. The abstract does not separate all modeled zinc-binding predictions from experimental protein-expression findings, so the specific structural mechanism remains qualified.
    evidence:
    - reference: PMID:33555497
      reference_title: A novel homozygous nonsense mutation in the CA2 gene (c.368G>A, p.W123X) linked to carbonic anhydrase II deficiency syndrome in a Chinese family.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Sequencing and genetic analysis have revealed the homozygous nonsense mutation of CA2 gene (c.368G > A, p.W123X) in the exon 4 of chromosome 8 of the proband
      explanation: Identification of the variant in the proband and its absence in unaffected relatives.
    - reference: PMID:33555497
      reference_title: A novel homozygous nonsense mutation in the CA2 gene (c.368G>A, p.W123X) linked to carbonic anhydrase II deficiency syndrome in a Chinese family.
      supports: SUPPORT
      evidence_source: COMPUTATIONAL
      snippet: Properties and structure of mutated CA2 proteins changed after mutation, especially in change of protein modification and hindrance of zinc ions binding
      explanation: Used for the structural-analysis hypothesis about zinc binding, not as a direct zinc-binding assay or a quantitative protein-expression result. The connection to enzyme dysfunction is inferential.
      directness: INDIRECT
  - name: c.753delG, p.Asn252Thrfs*14
    type: frameshift
    clinical_significance: LIKELY_PATHOGENIC
    description: Homozygous exon 7 deletion reported in the Brazilian woman with generalized hypoplastic enamel and other CA II-deficiency findings. Her mother was heterozygous; the father was unavailable for testing. The report notes prior observation in two Mexican siblings.
    evidence:
    - reference: PMID:37662627
      reference_title: Carbonic anhydrase II deficiency syndrome with amelogenesis imperfecta linked to a homozygous CA2 deletion.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Molecular analysis revealed a CA2 homozygous deletion (c.753delG, p.Asn252Thrfs*14) and confirmed the clinical diagnosis.
      explanation: The molecularly characterized Brazilian dental case carried this homozygous frameshift; the full report classified it as likely pathogenic.
  evidence:
  - reference: PMID:15300855
    reference_title: 'Carbonic anhydrase II deficiency syndrome (osteopetrosis with renal tubular acidosis and brain calcification): novel mutations in CA2 identified by direct sequencing expand the opportunity for genotype-phenotype correlation.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: With one exception, all patients with osteopetrosis and renal tubular acidosis examined have proven to have CA II deficiency. All CA II-deficient patients analyzed have been found to have mutations in the CA2 gene.
    explanation: The bidirectional gene-disease specificity statement, and the reason this is curated as a single disease rather than a grouping.
  - reference: PMID:6410391
    reference_title: 'Polymorphic gene for human carbonic anhydrase II: a molecular disease marker located on chromosome 8.'
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: Segregation analysis of the molecular marker and chromosomes in cell hybrids indicated a clear correlation between the presence of chromosome 8 and the human CA II gene (CA2).
    explanation: Somatic cell hybrid mapping of CA2 to chromosome 8, the step that made molecular diagnosis possible.
  - reference: PMID:15300855
    reference_title: 'Carbonic anhydrase II deficiency syndrome (osteopetrosis with renal tubular acidosis and brain calcification): novel mutations in CA2 identified by direct sequencing expand the opportunity for genotype-phenotype correlation.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: These mutations were scattered over the genome from exon 2 to 7.
    explanation: Establishes that pathogenic variants are distributed across the gene rather than clustering in one domain, which is why diagnostic sequencing covers all seven exons.
  - reference: PMID:31542996
    reference_title: Molecular modelling and dynamics of CA2 missense mutations causative to carbonic anhydrase 2 deficiency syndrome.
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: Free energy dynamics of tertiary structure models of CA2 mutants with DUET, mCSM, and SDM based consensus methods predicted only 50% of the variants as destabilizing.
    explanation: 'Bounds what in silico prediction can do for the missense class: consensus free-energy methods called only half of already-known disease-causing CA2 missense variants destabilizing, so a non-destabilizing prediction is not evidence against pathogenicity in this gene.'
  - reference: PMID:31542996
    reference_title: Molecular modelling and dynamics of CA2 missense mutations causative to carbonic anhydrase 2 deficiency syndrome.
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: Superimposition of native and mutant CA2 models revealed the minor structural fluctuations at the amino acid residue level but not at the whole protein structure level.
    explanation: 'Gives the reason the free-energy methods underperform: the perturbation is residue-local rather than global, which is the signal whole-protein stability scores are least able to see.'
biochemical:
- name: Erythrocyte carbonic anhydrase II
  biomarker_term:
    preferred_term: carbonic anhydrase II (erythrocyte isoenzyme II)
    term:
      id: NCIT:C183174
      label: Carbonic Anhydrase 2
  presence: Virtually absent in affected individuals by HPLC peak, esterase activity, CO2 hydratase activity and immunoprecipitation; approximately half-normal in asymptomatic obligate heterozygotes; carbonic anhydrase I unaffected.
  specificity: Isoenzyme-specific. CA I levels are normal, so the assay distinguishes this disorder from a general carbonic anhydrase deficiency.
  cell_types:
  - preferred_term: erythrocyte
    term:
      id: CL:0000232
      label: erythrocyte
  readouts:
  - target: pathophysiology#Biallelic CA2 Loss of Function
    relationship: READOUT_OF
    direction: PRESENT_ABSENT
    interpretation: Absence of erythrocyte CA II establishes the enzymatic diagnosis; the half-normal carrier level makes the same assay a carrier test.
    description: The erythrocyte is not an affected tissue in this disease, but it is the accessible one, and the enzyme is the same gene product.
    evidence:
    - reference: PMID:6405388
      reference_title: Carbonic anhydrase II deficiency identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: We report here biochemical and immunological evidence for the virtual absence of CA II in erythrocytes of patients affected with this condition, whereas CA I level is not reduced.
      explanation: The measurement behind this readout, and the isoenzyme specificity that makes an absent result interpretable.
  evidence:
  - reference: PMID:3925334
    reference_title: Carbonic anhydrase II deficiency in 12 families with the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Reduced levels of isozyme II were found in obligate heterozygotes.
    explanation: Supports the carrier-detection use of the assay.
  - reference: PMID:3925334
    reference_title: Carbonic anhydrase II deficiency in 12 families with the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Virtual absence of the carbonic anhydrase II peak on high-performance liquid chromatography, of the esterase and carbon dioxide hydratase activities of carbonic anhydrase II, and of immunoprecipitable isozyme II was demonstrated on extracts of erythrocyte hemolysates from all patients studied.
    explanation: Establishes absence of the enzyme by four concordant measurements in all patients studied.
animal_models:
- name: Car-2 null mouse (ENU-induced)
  species: Mouse
  genotype: Car-2 null (ENU-induced, homozygous)
  background: (C57BL/6J x DBA/2J)F1-derived
  publication: PMID:3126501
  description: ENU-induced Car2-null mice lack detectable CA II, develop growth restriction and renal tubular acidosis, and did not show osteopetrosis even in aged homozygotes. This establishes a model of selected renal and growth findings with an important skeletal divergence.
  modeled_mechanisms:
  - target: Systemic Hyperchloremic Metabolic Acidosis
    relationship: RECAPITULATES
    fidelity: MODERATE
    model_scale: ORGANISM
    description: Homozygous null mice develop renal tubular acidosis, as humans with the same enzyme defect do.
    limitations: Renal tubular acidosis is reproduced, but the founding phenotype does not by itself resolve the proximal and distal contributions. Later kidney studies show collecting-duct cellular and transporter changes whose extent in human CA2 deficiency remains unestablished.
    readouts:
    - name: Renal tubular acidosis in homozygous null mice
      target: Systemic Hyperchloremic Metabolic Acidosis
      direction: ALTERED
      interpretation: The acid-base phenotype of the model matches the human renal arm.
      evidence:
      - reference: PMID:3126501
        reference_title: 'N-ethyl-N-nitrosourea-induced null mutation at the mouse Car-2 locus: an animal model for human carbonic anhydrase II deficiency syndrome.'
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: Like humans with the same inherited enzyme defect, animals homozygous for the new null allele are runted and have renal tubular acidosis.
        explanation: Reports the renal tubular acidosis in the null mouse.
    evidence:
    - reference: PMID:3126501
      reference_title: 'N-ethyl-N-nitrosourea-induced null mutation at the mouse Car-2 locus: an animal model for human carbonic anhydrase II deficiency syndrome.'
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: The CA II protein, which is normally expressed in most tissues, could not be detected by immunodiffusion analysis in any tissues of the CA II-deficient mice
      explanation: Confirms the model is a true protein null, so it is informative for the consequences of losing the enzyme.
  - target: Impaired Distal Tubular Proton Secretion
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    model_scale: CELLULAR
    description: Collecting-duct cellular and transporter changes support disruption of distal acid-base transport in Car2-null mice.
    limitations: Intercalated-cell markers and transporter abundance are proxy readouts, not direct measurements of proton secretion. The same cellular depletion has not been established in human kidney.
    readouts:
    - name: Collecting-duct intercalated cell depletion
      target: Impaired Distal Tubular Proton Secretion
      direction: DECREASED
      interpretation: Beyond loss of enzyme activity, the acid-secreting cell population itself is depleted and replaced by principal cells - a cell-population phenotype, not only an enzymatic one.
      evidence:
      - reference: PMID:8594870
        reference_title: Depletion of intercalated cells from collecting ducts of carbonic anhydrase II-deficient (CAR2 null) mice.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: These results suggest that intercalated cells are severely depleted and are replaced by principal cells in CAII-deficient mice.
        explanation: Immunocytochemical quantification of intercalated-cell loss in the CAR2-null collecting duct.
    - name: Collecting-duct bicarbonate transporter expression
      target: Impaired Distal Tubular Proton Secretion
      direction: DECREASED
      interpretation: Pendrin, Slc26a7 and AE1 expression all fall, consistent with loss of the intercalated-cell phenotype rather than an isolated enzyme deficit.
      evidence:
      - reference: PMID:18209476
        reference_title: Decreased expression of Slc26a4 (Pendrin) and Slc26a7 in the kidneys of carbonic anhydrase II-deficient mice.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: CAII deficiency results in a significant decrease in the gene and protein expression of bicarbonate transport proteins from Slc26 gene family - Slc26a4 (pendrin) and Slc26a7.
        explanation: Quantifies the transporter expression changes in the null kidney.
    evidence:
    - reference: PMID:8594870
      reference_title: Depletion of intercalated cells from collecting ducts of carbonic anhydrase II-deficient (CAR2 null) mice.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: These results suggest that intercalated cells are severely depleted and are replaced by principal cells in CAII-deficient mice.
      explanation: Immunocytochemical quantification of intercalated-cell loss in the CAR2-null collecting duct.
    - reference: PMID:18209476
      reference_title: Decreased expression of Slc26a4 (Pendrin) and Slc26a7 in the kidneys of carbonic anhydrase II-deficient mice.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: CAII deficiency results in a significant decrease in the gene and protein expression of bicarbonate transport proteins from Slc26 gene family - Slc26a4 (pendrin) and Slc26a7.
      explanation: Quantifies the transporter expression changes in the null kidney.
  - target: Accumulation of Unresorbed Sclerotic Bone with Preserved Osteoclast Number
    relationship: FAILS_TO_RECAPITULATE
    fidelity: LOW
    model_scale: ORGANISM
    description: The defining human phenotype is absent from the mouse null. Osteopetrosis could not be detected even in very old homozygotes.
    limitations: The model does not reproduce overt human osteopetrosis, limiting its use for that skeletal endpoint. This negative organism-level finding does not demonstrate normality of every osteoclast process; the basis of the species difference remains unresolved.
    readouts:
    - name: Skeletal density in aged homozygous null mice
      target: Accumulation of Unresorbed Sclerotic Bone with Preserved Osteoclast Number
