3-Methylcrotonyl-CoA Carboxylase Deficiency

Mendelian MONDO:0018950 Pathograph 21 Show in embeddings browser Organic Acidemia Inborn Error of Metabolism

3-methylcrotonyl-CoA carboxylase deficiency (3-MCCD) is an autosomal recessive disorder of leucine catabolism caused by biallelic pathogenic variants in MCCC1 or MCCC2, which encode the alpha and beta subunits of the mitochondrial, biotin-dependent methylcrotonyl-CoA carboxylase complex. Reduced enzyme activity impairs conversion of 3-methylcrotonyl-CoA to 3-methylglutaconyl-CoA and produces elevated C5OH acylcarnitine and urinary 3-hydroxyisovaleric acid and 3-methylcrotonylglycine. Clinical penetrance is low: most newborn-screening-identified individuals remain asymptomatic, while a minority can develop acute organic-aciduria-like decompensation during intercurrent illness or other catabolic stress. C5OH concentration, biochemical severity, and genotype do not reliably predict outcome, and chronic nonspecific neurologic findings require evaluation for alternative causes.

Ask OpenScientist

Ask a research question about 3-Methylcrotonyl-CoA Carboxylase Deficiency. OpenScientist will conduct autonomous deep research using the Disorder Mechanisms Knowledge Base and PubMed literature (typically 10-30 minutes).

Submitting...

Do not include personal health information in your question. Questions and results are cached in your browser's local storage.

1
Inheritance
5
Pathophys.
6
Phenotypes
5
Gaps
21
Pathograph
2
Genes
3
Medical Actions
4
Differentials
20
References
2
Deep Research
👪

Inheritance

1
Autosomal recessive inheritance with incomplete clinical penetrance HP:0000007
Canonical 3-MCCD results from biallelic MCCC1 or MCCC2 variants. Clinical expression is incompletely penetrant and highly variable even among people with a marked biochemical or enzymatic defect.
Autosomal recessive inheritance Penetrance: INCOMPLETE Expressivity: VARIABLE
Show evidence (2 references)
PMID:22642865 SUPPORT Human Clinical
"autosomal recessive disorder of leucine metabolism caused by mutations in MCCC1 or MCCC2"
The 88-person cohort defines the canonical recessive MCCC1/MCCC2 disorder.
PMID:22642865 SUPPORT Human Clinical
"MCC deficiency, despite low penetrance, may lead to a severe clinical phenotype"
The cohort directly supports incomplete penetrance and variable clinical expression.
?

Discussions and Knowledge Gaps

5
Which genetic, metabolic, or environmental modifiers distinguish the small subgroup at risk of specific metabolic crises from the much larger asymptomatic biochemically affected population?
KNOWLEDGE GAP OPEN gap_penetrance_and_risk_stratification
Attached to
inheritance#Autosomal recessive inheritance with incomplete clinical penetrance progression#Occasional catabolic-stress decompensation pathophysiology#Catabolic-stress acute metabolic decompensation
Neither genotype, residual biochemical phenotype, nor newborn-screening C5OH identifies who will decompensate. Prospective, uniformly confirmed cohorts with untreated natural-history comparators are lacking.
Show evidence (2 references)
PMID:22642865 SUPPORT Human Clinical
"neither the genotype nor the biochemical phenotype is helpful in predicting the clinical course."
Directly states the current failure of genotype and biochemical risk stratification.
PMID:27033733 SUPPORT Human Clinical
"There was no correlation between newborn screening (NBS) C5OH level and presence of metabolic, newborn, later-life or developmental abnormalities"
Shows that initial C5OH concentration also fails as a prognostic marker.
Does population newborn screening for 3-MCCD prevent enough severe morbidity to justify false-positive recalls, detection of clinically insignificant biochemical phenotypes, chronic surveillance, and family psychological burden?
CONTROVERSY OPEN controversy_newborn_screening_benefit_and_burden
Attached to
diagnosis#C5OH newborn-screening signal with infant-mother evaluation progression#Predominantly asymptomatic course after newborn-screening detection
Programs identify many asymptomatic infants and adults, C5OH is nonspecific, and comparative evidence for prevention of crises is absent. Recent cohorts nonetheless document treatable illness events and parents who support screening, so benefit and burden remain contested.
Show evidence (4 references)
PMID:24103308 SUPPORT Human Clinical
"a significant number of individuals receiving treatment for 3-MCCD may not have a clinically significant condition."
Captures the overdiagnosis and overtreatment concern in a large state program.
PMID:26566957 SUPPORT Human Clinical
"we suggest to exclude this disorder from NBS programs."
Provides one published program-level argument against continued screening.
DOI:10.3390/ijns11040115 SUPPORT Human Clinical
"Psychological assessments revealed an initial high level of parental psychological impact, which decreased over time."
Adds recent direct evidence about an important screening burden.
+ 1 more reference
Which asymptomatic or symptomatic individuals benefit clinically from fasting precautions, carnitine replacement, or leucine restriction, and what biochemical or clinical thresholds should trigger each intervention?
KNOWLEDGE GAP OPEN gap_treatment_thresholds_and_effectiveness
Attached to
treatments#Sick-day emergency regimen and avoidance of prolonged fasting treatments#Carnitine supplementation for documented deficiency treatments#Individualized dietary management
Management recommendations are largely expert opinion or small observational studies. Carnitine clearly changes carnitine concentrations, but clinical benefit and the effects of fasting prevention or diet remain unresolved.
Show evidence (3 references)
PMID:18155630 SUPPORT Other
"Grade D consensus recommendations were made in each of these three areas."
Establishes that the consensus protocol rested on expert-opinion-level evidence.
PMID:25732994 SUPPORT Human Clinical
"a more general recommendation cannot be given."
Directly identifies the limit of the small carnitine study.
PMID:27033733 SUPPORT Human Clinical
"The role that prevention of fasting plays in outcome cannot be ascertained."
Directly identifies the unresolved effect of the main preventive strategy.
Do the mitochondrial and oxidative-stress effects observed in two affected fibroblast lines and metabolite-exposed rat cortex preparations operate in vivo, and do they explain any human clinical outcome?
HUMAN MODEL MISMATCH OPEN gap_translation_of_experimental_oxidative_stress
Attached to
pathophysiology#Experimental mitochondrial and oxidative stress
The experimental findings are mechanistically interesting but derive from very small in-vitro systems. The branch is intentionally detached from patient phenotypes until human tissue, biomarker, or intervention data establish translation.
Proposed experiments
Genotype-confirmed human metabolic-stress translation study
exp_3mccd_human_metabolic_stress_translation
Compare isogenic control and MCCC1- or MCCC2-deficient human iPSC-derived hepatocytes under basal and catabolic-stress conditions, measuring MCC flux, C5OH and organic-acid release, free carnitine, mitochondrial respiration, and redox markers. In parallel, test whether the same marker signature is detectable prospectively during well and illness visits in molecularly confirmed patients, including asymptomatic and crisis-prone groups. Concordant, crisis-associated human-cell and patient signatures would support translation; their absence would argue that the fibroblast and rat-preparation findings are model-specific.
Show evidence (2 references)
PMID:27417235 SUPPORT In Vitro
"immortalized cultured skin fibroblast cells of two clinically affected MCC deficient patients and two healthy individuals"
Shows the limited sample and surrogate cell type underlying the human-cell hypothesis.
PMID:23053545 SUPPORT In Vitro
"It is therefore presumed that alterations of the cellular redox homeostasis"
The study itself frames translation from the rat preparation as a presumption.
Should MCCC1 p.Arg385Ser be treated as a clinically relevant dominant-negative, biotin-responsive exception, or as a low-penetrance biochemical allele whose reported two-patient response does not justify changing canonical recessive inheritance or routine treatment?
INTERPRETATION OPEN interpretation_mccc1_arg385ser_and_biotin_response
Attached to
genetic#MCCC1
An early report described partial deficiency, heterozygous dominant-negative behavior, and biotin responsiveness in two people. A larger later cohort found the allele in both severely affected and asymptomatic individuals. This exceptional literature should not support routine biotin therapy without specialist review.
Show evidence (2 references)
PMID:15868465 SUPPORT Human Clinical
"Both showed evidence of biotin responsiveness and were heterozygous for the missense mutation MCCA-R385S."
Captures the two-patient observation behind the exceptional biotin-responsive claim.
PMID:22642865 SUPPORT Human Clinical
"p.R385S appears not to be a predictor of a particular phenotype"
Later cohort evidence weakens any simple clinical interpretation of the allele.
⚙

Pathophysiology

5
MCCC1/MCCC2-dependent methylcrotonyl-CoA carboxylase deficiency
Biallelic pathogenic variants in MCCC1 or MCCC2 reduce activity of the mitochondrial alpha6-beta6 methylcrotonyl-CoA carboxylase holoenzyme. The biotin-dependent enzyme catalyzes the fourth step of leucine catabolism, converting 3-methylcrotonyl-CoA toward 3-methylglutaconyl-CoA.
MCCC1 hgnc:6936 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves MCCC1 (hgnc:6936). hgnc:6936 is a gene from the HUGO Gene Nomenclature Committee. MCCC2 hgnc:6937 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves MCCC2 (hgnc:6937). hgnc:6937 is a gene from the HUGO Gene Nomenclature Committee.
L-leucine catabolic process GO:0006552 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased L-leucine catabolic process (GO:0006552). GO:0006552 is a biological process from the Gene Ontology. ↓ DECREASED
methylcrotonoyl-CoA carboxylase activity GO:0004485 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased methylcrotonoyl-CoA carboxylase activity (GO:0004485). GO:0004485 is a molecular function from the Gene Ontology. ↓ DECREASED
mitochondrial matrix GO:0005759 Gene Ontology (GO) Relation: this pathophysiological event involves this cellular component This pathophysiological event involves mitochondrial matrix (GO:0005759). GO:0005759 is a cellular component from the Gene Ontology.
Downstream
Leucine-pathway metabolite shunting Reduced MCC activity blocks normal leucine-catabolic flux and diverts upstream carbon into characteristic organic acids and C5OH acylcarnitine.
Show evidence (2 references)
PMID:22642865 SUPPORT Human Clinical
"The mitochondrial enzyme MCC (EC 6.4.1.4) catalyzes the fourth step in the leucine catabolic pathway"
Defines the enzyme, compartment, and blocked pathway step.
PMID:22642865 SUPPORT Human Clinical
"MCC consists of an alpha and a beta subunit assembled into a α6β6 dodecamer."
Supports the two-subunit holoenzyme architecture encoded by MCCC1 and MCCC2.
Leucine-pathway metabolite shunting
The blocked reaction is associated with accumulation and excretion of 3-hydroxyisovaleric acid, 3-methylcrotonylglycine, and 3-hydroxyisovalerylcarnitine (C5OH). Clinical toxicity and any associated energy deficit are incompletely established: very high metabolite excretion is often compatible with an asymptomatic course, so this node is not mapped to the module's toxic-metabolite/energy-deficit state.
Downstream
Secondary carnitine depletion Formation and excretion of hydroxyisovalerylcarnitine is associated with depletion of free carnitine in a subset of affected people.
Catabolic-stress acute metabolic decompensation In a minority, infection or another catabolic stress coincides with an organic-aciduria-like crisis; the factors separating tolerant from vulnerable individuals remain unknown.
Experimental mitochondrial and oxidative stress Cell and tissue-preparation experiments suggest that chronic metabolite exposure may perturb mitochondrial and redox homeostasis, but this has not been shown to cause human symptoms.
Show evidence (2 references)
PMID:36822454 SUPPORT Human Clinical
"94 % (50/53) of the patients had markedly increased urinary 3-hydroxyisovaleric acid and 3-methylcrotonylglycine."
Quantifies the characteristic urinary metabolite pattern in a screened cohort.
PMID:36822454 SUPPORT Human Clinical
"All these 53 patients exhibited increased C5OH concentrations in blood."
Supports C5OH elevation in the ascertained cohort without claiming diagnostic specificity.
Secondary carnitine depletion
Free carnitine can be low in blood and tissue. This is a secondary and variably present consequence rather than correction of, or a proxy for, residual MCC enzyme activity.
Show evidence (2 references)
PMID:25732994 SUPPORT Human Clinical
"the 3-MCCd patients (n= 13) had low plasma and muscle free carnitine levels"
Documents low free carnitine in a small Faroese founder cohort without supplementation.
PMID:36822454 SUPPORT Human Clinical
"Twenty-three of 53 patients had secondary carnitine deficiency."
Demonstrates that depletion is common but not universal in a larger screened cohort.
Catabolic-stress acute metabolic decompensation
Rare crises during infection, fasting, or another catabolic stress can resemble classical organic acidurias, with vomiting, metabolic acidosis, ketotic hypoglycemia, mild hyperammonemia, seizures, or coma. This branch is not an inevitable consequence of the biochemical defect.
hepatocyte CL:0000182 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves hepatocyte (CL:0000182). CL:0000182 is a cell type from the Cell Ontology.
response to starvation GO:0042594 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves response to starvation (GO:0042594). GO:0042594 is a biological process from the Gene Ontology.
Downstream
Metabolic acidosis Organic-aciduria-like decompensation can produce metabolic acidosis.
Hypoglycemia Acute decompensation can include hypoglycemia.
Hyperammonemia Mild hyperammonemia can accompany severe biochemical decompensation.
Vomiting Vomiting can be an early clinical manifestation of acute crisis.
Seizures Seizures can occur during severe acute decompensation.
Coma Severe decompensation can progress to coma.
Show evidence (1 reference)
PMID:22642865 SUPPORT Human Clinical
"12 patients (5 of 53 identified by newborn screening) presented with acute metabolic decompensations."
Establishes that crisis occurs in a minority, including some prospectively screened individuals.
Experimental mitochondrial and oxidative stress
Immortalized fibroblasts from two clinically affected individuals and in vitro exposure of young-rat cerebral cortex preparations to accumulating metabolites showed mitochondrial, energy-homeostasis, or oxidative-stress abnormalities. These experiments generate a mechanism hypothesis but do not establish a causal route to neurologic findings in affected people.
fibroblast CL:0000057 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves fibroblast (CL:0000057). CL:0000057 is a cell type from the Cell Ontology.
response to oxidative stress GO:0006979 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased response to oxidative stress (GO:0006979). GO:0006979 is a biological process from the Gene Ontology. ↑ INCREASED oxidative phosphorylation GO:0006119 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal oxidative phosphorylation (GO:0006119). GO:0006119 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Show evidence (3 references)
PMID:27417235 SUPPORT In Vitro
"immortalized cultured skin fibroblast cells of two clinically affected MCC deficient patients and two healthy individuals"
Defines the very small experimental sample and cellular context.
PMID:27417235 SUPPORT In Vitro
"mitochondrial dysfunction, decreased antioxidant response and disruption of energy homeostasis"
Supports the observed fibroblast stress signature.
PMID:23053545 SUPPORT In Vitro
"3MCG and 3MCA significantly increased TBA-RS and carbonyl formation"
Supports oxidative effects after direct metabolite exposure in rat cortex preparations.
⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for 3-Methylcrotonyl-CoA Carboxylase Deficiency Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
●