      direction: UNCHANGED
      interpretation: 'A genuine negative result: no osteopetrosis at any age examined.'
      evidence:
      - reference: PMID:3126501
        reference_title: 'N-ethyl-N-nitrosourea-induced null mutation at the mouse Car-2 locus: an animal model for human carbonic anhydrase II deficiency syndrome.'
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: However, the prominent osteopetrosis found in humans with CA II deficiency could not be detected even in very old homozygous null mice.
        explanation: Direct statement of the negative skeletal finding.
    evidence:
    - reference: PMID:3126501
      reference_title: 'N-ethyl-N-nitrosourea-induced null mutation at the mouse Car-2 locus: an animal model for human carbonic anhydrase II deficiency syndrome.'
      supports: REFUTE
      evidence_source: MODEL_ORGANISM
      snippet: However, the prominent osteopetrosis found in humans with CA II deficiency could not be detected even in very old homozygous null mice.
      explanation: The negative skeletal finding demonstrates non-recapitulation of overt osteopetrosis, not exclusion of all mechanistic value of the model.
  evidence:
  - reference: PMID:2495727
    reference_title: Mice carrying a CAR-2 null allele lack carbonic anhydrase II immunohistochemically and show vascular calcification.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: The CA II-deficient mice differed from controls in evidencing an age dependent medial calcification of small arteries in a number of organs.
    explanation: The model does develop an age-dependent ectopic calcification, though arterial rather than cerebral. Recorded here as a characterised feature of the model, not as a model of the human brain phenotype - see the cerebral calcification discussion.
- name: CA II-deficient mice in recombinant growth hormone studies
  species: Mouse
  publication: PMID:9688969
  description: CA II-deficient acidotic mice were studied for growth-hormone/IGF physiology and randomized recombinant growth hormone response. Reduced GH secretion was observed in males, not females. Treatment increased growth despite persistent acidosis, and the follow-up time-course analysis showed a progressively increasing response. These experiments do not establish human GH deficiency or justify clinical efficacy claims.
  evidence:
  - reference: PMID:9688969
    reference_title: Disturbances of growth hormone-insulin-like growth factor axis and response to growth hormone in acidosis.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Randomized administration of rGH from approximately 5 to approximately 12 wk to CAD mice improved food efficiency and increased serum IGF-I levels, final length, and weight compared with placebo without affecting blood pH.
    explanation: Mouse intervention improves growth without correcting acidosis; it is not human treatment evidence.
  - reference: PMID:9688969
    reference_title: Disturbances of growth hormone-insulin-like growth factor axis and response to growth hormone in acidosis.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Thus growth failure in CAD mice is associated with a decrease in GH secretion in males but not in females.
    explanation: Sex-specific finding limits extrapolation to a universal endocrine mechanism.
  - reference: PMID:10975300
    reference_title: Time course of the response to recombinant growth hormone in acidotic mice.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Administration of GH increased standardized growth progressively up to 5-9 weeks.
    explanation: Follow-up analysis emphasizes duration of exposure in the mouse experiment.
treatments:
- name: Oral alkali therapy
  description: Oral bicarbonate or citrate treats the acid-base disturbance. In the child reported with obstructive sleep apnea, sodium bicarbonate and potassium citrate improved blood acidosis and bicarbonate. Earlier treatment was proposed to explain milder outcomes in a separate cohort, but that comparison does not establish a disease-modifying neurological effect.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: sodium bicarbonate
      term:
        id: CHEBI:32139
        label: sodium hydrogencarbonate
  target_mechanisms:
  - target: Systemic Hyperchloremic Metabolic Acidosis
    treatment_effect: MODULATES
    description: Exogenous alkali can improve systemic acidosis without correcting the underlying genotype.
    evidence:
    - reference: PMID:30109220
      reference_title: 'Carbonic Anhydrase II Deficiency: A Rare Case of Severe Obstructive Sleep Apnea.'
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Oral therapy with sodium bicarbonate and potassium citrate was started with improvement of blood acidosis (Table 1) and blood HCO3-.
      explanation: Direct clinical response supports correction of acidosis; it does not establish normalization of all disease manifestations.
  evidence:
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Several mouse models of CA II deficiency have shown growth hormone deficiency, yet currently there is no standard pharmacologic therapy for patients.
    explanation: Establishes that no pharmacologic therapy is standardised for this disease - the reason alkali therapy is described here as the mainstay for one arm rather than as a protocol.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
  - reference: PMID:30109220
    reference_title: 'Carbonic Anhydrase II Deficiency: A Rare Case of Severe Obstructive Sleep Apnea.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Oral therapy with sodium bicarbonate and potassium citrate was started with improvement of blood acidosis (Table 1) and blood HCO3-.
    explanation: Direct clinical response supports correction of acidosis; it does not establish normalization of all disease manifestations.
- name: Allogeneic hematopoietic stem cell transplantation
  description: Allogeneic transplantation supplies donor hematopoietic precursors and can resolve osteopetrosis. In two related Irish families, two transplanted children had histological and radiographic skeletal improvement with stabilized hearing and vision at two and three years. Renal tubular acidosis and developmental delay persisted; the older transplanted child subsequently developed cerebral calcification. The authors suggested delayed calcification, which is distinct from proving neurological correction or proving that transplantation has no cerebral effect. A later Iranian child had improving vision and complete donor chimerism at twelve months. These small reports do not establish routine use for every patient.
  therapeutic_modality: CELL_THERAPY
  treatment_term:
    preferred_term: allogeneic hematopoietic stem cell transplantation
    term:
      id: NCIT:C15431
      label: Hematopoietic Cell Transplantation
  target_mechanisms:
  - target: Osteoclast Resorption Lacuna Acidification Failure
    treatment_effect: RESTORES
    description: Donor hematopoietic precursors can restore the osteoclast lineage; skeletal histology and imaging support correction of bone disease rather than a direct measurement of lacunar pH.
    evidence:
    - reference: PMID:11264157
      reference_title: Bone marrow transplantation corrects osteopetrosis in the carbonic anhydrase II deficiency syndrome.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Allogeneic bone marrow stem cell replacement cures the osteoclast component of CAII deficiency and retards the development of cerebral calcification, but it appears to have little or no effect on the renal lesions.
      explanation: States precisely which arm the transplant corrects and which it does not, which is what makes this an arm-specific mechanism link.
  evidence:
  - reference: PMID:11264157
    reference_title: Bone marrow transplantation corrects osteopetrosis in the carbonic anhydrase II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Post-transplantation evaluation at 2 and 3 years demonstrates histologic and radiologic resolution of their osteopetrosis with stabilization of hearing and vision.
    explanation: Documents resolution of the skeletal disease after transplantation.
  - reference: PMID:38655726
    reference_title: 'Allogenic hematopoietic stem cell transplantation in an Iranian patient with osteopetrosis caused by carbonic anhydrase II deficiency: A case report.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: He Engrafted on day +13, and 95% chimerism was achieved.
    explanation: A recent worked case with matched related donor engraftment, confirming feasibility in this specific genotype.
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Although CA II deficiency is an "osteoclast-rich" OPT, and therefore transplantation of healthy osteoclasts can improve the skeletal disease, the RTA and central nervous system difficulties persist.
    explanation: The review summarizes skeletal improvement with persistent renal and central nervous system difficulties; it does not establish that every extra-skeletal outcome is unchanged.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
  - reference: PMID:11264157
    reference_title: Bone marrow transplantation corrects osteopetrosis in the carbonic anhydrase II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Both children remain developmentally delayed and continue to have RTA, and the older child has now developed cerebral calcification.
    explanation: Documents persistent extra-skeletal findings despite skeletal response.
  - reference: PMID:38655726
    reference_title: 'Allogenic hematopoietic stem cell transplantation in an Iranian patient with osteopetrosis caused by carbonic anhydrase II deficiency: A case report.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: He is currently doing well without immunosuppressive therapy, now 12 months post HSCT, with normal calcium level and improving visual quality and FISH analysis revealed complete donor chimerism.
    explanation: The later report provides twelve-month clinical and chimerism outcomes, not proof of lifelong multisystem cure.
- name: Continuous positive airway pressure for obstructive sleep apnea
  description: CPAP for craniofacial obstructive sleep apnea. Reported in a single child in whom adherence built up gradually from a minimal starting pressure. Relevant because the airway obstruction in this disease is skeletal in origin, so adenotonsillectomy alone may not resolve it - in the reported family the twin had a severe respiratory complication after that surgery.
  therapeutic_modality: DEVICE
  treatment_term:
    preferred_term: continuous positive airway pressure
    term:
      id: NCIT:C124040
      label: Continuous Positive Airway Pressure
  target_phenotypes:
  - preferred_term: Obstructive sleep apnea
    term:
      id: HP:0002870
      label: Obstructive sleep apnea
  evidence:
  - reference: PMID:30109220
    reference_title: 'Carbonic Anhydrase II Deficiency: A Rare Case of Severe Obstructive Sleep Apnea.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Herein we report a case of osteopetrosis with RTA associated with severe OSAS successfully treated with continuous positive airway pressure (CPAP).
    explanation: The single report of CPAP use in this disease.
- name: Ophthalmologic surveillance for compressive optic neuropathy
  description: Ophthalmologic assessment is relevant because optic nerve involvement can accompany canal narrowing, and progressive visual loss prompted transplantation in reported patients. The clinical series supports the indication but does not establish a surveillance interval.
  treatment_term:
    preferred_term: serial ophthalmologic examination and optic canal assessment
    term:
      id: NCIT:C38060
      label: Eye Examination
  target_phenotypes:
  - preferred_term: Optic atrophy
    term:
      id: HP:0000648
      label: Optic atrophy
  evidence:
  - reference: PMID:22120147
    reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Globes and retinae were normal, but optic nerve involvement was present in 23/46 eyes and was variable in severity, random in occurrence and statistically correlated with degree of optic canal narrowing
    explanation: 'The case for surveillance rather than symptom-driven referral: half of eyes affected, occurrence random so not predictable from the rest of the phenotype, and a measurable bony parameter that tracks it.'
  - reference: PMID:39667299
    reference_title: The correlation of intracranial parenchymal calcium score and the severity of neurological clinical presentation in carbonic anhydrase deficiency type 2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Optic nerve atrophy was observed in 22.2 %, while brain calcifications were present in 70.0 % of cases.
    explanation: Frequency of optic nerve atrophy in a consecutive pediatric series, which is the prevalence that justifies routine rather than selective assessment.
  - reference: PMID:11264157
    reference_title: Bone marrow transplantation corrects osteopetrosis in the carbonic anhydrase II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Two children, one from each family, have undergone allogeneic bone marrow transplantation because of severe progressive visual and hearing loss.
    explanation: 'Surveillance has a consequence here: detected progressive visual loss was the stated indication for escalating to transplantation in these families.'
  - reference: PMID:39667299
    reference_title: The correlation of intracranial parenchymal calcium score and the severity of neurological clinical presentation in carbonic anhydrase deficiency type 2.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Follow-up brain imaging may not aid in prognosis or severity assessment, highlighting the need for further research.