Phenotypes

6
Digestive 1
Vomiting HP:0002013 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Vomiting (HP:0002013). HP:0002013 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22642865 SUPPORT Human Clinical
"vomiting, opisthotonus, involuntary movements"
Directly supports vomiting as a crisis manifestation.
Metabolism 3
Metabolic acidosis HP:0001942 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Metabolic acidosis (HP:0001942). HP:0001942 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22642865 SUPPORT Human Clinical
"acidosis, hypoglycemia and in some cases mild"
Directly supports metabolic acidosis during acute crises.
Hypoglycemia HP:0001943 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypoglycemia (HP:0001943). HP:0001943 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22642865 SUPPORT Human Clinical
"acidosis, hypoglycemia and in some cases mild"
Directly supports hypoglycemia during acute crises.
Hyperammonemia HP:0001987 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hyperammonemia (HP:0001987). HP:0001987 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22642865 SUPPORT Human Clinical
"hyperammonemia [3,7,23-27]"
Directly supports occasional hyperammonemia during acute crises.
Nervous System 2
Seizures HP:0001250 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Seizure (HP:0001250). HP:0001250 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:22642865 SUPPORT Human Clinical
"Neurologic symptoms like seizures"
Directly supports seizures in the acute crisis spectrum.
PMID:25356967 SUPPORT Human Clinical
"Nonspecific symptoms, which are not directly related to leucine metabolism, included developmental delay, intellectual disability, seizures"
Supports caution in attributing a chronic seizure phenotype to the MCC defect.
Coma HP:0001259 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Coma (HP:0001259). HP:0001259 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:25356967 SUPPORT Human Clinical
"Specific symptoms included ketoacidosis, hypoglycemia, hyperammonemia, coma"
Classifies coma among findings directly correlated with defective leucine catabolism.
🧬

Genetic Associations

2
MCCC1 (Biallelic pathogenic MCCC1 variants cause the MCCA complementation form of 3-MCCD.)
Gene: MCCC1 hgnc:6936 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is MCCC1 (hgnc:6936). hgnc:6936 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (2 references)
CGGV:assertion_6bd0c545-9ef8-4005-a124-a73be6178745-2019-10-25T160000.000Z SUPPORT Other
"MCCC1 | HGNC:6936 | 3-methylcrotonyl-CoA carboxylase deficiency | MONDO:0018950 | AR | Definitive"
ClinGen classifies this autosomal-recessive gene-disease relationship as definitive.
PMID:22642865 SUPPORT Human Clinical
"neither the genotype nor the biochemical phenotype is helpful in predicting the clinical course."
Supports the absence of a reliable genotype-phenotype correlation.
MCCC2 (Biallelic pathogenic MCCC2 variants cause the MCCB complementation form of 3-MCCD.)
Gene: MCCC2 hgnc:6937 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is MCCC2 (hgnc:6937). hgnc:6937 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (2 references)
CGGV:assertion_3653ea30-b630-499a-a6f0-65152106ad8c-2019-10-25T160000.000Z SUPPORT Other
"MCCC2 | HGNC:6937 | 3-methylcrotonyl-CoA carboxylase deficiency | MONDO:0018950 | AR | Definitive"
ClinGen classifies this autosomal-recessive gene-disease relationship as definitive.
PMID:22642865 SUPPORT Human Clinical
"neither the genotype nor the biochemical phenotype is helpful in predicting the clinical course."
Supports the absence of a reliable genotype-phenotype correlation.
💊

Medical Actions

3
Sick-day emergency regimen and avoidance of prolonged fasting
Category: Therapeutic Action: supportive careNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is supportive care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. Ontology label: Supportive Care NCIT:C15747
A metabolic-specialist sick-day plan and precautions against prolonged fasting are commonly used with the aim of reducing catabolic stress. Observational evidence documents this practice, but its preventive effect has not been established in a controlled study.
Mechanism Target:
MODULATES Catabolic-stress acute metabolic decompensation — Reducing catabolism is intended to lower the risk or severity of an illness-associated crisis.
Show evidence (1 reference)
PMID:27033733 SUPPORT Human Clinical
"The role that prevention of fasting plays in outcome cannot be ascertained."
Supports the clinical rationale while explicitly showing that effectiveness remains unresolved.
Show evidence (2 references)
PMID:22642865 SUPPORT Human Clinical
"an emergency regimen during intercurrent illness"
The cohort authors cautiously recommend an illness regimen rather than asserting proven efficacy.
DOI:10.3390/ijns11040115 SUPPORT Human Clinical
"An emergency protocol was provided to all affected children to manage intercurrent illnesses."
A recent cohort documents contemporary use of an emergency protocol.
Carnitine supplementation for documented deficiency
Category: Therapeutic Action: carnitine supplementationNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is carnitine supplementation, annotated with Nutritional Support (NCIT:C15433). NCIT:C15433 is a clinical intervention from the NCI Thesaurus. Ontology label: Nutritional Support NCIT:C15433
Agent: carnitine CHEBI:17126 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses carnitine (CHEBI:17126). CHEBI:17126 is a therapeutic agent from Chemical Entities of Biological Interest.
Oral L-carnitine can restore low free carnitine in people with documented secondary deficiency and may help selected symptomatic individuals. It does not correct the MCC block, and available data do not support universal supplementation.
Mechanism Target:
RESTORES Secondary carnitine depletion — Supplementation replenishes depleted plasma and tissue free carnitine.
Show evidence (1 reference)
PMID:25732994 SUPPORT Human Clinical
"L-Carnitine supplementation increased muscle and plasma carnitine levels to a low-normal range"
Directly supports biochemical restoration in the 13-person Faroese study.
Show evidence (2 references)
PMID:22642865 SUPPORT Human Clinical
"monitoring of free carnitine concentrations"
Supports monitoring-first, deficiency-directed replacement.
PMID:25732994 SUPPORT Human Clinical
"Some symptomatic 3-MCCd patients may benefit biochemically and clinically from L-carnitine supplementation, a more general recommendation cannot be given."
Limits the possible benefit to selected individuals and rejects a general recommendation.
Individualized dietary management
Category: Therapeutic Action: dietary interventionNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is dietary intervention (NCIT:C15447). NCIT:C15447 is a clinical intervention from the NCI Thesaurus. Ontology label: Dietary Intervention NCIT:C15447
Routine leucine or protein restriction is usually unnecessary. A metabolic specialist may consider a modest, individualized leucine restriction for a person with recurrent or severe specific crises, but clinical efficacy is unproven and nutritional adequacy must be protected.
Mechanism Target:
MODULATES Leucine-pathway metabolite shunting — Reducing leucine substrate is intended to reduce upstream metabolite burden, but outcome benefit has not been demonstrated.
Show evidence (1 reference)
PMID:27033733 SUPPORT Human Clinical
"At present, the role of dietary leucine restriction remains unproven"
Directly identifies the proposed intervention and uncertainty in efficacy.
Show evidence (2 references)
PMID:22642865 SUPPORT Human Clinical
"Dietetic treatment is usually not required."
Supports avoiding routine restriction for most affected people.
PMID:22642865 SUPPORT Human Clinical
"but the efficacy of these approaches is unproven"
Supports selective use only with an explicit evidence limitation.
🔬

Biochemical Markers

5
Elevated C5OH acylcarnitine (INCREASED)
Context: C5OH is the primary tandem-mass-spectrometry screening signal, but it is not specific for 3-MCCD and its concentration does not predict the clinical course.
Pathograph Readouts
Readout Of Leucine-pathway metabolite shunting Positive Diagnostic
Elevated C5OH reports abnormal acyl-group handling but does not by itself establish 3-MCCD.
Show evidence (2 references)
PMID:36822454 SUPPORT Human Clinical
"All these 53 patients exhibited increased C5OH concentrations in blood."
Supports C5OH elevation among screened, confirmed cases.
PMID:39484073 SUPPORT Human Clinical
"relying solely on C5OH levels from newborn screening is insufficient for making accurate diagnoses or predictions"
Directly limits the diagnostic and prognostic interpretation of the screening marker.
Increased urinary 3-hydroxyisovaleric acid (INCREASED)
Context: Urinary 3-hydroxyisovaleric acid is a characteristic downstream metabolite, but it can be absent or only mildly elevated and must be interpreted with the full acylcarnitine, organic-acid, enzymatic, and molecular profile.
Pathograph Readouts
Readout Of Leucine-pathway metabolite shunting Positive Diagnostic
Increased urinary 3-hydroxyisovaleric acid supports a leucine-pathway block.
Show evidence (1 reference)
PMID:36822454 SUPPORT Human Clinical
"94 % (50/53) of the patients had markedly increased urinary 3-hydroxyisovaleric acid and 3-methylcrotonylglycine."
Provides a defined denominator and shows the marker is characteristic but not universal.
Increased urinary 3-methylcrotonylglycine (INCREASED)
Context: Urinary 3-methylcrotonylglycine accompanies 3-hydroxyisovaleric acid in the characteristic organic-acid profile; the analyte is kept separate because it does not share the 3-hydroxyisovaleric-acid ontology binding.
Pathograph Readouts
Readout Of Leucine-pathway metabolite shunting Positive Diagnostic
Increased urinary 3-methylcrotonylglycine supports abnormal handling of 3-methylcrotonyl-CoA.
Show evidence (1 reference)
PMID:36822454 SUPPORT Human Clinical
"94 % (50/53) of the patients had markedly increased urinary 3-hydroxyisovaleric acid and 3-methylcrotonylglycine."
Quantifies the combined urinary metabolite pattern.
Decreased free carnitine (DECREASED)
Context: Secondary carnitine deficiency is variably present and is a monitoring and treatment target rather than a measure of residual MCC activity.
Pathograph Readouts
Readout Of Secondary carnitine depletion Negative Monitoring
Low plasma free carnitine identifies secondary depletion that may warrant replacement.
Show evidence (1 reference)
PMID:36822454 SUPPORT Human Clinical
"Twenty-three of 53 patients had secondary carnitine deficiency."
Shows depletion in a substantial but non-universal subset.
Isolated reduction of MCC enzyme activity (DECREASED)
Context: Demonstration of severely reduced MCC activity with preserved propionyl-CoA carboxylase activity in cultured fibroblasts supports isolated MCC deficiency and helps distinguish it from multiple-carboxylase defects.
Pathograph Readouts
Readout Of MCCC1/MCCC2-dependent methylcrotonyl-CoA carboxylase deficiency Negative Diagnostic
Low MCC with normal PCC directly measures the isolated enzyme defect.
Show evidence (1 reference)
PMID:22642865 SUPPORT In Vitro
"In all but two individuals MCC activity in fibroblasts was severely reduced combined with normal activity of PCC."
Directly supports the isolated fibroblast enzyme-activity pattern.
🔬

Diagnosis

4
C5OH newborn-screening signal with infant-mother evaluation
In newborn-screening programs, elevated C5OH is followed by repeat infant biochemical testing and evaluation of a possible maternal source. C5OH alone neither establishes 3-MCCD nor predicts which person will become symptomatic.
disease screening NCIT:C15419 NCI Thesaurus (NCIT)
Results: Persistent infant C5OH supports an infant biochemical phenotype; infant normalization with a persistent maternal pattern supports maternal origin. Neither is a final diagnosis.
Show evidence (3 references)
PMID:27033733 SUPPORT Human Clinical
"C5OH level found on newborn screening by itself is not sufficient for diagnostic or predictive purposes."
Directly limits the interpretation of C5OH screening concentration.
PMID:26566957 SUPPORT Human Clinical
"several asymptomatic 3MCCD mothers were initially identified following abnormal screening of their healthy babies"
Supports evaluation for a maternal source of an infant screening signal.
PMID:18155630 SUPPORT Other
"initial evaluation of the screen-positive infant-mother dyad"
Expert consensus explicitly frames initial evaluation around the infant-mother dyad, although all recommendations were Grade D.
Confirmatory acylcarnitine and urine organic-acid testing
Repeat plasma or dried-blood-spot acylcarnitines and urine organic acids define the biochemical pattern. The characteristic combination is supportive but was paired with molecular or enzymatic confirmation in well-characterized cohorts when the diagnosis was uncertain.
diagnostic procedure NCIT:C18020 NCI Thesaurus (NCIT)
Results: Elevated C5OH plus urinary 3-hydroxyisovaleric acid and 3-methylcrotonylglycine.
Show evidence (2 references)
PMID:27033733 SUPPORT Human Clinical
"Follow-up confirmatory testing included plasma acylcarnitine and urine organic acid analyses."
Documents the core confirmatory biochemical studies used after screening.
PMID:36822454 SUPPORT Human Clinical
"recalled for repeated testing, urine organic acid analysis and molecular genetic analysis"
Provides an independent staged confirmatory workflow.
MCCC1 and MCCC2 molecular genetic testing
Identification of biallelic pathogenic variants in either MCCC1 or MCCC2 confirms canonical autosomal-recessive 3-MCCD and enables family testing. A single MCCC1 or MCCC2 variant can accompany a mild biochemical phenotype and does not meet this canonical biallelic criterion. Molecular results do not by themselves predict clinical severity.
molecular genetic testing NCIT:C19770 NCI Thesaurus (NCIT)
Results: Biallelic pathogenic variants in MCCC1 or MCCC2.
Show evidence (3 references)
CGGV:assertion_6bd0c545-9ef8-4005-a124-a73be6178745-2019-10-25T160000.000Z SUPPORT Other
"MCCC1 | HGNC:6936 | 3-methylcrotonyl-CoA carboxylase deficiency | MONDO:0018950 | AR | Definitive"
Supports MCCC1 as a definitive molecular cause.
CGGV:assertion_3653ea30-b630-499a-a6f0-65152106ad8c-2019-10-25T160000.000Z SUPPORT Other
"MCCC2 | HGNC:6937 | 3-methylcrotonyl-CoA carboxylase deficiency | MONDO:0018950 | AR | Definitive"
Supports MCCC2 as a definitive molecular cause.
PMID:22642865 SUPPORT Human Clinical
"carriers of a single mutation at the MCCC1 locus"
Supports caution that a single variant can accompany a mild biochemical phenotype without establishing canonical biallelic disease.
Isolated MCC enzyme activity assay
When molecular findings are incomplete or require functional confirmation, MCC and propionyl-CoA carboxylase activities can be assayed in cultured fibroblasts. Severe MCC reduction with preserved PCC supports isolated 3-MCCD.
diagnostic procedure NCIT:C18020 NCI Thesaurus (NCIT)
Results: Severely reduced MCC activity with normal PCC activity and an increased PCC-to-MCC activity ratio.
Show evidence (1 reference)
PMID:22642865 SUPPORT In Vitro
"Diagnosis of MCC deficiency was confirmed by assaying MCC and PCC activities in fibroblasts"
Directly supports paired fibroblast enzyme assays for confirmation.
📈

Progression

3
Predominantly asymptomatic course after newborn-screening detection
Age: Infancy through reported follow-up
Most people identified prospectively through newborn screening remain clinically well during available follow-up. Follow-up remains relatively short in many cohorts, so absence of childhood symptoms does not establish lifelong absence of risk.
Show evidence (2 references)
PMID:36822454 SUPPORT Human Clinical
"All these 53 patients did not present any clinical symptom."
All 53 screened cases were asymptomatic during reported follow-up.
PMID:26566957 SUPPORT Human Clinical
"Most of the primary 3MCCD individuals were asymptomatic"
The Israeli program likewise found a predominantly asymptomatic screened phenotype.
Occasional catabolic-stress decompensation
Age: Infancy and childhood, rarely later
A minority develop acute metabolic crises, commonly in association with infection or another catabolic stressor. The largest classic cohort mixed newborn-screened, symptom-referred, family-screened, and maternal cases and is therefore vulnerable to ascertainment bias.
Show evidence (2 references)
PMID:22642865 SUPPORT Human Clinical
"12 patients (5 of 53 identified by newborn screening) presented with acute metabolic decompensations."
Quantifies acute crises in the mixed 88-person cohort and its newborn-screened subset.
DOI:10.3390/ijns11040115 SUPPORT Human Clinical
"one patient experienced metabolic decompensation during an intercurrent illness, which was promptly treated."
A recent nine-child cohort documents one illness-associated crisis during mean follow-up to 4.2 years.
Uncertain attribution of chronic nonspecific findings
Age: Childhood through adulthood
Developmental delay, hypotonia, seizures, failure to thrive, and fatigue have been reported, but they are not specific to defective leucine catabolism. A second diagnosis or other contributor should be sought when chronic neurologic or multisystem findings dominate.
Show evidence (2 references)
PMID:25356967 SUPPORT Human Clinical
"For 5 of these 10 individuals, we discovered a homozygous damaging mutation in a disease gene"
Alternative homozygous disease-gene variants plausibly explained nonspecific findings in half of the examined cases.
PMID:27033733 SUPPORT Human Clinical
"15% of those with available developmental information had recorded developmental disabilities not clearly attributable to other causes."
Registry data show a small developmental-signal estimate while retaining uncertainty about attribution.
📊

Prevalence

3
California newborn-screening cohort
Birth Prevalence 2.399 per 100,000 1–9 per 100,000
The California program reported 71 diagnosed infants among 2,959,108 screened (1 in 41,676). Incomplete confirmatory data and many mild or unclassified biochemical phenotypes limit generalization beyond this screening program.
Show evidence (1 reference)
PMID:24103308 SUPPORT Human Clinical
"2,959,108 infants were screened and 71 infants were diagnosed with 3-MCCD for an overall incidence of 1:41,676."
Provides the regional newborn-screening yield used to calculate the rate.
Zhejiang Province newborn-screening cohort
Birth Prevalence 1.204 per 100,000 1–9 per 100,000
Screening of 4,402,587 newborns from 2009 through August 2022 identified 53 cases, corresponding to 1 in 83,068 newborns in Zhejiang Province.
Show evidence (1 reference)
PMID:36822454 SUPPORT Human Clinical
"The estimated incidence of 3-MCCD in Zhejiang Province was 1 in 83,068 newborns."
Provides a large regional newborn-screening estimate.
Quanzhou newborn-screening cohort
Birth Prevalence 2.641 per 100,000 1–9 per 100,000
Screening of 643,606 newborns from 2014 through 2022 identified 17 neonatal cases, corresponding to 1 in 37,859 newborns in the Quanzhou study population. Maternal and paternal cases were reported separately.
Show evidence (1 reference)
PMID:39188588 SUPPORT Human Clinical
"Its incidence in the Quanzhou study population was 1/37,859 newborns."
Provides a second recent regional newborn-screening estimate.
🔀

Differential Diagnoses

4

Conditions with similar clinical presentations that must be differentiated from 3-Methylcrotonyl-CoA Carboxylase Deficiency:

Maternal 3-methylcrotonyl-CoA carboxylase deficiency
Overlapping Features Placental transfer of maternal C5OH-related metabolites can produce an abnormal newborn result in an unaffected infant. Repeat infant studies can normalize while maternal acylcarnitines remain abnormal; maternal molecular or enzyme confirmation then identifies the source.
Distinguishing Features
  • Resolution of the infant C5OH elevation on repeat testing.
  • Persistent abnormal maternal acylcarnitines or urine organic acids.
  • Maternal biallelic MCCC1/MCCC2 variants or isolated MCC enzyme deficiency.
Show evidence (1 reference)
PMID:25356967 SUPPORT Human Clinical
"18 asymptomatic mothers who were discovered to have MCCD only by detection of abnormal C5OH-carnitine in the newborn screening sample"
Documents maternal 3-MCCD discovered through an infant's transient screening abnormality.
Multiple-carboxylase deficiencies
Overlapping Features Biotinidase deficiency and holocarboxylase synthetase deficiency can elevate C5OH but affect multiple biotin-dependent carboxylases rather than isolated MCC. Serum biotinidase activity, the broader metabolite pattern, clinical context, and BTD or HLCS testing distinguish these treatable disorders.
Distinguishing Features
  • Reduced biotinidase activity in biotinidase deficiency.
  • Biochemical evidence of more than one carboxylase defect.
  • Biallelic BTD or HLCS variants rather than MCCC1/MCCC2 variants.
Show evidence (2 references)
PMID:37373384 SUPPORT Human Clinical
"3-hydroxyisovaleryl-carnitine in plasma"
Directly supports C5OH elevation in biotinidase deficiency.
PMID:9350481 SUPPORT Other
"lead to deficiency of the 4 biotin-dependent carboxylases"
Distinguishes biotin-metabolism disorders from an isolated MCC enzyme defect.
HMG-CoA lyase deficiency
Overlapping Features HMG-CoA lyase deficiency also elevates C5OH but produces a broader urinary leucine-pathway profile and a clinically important hypoketotic-hypoglycemia crisis phenotype. Urine organic acids and HMGCL testing distinguish it from isolated 3-MCCD.
Distinguishing Features
  • Urinary 3-hydroxy-3-methylglutaric, 3-methylglutaconic, and 3-methylglutaric acids.
  • Hypoketotic hypoglycemia rather than a usually asymptomatic screened course.
  • Biallelic HMGCL variants.
Show evidence (3 references)
PMID:32685354 SUPPORT Human Clinical
"Plasma of these patients contains elevated levels of 3‐hydroxyisovalerylcarnitine"
Directly supports C5OH elevation in HMG-CoA lyase deficiency.
PMID:32685354 SUPPORT Human Clinical
"acidosis with hypoketotic hypoglycemia"
Supports the distinguishing acute crisis pattern.
PMID:32685354 SUPPORT Human Clinical
"3‐hydroxy‐3‐methylglutaric acid (3H3MG‐A), 3‐methylglutaric acid (3MG‐A)"
Supports the broader urine organic-acid profile that distinguishes HMG-CoA lyase deficiency.
Beta-ketothiolase deficiency
Overlapping Features Beta-ketothiolase deficiency can elevate C5OH during an acute crisis, but the acyl group is 2-methyl-3-hydroxybutyryl rather than 3-hydroxyisovaleryl. Concurrent C5:1 and characteristic isoleucine-derived urine metabolites support ACAT1 deficiency.
Distinguishing Features
  • Elevated C5:1 tiglylcarnitine with C5OH.
  • Urinary 2-methyl-3-hydroxybutyrate, tiglylglycine, and 2-methylacetoacetate.
  • Biallelic ACAT1 variants.
Show evidence (2 references)
PMID:23958592 SUPPORT Human Clinical
"(2-methyl-3-hydroxybutyryl) carnitine and C5:1 (tiglyl) carnitine were elevated"
Directly supports the distinguishing C5OH-plus-C5:1 crisis profile.
PMID:23958592 SUPPORT Human Clinical
"large amounts of 2-methyl-3-hydroxybutyrate, tiglylglycine, and 2-methylacetoacetate were excreted."
Supports the characteristic urinary isoleucine-derived metabolites.
{ }