    explanation: Scopes the surveillance claim to the eye. The same series found serial brain imaging unhelpful for prognosis, so this entry recommends ophthalmologic monitoring and does not recommend routine repeat neuroimaging. Indirect because it argues for the scope of surveillance by excluding a different modality rather than by assessing eye examination.
  - reference: PMID:11938359
    reference_title: Long-term follow up of carbonic anhydrase II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Close clinical and neurological assessment of these patients is mandatory to early detect and manage potential serious complications.
    explanation: The longitudinal cohort explicitly recommends close assessment, without a defined interval.
- name: Dental and orthodontic care
  description: Dental evaluation and appropriate restorative or orthodontic care address eruption disturbances, malocclusion and enamel defects. The clinical evidence characterizes the indication; it does not establish a tested CA2-specific dental treatment protocol.
  treatment_term:
    preferred_term: restorative and orthodontic dental procedures
    term:
      id: NCIT:C38052
      label: Dental Procedure
  target_phenotypes:
  - preferred_term: Amelogenesis imperfecta
    term:
      id: HP:0000705
      label: Amelogenesis imperfecta
  - preferred_term: Delayed eruption of teeth
    term:
      id: HP:0000684
      label: Delayed eruption of teeth
  evidence:
  - reference: PMID:37662627
    reference_title: Carbonic anhydrase II deficiency syndrome with amelogenesis imperfecta linked to a homozygous CA2 deletion.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The oral manifestations included anterior open bite, posterior crossbite, tooth eruption impairment, and hypoplastic amelogenesis imperfecta (AI).
    explanation: The specific oral findings this care addresses - two malocclusion patterns, impaired eruption, and an enamel defect.
  - reference: PMID:37662627
    reference_title: Carbonic anhydrase II deficiency syndrome with amelogenesis imperfecta linked to a homozygous CA2 deletion.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: This study suggests that AI can be another feature of CAII deficiency syndrome.
    explanation: The authors' own hedged statement that amelogenesis imperfecta belongs to this syndrome. Quoted as hedged because it rests on one patient.
- name: Genetic counseling, carrier testing and prenatal diagnosis
  description: Counseling can explain recessive inheritance and support informed reproductive choices. Familial CA2 variant testing enables carrier and prenatal diagnosis. Historical erythrocyte enzyme studies also detected reduced activity in obligate carriers.
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Prenatal diagnosis requires mutational analysis of CA2.
    explanation: Establishes that prenatal diagnosis is molecular.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
  - reference: PMID:15300855
    reference_title: 'Carbonic anhydrase II deficiency syndrome (osteopetrosis with renal tubular acidosis and brain calcification): novel mutations in CA2 identified by direct sequencing expand the opportunity for genotype-phenotype correlation.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: In two opportunities for prenatal diagnosis, one from cultured amniocytes and one from chorionic villus biopsy, we demonstrated the general utility of the direct sequencing method for prenatal DNA diagnosis.
    explanation: Documents prenatal diagnosis on both sample types.
  - reference: PMID:9580777
    reference_title: Clinical and molecular heterogeneity in carbonic anhydrase II deficiency and prenatal diagnosis in an Italian family.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The identification of the mutation in this family provided the opportunity for prenatal diagnosis in a subsequent pregnancy.
    explanation: A worked instance of family-specific prenatal diagnosis.
- name: Closed fracture reduction and cast immobilization
  description: Closed reduction with an above-knee cast was used after medical stabilization in the 2024 child with bilateral tibial fractures. The report describes subsequent healing; it does not establish a comparative orthopedic protocol.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: closed fracture reduction and cast immobilization
    term:
      id: NCIT:C49236
      label: Therapeutic Procedure
  target_phenotypes:
  - preferred_term: Recurrent fractures
    term:
      id: HP:0002757
      label: Recurrent fractures
  evidence:
  - reference: PMID:38328522
    reference_title: 'Carbonic Anhydrase II Deficiency: Unusual Presentation of the Arabic Mutation. A Case Report.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: A closed reduction and above-knee cast was applied after stabilization.
    explanation: Directly reported orthopedic treatment in this patient.
  notes: The NCIT query `runoak -i sqlite:obo:ncit search t~fracture` did not return a fracture-reduction procedure; NCIT:C206538 is Loss of Fracture Reduction and does not represent the intervention. The parent Therapeutic Procedure is used with the intervention specified in preferred_term.
- name: Potassium replacement for hypokalemia
  description: Potassium chloride was given for recurrent hypokalemia during the 2024 admission. This is correction of an electrolyte complication, not evidence of modifying the underlying CA2 disorder.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: potassium chloride
      term:
        id: CHEBI:32588
        label: potassium chloride
  evidence:
  - reference: PMID:38328522
    reference_title: 'Carbonic Anhydrase II Deficiency: Unusual Presentation of the Arabic Mutation. A Case Report.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: He was started on potassium chloride (KCl) 30 mEq/L added to intravenous (IV) fluid.
    explanation: Documents use in the individual report; no general efficacy estimate is inferred.
- name: Vitamin D supplementation
  description: Vitamin D3 supplementation was included in the Jordanian child’s discharge treatment. The report does not establish a CA2-specific skeletal or neurological benefit.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: vitamin D3
      term:
        id: CHEBI:28940
        label: calciol
  evidence:
  - reference: PMID:35035649
    reference_title: 'Blessing in disguise: when head trauma solves the riddle of carbonic anhydrase II deficiency.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The patient was discharged on sodium bicarbonate therapy, lactulose and vitamin D3 supplements and has been followed up regularly.
    explanation: Documents use in the individual report; no general efficacy estimate is inferred.
discussions:
- discussion_id: ca2_cerebral_calcification_mechanism
  prompt: By what route does loss of cytosolic carbonic anhydrase II produce parenchymal calcium deposition in basal ganglia and thalami, and why does the calcium burden not track neurodevelopmental outcome?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Intracranial Parenchymal Calcium Deposition
  - phenotypes#Cerebral calcification
  - phenotypes#Intellectual disability
  rationale: Intracranial calcification and cognitive impairment are observed, but their causal relationship and their route from CA II deficiency remain unresolved. Calcification can be absent during childhood. In a ten-patient series, calcium score did not significantly associate with age, developmental quotient or optic canal diameter; this small negative study does not prove that calcium burden can never affect neurological outcome. The early transplant series suggested delayed calcification while developmental impairment persisted.
  evidence:
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Although this enzymopathy reveals how CA II is important for the skeleton and kidney tubule, the pathogenesis of the mental subnormality and cerebral calcification is less well understood.
    explanation: The explicit statement that this arm is the unexplained one, contrasted with the two arms that are explained.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
  - reference: PMID:39667299
    reference_title: The correlation of intracranial parenchymal calcium score and the severity of neurological clinical presentation in carbonic anhydrase deficiency type 2.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Correlation analysis revealed no significant associations between intracranial parenchymal calcium score and age, DQ score, or optic canal diameter.
    explanation: The negative correlation that blocks the obvious inference from calcium burden to cognitive outcome.
  - reference: PMID:22120147
    reference_title: The neurology of carbonic anhydrase type II deficiency syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Variability of brain calcification, cognitive disturbance and optic nerve involvement may imply additional genetic or epigenetic influences affecting the course of the disease.
    explanation: Genotype-identical patients diverge on exactly the features in question, which is what makes this a mechanism gap rather than a sampling problem.
- discussion_id: ca2_mouse_null_lacks_osteopetrosis
  prompt: Why does the Car-2 null mouse reproduce the renal and growth arms of carbonic anhydrase II deficiency but not the osteopetrosis, and what model is therefore valid for studying the human bone arm?
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Osteoclast Resorption Lacuna Acidification Failure
  - pathophysiology#Impaired Osteoclastic Bone Resorption
  - animal_models#Mouse
  rationale: Car2-null mice reproduce growth restriction and renal tubular acidosis but did not develop osteopetrosis even at old ages. The reason for this species difference is unresolved. It limits translation of skeletal endpoints from this mouse, while leaving the model useful for renal physiology and for investigating compensation. The negative skeletal result should not be generalized to every cellular resorption assay.
  evidence:
  - reference: PMID:3126501
    reference_title: 'N-ethyl-N-nitrosourea-induced null mutation at the mouse Car-2 locus: an animal model for human carbonic anhydrase II deficiency syndrome.'
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: However, the prominent osteopetrosis found in humans with CA II deficiency could not be detected even in very old homozygous null mice.
    explanation: The negative skeletal result in a confirmed protein null, which is the mismatch itself.
  - reference: PMID:3126501
    reference_title: 'N-ethyl-N-nitrosourea-induced null mutation at the mouse Car-2 locus: an animal model for human carbonic anhydrase II deficiency syndrome.'
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Like humans with the same inherited enzyme defect, animals homozygous for the new null allele are runted and have renal tubular acidosis.
    explanation: The same animal reproduces the renal and growth arms, so the failure is specific to bone rather than a failed model overall.
- discussion_id: ca2_growth_failure_attribution
  prompt: Is the short stature of carbonic anhydrase II deficiency caused by chronic metabolic acidosis, by the skeletal disease, or by growth hormone deficiency?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - phenotypes#Short stature
  - pathophysiology#Systemic Hyperchloremic Metabolic Acidosis
  rationale: Short stature is well documented clinically, but contributions from acidosis, skeletal disease and endocrine dysfunction remain unresolved. In CA II-deficient mice, low GH secretion was male-specific, and randomized GH administration increased linear growth despite persistent acidosis. This demonstrates a modifiable mouse growth phenotype, not an established human GH-deficiency mechanism or clinical treatment effect.
  evidence:
  - reference: PMID:36709914
    reference_title: Carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Several mouse models of CA II deficiency have shown growth hormone deficiency, yet currently there is no standard pharmacologic therapy for patients.
    explanation: Introduces the growth-hormone candidate, and in the same sentence records that no therapy is standardised.
    quote_role: REVIEW_SYNTHESIS
    directness: INDIRECT
  - reference: PMID:3126501
    reference_title: 'N-ethyl-N-nitrosourea-induced null mutation at the mouse Car-2 locus: an animal model for human carbonic anhydrase II deficiency syndrome.'
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Like humans with the same inherited enzyme defect, animals homozygous for the new null allele are runted and have renal tubular acidosis.
    explanation: Growth restriction and acidosis co-occur in the null mouse, which is the observation behind the acidosis hypothesis - but it is co-occurrence, not a demonstrated mechanism.
  - reference: PMID:9688969
    reference_title: Disturbances of growth hormone-insulin-like growth factor axis and response to growth hormone in acidosis.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Randomized administration of rGH from approximately 5 to approximately 12 wk to CAD mice improved food efficiency and increased serum IGF-I levels, final length, and weight compared with placebo without affecting blood pH.
    explanation: Mouse intervention improves growth without correcting acidosis; it is not human treatment evidence.
  - reference: PMID:9688969
    reference_title: Disturbances of growth hormone-insulin-like growth factor axis and response to growth hormone in acidosis.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Thus growth failure in CAD mice is associated with a decrease in GH secretion in males but not in females.