Source YAML

click to show 
name: 3-Methylcrotonyl-CoA Carboxylase Deficiency
category: Mendelian
creation_date: '2026-05-03T00:00:00Z'
synonyms:
- 3-MCC deficiency
- 3-MCCD
- 3-methylcrotonylglycinuria
- MCC deficiency
- MCCD
description: >
  3-methylcrotonyl-CoA carboxylase deficiency (3-MCCD) is an autosomal
  recessive disorder of leucine catabolism caused by biallelic pathogenic
  variants in MCCC1 or MCCC2, which encode the alpha and beta subunits of the
  mitochondrial, biotin-dependent methylcrotonyl-CoA carboxylase complex.
  Reduced enzyme activity impairs conversion of 3-methylcrotonyl-CoA to
  3-methylglutaconyl-CoA and produces elevated C5OH acylcarnitine and urinary
  3-hydroxyisovaleric acid and 3-methylcrotonylglycine. Clinical penetrance is
  low: most newborn-screening-identified individuals remain asymptomatic, while
  a minority can develop acute organic-aciduria-like decompensation during
  intercurrent illness or other catabolic stress. C5OH concentration,
  biochemical severity, and genotype do not reliably predict outcome, and
  chronic nonspecific neurologic findings require evaluation for alternative
  causes.
disease_term:
  preferred_term: 3-methylcrotonyl-CoA carboxylase deficiency
  term:
    id: MONDO:0018950
    label: 3-methylcrotonyl-CoA carboxylase deficiency
parents:
- Organic Acidemia
- Inborn Error of Metabolism
inheritance:
- name: Autosomal recessive inheritance with incomplete clinical penetrance
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  penetrance: INCOMPLETE
  expressivity: VARIABLE
  description: >
    Canonical 3-MCCD results from biallelic MCCC1 or MCCC2 variants. Clinical
    expression is incompletely penetrant and highly variable even among people
    with a marked biochemical or enzymatic defect.
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "autosomal recessive disorder of leucine metabolism caused by mutations in MCCC1 or MCCC2"
    explanation: The 88-person cohort defines the canonical recessive MCCC1/MCCC2 disorder.
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "MCC deficiency, despite low penetrance, may lead to a severe clinical phenotype"
    explanation: The cohort directly supports incomplete penetrance and variable clinical expression.
prevalence:
- population: California newborn-screening cohort
  measure_type: BIRTH_PREVALENCE
  prevalence_class: BAND_1_9_PER_100000
  rate_per_100000: 2.399
  notes: >
    The California program reported 71 diagnosed infants among 2,959,108
    screened (1 in 41,676). Incomplete confirmatory data and many mild or
    unclassified biochemical phenotypes limit generalization beyond this
    screening program.
  evidence:
  - reference: PMID:24103308
    reference_title: Analysis of cases of 3-methylcrotonyl CoA carboxylase deficiency (3-MCCD) in the California newborn screening program reported in the state database.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "2,959,108 infants were screened and 71 infants were diagnosed with 3-MCCD for an overall incidence of 1:41,676."
    explanation: Provides the regional newborn-screening yield used to calculate the rate.
- population: Zhejiang Province newborn-screening cohort
  measure_type: BIRTH_PREVALENCE
  prevalence_class: BAND_1_9_PER_100000
  rate_per_100000: 1.204
  notes: >
    Screening of 4,402,587 newborns from 2009 through August 2022 identified 53
    cases, corresponding to 1 in 83,068 newborns in Zhejiang Province.
  evidence:
  - reference: PMID:36822454
    reference_title: "Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency in Zhejiang province, China."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The estimated incidence of 3-MCCD in Zhejiang Province was 1 in 83,068 newborns."
    explanation: Provides a large regional newborn-screening estimate.
- population: Quanzhou newborn-screening cohort
  measure_type: BIRTH_PREVALENCE
  prevalence_class: BAND_1_9_PER_100000
  rate_per_100000: 2.641
  notes: >
    Screening of 643,606 newborns from 2014 through 2022 identified 17 neonatal
    cases, corresponding to 1 in 37,859 newborns in the Quanzhou study
    population. Maternal and paternal cases were reported separately.
  evidence:
  - reference: PMID:39188588
    reference_title: "Newborn screening and genetic diagnosis of 3-methylcrotonyl-CoA carboxylase deficiency in Quanzhou,China."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Its incidence in the Quanzhou study population was 1/37,859 newborns."
    explanation: Provides a second recent regional newborn-screening estimate.
progression:
- phase: Predominantly asymptomatic course after newborn-screening detection
  age_range: Infancy through reported follow-up
  notes: >
    Most people identified prospectively through newborn screening remain
    clinically well during available follow-up. Follow-up remains relatively
    short in many cohorts, so absence of childhood symptoms does not establish
    lifelong absence of risk.
  evidence:
  - reference: PMID:36822454
    reference_title: "Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency in Zhejiang province, China."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All these 53 patients did not present any clinical symptom."
    explanation: All 53 screened cases were asymptomatic during reported follow-up.
  - reference: PMID:26566957
    reference_title: "Primary and maternal 3-methylcrotonyl-CoA carboxylase deficiency: insights from the Israel newborn screening program."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Most of the primary 3MCCD individuals were asymptomatic"
    explanation: The Israeli program likewise found a predominantly asymptomatic screened phenotype.
- phase: Occasional catabolic-stress decompensation
  age_range: Infancy and childhood, rarely later
  notes: >
    A minority develop acute metabolic crises, commonly in association with
    infection or another catabolic stressor. The largest classic cohort mixed
    newborn-screened, symptom-referred, family-screened, and maternal cases and
    is therefore vulnerable to ascertainment bias.
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "12 patients (5 of 53 identified by newborn screening) presented with acute metabolic decompensations."
    explanation: Quantifies acute crises in the mixed 88-person cohort and its newborn-screened subset.
  - reference: DOI:10.3390/ijns11040115
    reference_title: "Psychological Impact of Newborn Screening for 3-Methylcrotonyl-CoA Carboxylase Deficiency: The Parental Experience"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "one patient experienced metabolic decompensation during an intercurrent illness, which was promptly treated."
    explanation: A recent nine-child cohort documents one illness-associated crisis during mean follow-up to 4.2 years.
- phase: Uncertain attribution of chronic nonspecific findings
  age_range: Childhood through adulthood
  notes: >
    Developmental delay, hypotonia, seizures, failure to thrive, and fatigue
    have been reported, but they are not specific to defective leucine
    catabolism. A second diagnosis or other contributor should be sought when
    chronic neurologic or multisystem findings dominate.
  evidence:
  - reference: PMID:25356967
    reference_title: Consanguinity and rare mutations outside of MCCC genes underlie nonspecific phenotypes of MCCD.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "For 5 of these 10 individuals, we discovered a homozygous damaging mutation in a disease gene"
    explanation: Alternative homozygous disease-gene variants plausibly explained nonspecific findings in half of the examined cases.
  - reference: PMID:27033733
    reference_title: Outcomes of cases with 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency - Report from the Inborn Errors of Metabolism Information System.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "15% of those with available developmental information had recorded developmental disabilities not clearly attributable to other causes."
    explanation: Registry data show a small developmental-signal estimate while retaining uncertainty about attribution.
pathophysiology:
- name: MCCC1/MCCC2-dependent methylcrotonyl-CoA carboxylase deficiency
  conforms_to: "metabolic_intoxication_decompensation#Enzymatic Block in Intermediary Metabolism"
  description: >
    Biallelic pathogenic variants in MCCC1 or MCCC2 reduce activity of the
    mitochondrial alpha6-beta6 methylcrotonyl-CoA carboxylase holoenzyme. The
    biotin-dependent enzyme catalyzes the fourth step of leucine catabolism,
    converting 3-methylcrotonyl-CoA toward 3-methylglutaconyl-CoA.
  genes:
  - preferred_term: MCCC1
    term:
      id: hgnc:6936
      label: MCCC1
  - preferred_term: MCCC2
    term:
      id: hgnc:6937
      label: MCCC2
  molecular_functions:
  - preferred_term: methylcrotonoyl-CoA carboxylase activity
    term:
      id: GO:0004485
      label: methylcrotonoyl-CoA carboxylase activity
    modifier: DECREASED
  biological_processes:
  - preferred_term: L-leucine catabolic process
    term:
      id: GO:0006552
      label: L-leucine catabolic process
    modifier: DECREASED
  cellular_components:
  - preferred_term: mitochondrial matrix
    term:
      id: GO:0005759
      label: mitochondrial matrix
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The mitochondrial enzyme MCC (EC 6.4.1.4) catalyzes the fourth step in the leucine catabolic pathway"
    explanation: Defines the enzyme, compartment, and blocked pathway step.
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "MCC consists of an alpha and a beta subunit assembled into a α6β6 dodecamer."
    explanation: Supports the two-subunit holoenzyme architecture encoded by MCCC1 and MCCC2.
  downstream:
  - target: Leucine-pathway metabolite shunting
    description: Reduced MCC activity blocks normal leucine-catabolic flux and diverts upstream carbon into characteristic organic acids and C5OH acylcarnitine.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:27033733
      reference_title: Outcomes of cases with 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency - Report from the Inborn Errors of Metabolism Information System.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "A genetic defect in 3-methylcrotonyl-CoA carboxylase leads to elevation of 3-methylcrotonyl CoA, 3-methylcrotonyl glycine and 3-OH isovaleric acid"
      explanation: Directly connects the enzyme defect to upstream metabolite elevation.
- name: Leucine-pathway metabolite shunting
  description: >
    The blocked reaction is associated with accumulation and excretion of
    3-hydroxyisovaleric acid, 3-methylcrotonylglycine, and
    3-hydroxyisovalerylcarnitine (C5OH). Clinical toxicity and any associated
    energy deficit are incompletely established: very high metabolite excretion
    is often compatible with an asymptomatic course, so this node is not mapped
    to the module's toxic-metabolite/energy-deficit state.
  chemical_entities:
  - preferred_term: 3-hydroxyisovaleric acid
    term:
      id: CHEBI:37084
      label: 3-hydroxyisovaleric acid
    modifier: INCREASED
  - preferred_term: 3-methylcrotonylglycine
    modifier: INCREASED
  - preferred_term: 3-hydroxyisovalerylcarnitine
    term:
      id: CHEBI:73027
      label: 3-hydroxyisovalerylcarnitine
    modifier: INCREASED
  evidence:
  - reference: PMID:36822454
    reference_title: "Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency in Zhejiang province, China."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "94 % (50/53) of the patients had markedly increased urinary 3-hydroxyisovaleric acid and 3-methylcrotonylglycine."
    explanation: Quantifies the characteristic urinary metabolite pattern in a screened cohort.
  - reference: PMID:36822454
    reference_title: "Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency in Zhejiang province, China."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All these 53 patients exhibited increased C5OH concentrations in blood."
    explanation: Supports C5OH elevation in the ascertained cohort without claiming diagnostic specificity.
  downstream:
  - target: Secondary carnitine depletion
    description: Formation and excretion of hydroxyisovalerylcarnitine is associated with depletion of free carnitine in a subset of affected people.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:36822454
      reference_title: "Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency in Zhejiang province, China."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Twenty-three of 53 patients had secondary carnitine deficiency."
      explanation: Establishes the association between 3-MCCD and secondary carnitine deficiency.
  - target: Catabolic-stress acute metabolic decompensation
    description: In a minority, infection or another catabolic stress coincides with an organic-aciduria-like crisis; the factors separating tolerant from vulnerable individuals remain unknown.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:22642865
      reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Some patients develop an acute metabolic crisis usually triggered by intercurrent infections"
      explanation: Supports the stress-associated clinical branch while limiting it to a subset.
  - target: Experimental mitochondrial and oxidative stress
    description: Cell and tissue-preparation experiments suggest that chronic metabolite exposure may perturb mitochondrial and redox homeostasis, but this has not been shown to cause human symptoms.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:23053545
      reference_title: Neurochemical evidence that the metabolites accumulating in 3-methylcrotonyl-CoA carboxylase deficiency induce oxidative damage in cerebral cortex of young rats.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "We investigated the in vitro effects of 3MCG and 3MCA on important parameters of oxidative stress"
      explanation: Provides experimental evidence for metabolite-induced oxidative effects, not a patient-level causal link.
- name: Secondary carnitine depletion
  description: >
    Free carnitine can be low in blood and tissue. This is a secondary and
    variably present consequence rather than correction of, or a proxy for,
    residual MCC enzyme activity.
  chemical_entities:
  - preferred_term: carnitine
    term:
      id: CHEBI:17126
      label: carnitine
    modifier: DECREASED
  evidence:
  - reference: PMID:25732994
    reference_title: "Is L-Carnitine Supplementation Beneficial in 3-Methylcrotonyl-CoA Carboxylase Deficiency?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the 3-MCCd patients (n= 13) had low plasma and muscle free carnitine levels"
    explanation: Documents low free carnitine in a small Faroese founder cohort without supplementation.
  - reference: PMID:36822454
    reference_title: "Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency in Zhejiang province, China."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Twenty-three of 53 patients had secondary carnitine deficiency."
    explanation: Demonstrates that depletion is common but not universal in a larger screened cohort.
- name: Catabolic-stress acute metabolic decompensation
  conforms_to: "metabolic_intoxication_decompensation#Acute Metabolic Decompensation"
  description: >
    Rare crises during infection, fasting, or another catabolic stress can
    resemble classical organic acidurias, with vomiting, metabolic acidosis,
    ketotic hypoglycemia, mild hyperammonemia, seizures, or coma. This branch is
    not an inevitable consequence of the biochemical defect.
  biological_processes:
  - preferred_term: response to starvation
    term:
      id: GO:0042594
      label: response to starvation
  cell_types:
  - preferred_term: hepatocyte
    term:
      id: CL:0000182
      label: hepatocyte
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "12 patients (5 of 53 identified by newborn screening) presented with acute metabolic decompensations."
    explanation: Establishes that crisis occurs in a minority, including some prospectively screened individuals.
  downstream:
  - target: Metabolic acidosis
    description: Organic-aciduria-like decompensation can produce metabolic acidosis.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:22642865
      reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "acidosis, hypoglycemia and in some cases mild"
      explanation: Directly describes acidosis during reported crises.
  - target: Hypoglycemia
    description: Acute decompensation can include hypoglycemia.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:22642865
      reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "acidosis, hypoglycemia and in some cases mild"
      explanation: Directly describes hypoglycemia during reported crises.
  - target: Hyperammonemia
    description: Mild hyperammonemia can accompany severe biochemical decompensation.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:22642865
      reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "hyperammonemia [3,7,23-27]"
      explanation: Limits hyperammonemia to some acute crises rather than treating it as a frequent baseline feature.
  - target: Vomiting
    description: Vomiting can be an early clinical manifestation of acute crisis.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:22642865
      reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "vomiting, opisthotonus, involuntary movements"
      explanation: Directly lists vomiting among crisis symptoms.
  - target: Seizures
    description: Seizures can occur during severe acute decompensation.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:22642865
      reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Neurologic symptoms like seizures"
      explanation: Directly lists seizures among crisis symptoms.
  - target: Coma
    description: Severe decompensation can progress to coma.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:25356967
      reference_title: Consanguinity and rare mutations outside of MCCC genes underlie nonspecific phenotypes of MCCD.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Specific symptoms included ketoacidosis, hypoglycemia, hyperammonemia, coma"
      explanation: Classifies coma among severe findings directly correlated with defective leucine catabolism.
- name: Experimental mitochondrial and oxidative stress
  mechanism_confidence: PROVISIONAL
  description: >
    Immortalized fibroblasts from two clinically affected individuals and in
    vitro exposure of young-rat cerebral cortex preparations to accumulating
    metabolites showed mitochondrial, energy-homeostasis, or oxidative-stress
    abnormalities. These experiments generate a mechanism hypothesis but do
    not establish a causal route to neurologic findings in affected people.
  biological_processes:
  - preferred_term: response to oxidative stress
    term:
      id: GO:0006979
      label: response to oxidative stress
    modifier: INCREASED
  - preferred_term: oxidative phosphorylation
    term:
      id: GO:0006119
      label: oxidative phosphorylation
    modifier: ABNORMAL
  cell_types:
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  evidence:
  - reference: PMID:27417235
    reference_title: A 3-methylcrotonyl-CoA carboxylase deficient human skin fibroblast transcriptome reveals underlying mitochondrial dysfunction and oxidative stress.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "immortalized cultured skin fibroblast cells of two clinically affected MCC deficient patients and two healthy individuals"
    explanation: Defines the very small experimental sample and cellular context.
  - reference: PMID:27417235
    reference_title: A 3-methylcrotonyl-CoA carboxylase deficient human skin fibroblast transcriptome reveals underlying mitochondrial dysfunction and oxidative stress.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "mitochondrial dysfunction, decreased antioxidant response and disruption of energy homeostasis"
    explanation: Supports the observed fibroblast stress signature.
  - reference: PMID:23053545
    reference_title: Neurochemical evidence that the metabolites accumulating in 3-methylcrotonyl-CoA carboxylase deficiency induce oxidative damage in cerebral cortex of young rats.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "3MCG and 3MCA significantly increased TBA-RS and carbonyl formation"
    explanation: Supports oxidative effects after direct metabolite exposure in rat cortex preparations.
phenotypes:
- name: Metabolic acidosis
  description: A crisis-associated finding in a minority of affected people, not a typical baseline feature of newborn-screened 3-MCCD.
  phenotype_term:
    preferred_term: Metabolic acidosis
    term:
      id: HP:0001942
      label: Metabolic acidosis
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "acidosis, hypoglycemia and in some cases mild"
    explanation: Directly supports metabolic acidosis during acute crises.
- name: Hypoglycemia
  description: A crisis-associated finding whose presence and recurrence are not predicted by newborn-screening C5OH concentration.
  phenotype_term:
    preferred_term: Hypoglycemia
    term:
      id: HP:0001943
      label: Hypoglycemia
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "acidosis, hypoglycemia and in some cases mild"
    explanation: Directly supports hypoglycemia during acute crises.
- name: Hyperammonemia
  description: Mild hyperammonemia is reported in some acute crises and should not be assigned a general disease-wide frequency.
  phenotype_term:
    preferred_term: Hyperammonemia
    term:
      id: HP:0001987
      label: Hyperammonemia
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "hyperammonemia [3,7,23-27]"
    explanation: Directly supports occasional hyperammonemia during acute crises.
- name: Vomiting
  description: Vomiting can occur during acute metabolic decompensation.
  phenotype_term:
    preferred_term: Vomiting
    term:
      id: HP:0002013
      label: Vomiting
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "vomiting, opisthotonus, involuntary movements"
    explanation: Directly supports vomiting as a crisis manifestation.
- name: Seizures
  description: Seizures can occur during severe crisis, but chronic seizure disorders require evaluation for other causes.
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Neurologic symptoms like seizures"
    explanation: Directly supports seizures in the acute crisis spectrum.
  - reference: PMID:25356967
    reference_title: Consanguinity and rare mutations outside of MCCC genes underlie nonspecific phenotypes of MCCD.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Nonspecific symptoms, which are not directly related to leucine metabolism, included developmental delay, intellectual disability, seizures"
    explanation: Supports caution in attributing a chronic seizure phenotype to the MCC defect.
- name: Coma
  description: Coma is a rare severe endpoint of acute biochemical decompensation.
  phenotype_term:
    preferred_term: Coma
    term:
      id: HP:0001259
      label: Coma
  evidence:
  - reference: PMID:25356967
    reference_title: Consanguinity and rare mutations outside of MCCC genes underlie nonspecific phenotypes of MCCD.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Specific symptoms included ketoacidosis, hypoglycemia, hyperammonemia, coma"
    explanation: Classifies coma among findings directly correlated with defective leucine catabolism.
biochemical:
- name: Elevated C5OH acylcarnitine
  presence: INCREASED
  context: >
    C5OH is the primary tandem-mass-spectrometry screening signal, but it is not
    specific for 3-MCCD and its concentration does not predict the clinical
    course.
  biomarker_term:
    preferred_term: 3-hydroxyisovalerylcarnitine
    term:
      id: CHEBI:73027
      label: 3-hydroxyisovalerylcarnitine
  readouts:
  - target: Leucine-pathway metabolite shunting
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Elevated C5OH reports abnormal acyl-group handling but does not by itself establish 3-MCCD.
  evidence:
  - reference: PMID:36822454
    reference_title: "Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency in Zhejiang province, China."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All these 53 patients exhibited increased C5OH concentrations in blood."
    explanation: Supports C5OH elevation among screened, confirmed cases.
  - reference: PMID:39484073
    reference_title: Outcomes of cases with elevated 3-hydroxyisovaleryl carnitine report from the newborn screening program.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "relying solely on C5OH levels from newborn screening is insufficient for making accurate diagnoses or predictions"
    explanation: Directly limits the diagnostic and prognostic interpretation of the screening marker.
- name: Increased urinary 3-hydroxyisovaleric acid
  presence: INCREASED
  context: >
    Urinary 3-hydroxyisovaleric acid is a characteristic downstream metabolite,
    but it can be absent or only mildly elevated and must be interpreted with
    the full acylcarnitine, organic-acid, enzymatic, and molecular profile.
  biomarker_term:
    preferred_term: 3-hydroxyisovaleric acid
    term:
      id: CHEBI:37084
      label: 3-hydroxyisovaleric acid
  readouts:
  - target: Leucine-pathway metabolite shunting
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Increased urinary 3-hydroxyisovaleric acid supports a leucine-pathway block.
  evidence:
  - reference: PMID:36822454
    reference_title: "Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency in Zhejiang province, China."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "94 % (50/53) of the patients had markedly increased urinary 3-hydroxyisovaleric acid and 3-methylcrotonylglycine."
    explanation: Provides a defined denominator and shows the marker is characteristic but not universal.
- name: Increased urinary 3-methylcrotonylglycine
  presence: INCREASED
  context: >
    Urinary 3-methylcrotonylglycine accompanies 3-hydroxyisovaleric acid in the
    characteristic organic-acid profile; the analyte is kept separate because
    it does not share the 3-hydroxyisovaleric-acid ontology binding.
  readouts:
  - target: Leucine-pathway metabolite shunting
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Increased urinary 3-methylcrotonylglycine supports abnormal handling of 3-methylcrotonyl-CoA.
  evidence:
  - reference: PMID:36822454
    reference_title: "Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency in Zhejiang province, China."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "94 % (50/53) of the patients had markedly increased urinary 3-hydroxyisovaleric acid and 3-methylcrotonylglycine."
    explanation: Quantifies the combined urinary metabolite pattern.
- name: Decreased free carnitine
  presence: DECREASED
  context: >
    Secondary carnitine deficiency is variably present and is a monitoring and
    treatment target rather than a measure of residual MCC activity.
  biomarker_term:
    preferred_term: carnitine
    term:
      id: CHEBI:17126
      label: carnitine
  readouts:
  - target: Secondary carnitine depletion
    relationship: READOUT_OF
    direction: NEGATIVE
    endpoint_context: MONITORING
    interpretation: Low plasma free carnitine identifies secondary depletion that may warrant replacement.
  evidence:
  - reference: PMID:36822454
    reference_title: "Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency in Zhejiang province, China."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Twenty-three of 53 patients had secondary carnitine deficiency."
    explanation: Shows depletion in a substantial but non-universal subset.
- name: Isolated reduction of MCC enzyme activity
  presence: DECREASED
  context: >
    Demonstration of severely reduced MCC activity with preserved
    propionyl-CoA carboxylase activity in cultured fibroblasts supports isolated
    MCC deficiency and helps distinguish it from multiple-carboxylase defects.
  readouts:
  - target: MCCC1/MCCC2-dependent methylcrotonyl-CoA carboxylase deficiency
    relationship: READOUT_OF
    direction: NEGATIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Low MCC with normal PCC directly measures the isolated enzyme defect.
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In all but two individuals MCC activity in fibroblasts was severely reduced combined with normal activity of PCC."
    explanation: Directly supports the isolated fibroblast enzyme-activity pattern.
genetic:
- name: MCCC1
  gene_term:
    preferred_term: MCCC1
    term:
      id: hgnc:6936
      label: MCCC1
  association: Biallelic pathogenic MCCC1 variants cause the MCCA complementation form of 3-MCCD.
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  features: >
    MCCC1 encodes the biotin-containing alpha subunit. Variant type and residual
    biochemical phenotype do not reliably predict clinical severity.
  evidence:
  - reference: CGGV:assertion_6bd0c545-9ef8-4005-a124-a73be6178745-2019-10-25T160000.000Z
    reference_title: "MCCC1 / 3-methylcrotonyl-CoA carboxylase deficiency (Definitive)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "MCCC1 | HGNC:6936 | 3-methylcrotonyl-CoA carboxylase deficiency | MONDO:0018950 | AR | Definitive"
    explanation: ClinGen classifies this autosomal-recessive gene-disease relationship as definitive.
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "neither the genotype nor the biochemical phenotype is helpful in predicting the clinical course."
    explanation: Supports the absence of a reliable genotype-phenotype correlation.
- name: MCCC2
  gene_term:
    preferred_term: MCCC2
    term:
      id: hgnc:6937
      label: MCCC2
  association: Biallelic pathogenic MCCC2 variants cause the MCCB complementation form of 3-MCCD.
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  features: >
    MCCC2 encodes the beta subunit. Variant type and residual biochemical
    phenotype do not reliably predict clinical severity.
  evidence:
  - reference: CGGV:assertion_3653ea30-b630-499a-a6f0-65152106ad8c-2019-10-25T160000.000Z
    reference_title: "MCCC2 / 3-methylcrotonyl-CoA carboxylase deficiency (Definitive)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "MCCC2 | HGNC:6937 | 3-methylcrotonyl-CoA carboxylase deficiency | MONDO:0018950 | AR | Definitive"
    explanation: ClinGen classifies this autosomal-recessive gene-disease relationship as definitive.
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "neither the genotype nor the biochemical phenotype is helpful in predicting the clinical course."
    explanation: Supports the absence of a reliable genotype-phenotype correlation.
diagnosis:
- name: C5OH newborn-screening signal with infant-mother evaluation
  diagnosis_term:
    preferred_term: disease screening
    term:
      id: NCIT:C15419
      label: Disease Screening
  description: >
    In newborn-screening programs, elevated C5OH is followed by repeat infant
    biochemical testing and evaluation of a possible maternal source. C5OH
    alone neither establishes 3-MCCD nor predicts which person will become
    symptomatic.
  results: Persistent infant C5OH supports an infant biochemical phenotype; infant normalization with a persistent maternal pattern supports maternal origin. Neither is a final diagnosis.
  evidence:
  - reference: PMID:27033733
    reference_title: Outcomes of cases with 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency - Report from the Inborn Errors of Metabolism Information System.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "C5OH level found on newborn screening by itself is not sufficient for diagnostic or predictive purposes."
    explanation: Directly limits the interpretation of C5OH screening concentration.
  - reference: PMID:26566957
    reference_title: "Primary and maternal 3-methylcrotonyl-CoA carboxylase deficiency: insights from the Israel newborn screening program."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "several asymptomatic 3MCCD mothers were initially identified following abnormal screening of their healthy babies"
    explanation: Supports evaluation for a maternal source of an infant screening signal.
  - reference: PMID:18155630
    reference_title: A Delphi-based consensus clinical practice protocol for the diagnosis and management of 3-methylcrotonyl CoA carboxylase deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "initial evaluation of the screen-positive infant-mother dyad"
    explanation: Expert consensus explicitly frames initial evaluation around the infant-mother dyad, although all recommendations were Grade D.
- name: Confirmatory acylcarnitine and urine organic-acid testing
  diagnosis_term:
    preferred_term: diagnostic procedure
    term:
      id: NCIT:C18020
      label: Diagnostic Procedure
  description: >
    Repeat plasma or dried-blood-spot acylcarnitines and urine organic acids
    define the biochemical pattern. The characteristic combination is
    supportive but was paired with molecular or enzymatic confirmation in
    well-characterized cohorts when the diagnosis was uncertain.
  results: Elevated C5OH plus urinary 3-hydroxyisovaleric acid and 3-methylcrotonylglycine.
  evidence:
  - reference: PMID:27033733
    reference_title: Outcomes of cases with 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency - Report from the Inborn Errors of Metabolism Information System.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Follow-up confirmatory testing included plasma acylcarnitine and urine organic acid analyses."
    explanation: Documents the core confirmatory biochemical studies used after screening.
  - reference: PMID:36822454
    reference_title: "Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency in Zhejiang province, China."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "recalled for repeated testing, urine organic acid analysis and molecular genetic analysis"
    explanation: Provides an independent staged confirmatory workflow.
- name: MCCC1 and MCCC2 molecular genetic testing
  diagnosis_term:
    preferred_term: molecular genetic testing
    term:
      id: NCIT:C19770
      label: Molecular Analysis
  description: >
    Identification of biallelic pathogenic variants in either MCCC1 or MCCC2
    confirms canonical autosomal-recessive 3-MCCD and enables family testing.
    A single MCCC1 or MCCC2 variant can accompany a mild biochemical phenotype
    and does not meet this canonical biallelic criterion. Molecular results do
    not by themselves predict clinical severity.
  results: Biallelic pathogenic variants in MCCC1 or MCCC2.
  evidence:
  - reference: CGGV:assertion_6bd0c545-9ef8-4005-a124-a73be6178745-2019-10-25T160000.000Z
    reference_title: "MCCC1 / 3-methylcrotonyl-CoA carboxylase deficiency (Definitive)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "MCCC1 | HGNC:6936 | 3-methylcrotonyl-CoA carboxylase deficiency | MONDO:0018950 | AR | Definitive"
    explanation: Supports MCCC1 as a definitive molecular cause.
  - reference: CGGV:assertion_3653ea30-b630-499a-a6f0-65152106ad8c-2019-10-25T160000.000Z
    reference_title: "MCCC2 / 3-methylcrotonyl-CoA carboxylase deficiency (Definitive)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "MCCC2 | HGNC:6937 | 3-methylcrotonyl-CoA carboxylase deficiency | MONDO:0018950 | AR | Definitive"
    explanation: Supports MCCC2 as a definitive molecular cause.
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "carriers of a single mutation at the MCCC1 locus"
    explanation: Supports caution that a single variant can accompany a mild biochemical phenotype without establishing canonical biallelic disease.
- name: Isolated MCC enzyme activity assay
  diagnosis_term:
    preferred_term: diagnostic procedure
    term:
      id: NCIT:C18020
      label: Diagnostic Procedure
  description: >
    When molecular findings are incomplete or require functional confirmation,
    MCC and propionyl-CoA carboxylase activities can be assayed in cultured
    fibroblasts. Severe MCC reduction with preserved PCC supports isolated
    3-MCCD.
  results: Severely reduced MCC activity with normal PCC activity and an increased PCC-to-MCC activity ratio.
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Diagnosis of MCC deficiency was confirmed by assaying MCC and PCC activities in fibroblasts"
    explanation: Directly supports paired fibroblast enzyme assays for confirmation.
differential_diagnoses:
- name: Maternal 3-methylcrotonyl-CoA carboxylase deficiency
  description: >
    Placental transfer of maternal C5OH-related metabolites can produce an
    abnormal newborn result in an unaffected infant. Repeat infant studies can
    normalize while maternal acylcarnitines remain abnormal; maternal molecular
    or enzyme confirmation then identifies the source.
  distinguishing_features:
  - Resolution of the infant C5OH elevation on repeat testing.
  - Persistent abnormal maternal acylcarnitines or urine organic acids.
  - Maternal biallelic MCCC1/MCCC2 variants or isolated MCC enzyme deficiency.
  evidence:
  - reference: PMID:25356967
    reference_title: Consanguinity and rare mutations outside of MCCC genes underlie nonspecific phenotypes of MCCD.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "18 asymptomatic mothers who were discovered to have MCCD only by detection of abnormal C5OH-carnitine in the newborn screening sample"
    explanation: Documents maternal 3-MCCD discovered through an infant's transient screening abnormality.
- name: Multiple-carboxylase deficiencies
  description: >
    Biotinidase deficiency and holocarboxylase synthetase deficiency can elevate
    C5OH but affect multiple biotin-dependent carboxylases rather than isolated
    MCC. Serum biotinidase activity, the broader metabolite pattern, clinical
    context, and BTD or HLCS testing distinguish these treatable disorders.
  distinguishing_features:
  - Reduced biotinidase activity in biotinidase deficiency.
  - Biochemical evidence of more than one carboxylase defect.
  - Biallelic BTD or HLCS variants rather than MCCC1/MCCC2 variants.
  evidence:
  - reference: PMID:37373384
    reference_title: "Delayed Biotin Therapy in a Child with Atypical Profound Biotinidase Deficiency: Late Arrival of the Truth and a Lesson Worth Thinking."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "3-hydroxyisovaleryl-carnitine in plasma"
    explanation: Directly supports C5OH elevation in biotinidase deficiency.
  - reference: PMID:9350481
    reference_title: "Multiple carboxylase deficiency: inherited and acquired disorders of biotin metabolism."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "lead to deficiency of the 4 biotin-dependent carboxylases"
    explanation: Distinguishes biotin-metabolism disorders from an isolated MCC enzyme defect.
- name: HMG-CoA lyase deficiency
  description: >
    HMG-CoA lyase deficiency also elevates C5OH but produces a broader urinary
    leucine-pathway profile and a clinically important hypoketotic-hypoglycemia
    crisis phenotype. Urine organic acids and HMGCL testing distinguish it from
    isolated 3-MCCD.
  distinguishing_features:
  - Urinary 3-hydroxy-3-methylglutaric, 3-methylglutaconic, and 3-methylglutaric acids.
  - Hypoketotic hypoglycemia rather than a usually asymptomatic screened course.
  - Biallelic HMGCL variants.
  evidence:
  - reference: PMID:32685354
    reference_title: A newborn screening approach to diagnose 3-hydroxy-3-methylglutaryl-CoA lyase deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Plasma of these patients contains elevated levels of 3‐hydroxyisovalerylcarnitine"
    explanation: Directly supports C5OH elevation in HMG-CoA lyase deficiency.