    explanation: The endocrine observation is sex-specific in the animal study.
- discussion_id: ca2_residual_renal_carbonic_anhydrase
  prompt: If CA II is the only soluble carbonic anhydrase isoenzyme in the kidney, why do CA II-deficient patients still respond to intravenous acetazolamide with a rise in urinary pH and bicarbonate excretion?
  kind: OPEN_QUESTION
  status: OPEN
  attaches_to:
  - pathophysiology#Impaired Proximal Tubular Bicarbonate Reclamation
  rationale: 'Patients with undetectable erythrocyte CA II nevertheless show a normal renal response to acetazolamide. The original authors set out three explanations - that the deficiency is not expressed in kidney, that another acetazolamide-sensitive soluble isoenzyme is expressed there in these patients, or that the response reflects the membrane-bound luminal carbonic anhydrase, a different gene product unaffected by the CA2 mutation - and favoured the third. This is recorded as an open question rather than settled because the entry''s renal node depends on it: if luminal membrane-bound activity is intact, the human renal defect is a partial loss of tubular carbonic anhydrase function, not a total one, and the residual activity is a potential target. It also bears on the interpretation of acetazolamide challenge as a diagnostic test, which would be uninformative here.'
  evidence:
  - reference: PMID:3932950
    reference_title: Positive renal response to intravenous acetazolamide in patients with carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Following acetazolamide infusion, the CA II-deficient patients exhibited a prompt rise in urinary pH and HCO3- excretion similar to the response seen in control subjects.
    explanation: The observation that creates the question.
  - reference: PMID:3932950
    reference_title: Positive renal response to intravenous acetazolamide in patients with carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: These findings indicate that CA II-deficient patients, who lack detectable CA II in their erythrocytes, still expressed an acetazolamide-inhibitable CA activity in their kidneys.
    explanation: The authors' inference that residual acetazolamide-sensitive activity persists in the CA II-deficient kidney.
- discussion_id: ca2_osteopetrosis_rta_phenocopy
  prompt: Does the combination of osteopetrosis and renal tubular acidosis always mean CA II deficiency, given that coinherited TCIRG1 and ATP6V1B1 defects can reproduce it?
  kind: INTERPRETATION
  status: OPEN
  attaches_to:
  - phenotypes#Osteopetrosis
  - phenotypes#Renal tubular acidosis
  - genetic#CA2
  rationale: The clinical triad is a very strong pointer to CA2 and is often treated as diagnostic, but it is not pathognomonic, and the documented exception is instructive rather than merely pedantic. One kindred with osteopetrosis and distal renal tubular acidosis had normal CA II levels and no CA2 defect; instead two separate recessive disorders were segregating, a TCIRG1 deletion producing the osteopetrosis and an ATP6V1B1 mutation producing the distal RTA. Both genes encode subunits of the same proton pump, in tissue-specific isoforms. The practical consequence is that the triad justifies CA2 testing and the enzyme assay, but a negative CA2 result in a patient with the full phenotype should prompt a search for coinherited pump subunit defects rather than a presumption of a cryptic CA2 variant.
  evidence:
  - reference: PMID:12566520
    reference_title: 'A phenocopy of CAII deficiency: a novel genetic explanation for inherited infantile osteopetrosis with distal renal tubular acidosis.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: In this kindred, two separate recessive disorders are penetrant, each affecting a different, tissue specific subunit of the vacuolar proton pump (H(+)-ATPase), providing a highly unusual, novel genetic explanation for the coexistence of osteopetrosis and dRTA.
    explanation: The documented genetic phenocopy of the CA II deficiency phenotype.
references:
- reference: PMID:36709914
  title: Carbonic anhydrase II deficiency.
- reference: PMID:6405388
  title: Carbonic anhydrase II deficiency identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.
- reference: PMID:3925334
  title: Carbonic anhydrase II deficiency in 12 families with the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.
- reference: PMID:22120147
  title: The neurology of carbonic anhydrase type II deficiency syndrome.
- reference: PMID:38328522
  title: 'Carbonic Anhydrase II Deficiency: Unusual Presentation of the Arabic Mutation. A Case Report.'
- reference: PMID:11938359
  title: Long-term follow up of carbonic anhydrase II deficiency syndrome.
- reference: PMID:9688969
  title: Disturbances of growth hormone-insulin-like growth factor axis and response to growth hormone in acidosis.
- reference: PMID:10975300
  title: Time course of the response to recombinant growth hormone in acidotic mice.
diagnosis:
- name: Clinical and radiographic recognition
  description: Osteopetrosis, renal tubular acidosis and cerebral calcification form the classic clinical pattern. The complete triad is not required at assessment, and the pattern alone is not fully gene-specific. Skeletal imaging, acid-base studies and brain imaging characterize the presentation.
  evidence:
  - reference: PMID:30109220
    reference_title: 'Carbonic Anhydrase II Deficiency: A Rare Case of Severe Obstructive Sleep Apnea.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The deficiency of carbonic anhydrase II causes a moderate form, presenting classically as a triad of osteopetrosis, renal tubular acidosis (RTA), and cerebral calcification.
    explanation: States the classical triad.
  - reference: PMID:8128957
    reference_title: 'Carbonic anhydrase II deficiency: single-base deletion in exon 7 is the predominant mutation in Caribbean Hispanic patients.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The proband was a 2 1/2-year-old Hispanic girl of Puerto Rican ancestry who was unique clinically, in that she had no evidence of renal tubular acidosis, even though she did have osteopetrosis, developmental delay, and cerebral calcification.
    explanation: The clinical triad is incomplete in this molecularly confirmed patient, so all three features are not required for recognition.
- name: Molecular confirmation
  description: Identify biallelic disease-causing CA2 variants and interpret them with the clinical findings. Published methods include sequencing all seven exons and whole-exome sequencing; splice and truncating alleles require variant-specific assessment.
  evidence:
  - reference: PMID:15300855
    reference_title: 'Carbonic anhydrase II deficiency syndrome (osteopetrosis with renal tubular acidosis and brain calcification): novel mutations in CA2 identified by direct sequencing expand the opportunity for genotype-phenotype correlation.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: we amplified all seven exons by PCR from genomic DNA and directly sequenced the amplified products
    explanation: The molecular confirmation method, covering all seven exons.
  - reference: PMID:1542674
    reference_title: Molecular basis of human carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The three affected siblings from this family are compound heterozygotes, each having inherited two different mutations in the structural gene for carbonic anhydrase II.
    explanation: Molecular diagnosis includes compound heterozygosity, not only homozygous variants.
- name: Erythrocyte CA II assay
  description: Marked reduction of CA II protein and activity with preserved CA I supports the enzymatic diagnosis. Erythrocyte measurements do not establish enzyme activity in every tissue.
  evidence:
  - reference: PMID:6405388
    reference_title: Carbonic anhydrase II deficiency identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: We report here biochemical and immunological evidence for the virtual absence of CA II in erythrocytes of patients affected with this condition, whereas CA I level is not reduced.
    explanation: The measurement behind this readout, and the isoenzyme specificity that makes an absent result interpretable.
experimental_models:
- name: Recombinant His107Tyr CA II
  experimental_model_type: OTHER
  description: Bacterially expressed mutant CA II retains detectable but greatly reduced catalytic activity.
  publication: PMID:1542674
  modeled_mechanisms:
  - target: Biallelic CA2 Loss of Function
    relationship: PARTIALLY_RECAPITULATES
    model_scale: MOLECULAR
    description: Bacterially expressed mutant CA II retains detectable but greatly reduced catalytic activity.
    limitations: Residual enzyme activity in a heterologous system does not measure patient-tissue protein abundance or establish the cause of milder cognition.
    evidence:
    - reference: PMID:1542674
      reference_title: Molecular basis of human carbonic anhydrase II deficiency.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: We show that the mutant enzyme expressed in bacteria from the cDNA containing the His-107----Tyr mutation has detectable, though greatly reduced, activity.
      explanation: Direct biochemical activity assay of the recombinant allele.
  evidence:
  - reference: PMID:1542674
    reference_title: Molecular basis of human carbonic anhydrase II deficiency.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: We show that the mutant enzyme expressed in bacteria from the cDNA containing the His-107----Tyr mutation has detectable, though greatly reduced, activity.
    explanation: Direct biochemical activity assay of the recombinant allele.
- name: CA2 exon 7 frameshift expression in COS cells
  experimental_model_type: CELL_LINE
  description: The patient-associated exon 7 frameshift produces a truncated, inactive protein with aggregation and accelerated degradation in transfected COS cells.
  publication: PMID:8128957
  modeled_mechanisms:
  - target: Biallelic CA2 Loss of Function
    relationship: PARTIALLY_RECAPITULATES
    model_scale: MOLECULAR
    description: The patient-associated exon 7 frameshift produces a truncated, inactive protein with aggregation and accelerated degradation in transfected COS cells.
    limitations: Overexpression in COS cells measures allele-specific processing and activity, not human osteoclast resorption or renal transport.
    evidence:
    - reference: PMID:8128957
      reference_title: 'Carbonic anhydrase II deficiency: single-base deletion in exon 7 is the predominant mutation in Caribbean Hispanic patients.'
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: The 27-kD truncated enzyme produced when the mutant cDNA was expressed in COS cells was enzymatically inactive, present mainly in insoluble aggregates, and detectable immunologically at only 5% the level of the 29-kD normal carbonic anhydrase II expressed from the wild-type cDNA.
      explanation: Direct expression and biochemical measurements in a heterologous cell line.
  evidence:
  - reference: PMID:8128957
    reference_title: 'Carbonic anhydrase II deficiency: single-base deletion in exon 7 is the predominant mutation in Caribbean Hispanic patients.'
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: The 27-kD truncated enzyme produced when the mutant cDNA was expressed in COS cells was enzymatically inactive, present mainly in insoluble aggregates, and detectable immunologically at only 5% the level of the 29-kD normal carbonic anhydrase II expressed from the wild-type cDNA.
    explanation: Direct expression and biochemical measurements in a heterologous cell line.
  - reference: PMID:8128957
    reference_title: 'Carbonic anhydrase II deficiency: single-base deletion in exon 7 is the predominant mutation in Caribbean Hispanic patients.'
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: Metabolic labeling revealed that this 27-kD mutant protein has an accelerated rate of degradation.
    explanation: Pulse-labeling provides evidence of altered turnover in the cell-expression model.
- name: Purified Gly145Arg and His94Tyr CA II proteins
  experimental_model_type: OTHER
  description: Site-directed mutant proteins were expressed and purified for spectroscopic stability and activity studies; both showed local structural changes and reduced native-state stability.
  publication: PMID:31778702
  modeled_mechanisms:
  - target: Biallelic CA2 Loss of Function
    relationship: PARTIALLY_RECAPITULATES
    model_scale: MOLECULAR
    description: Site-directed mutant proteins were expressed and purified for spectroscopic stability and activity studies; both showed local structural changes and reduced native-state stability.
    limitations: Protein stability assays do not reproduce tissue-selective clinical disease and should not be treated as measurements in patient organs.
    evidence:
    - reference: PMID:31778702
      reference_title: Effect of disease-linked mutations on the structure, function, stability and aggregation of human carbonic anhydrase II.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: Studies suggest that the mutant proteins undergo local structural perturbations and have compromised native state stability.
      explanation: Purified-protein studies used circular dichroism, fluorescence, NMR and activity assays.
  evidence:
  - reference: PMID:31778702
    reference_title: Effect of disease-linked mutations on the structure, function, stability and aggregation of human carbonic anhydrase II.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: Studies suggest that the mutant proteins undergo local structural perturbations and have compromised native state stability.
    explanation: Purified-protein studies used circular dichroism, fluorescence, NMR and activity assays.