  - reference: PMID:32685354
    reference_title: A newborn screening approach to diagnose 3-hydroxy-3-methylglutaryl-CoA lyase deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "acidosis with hypoketotic hypoglycemia"
    explanation: Supports the distinguishing acute crisis pattern.
  - reference: PMID:32685354
    reference_title: A newborn screening approach to diagnose 3-hydroxy-3-methylglutaryl-CoA lyase deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "3‐hydroxy‐3‐methylglutaric acid (3H3MG‐A), 3‐methylglutaric acid (3MG‐A)"
    explanation: Supports the broader urine organic-acid profile that distinguishes HMG-CoA lyase deficiency.
- name: Beta-ketothiolase deficiency
  description: >
    Beta-ketothiolase deficiency can elevate C5OH during an acute crisis, but
    the acyl group is 2-methyl-3-hydroxybutyryl rather than
    3-hydroxyisovaleryl. Concurrent C5:1 and characteristic isoleucine-derived
    urine metabolites support ACAT1 deficiency.
  distinguishing_features:
  - Elevated C5:1 tiglylcarnitine with C5OH.
  - Urinary 2-methyl-3-hydroxybutyrate, tiglylglycine, and 2-methylacetoacetate.
  - Biallelic ACAT1 variants.
  evidence:
  - reference: PMID:23958592
    reference_title: "Metabolic encephalopathy in beta-ketothiolase deficiency: the first report from India."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "(2-methyl-3-hydroxybutyryl) carnitine and C5:1 (tiglyl) carnitine were elevated"
    explanation: Directly supports the distinguishing C5OH-plus-C5:1 crisis profile.
  - reference: PMID:23958592
    reference_title: "Metabolic encephalopathy in beta-ketothiolase deficiency: the first report from India."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "large amounts of 2-methyl-3-hydroxybutyrate, tiglylglycine, and 2-methylacetoacetate were excreted."
    explanation: Supports the characteristic urinary isoleucine-derived metabolites.
treatments:
- name: Sick-day emergency regimen and avoidance of prolonged fasting
  action_category: THERAPEUTIC
  description: >
    A metabolic-specialist sick-day plan and precautions against prolonged
    fasting are commonly used with the aim of reducing catabolic stress.
    Observational evidence documents this practice, but its preventive effect
    has not been established in a controlled study.
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  target_mechanisms:
  - target: Catabolic-stress acute metabolic decompensation
    treatment_effect: MODULATES
    description: Reducing catabolism is intended to lower the risk or severity of an illness-associated crisis.
    evidence:
    - reference: PMID:27033733
      reference_title: Outcomes of cases with 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency - Report from the Inborn Errors of Metabolism Information System.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The role that prevention of fasting plays in outcome cannot be ascertained."
      explanation: Supports the clinical rationale while explicitly showing that effectiveness remains unresolved.
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "an emergency regimen during intercurrent illness"
    explanation: The cohort authors cautiously recommend an illness regimen rather than asserting proven efficacy.
  - reference: DOI:10.3390/ijns11040115
    reference_title: "Psychological Impact of Newborn Screening for 3-Methylcrotonyl-CoA Carboxylase Deficiency: The Parental Experience"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "An emergency protocol was provided to all affected children to manage intercurrent illnesses."
    explanation: A recent cohort documents contemporary use of an emergency protocol.
- name: Carnitine supplementation for documented deficiency
  action_category: THERAPEUTIC
  description: >
    Oral L-carnitine can restore low free carnitine in people with documented
    secondary deficiency and may help selected symptomatic individuals. It
    does not correct the MCC block, and available data do not support universal
    supplementation.
  treatment_term:
    preferred_term: carnitine supplementation
    term:
      id: NCIT:C15433
      label: Nutritional Support
    therapeutic_agent:
    - preferred_term: carnitine
      term:
        id: CHEBI:17126
        label: carnitine
  target_mechanisms:
  - target: Secondary carnitine depletion
    treatment_effect: RESTORES
    description: Supplementation replenishes depleted plasma and tissue free carnitine.
    evidence:
    - reference: PMID:25732994
      reference_title: "Is L-Carnitine Supplementation Beneficial in 3-Methylcrotonyl-CoA Carboxylase Deficiency?"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "L-Carnitine supplementation increased muscle and plasma carnitine levels to a low-normal range"
      explanation: Directly supports biochemical restoration in the 13-person Faroese study.
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "monitoring of free carnitine concentrations"
    explanation: Supports monitoring-first, deficiency-directed replacement.
  - reference: PMID:25732994
    reference_title: "Is L-Carnitine Supplementation Beneficial in 3-Methylcrotonyl-CoA Carboxylase Deficiency?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Some symptomatic 3-MCCd patients may benefit biochemically and clinically from L-carnitine supplementation, a more general recommendation cannot be given."
    explanation: Limits the possible benefit to selected individuals and rejects a general recommendation.
- name: Individualized dietary management
  therapeutic_modality: BEHAVIORAL
  action_category: THERAPEUTIC
  description: >
    Routine leucine or protein restriction is usually unnecessary. A metabolic
    specialist may consider a modest, individualized leucine restriction for a
    person with recurrent or severe specific crises, but clinical efficacy is
    unproven and nutritional adequacy must be protected.
  treatment_term:
    preferred_term: dietary intervention
    term:
      id: NCIT:C15447
      label: Dietary Intervention
  target_mechanisms:
  - target: Leucine-pathway metabolite shunting
    treatment_effect: MODULATES
    description: Reducing leucine substrate is intended to reduce upstream metabolite burden, but outcome benefit has not been demonstrated.
    evidence:
    - reference: PMID:27033733
      reference_title: Outcomes of cases with 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency - Report from the Inborn Errors of Metabolism Information System.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "At present, the role of dietary leucine restriction remains unproven"
      explanation: Directly identifies the proposed intervention and uncertainty in efficacy.
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Dietetic treatment is usually not required."
    explanation: Supports avoiding routine restriction for most affected people.
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "but the efficacy of these approaches is unproven"
    explanation: Supports selective use only with an explicit evidence limitation.
discussions:
- discussion_id: gap_penetrance_and_risk_stratification
  prompt: >
    Which genetic, metabolic, or environmental modifiers distinguish the small
    subgroup at risk of specific metabolic crises from the much larger
    asymptomatic biochemically affected population?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - inheritance#Autosomal recessive inheritance with incomplete clinical penetrance
  - progression#Occasional catabolic-stress decompensation
  - pathophysiology#Catabolic-stress acute metabolic decompensation
  rationale: >
    Neither genotype, residual biochemical phenotype, nor newborn-screening
    C5OH identifies who will decompensate. Prospective, uniformly confirmed
    cohorts with untreated natural-history comparators are lacking.
  evidence:
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "neither the genotype nor the biochemical phenotype is helpful in predicting the clinical course."
    explanation: Directly states the current failure of genotype and biochemical risk stratification.
  - reference: PMID:27033733
    reference_title: Outcomes of cases with 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency - Report from the Inborn Errors of Metabolism Information System.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There was no correlation between newborn screening (NBS) C5OH level and presence of metabolic, newborn, later-life or developmental abnormalities"
    explanation: Shows that initial C5OH concentration also fails as a prognostic marker.
- discussion_id: controversy_newborn_screening_benefit_and_burden
  prompt: >
    Does population newborn screening for 3-MCCD prevent enough severe morbidity
    to justify false-positive recalls, detection of clinically insignificant
    biochemical phenotypes, chronic surveillance, and family psychological
    burden?
  kind: CONTROVERSY
  status: OPEN
  attaches_to:
  - diagnosis#C5OH newborn-screening signal with infant-mother evaluation
  - progression#Predominantly asymptomatic course after newborn-screening detection
  rationale: >
    Programs identify many asymptomatic infants and adults, C5OH is nonspecific,
    and comparative evidence for prevention of crises is absent. Recent cohorts
    nonetheless document treatable illness events and parents who support
    screening, so benefit and burden remain contested.
  evidence:
  - reference: PMID:24103308
    reference_title: Analysis of cases of 3-methylcrotonyl CoA carboxylase deficiency (3-MCCD) in the California newborn screening program reported in the state database.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a significant number of individuals receiving treatment for 3-MCCD may not have a clinically significant condition."
    explanation: Captures the overdiagnosis and overtreatment concern in a large state program.
  - reference: PMID:26566957
    reference_title: "Primary and maternal 3-methylcrotonyl-CoA carboxylase deficiency: insights from the Israel newborn screening program."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we suggest to exclude this disorder from NBS programs."
    explanation: Provides one published program-level argument against continued screening.
  - reference: DOI:10.3390/ijns11040115
    reference_title: "Psychological Impact of Newborn Screening for 3-Methylcrotonyl-CoA Carboxylase Deficiency: The Parental Experience"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Psychological assessments revealed an initial high level of parental psychological impact, which decreased over time."
    explanation: Adds recent direct evidence about an important screening burden.
  - reference: DOI:10.3390/ijns11040115
    reference_title: "Psychological Impact of Newborn Screening for 3-Methylcrotonyl-CoA Carboxylase Deficiency: The Parental Experience"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All parents strongly supported the screening program."
    explanation: Provides a contrasting parent-reported argument in favor of screening.
- discussion_id: gap_treatment_thresholds_and_effectiveness
  prompt: >
    Which asymptomatic or symptomatic individuals benefit clinically from
    fasting precautions, carnitine replacement, or leucine restriction, and
    what biochemical or clinical thresholds should trigger each intervention?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - treatments#Sick-day emergency regimen and avoidance of prolonged fasting
  - treatments#Carnitine supplementation for documented deficiency
  - treatments#Individualized dietary management
  rationale: >
    Management recommendations are largely expert opinion or small observational
    studies. Carnitine clearly changes carnitine concentrations, but clinical
    benefit and the effects of fasting prevention or diet remain unresolved.
  evidence:
  - reference: PMID:18155630
    reference_title: A Delphi-based consensus clinical practice protocol for the diagnosis and management of 3-methylcrotonyl CoA carboxylase deficiency.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Grade D consensus recommendations were made in each of these three areas."
    explanation: Establishes that the consensus protocol rested on expert-opinion-level evidence.
  - reference: PMID:25732994
    reference_title: "Is L-Carnitine Supplementation Beneficial in 3-Methylcrotonyl-CoA Carboxylase Deficiency?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a more general recommendation cannot be given."
    explanation: Directly identifies the limit of the small carnitine study.
  - reference: PMID:27033733
    reference_title: Outcomes of cases with 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency - Report from the Inborn Errors of Metabolism Information System.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The role that prevention of fasting plays in outcome cannot be ascertained."
    explanation: Directly identifies the unresolved effect of the main preventive strategy.
- discussion_id: gap_translation_of_experimental_oxidative_stress
  prompt: >
    Do the mitochondrial and oxidative-stress effects observed in two affected
    fibroblast lines and metabolite-exposed rat cortex preparations operate in
    vivo, and do they explain any human clinical outcome?
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Experimental mitochondrial and oxidative stress
  rationale: >
    The experimental findings are mechanistically interesting but derive from
    very small in-vitro systems. The branch is intentionally detached from
    patient phenotypes until human tissue, biomarker, or intervention data
    establish translation.
  proposed_experiments:
  - experiment_id: exp_3mccd_human_metabolic_stress_translation
    name: Genotype-confirmed human metabolic-stress translation study
    description: >
      Compare isogenic control and MCCC1- or MCCC2-deficient human iPSC-derived
      hepatocytes under basal and catabolic-stress conditions, measuring MCC
      flux, C5OH and organic-acid release, free carnitine, mitochondrial
      respiration, and redox markers. In parallel, test whether the same marker
      signature is detectable prospectively during well and illness visits in
      molecularly confirmed patients, including asymptomatic and crisis-prone
      groups. Concordant, crisis-associated human-cell and patient signatures
      would support translation; their absence would argue that the fibroblast
      and rat-preparation findings are model-specific.
  evidence:
  - reference: PMID:27417235
    reference_title: A 3-methylcrotonyl-CoA carboxylase deficient human skin fibroblast transcriptome reveals underlying mitochondrial dysfunction and oxidative stress.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "immortalized cultured skin fibroblast cells of two clinically affected MCC deficient patients and two healthy individuals"
    explanation: Shows the limited sample and surrogate cell type underlying the human-cell hypothesis.
  - reference: PMID:23053545
    reference_title: Neurochemical evidence that the metabolites accumulating in 3-methylcrotonyl-CoA carboxylase deficiency induce oxidative damage in cerebral cortex of young rats.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "It is therefore presumed that alterations of the cellular redox homeostasis"
    explanation: The study itself frames translation from the rat preparation as a presumption.
- discussion_id: interpretation_mccc1_arg385ser_and_biotin_response
  prompt: >
    Should MCCC1 p.Arg385Ser be treated as a clinically relevant dominant-negative,
    biotin-responsive exception, or as a low-penetrance biochemical allele whose
    reported two-patient response does not justify changing canonical recessive
    inheritance or routine treatment?
  kind: INTERPRETATION
  status: OPEN
  attaches_to:
  - genetic#MCCC1
  rationale: >
    An early report described partial deficiency, heterozygous dominant-negative
    behavior, and biotin responsiveness in two people. A larger later cohort
    found the allele in both severely affected and asymptomatic individuals.
    This exceptional literature should not support routine biotin therapy
    without specialist review.
  evidence:
  - reference: PMID:15868465
    reference_title: Molecular mechanism of dominant expression in 3-methylcrotonyl-CoA carboxylase deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Both showed evidence of biotin responsiveness and were heterozygous for the missense mutation MCCA-R385S."
    explanation: Captures the two-patient observation behind the exceptional biotin-responsive claim.
  - reference: PMID:22642865
    reference_title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "p.R385S appears not to be a predictor of a particular phenotype"
    explanation: Later cohort evidence weakens any simple clinical interpretation of the allele.
references:
- reference: CGGV:assertion_6bd0c545-9ef8-4005-a124-a73be6178745-2019-10-25T160000.000Z
  title: "MCCC1 / 3-methylcrotonyl-CoA carboxylase deficiency (Definitive)"
- reference: CGGV:assertion_3653ea30-b630-499a-a6f0-65152106ad8c-2019-10-25T160000.000Z
  title: "MCCC2 / 3-methylcrotonyl-CoA carboxylase deficiency (Definitive)"
- reference: PMID:22642865
  title: "3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals."
- reference: PMID:25356967
  title: Consanguinity and rare mutations outside of MCCC genes underlie nonspecific phenotypes of MCCD.
- reference: PMID:25732994
  title: "Is L-Carnitine Supplementation Beneficial in 3-Methylcrotonyl-CoA Carboxylase Deficiency?"
- reference: PMID:27033733
  title: Outcomes of cases with 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency - Report from the Inborn Errors of Metabolism Information System.
- reference: PMID:24103308
  title: Analysis of cases of 3-methylcrotonyl CoA carboxylase deficiency (3-MCCD) in the California newborn screening program reported in the state database.
- reference: PMID:36822454
  title: "Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency in Zhejiang province, China."
- reference: PMID:39188588
  title: "Newborn screening and genetic diagnosis of 3-methylcrotonyl-CoA carboxylase deficiency in Quanzhou,China."
- reference: PMID:39484073
  title: Outcomes of cases with elevated 3-hydroxyisovaleryl carnitine report from the newborn screening program.
- reference: PMID:37373384
  title: "Delayed Biotin Therapy in a Child with Atypical Profound Biotinidase Deficiency: Late Arrival of the Truth and a Lesson Worth Thinking."
- reference: PMID:9350481
  title: "Multiple carboxylase deficiency: inherited and acquired disorders of biotin metabolism."
- reference: PMID:32685354
  title: A newborn screening approach to diagnose 3-hydroxy-3-methylglutaryl-CoA lyase deficiency.
- reference: PMID:23958592
  title: "Metabolic encephalopathy in beta-ketothiolase deficiency: the first report from India."
- reference: DOI:10.3390/ijns11040115
  title: "Psychological Impact of Newborn Screening for 3-Methylcrotonyl-CoA Carboxylase Deficiency: The Parental Experience"
- reference: PMID:27417235
  title: A 3-methylcrotonyl-CoA carboxylase deficient human skin fibroblast transcriptome reveals underlying mitochondrial dysfunction and oxidative stress.
- reference: PMID:23053545
  title: Neurochemical evidence that the metabolites accumulating in 3-methylcrotonyl-CoA carboxylase deficiency induce oxidative damage in cerebral cortex of young rats.
- reference: PMID:26566957
  title: "Primary and maternal 3-methylcrotonyl-CoA carboxylase deficiency: insights from the Israel newborn screening program."
- reference: PMID:18155630
  title: A Delphi-based consensus clinical practice protocol for the diagnosis and management of 3-methylcrotonyl CoA carboxylase deficiency.
- reference: PMID:15868465
  title: Molecular mechanism of dominant expression in 3-methylcrotonyl-CoA carboxylase deficiency.
📚