📚

References & Deep Research

References

8

PMID:36709914

Carbonic anhydrase II deficiency.

No top-level findings curated for this source.

PMID:6405388

Carbonic anhydrase II deficiency identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.

No top-level findings curated for this source.

PMID:3925334

Carbonic anhydrase II deficiency in 12 families with the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification.

No top-level findings curated for this source.

PMID:22120147

The neurology of carbonic anhydrase type II deficiency syndrome.

No top-level findings curated for this source.

PMID:38328522

Carbonic Anhydrase II Deficiency: Unusual Presentation of the Arabic Mutation. A Case Report.

No top-level findings curated for this source.

PMID:11938359

Long-term follow up of carbonic anhydrase II deficiency syndrome.

No top-level findings curated for this source.

PMID:9688969

Disturbances of growth hormone-insulin-like growth factor axis and response to growth hormone in acidosis.

No top-level findings curated for this source.

PMID:10975300

Time course of the response to recombinant growth hormone in acidotic mice.

No top-level findings curated for this source.

Deep Research

1

Deep research results are used as seeds for research; they do not undergo the same validation as the main records and may contain errors. How we use deep research.

Evaluations and curation notes (1)

Create: Autosomal_Recessive_Osteopetrosis_3 · 2026-09-10T17:29:03Z · View source

Created kb/disorders/Autosomal_Recessive_Osteopetrosis_3.yaml (MONDO:0009818, carbonic anhydrase II deficiency / CA2, OMIM 259730) as a new DISEASE entry. Lump/split: single DISEASE — one enzymatic lesion, conserved three-arm pathograph, near-absolute bidirectional gene-disease specificity (PMID:15300855, PMID:12566520); no grouping built and no parent subtype attachment. Verified before curating that neither kb/disorders/Osteopetrosis.yaml nor Autosomal_Recessive_Osteopetrosis.yaml nor either autosomal dominant entry mentioned CA2, carbonic anhydrase or renal tubular acidosis, and that no kb/ file bound hgnc:1373, on the working tree and on origin/main. Deep research: 'just research-disorder falcon' returned HTTP 402 (Edison account out of credits); re-run once with dr_fallback='--fallback', which fell back to openscientist and recorded fell_back/requested_provider/provider_attempts in the report frontmatter. Report used: research/Autosomal_Recessive_Osteopetrosis_3-deep-research-openscientist.md (18/18 references resolved, 17/18 on topic; term_validation needs_review=true). Acted on the report's Term Validation: did NOT bind HP:0031815 for cranial nerve compression (it resolves to 'Abnormal oral physiology'); bound independently looked-up HP:0001293 instead. Also did not reuse the report's GO:0001618 'ruffled border' suggestion (that CURIE is virus receptor activity) or its GO:0051452-labelled 'regulation of intracellular pH'. just preflight-dr emitted WARN on two counts, both false positives: the 'rival gene HP' signal is HPO CURIE prefixes being read as the haptoglobin symbol, and the OMIM mismatch is the report citing the CA2 gene OMIM 611492 alongside the disease OMIM 259730 that MONDO xrefs. Gene binding verified bidirectionally against the HGNC REST API in the same step it was written (CA2 -> HGNC:1373 and HGNC:1373 -> CA2); hgnc:13733 is CDH23, which is the near-miss the task flagged. Pathograph: 10 nodes, each with biological_scale, running from Biallelic CA2 Loss of Function through Loss of Cytosolic Proton and Bicarbonate Supply, which branches into the osteoclast, renal and cerebral arms; the cerebral edge is typed INDIRECT_UNKNOWN_INTERMEDIATES and its node marked mechanism_confidence HYPOTHETICAL. No conforms_to: osteoporosis_bone_resorption is the mechanistic inverse, defective_skeletal_mineralization covers hypomineralization, and no osteoclast-acidification module exists yet — recorded in notes as a follow-up. 18 phenotypes with HP bindings, including two deliberate SUPPORT+REFUTE pairs where cohorts genuinely disagree (renal tubular acidosis as obligate, and hearing impairment). Car-2 null mouse recorded as RECAPITULATES for the renal arm and FAILS_TO_RECAPITULATE for the bone arm, with a HUMAN_MODEL_MISMATCH discussion. Five discussions: cerebral-calcification mechanism (KNOWLEDGE_GAP), mouse model mismatch (HUMAN_MODEL_MISMATCH), growth-failure attribution (KNOWLEDGE_GAP), residual renal carbonic anhydrase (OPEN_QUESTION), osteopetrosis+RTA phenocopy (INTERPRETATION). Deliberately omitted: interferon-gamma and vitamin D/calcium (general-osteopetrosis therapy, not CA2), datasets (GEO has no CA2-deficiency dataset; the three human osteopetrosis GEO series are CLCN7/ADO2, a different disease), and ISDS group membership beyond the group's own stated scope (the cached 2023 nosology record does not name CA2). No GeneReviews chapter exists for CA2-related osteopetrosis (PubMed carries TCIRG1 and CLCN7 chapters only), so that baseline could not be applied. Validators run and their results: just validate PASS (schema, terms, references); just validate-terms PASS; just count-verified-snippets 112/112 verified; just check-duplicate-keys PASS; just check-entity-refs PASS; just check-causal-targets PASS (no new dangling targets); just check-qualifier-terms PASS; just check-enum-values PASS; just normalize-cache clean; just check-term-cache-integrity PASS; just validate-disorders PASS (batched authoritative gate, 112/112); whole-KB just check-folded-hyphens, check-snippet-length, check-title-snippets and check-snippet-grading all PASS with no new findings. just compliance 95.2%. One label divergence handled explicitly: HP:0000787 is 'Kidney stone' via OLS and 'Nephrolithiasis' in cache/hp/terms.csv; the cached form is bound because that is what term validation compares against, and the divergence is recorded in the phenotype notes.

OpenScientist ▸

1. Disease Information

openscientist-autonomous 18 citations 2026-09-10T16:52:35.507275

1. Disease Information

Overview. OPTB3 / CA II deficiency syndrome was originally described as "osteopetrosis with renal tubular acidosis and cerebral calcification syndrome" (also historically "marble brain disease"). It reveals a critical, non-redundant role for carbonic anhydrase II in both osteoclast and renal tubule function. As the authoritative review states, it "reveals an important role for the enzyme carbonic anhydrase II (CA II) in osteoclast and renal tubule function" PMID: 36709914. More than 100 affected individuals have been reported worldwide, with a marked predominance in the Middle East and Mediterranean basin, reflecting consanguinity and a founder allele.

Key identifiers.

Resource	Identifier
MONDO	MONDO:0009818
OMIM	#259730
Gene (OMIM)	CA2, 611492
MeSH	Osteopetrosis; Carbonic Anhydrase II deficiency
ICD-10	Q78.2 (Osteopetrosis)
Orphanet	ORPHA:2785 (Osteopetrosis with renal tubular acidosis)

Synonyms / alternative names. Carbonic anhydrase II deficiency syndrome; CA2 deficiency syndrome; osteopetrosis with renal tubular acidosis and cerebral calcification (OP-RTA); marble brain disease; Guibaud–Vainsel syndrome; osteopetrosis type 3, autosomal recessive.

Data provenance. Information is derived from aggregated disease-level resources (OMIM, Orphanet) and primary literature consisting largely of case reports and small consanguineous-family case series, rather than from EHR-scale individual-patient datasets.

2. Etiology

Disease causal factors. The disease is entirely genetic: bi-allelic loss-of-function mutations of CA2, which encodes carbonic anhydrase II. As stated directly in the authoritative review, "The etiology is bi-allelic loss-of-function mutations of CA2 that encodes CA II" PMID: 36709914. There is no environmental or infectious cause.

Genetic risk factors. The single necessary and sufficient risk factor is inheriting two loss-of-function CA2 alleles. Consanguinity is the dominant epidemiological risk factor because it raises the probability of homozygosity for a recessive allele; nearly all reported large series come from consanguineous unions. The intron 2 splice-site "Arabic mutation" is a founder allele confined to patients of Arab background: "This mutation is found exclusively in patients with an Arabic background and thus may be confined to this ethnic group" PMID: 7959703.