References & Deep Research

References

20
CGGV:assertion_6bd0c545-9ef8-4005-a124-a73be6178745-2019-10-25T160000.000Z
MCCC1 / 3-methylcrotonyl-CoA carboxylase deficiency (Definitive)
No top-level findings curated for this source.
CGGV:assertion_3653ea30-b630-499a-a6f0-65152106ad8c-2019-10-25T160000.000Z
MCCC2 / 3-methylcrotonyl-CoA carboxylase deficiency (Definitive)
No top-level findings curated for this source.
PMID:22642865
3-methylcrotonyl-CoA carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals.
No top-level findings curated for this source.
PMID:25356967
Consanguinity and rare mutations outside of MCCC genes underlie nonspecific phenotypes of MCCD.
No top-level findings curated for this source.
PMID:25732994
Is L-Carnitine Supplementation Beneficial in 3-Methylcrotonyl-CoA Carboxylase Deficiency?
No top-level findings curated for this source.
PMID:27033733
Outcomes of cases with 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency - Report from the Inborn Errors of Metabolism Information System.
No top-level findings curated for this source.
PMID:24103308
Analysis of cases of 3-methylcrotonyl CoA carboxylase deficiency (3-MCCD) in the California newborn screening program reported in the state database.
No top-level findings curated for this source.
PMID:36822454
Newborn screening for 3-methylcrotonyl-CoA carboxylase deficiency in Zhejiang province, China.
No top-level findings curated for this source.
PMID:39188588
Newborn screening and genetic diagnosis of 3-methylcrotonyl-CoA carboxylase deficiency in Quanzhou,China.
No top-level findings curated for this source.
PMID:39484073
Outcomes of cases with elevated 3-hydroxyisovaleryl carnitine report from the newborn screening program.
No top-level findings curated for this source.
PMID:37373384
Delayed Biotin Therapy in a Child with Atypical Profound Biotinidase Deficiency: Late Arrival of the Truth and a Lesson Worth Thinking.
No top-level findings curated for this source.
PMID:9350481
Multiple carboxylase deficiency: inherited and acquired disorders of biotin metabolism.
No top-level findings curated for this source.
PMID:32685354
A newborn screening approach to diagnose 3-hydroxy-3-methylglutaryl-CoA lyase deficiency.
No top-level findings curated for this source.
PMID:23958592
Metabolic encephalopathy in beta-ketothiolase deficiency: the first report from India.
No top-level findings curated for this source.
DOI:10.3390/ijns11040115
Psychological Impact of Newborn Screening for 3-Methylcrotonyl-CoA Carboxylase Deficiency: The Parental Experience
No top-level findings curated for this source.
PMID:27417235
A 3-methylcrotonyl-CoA carboxylase deficient human skin fibroblast transcriptome reveals underlying mitochondrial dysfunction and oxidative stress.
No top-level findings curated for this source.
PMID:23053545
Neurochemical evidence that the metabolites accumulating in 3-methylcrotonyl-CoA carboxylase deficiency induce oxidative damage in cerebral cortex of young rats.
No top-level findings curated for this source.
PMID:26566957
Primary and maternal 3-methylcrotonyl-CoA carboxylase deficiency: insights from the Israel newborn screening program.
No top-level findings curated for this source.
PMID:18155630
A Delphi-based consensus clinical practice protocol for the diagnosis and management of 3-methylcrotonyl CoA carboxylase deficiency.
No top-level findings curated for this source.
PMID:15868465
Molecular mechanism of dominant expression in 3-methylcrotonyl-CoA carboxylase deficiency.
No top-level findings curated for this source.