Environmental / lifestyle / protective factors. None established. Because the disease is monogenic and fully penetrant for the biochemical enzyme defect, there are no recognized environmental risk factors, protective dietary/lifestyle factors, or protective genetic variants. Gene–environment interactions are not a described feature. (Not applicable / no data.)

3. Phenotypes

The phenotype spectrum with approximate frequencies and suggested HPO terms is summarized below. Frequencies are drawn from case series, notably a 10-patient Saudi series and a 23-patient Arabic-mutation cohort.

Phenotype	HPO term	Frequency	Type
Osteopetrosis (increased bone density)	HP:0011002	~100%	Physical/radiologic
Renal tubular acidosis	HP:0001947	~100%	Laboratory
Cerebral / basal-ganglia calcification	HP:0002514	~70% (near-universal later)	Radiologic
Global developmental delay	HP:0001263	~60%	Clinical sign
Intellectual disability / mental retardation	HP:0001249	~2/3	Clinical sign
Short stature	HP:0004322	Common	Physical
Recurrent fractures	HP:0002757	Common	Physical
Craniofacial disproportion / broad forehead	HP:0000929 / HP:0011220	Common	Physical
Optic nerve atrophy / visual impairment	HP:0000648	~22%	Clinical sign
Congenital nystagmus	HP:0000639	Reported	Clinical sign
Cranial nerve compression	HP:0031815	Variable	Clinical sign
Amelogenesis imperfecta / dental defects	HP:0000705 / HP:0006297	Reported	Physical
Obstructive sleep apnea	HP:0002870	Reported	Clinical sign
Nephrocalcinosis / urolithiasis / hypercalciuria	HP:0000121	Some patients	Laboratory/imaging

Neurodevelopment. In a modern series, "60.0% of patients presented with global developmental delay, 20.0% had intellectual disability, and the remaining 20.0% had normal development" PMID: 39667299. The same series reported "Optic nerve atrophy was observed in 22.2%, while brain calcifications were present in 70.0% of cases" PMID: 39667299.

Optic nerve involvement. In the 23-patient Arabic-mutation cohort, "optic nerve involvement was present in 23/46 eyes and was variable in severity, random in occurrence and statistically correlated with degree of optic canal narrowing" PMID: 22120147 — implicating bony compression of the optic canal rather than a primary neuropathy.

Dental/oral. "The oral manifestations included anterior open bite, posterior crossbite, tooth eruption impairment, and hypoplastic amelogenesis imperfecta (AI)" PMID: 37662627.

Airway. Craniofacial dysmorphism "leads to specific craniofacial dysmorphisms associated with upper airway obstruction that may result in obstructive sleep apnea" PMID: 30109220.

Onset, severity, progression. Onset is typically late infancy to early childhood. Severity is variable even within families sharing the same mutation. Unlike malignant infantile osteopetrosis, hematologic marrow findings are usually mild or absent, and the skeletal phenotype tends to stabilize or improve with age.

Quality-of-life impact. Per-phenotype QoL instruments (EQ-5D, SF-36) have not been formally applied in this rare disease; impact is inferred from the burden of intellectual disability, visual loss, recurrent fractures, dental morbidity, and (in some) sleep-disordered breathing. (Limited data.)

4. Genetic / Molecular Information

Causal gene. CA2 (carbonic anhydrase II; HGNC:1373), chromosome 8q21.2, OMIM 611492. Somatic-cell-hybrid mapping first localized human CA2 to chromosome 8 and provided "a molecular disease marker, because human CA II deficiency has recently been linked to an autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification" PMID: 6410391.

Gene–disease specificity. The relationship is near-perfect: "With one exception, all patients with osteopetrosis and renal tubular acidosis examined have proven to have CA II deficiency. All CA II-deficient patients analyzed have been found to have mutations in the CA2 gene" PMID: 15300855.

Pathogenic variants. - Founder splice variant: intron 2 5′ splice-site "Arabic mutation" (c.232+1G>T) — the dominant allele in Arab/Mediterranean patients PMID: 7959703. - Nonsense: e.g., c.368G>A, p.W123X, reported homozygously in a Chinese family; the mutant protein shows "change of protein modification and hindrance of zinc ions binding, which may lead to decreased protein expression level of CA2" PMID: 33555497. - Missense variants: multiple, with pathogenicity assessed computationally; only ~50% predicted destabilizing by consensus free-energy methods, and structural fluctuations occur at residue level rather than whole-protein level PMID: 31542996. - Direct sequencing of all seven exons identified "eleven new mutations in 21 patients" PMID: 15300855.

Variant classification / type. Reported classes: splice-site, nonsense, missense, and small indels — predominantly loss-of-function. Under ACMG/AMP framework, null variants (nonsense, canonical splice) are typically classified Pathogenic; missense variants require functional/structural corroboration.

Allele frequency & origin. All disease alleles are germline; there is no somatic component. Pathogenic alleles are rare in gnomAD-scale databases but enriched locally in consanguineous populations via founder effect.

Functional consequences. Loss of catalytic function and/or destabilization/impaired zinc binding of the enzyme PMID: 33555497, PMID: 31542996.

Modifier genes / epigenetics / chromosomal abnormalities. No established modifier genes, epigenetic mechanisms, or chromosomal abnormalities. Intra- and inter-familial variability exists despite identical genotypes, implying unidentified modifiers or stochastic effects PMID: 9453381. (No specific modifier locus identified.)

5. Environmental Information

Not applicable. CA II deficiency is a fully genetic monogenic disorder with no environmental, lifestyle, or infectious contributing factors. Notably, the disease can clinically mimic infection — infantile osteopetrosis has been misdiagnosed as congenital cytomegalovirus infection PMID: 41204604 — but this reflects overlapping phenotypes (hepatosplenomegaly, hematologic and optic-nerve abnormalities), not an infectious etiology.

6. Mechanism / Pathophysiology

Ordered causal chain

Bi-allelic loss-of-function mutation in CA2 → leads to absent or catalytically dead carbonic anhydrase II protein in all tissues (erythrocytes, osteoclasts, renal tubule, brain).
Loss of CA II → results in failure of the cytosolic reaction CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻, i.e., loss of a rapid intracellular proton (H⁺) and bicarbonate supply.

Branch A — Bone: 3a. Depleted H⁺ supply in the osteoclast → results in inability of the ruffled-border V-ATPase to acidify the sealed resorption lacuna to ~pH 4.5. 4a. Failure to acidify the pericellular milieu → leads to failure to dissolve hydroxyapatite mineral → impaired bone resorption. 5a. Impaired resorption with continued bone formation → results in dense, brittle, poorly remodeled bone → osteopetrosis, marrow-space encroachment (usually mild), and cranial-foramen narrowing → cranial nerve (esp. optic) compression.

Branch B — Kidney: 3b. Loss of CA II in proximal and distal tubular epithelium → impairs H⁺ secretion and HCO₃⁻ reclamation. 4b. Impaired renal acid handling → results in renal tubular acidosis (proximal, distal, or mixed) with hyperchloremic metabolic acidosis, and in some patients hypercalciuria → nephrocalcinosis/urolithiasis.

Branch C — Brain (less defined): 3c. Loss of CA II in brain (oligodendrocytes/choroid plexus) → is uniquely associated with early-childhood basal-ganglia/cerebral calcification (mechanism inferred, not fully demonstrated). 4c. Calcification plus optic-canal narrowing plus (possibly) systemic acidosis → contributes to developmental delay/intellectual disability and visual impairment.

Supporting evidence and detail

The osteoclast arm is stated directly: "CA II deficiency is the paradigm OPT featuring failure of osteoclasts to resorb bone due to inability to acidify their pericellular milieu" PMID: 36709914. The renal arm: "In CA II deficiency, OPT is uniquely accompanied by renal tubular acidosis (RTA) of proximal, distal, or combined type featuring hyperchloremic metabolic acidosis" PMID: 36709914.

Convergent pathway context. CA II sits within a broader "osteoclast acidification" module: other osteopetrosis genes — TCIRG1 (a3 V-ATPase subunit), CLCN7 (chloride channel), OSTM1, SNX10, RANK/TNFRSF11A, RANKL/TNFSF11 — converge on the same functional axis of osteoclast-mediated bone resorption. The differential diagnosis is "radiologic, supported by biochemical and genetic examination to identify mutations in the key genes involved in osteoclasts, TCIRG1, CLCN7, OSTM1, SNX10, RANK, and RANKL" PMID: 42096006. CA II supplies the protons; the V-ATPase pumps them; CLCN7 provides the counter-ion chloride. This places CA2 in the osteoclast-rich (osteoclast present but non-functional) category, distinct from osteoclast-poor forms (RANK/RANKL).

Molecular/cellular/subcellular annotations (suggested ontology terms): - Biological process (GO): carbonate dehydratase activity (GO:0004089); one-carbon metabolic process (GO:0006730); regulation of intracellular pH (GO:0051452); bone resorption (GO:0045453); ossification (GO:0001503). - Cellular component (GO): cytosol (GO:0005829); ruffled border (GO:0001618). - Cell types (CL): osteoclast (CL:0000092); kidney proximal/distal tubule epithelial cell (CL:1000838 / CL:1000849 / CL:1001108); erythrocyte (CL:0000232); oligodendrocyte (CL:0000128). - Chemical entities (CHEBI): carbon dioxide (CHEBI:16526); bicarbonate (CHEBI:17544); proton (CHEBI:24636); zinc(2+) (CHEBI:29105); hydroxyapatite.

Protein dysfunction. CA II is a cytosolic zinc-dependent carbonate dehydratase; pathogenic variants act by loss of catalytic activity and/or destabilization/impaired zinc coordination PMID: 33555497, PMID: 31542996.

Molecular profiling / advanced technologies. No transcriptomic, proteomic, metabolomic, single-cell, or CRISPR-screen datasets specific to OPTB3 were identified. (No data.)

7. Anatomical Structures Affected

Organ level. - Primary: skeleton/bone (UBERON:0002481 bone tissue; UBERON:0001474 bone element), kidney (UBERON:0002113), brain (UBERON:0000955, esp. basal ganglia UBERON:0002420). - Secondary: optic nerve (UBERON:0000941) and other cranial nerves via foraminal narrowing; teeth (UBERON:0001091) and jaw; upper airway (obstructive sleep apnea). - Body systems: skeletal, renal/urinary, central nervous, visual, craniofacial/dental, respiratory.

Tissue and cell level. Connective/mineralized tissue (bone); renal tubular epithelium; nervous tissue. Key cell populations: osteoclasts (CL:0000092), renal tubular epithelial cells, erythrocytes (used for diagnostic enzyme assay), oligodendrocytes.

Subcellular level. Cytosol (site of CA II) and, functionally coupled, the osteoclast ruffled-border/resorption lacuna and the V-ATPase proton pump apparatus. GO cellular component: cytosol (GO:0005829); ruffled border (GO:0001618).

Localization / lateralization. Skeletal involvement is generalized/systemic; brain calcification is characteristically symmetric and bilateral in the basal ganglia; optic-canal narrowing and nephrocalcinosis are typically bilateral.

8. Temporal Development

Onset. Typically late infancy to early childhood; onset is insidious/chronic rather than acute. Diagnosis is often triggered by failure to thrive, fractures, developmental concerns, or incidental radiographic findings — in one case, head trauma imaging revealed the diagnosis PMID: 35035649.

Progression. The course is chronic and lifelong at the biochemical level, but the skeletal phenotype tends to stabilize or improve with age: "The skeletal findings may improve by adult life, and CA II deficiency can be associated with a normal life-span. Therefore, it has been considered an 'intermediate' type of OPT" PMID: 36709914. Cerebral calcification tends to become more evident over childhood. RTA persists.

Patterns / critical periods. No spontaneous remission of the enzyme defect. Early childhood is the critical window for detecting and managing RTA (to protect growth and neurodevelopment) and for monitoring optic-canal narrowing (to preserve vision). HSCT, when pursued for the bone component, is typically considered in childhood.

9. Inheritance and Population

Inheritance. Autosomal recessive. The enzyme defect is fully penetrant; clinical expressivity is variable even within families sharing the identical mutation PMID: 9453381.

Epidemiology. A rare disease; >100 cases reported globally. Precise prevalence/incidence figures are not well established but fall within the rare-disease range (<1–5 per 10,000). Prevalence is elevated in populations with high consanguinity rates.

Penetrance / expressivity / anticipation / mosaicism. Penetrance of the biochemical defect is complete; clinical expressivity is variable (particularly neurodevelopmental severity). No genetic anticipation (not a repeat-expansion disorder). Germline mosaicism is not a described feature.

Founder effect / consanguinity / carrier frequency. The intron 2 splice "Arabic mutation" is a classic founder allele "found exclusively in patients with an Arabic background" PMID: 7959703. Consanguinity is the principal driver of homozygosity; the 23-patient neurology cohort comprised "10 unrelated consanguineous families with carbonic anhydrase type II deficiency syndrome due to homozygous intron 2 splice site mutation (the 'Arabic mutation')" PMID: 22120147. Carrier frequencies are population-specific and elevated in Middle Eastern/Mediterranean groups; precise gnomAD-based estimates are not established.

Population demographics. Predominantly Middle Eastern (Tunisian, Kuwaiti, Saudi) and Mediterranean patients for the Arabic mutation; additional cases reported in East Asian (Chinese) and other populations with distinct mutations. No strong sex bias is described. Age distribution centers on pediatric diagnosis.

10. Diagnostics

Diagnostic triad + biochemistry + genetics.

Test	Finding	Reference
Skeletal survey / X-ray	Diffuse increased bone density (osteopetrosis)	PMID: 36709914
Arterial blood gas / serum electrolytes	Hyperchloremic metabolic acidosis (RTA), abnormal urine pH	PMID: 35035649
Brain CT	Symmetric basal-ganglia/subcortical calcification	PMID: 39667299
Erythrocyte CA II enzyme assay	Markedly reduced/absent CA II activity	—
CA2 sequencing (7 exons)	Confirmatory pathogenic variant	PMID: 15300855

The triad can be confirmed at the bedside: "The suspicion of carbonic anhydrase II deficiency was confirmed by arterial blood gases revealing a marked metabolic acidosis fulfilling the diagnostic triad" PMID: 35035649.

Genetic testing. Molecular confirmation is by CA2 sequencing of all seven exons: "we amplified all seven exons by PCR from genomic DNA and directly sequenced the amplified products. Application of this method allowed identification of eleven new mutations in 21 patients referred for confirmation of the diagnosis of CA II deficiency" PMID: 15300855. Whole-exome/whole-genome sequencing is increasingly first-line and readily detects CA2 variants (e.g., intron 2 c.232+1G>T; p.W123X). Single-gene testing is highly efficient given near-perfect gene–disease specificity. Chromosomal microarray, karyotyping, FISH, mtDNA, and repeat-expansion testing are not applicable.

Imaging quantification. A CT parenchymal calcium score has been developed, but it did not clearly correlate with neurological severity in one study PMID: 39667299.

Prenatal diagnosis. Molecular: "Prenatal diagnosis requires mutational analysis of CA2" PMID: 36709914, feasible from cultured amniocytes or chorionic villus sampling.

Differential diagnosis. Other osteopetroses (TCIRG1, CLCN7, OSTM1, SNX10, RANK/RANKL) — distinguished by absence of RTA + cerebral calcification and by more severe hematologic failure in malignant infantile forms PMID: 42096006; congenital CMV infection (mimic) PMID: 41204604; other causes of RTA and of bilateral basal-ganglia calcification.

11. Outcome / Prognosis

Survival / life expectancy. Favorable relative to other osteopetroses: CA II deficiency is an "intermediate" osteopetrosis compatible with a normal lifespan, and skeletal findings may improve by adult life PMID: 36709914. This contrasts with malignant infantile osteopetrosis, whose "morbidity and mortality rates are extremely high" PMID: 41204604.

Morbidity / function. The principal long-term burdens are intellectual disability/developmental delay (~2/3), visual impairment from optic-nerve compression, recurrent fractures, dental morbidity, growth failure, and, in some, sleep-disordered breathing and nephrocalcinosis/urolithiasis.

Complications. Fractures; cranial-nerve palsies (optic atrophy, and potentially facial/auditory); dental malocclusion and amelogenesis imperfecta; obstructive sleep apnea from craniofacial dysmorphism; nephrocalcinosis/urolithiasis; persistent metabolic acidosis affecting growth.

Prognostic factors. Genotype (the Arabic splice allele is associated with mental retardation PMID: 7959703); degree of optic-canal narrowing predicts optic-nerve involvement PMID: 22120147; early recognition and correction of acidosis plausibly protects growth and neurodevelopment. No validated molecular prognostic biomarker exists. Quality-of-life instruments have not been formally applied.

12. Treatment

Overall strategy. Management is "largely the management of complications and includes vitamin D and calcium supplements, IFN-γ therapy, and hematopoietic stem cell transplantation (HSCT), the latter being the treatment of choice for most forms of osteopetrosis" PMID: 42096006.

Pharmacotherapy / supportive care (NCIT-annotatable interventions). - Alkali therapy for RTA — oral sodium/potassium bicarbonate or citrate to correct hyperchloremic metabolic acidosis and protect growth (NCIT: Sodium Bicarbonate; Potassium Citrate). - Vitamin D and calcium supplementation (general osteopetrosis management) PMID: 42096006 (NCIT: Vitamin D; Calcium). - Interferon-γ (IFN-γ) — used in osteopetrosis generally PMID: 42096006 (NCIT: Interferon Gamma). - Supportive care for fractures (orthopedics), dental/orthodontic care for amelogenesis imperfecta and malocclusion PMID: 37662627, ophthalmologic monitoring, and airway management/CPAP or surgery for OSA PMID: 30109220.

Advanced therapeutics — HSCT (NCIT: Hematopoietic Stem Cell Transplantation). Allogeneic HSCT can restore CA II-competent, hematopoietic-derived osteoclasts and thereby correct the bone component. It has been successfully applied in CA II deficiency: "we present the diagnosis and successful application of hematopoietic stem cell transplantation (HSCT) in a patient with osteopetrosis caused by carbonic anhydrase II deficiency" with the outcome that "He Engrafted on day +13, and 95% chimerism was achieved. He is currently doing well without immunosuppressive therapy" PMID: 38655726. Critical caveat: HSCT replaces the osteoclast lineage only and does not correct the intrinsic renal-tubular or neural CA II deficiency, so RTA and CNS features persist. Because CA II deficiency is an intermediate osteopetrosis with generally favorable skeletal prognosis, HSCT is reserved rather than routine.

Emerging / experimental. For osteopetrosis broadly, small interfering RNA (siRNA) therapy and gene/iPSC-based osteoclast reconstitution are under investigation PMID: 42466325. HSC-targeted gene therapy corrected many aspects of disease in a mouse model of infantile malignant osteopetrosis: "HSC-targeted gene therapy in a mouse model of infantile malignant osteopetrosis was recently shown to correct many aspects of the disease" PMID: 18241253. No CA II-specific gene-therapy trial is established.

Pharmacogenomics / personalized medicine. Not established for this disorder. Genotype (e.g., the Arabic allele) informs prognosis and counseling more than drug selection.

13. Prevention

Primary prevention. No environmental prevention is possible (genetic disease). Prevention operates through reproductive genetics: genetic counseling for consanguineous couples and known carrier families, carrier testing, prenatal diagnosis by CA2 mutation analysis PMID: 36709914, and preimplantation genetic testing where available.

Secondary prevention. Early detection and correction of RTA with alkali therapy; surveillance of optic-canal narrowing to preserve vision; dental and airway monitoring. Cascade genetic screening of at-risk relatives in founder-mutation populations.

Tertiary prevention. Fracture prevention, management of acidosis to protect growth/neurodevelopment, ophthalmologic and audiologic surveillance, dental care, and treatment of OSA to reduce complications.

Immunization / public health / prophylaxis. Not applicable beyond standard care. Genetic counseling is the central preventive tool, particularly in high-consanguinity communities harboring the Arabic founder allele.

14. Other Species / Natural Disease

Taxonomy / orthologs. Human CA2 has a well-conserved mouse ortholog, Car2 (mouse Car-2 locus, chromosome 3) PMID: 3126501. Carbonic anhydrase II is an evolutionarily conserved cytosolic zinc metalloenzyme across mammals.

Natural disease in other species. No prominent naturally occurring CA II-deficiency syndrome in companion animals or wildlife is documented in the reviewed literature. (No data.)

Comparative pathology. The mouse model reveals a striking species difference (see Section 15): mice reproduce the renal and growth phenotype but not osteopetrosis, indicating that CA II's non-redundant role in osteoclast acidification differs quantitatively between mouse and human (possible compensation by other carbonic anhydrase isoforms in mouse osteoclasts).

Zoonotic potential. Not applicable (non-infectious genetic disease).

15. Model Organisms

Primary model — Car2-null mouse. An ENU-induced null mutation at the mouse Car-2 locus produced homozygotes lacking CA II protein in all tissues. Crucially: "Like humans with the same inherited enzyme defect, animals homozygous for the new null allele are runted and have renal tubular acidosis. However, the prominent osteopetrosis found in humans with CA II deficiency could not be detected even in very old homozygous null mice" PMID: 3126501.

Feature	Human CA II deficiency	Car2-null mouse
Renal tubular acidosis	Yes	Yes (recapitulated)
Growth failure / runting	Yes (short stature)	Yes (recapitulated)
Osteopetrosis	Yes (cardinal)	No (not detected)
Cerebral calcification	Yes	Not reported

Phenotype recapitulation & limitations. The model faithfully reproduces the renal and growth arms but fails to reproduce the defining skeletal (osteopetrosis) and cerebral-calcification arms — a major limitation for studying bone pathogenesis in this disease. This dissociation is itself scientifically informative, suggesting mouse osteoclasts tolerate CA II loss (possibly via isozyme redundancy) whereas human osteoclasts do not.

Other systems. In vitro structural/computational models (3D protein modeling, molecular dynamics, minigene splicing assays) have characterized variant pathogenicity — impaired zinc binding and reduced expression for p.W123X PMID: 33555497 and residue-level destabilization for missense variants PMID: 31542996. HSC-targeted gene therapy has been demonstrated in a mouse model of infantile malignant osteopetrosis (a related but distinct condition) PMID: 18241253.

Resources. MGI (Car2); Cellosaurus/ATCC and HEK293T-based minigene assays for splicing/expression studies.

Mechanistic Model / Interpretation

Bi-allelic loss-of-function CA2 mutation (8q21.2)
          │
          ▼
   Absent / dead carbonic anhydrase II (cytosol)
          │
      Loss of CO2 + H2O <=> H2CO3 <=> H+ + HCO3-  (proton/bicarbonate supply)
          │
      ┌───────────────────┼─────────────────────────┐
      ▼                   ▼                          ▼
  OSTEOCLAST           RENAL TUBULE                BRAIN
  no H+ for            impaired H+ secretion /     (mechanism inferred)
  V-ATPase at          HCO3- reclamation
  ruffled border            │                          │
      │                     ▼                          ▼
  lacuna not          hyperchloremic            basal-ganglia
  acidified (pH4.5)   metabolic acidosis        calcification
      │                     │                          │
      ▼                     ▼                          ▼
  hydroxyapatite      RENAL TUBULAR ACIDOSIS     developmental delay /
  not dissolved       (± hypercalciuria ->       intellectual disability
      │                nephrocalcinosis)                │
      ▼                                                 ▼
  OSTEOPETROSIS --> foraminal narrowing --> optic / cranial-nerve compression
      │
      ▼
  fractures, craniofacial disproportion, dental defects, OSA

The model's central insight is a single upstream lesion (loss of cytosolic proton supply) producing three semi-independent downstream arms. This explains why HSCT — which replaces only the hematopoietic osteoclast lineage — rescues the bone arm but leaves the renal and neural arms intact, and why alkali therapy addresses the renal arm but not the bone or brain arms. It also frames CA II within the wider osteoclast-acidification module shared with TCIRG1, CLCN7, and OSTM1, differing chiefly by its additional renal and cerebral involvement.

Evidence Base

PMID	Title (abbrev.)	Contribution
36709914	Carbonic anhydrase II deficiency (review)	Anchor review: OMIM identity, LoF etiology, osteoclast/renal mechanism, intermediate prognosis, prenatal Dx
7959703	A unique mutation ... in patients of Arab descent	Establishes intron-2 "Arabic" founder allele + mental retardation link
22120147	The neurology of CA II deficiency	23-patient consanguineous cohort; optic-canal narrowing correlates with optic-nerve involvement
3126501	ENU-induced null Car-2 mouse	Model reproduces RTA + runting but NOT osteopetrosis
38655726	Allogeneic HSCT in CA II deficiency	HSCT feasible/effective for bone arm; engraftment day +13, 95% chimerism
15300855	Novel CA2 mutations by direct sequencing	7-exon sequencing method; near-perfect gene–disease specificity
6410391	CA2 gene marker on chromosome 8	Maps CA2 to chromosome 8; molecular disease marker
39667299	Calcium score vs neurological severity	Frequencies: 60% dev delay, 22.2% optic atrophy, 70% brain calcification
37662627	CA II deficiency with amelogenesis imperfecta	Dental/oral phenotype
30109220	Severe OSA in CA II deficiency	Craniofacial-related airway obstruction
33555497	Nonsense p.W123X in Chinese family	Impaired zinc binding, reduced expression
31542996	Molecular modelling of CA2 missense variants	Residue-level destabilization; ~50% destabilizing
42096006	Osteopetrosis: pathogenesis & therapies	Osteoclast-gene classification; treatment framework
9453381	Nephrocalcinosis/urolithiasis in CA II deficiency	Intra-/inter-familial variability; renal stones/hypercalciuria
35035649	Head trauma reveals CA II deficiency	Triad confirmation via blood-gas acidosis
18241253	Understanding & new therapeutics of osteopetrosis	HSC gene therapy in mouse model
42466325	Genetic bone diseases scoping review	siRNA therapy for osteopetrosis (emerging)
41204604	Osteopetrosis misdiagnosed as CMV	Differential-diagnosis mimic; malignant-form severity

Evidence source types: predominantly human clinical (case reports, consanguineous-family case series, review syntheses); model organism (ENU Car2-null mouse; malignant-osteopetrosis mouse gene therapy); in vitro/computational (minigene splicing assays, molecular dynamics of variants).

Limitations and Knowledge Gaps

Rare-disease evidence base. Findings rest on case reports and small consanguineous-family series; there are no large prospective cohorts, natural-history registries, or randomized trials. Prevalence/incidence figures are imprecise.
Brain-calcification mechanism unresolved. The causal route from CA II loss to basal-ganglia calcification and to intellectual disability is inferred, not experimentally demonstrated; calcium score did not correlate with neurological severity PMID: 39667299.
Model-organism gap. The Car2-null mouse does not reproduce osteopetrosis PMID: 3126501, limiting mechanistic and preclinical study of the bone arm and leaving CA-isozyme redundancy in mouse osteoclasts unexplained.
Genotype–phenotype correlation incomplete. Marked intra-familial variability despite identical genotypes PMID: 9453381 points to unidentified modifiers or stochastic factors; missense-variant pathogenicity prediction is imperfect PMID: 31542996.
No omics data. No transcriptomic, proteomic, metabolomic, or single-cell datasets specific to OPTB3 were identified.
Therapeutic gaps. HSCT addresses only the bone arm; no therapy corrects the systemic enzyme deficiency. QoL outcomes are unquantified.

Proposed Follow-up Experiments / Actions

Human osteoclast disease model. Generate patient-derived iPSC osteoclasts (or CRISPR CA2-knockout human osteoclasts) to recapitulate the resorption defect that the mouse lacks, dissect CA-isozyme redundancy, and serve as a gene-/enzyme-therapy testbed.
Mechanistic study of cerebral calcification. Use CA II-deficient brain organoids/choroid-plexus models and imaging cohorts to test whether calcification arises from local pH dysregulation, CSF handling, or systemic acidosis.
Natural-history registry. Establish an international OPTB3 registry (leveraging founder-mutation populations) to quantify prevalence, penetrance/expressivity, QoL (EQ-5D/PROMIS), and long-term outcomes with vs without alkali therapy and HSCT.
Genotype–modifier discovery. WGS + modifier screens across discordant siblings sharing the Arabic allele to identify factors governing neurodevelopmental variability.
Targeted therapy development. Evaluate CA II mRNA/gene-replacement or small-molecule chaperone strategies for destabilizing missense variants; test siRNA/gene-therapy approaches (proven in malignant-osteopetrosis mice) adapted to the CA II bone arm.
Preventive genetics rollout. Implement carrier and cascade screening plus prenatal/PGT counseling programs in high-consanguinity communities carrying the founder allele.

Report compiled from an autonomous multi-iteration literature investigation. All quoted statements are verbatim from the cited PubMed abstracts/records. Ontology suggestions (HPO, GO, CL, UBERON, CHEBI, NCIT, MONDO) are provided to support knowledge-base curation and should be verified against current ontology releases.

Artifacts

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome	Count
References checked	18
Resolved	18
Unresolved (possible confabulation)	0
Unverifiable	0
References weighed for topical relevance	18
On topic	17
Off topic	0

All extracted references resolved successfully.

Term Validation

Checked with linkml-term-validator 0.4.5, through the ols: adapter.

Outcome	Count
Terms checked	43
Resolved	41
Unresolved (possible confabulation)	0
Obsolete	0
Unverifiable	2
Terms whose name was checked	15
Terms named correctly	6
Terms named as a different term	2
Terms whose name is worth a second look	7

Terms the report names something else

These identifiers resolve, so nothing about them looks wrong, and the ontology calls them something unrelated to what the report calls them. That usually means the identifier is not the one the sentence needs:

MONDO:0009818 (2 mentions) - the report calls it "MONDO"; MONDO calls it autosomal recessive osteopetrosis 3
HP:0031815 (1 mention) - the report calls it "Cranial nerve compression"; HP calls it Abnormal oral physiology

Terms whose name is worth a second look

The report's name for these is recognisably related to the term's own name without being one of them. A loose paraphrase reads the same way as a citation of the wrong sibling term - and so does a related synonym, which the ontology records precisely because it names something adjacent rather than the same thing - so these are listed rather than judged:

HP:0011002 (1 mention) - the report calls it "Osteopetrosis (increased bone density)"; HP calls it Osteopetrosis
HP:0002514 (1 mention) - the report calls it "Cerebral / basal-ganglia calcification"; HP calls it Cerebral calcification
HP:0001249 (1 mention) - the report calls it "Intellectual disability / mental retardation"; HP calls it Intellectual disability
HP:0000648 (1 mention) - the report calls it "Optic nerve atrophy / visual impairment"; HP calls it Optic atrophy, and lists "Optic nerve atrophy" among its other names
HP:0000639 (1 mention) - the report calls it "Congenital nystagmus"; HP calls it Nystagmus
HP:0000121 (1 mention) - the report calls it "Nephrocalcinosis / urolithiasis / hypercalciuria"; HP calls it Nephrocalcinosis
UBERON:0000941 (1 mention) - the report calls it "Secondary: optic nerve"; UBERON calls it cranial nerve II*, and lists "optic nerve" among its other names

Prefixes with no resolver

Terms carrying these prefixes were not checked either way, because no configured ontology covers them. An unrecognised prefix may name an ontology this run could not reach as easily as one that does not exist, so nothing here is evidence of fabrication: ORPHA.

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Animal Models

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References

Deep Research

1. Disease Information

2. Etiology

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7. Anatomical Structures Affected

8. Temporal Development

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10. Diagnostics

11. Outcome / Prognosis

12. Treatment

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14. Other Species / Natural Disease

15. Model Organisms

Mechanistic Model / Interpretation

Evidence Base

Limitations and Knowledge Gaps

Proposed Follow-up Experiments / Actions

Artifacts

Reference Validation

Term Validation

Terms the report names something else

Terms whose name is worth a second look

Prefixes with no resolver