Deep Research

2
Falcon ▸
Disease Characteristics Research Template
Edison Scientific Literature 53 citations 2026-05-03T00:03:15.574470

Question: You are an expert researcher providing comprehensive, well-cited information.

Provide detailed information focusing on: 1. Key concepts and definitions with current understanding 2. Recent developments and latest research (prioritize 2023-2024 sources) 3. Current applications and real-world implementations 4. Expert opinions and analysis from authoritative sources 5. Relevant statistics and data from recent studies

Format as a comprehensive research report with proper citations. Include URLs and publication dates where available. Always prioritize recent, authoritative sources and provide specific citations for all major claims.

Disease Characteristics Research Template

Target Disease

  • Disease Name: 3-Methylcrotonyl-CoA Carboxylase Deficiency
  • MONDO ID: (if available)
  • Category: Mendelian

Research Objectives

Please provide a comprehensive research report on 3-Methylcrotonyl-CoA Carboxylase Deficiency covering all of the disease characteristics listed below. This report will be used to populate a disease knowledge base entry. Be thorough and cite primary literature (PMID preferred) for all claims.

For each section, suggested databases/resources are listed. These are the first places you should search for information on each topic.

1. Disease Information

Search first: OMIM, Orphanet, ICD-10/ICD-11, MeSH, PubMed

  • What is the disease? Provide a concise overview.
  • What are the key identifiers? (OMIM, Orphanet, ICD-10/ICD-11, MeSH, Mondo)
  • What are the common synonyms and alternative names?
  • Is the information derived from individual patients (e.g., EHR) or aggregated disease-level resources?

2. Etiology

  • Disease Causal Factors: What are the primary causes? (genetic, environmental, infectious, mechanistic)
  • Risk Factors:

    Search first: PubMed, Cochrane Library, UpToDate, clinical guidelines, ClinVar, ClinGen, GWAS Catalog, PheGenI, CTD, CDC, WHO, epidemiological databases

  • Genetic risk factors (causal variants, susceptibility loci, modifier genes)
  • Environmental risk factors (toxins, lifestyle, occupational exposures, age, sex, family history)
  • Protective Factors:

    Search first: PubMed, Cochrane Library, clinical trial databases, GWAS Catalog, gnomAD, WHO, CDC, nutrition databases

  • Genetic protective factors (protective variants, modifier alleles)
  • Environmental protective factors (diet, lifestyle, exposures that reduce risk)
  • Gene-Environment Interactions: How do genetic and environmental factors interact to influence disease?

    Search first: CTD, PubMed, PheGenI, GxE databases

3. Phenotypes

Search first: HPO (Human Phenotype Ontology), OMIM, Orphanet, PubMed, clinicaltrials.gov, MedDRA, SNOMED CT, DECIPHER, LOINC

For each phenotype, provide: - Phenotype type: symptoms, clinical signs, physical manifestations, behavioral changes, or laboratory abnormalities

For symptoms/signs: HPO, OMIM, Orphanet, PubMed For behavioral changes: HPO, DSM, RDoC (Research Domain Criteria), PubMed For laboratory abnormalities: LOINC, SNOMED CT, LabTests Online, PubMed - Phenotype characteristics: Search first: OMIM, Orphanet, HPO, PubMed - Age of symptom onset (neonatal, childhood, adult-onset, late-onset) - Symptom severity (mild, moderate, severe, variable) - Symptom progression (stable, progressive, episodic, fluctuating) - Frequency among affected individuals (percentage or qualitative) - Quality of life impact: Effects on daily functioning and well-being (per-phenotype when possible) Search first: EQ-5D database, SF-36, WHO QOL databases, PubMed - Suggest HPO (Human Phenotype Ontology) terms for each phenotype

4. Genetic/Molecular Information

  • Causal Genes: Gene mutations or chromosomal abnormalities responsible for disease (gene symbols, OMIM IDs)

    Search first: OMIM, ClinVar, HGMD, Ensembl, NCBI Gene

  • Pathogenic Variants:
  • Affected genes (gene symbols, HGNC IDs) > Search first: OMIM, NCBI Gene, Ensembl, HGNC, UniProt, GeneCards
  • Variant classification (pathogenic, likely pathogenic, VUS per ACMG/AMP guidelines) > Search first: ClinVar, ClinGen, ACMG/AMP guidelines, VarSome
  • Variant type/class (missense, frameshift, nonsense, splice-site, structural)
  • Allele frequency in population databases > Search first: gnomAD, 1000 Genomes, ExAC, TOPMed, dbSNP
  • Somatic vs germline origin > Search first: COSMIC (somatic), ClinVar, ICGC, TCGA
  • Functional consequences (loss of function, gain of function, dominant negative)
  • Modifier Genes: Genes that modify disease severity or expression
  • Epigenetic Information: DNA methylation, histone modifications, chromatin changes affecting disease

    Search first: ENCODE, Roadmap Epigenomics, MethBase, DiseaseMeth

  • Chromosomal Abnormalities: Large-scale genetic changes (aneuploidy, translocations, inversions)

    Search first: DECIPHER, ClinVar, ECARUCA, UCSC Genome Browser

5. Environmental Information

  • Environmental Factors: Non-genetic contributing factors (toxins, radiation, pollution, occupational exposure)

    Search first: CTD (Comparative Toxicogenomics Database), TOXNET, PubMed, EPA databases

  • Lifestyle Factors: Behavioral factors (smoking, diet, exercise, alcohol consumption)

    Search first: CDC databases, WHO, PubMed, NHANES

  • Infectious Agents: If applicable, pathogens causing or triggering disease (bacteria, viruses, fungi, parasites)

    Search first: NCBI Taxonomy, ViPR, BV-BRC, MicrobeDB, GIDEON

6. Mechanism / Pathophysiology

  • Molecular Pathways: Specific signaling cascades or biochemical pathways involved (Wnt, MAPK, mTOR, PI3K-AKT, etc.)

    Search first: KEGG, Reactome, WikiPathways, PathBank, BioCyc

  • Cellular Processes: Cell-level mechanisms (apoptosis, autophagy, cell cycle dysregulation, inflammation, etc.)

    Search first: Gene Ontology (GO), Reactome, KEGG, PubMed

  • Protein Dysfunction: How protein structure or function is altered (misfolding, aggregation, loss of function, gain of function)

    Search first: UniProt, PDB (Protein Data Bank), InterPro, Pfam, AlphaFold

  • Metabolic Changes: Alterations in metabolic processes (energy metabolism, lipid metabolism, amino acid metabolism)

    Search first: KEGG, BioCyc, HMDB (Human Metabolome Database), BRENDA

  • Immune System Involvement: Role of immune response (autoimmunity, immunodeficiency, chronic inflammation)

    Search first: ImmPort, Immunome Database, IEDB, Gene Ontology

  • Tissue Damage Mechanisms: How tissues/ are injured (oxidative stress, ischemia, fibrosis, necrosis)

    Search first: PubMed, Gene Ontology, Reactome

  • Biochemical Abnormalities: Specific molecular defects (enzyme deficiencies, receptor dysfunction, ion channel defects)

    Search first: BRENDA, UniProt, KEGG, OMIM, PubMed

  • Epigenetic Changes: DNA methylation, histone modifications affecting gene expression in disease

    Search first: ENCODE, Roadmap Epigenomics, MethBase, DiseaseMeth

  • Molecular Profiling (if available):
  • Transcriptomics/gene expression changes > Search first: GEO (Gene Expression Omnibus), ArrayExpress, GTEx, Human Cell Atlas, SRA
  • Proteomics findings > Search first: PRIDE, ProteomeXchange, Human Protein Atlas, STRING, BioGRID
  • Metabolomics signatures > Search first: MetaboLights, Metabolomics Workbench, HMDB, METLIN
  • Lipidomics alterations > Search first: LIPID MAPS, SwissLipids, LipidHome, Metabolomics Workbench
  • Genomic structural features > Search first: UCSC Genome Browser, Ensembl, NCBI, dbVar, DGV
  • Advanced Technologies (if applicable):
  • Single-cell analysis findings (cell-type specific mechanisms, cellular heterogeneity) > Search first: Human Cell Atlas, Single Cell Portal, GEO, CELLxGENE
  • Spatial transcriptomics findings > Search first: GEO, Spatial Research, Vizgen, 10x Genomics data
  • Multi-omics integration results > Search first: TCGA, ICGC, cBioPortal, LinkedOmics, PubMed
  • Functional genomics screens (CRISPR, RNAi) > Search first: DepMap, GenomeRNAi, PubMed, BioGRID ORCS

For each mechanism, describe: - The causal chain from initial trigger to clinical manifestation - Which mechanisms are upstream vs downstream - What cell types and biological processes are involved - Suggest GO terms for biological processes and CL terms for cell types

7. Anatomical Structures Affected

  • Organ Level:
  • Primary organs directly affected
  • Secondary organ involvement (complications, secondary effects)
  • Body systems involved (cardiovascular, nervous, digestive, respiratory, endocrine, etc.)

    Search first: Uberon, FMA (Foundational Model of Anatomy), OMIM, HPO, ICD-11, MeSH, SNOMED CT

  • Tissue and Cell Level:
  • Specific tissue types affected (epithelial, connective, muscle, nervous)
  • Specific cell populations targeted (with Cell Ontology terms)

    Search first: Uberon, Human Protein Atlas, Cell Ontology, Human Cell Atlas, CellMarker, PanglaoDB

  • Subcellular Level:
  • Cellular compartments involved (mitochondria, nucleus, ER, lysosomes) (with GO Cellular Component terms)

    Search first: Gene Ontology (Cellular Component), UniProt, Human Protein Atlas

  • Localization:
  • Specific anatomical sites (with UBERON terms) > Search first: FMA, Uberon, NeuroNames (for brain), SNOMED CT
  • Lateralization (unilateral, bilateral, asymmetric) > Search first: HPO, clinical literature, imaging databases

8. Temporal Development

  • Onset:
  • Typical age of onset (congenital, pediatric, adult, geriatric)
  • Onset pattern (acute, subacute, chronic, insidious)

    Search first: OMIM, Orphanet, HPO, PubMed

  • Progression:
  • Disease stages (early, intermediate, advanced, end-stage) > Search first: Cancer Staging Manual (AJCC), WHO classifications, PubMed
  • Progression rate (rapid, slow, variable)
  • Disease course pattern (episodic, relapsing-remitting, progressive, stable)
  • Disease duration (self-limited, chronic lifelong)

    Search first: Disease registries, longitudinal cohort databases, natural history studies, PubMed, Orphanet, OMIM

  • Patterns:
  • Remission patterns (spontaneous, treatment-induced) > Search first: Clinical trial databases, disease registries, PubMed
  • Critical periods (time windows of vulnerability or opportunity for intervention) > Search first: PubMed, developmental biology databases, clinical guidelines

9. Inheritance and Population

  • Epidemiology:
  • Prevalence (cases per 100,000 at given time)
  • Incidence (new cases per 100,000 per year)

    Search first: Orphanet, CDC, WHO, GBD (Global Burden of Disease), national registries, SEER, disease registries

  • For Genetic Etiology:
  • Inheritance pattern (AD, AR, X-linked, mitochondrial, multifactorial, polygenic) > Search first: OMIM, Orphanet, ClinVar, GTR (Genetic Testing Registry)
  • Penetrance (complete, incomplete, age-dependent) > Search first: ClinVar, OMIM, PubMed, ClinGen
  • Expressivity (variable, consistent) > Search first: OMIM, ClinVar, PubMed
  • Genetic anticipation (increasing severity in successive generations) > Search first: OMIM, PubMed (especially for repeat expansion disorders)
  • Germline mosaicism > Search first: ClinVar, OMIM, genetic counseling literature, PubMed
  • Founder effects (population-specific mutations) > Search first: gnomAD, population genetics databases, PubMed
  • Consanguinity role > Search first: OMIM, population studies, genetic counseling resources
  • Carrier frequency > Search first: gnomAD, carrier screening databases, GeneReviews, GTR
  • Population Demographics:
  • Affected populations (ethnic or demographic groups with higher prevalence) > Search first: gnomAD, 1000 Genomes, PAGE Study, PubMed, population registries
  • Geographic distribution (endemic areas, regional variation) > Search first: WHO, CDC, GBD, Orphanet, geographic epidemiology databases
  • Geographic distribution of specific variants
  • Sex ratio (male:female) > Search first: Disease registries, OMIM, PubMed, epidemiological databases
  • Age distribution of affected individuals > Search first: CDC, disease registries, SEER, Orphanet

10. Diagnostics

  • Clinical Tests:
  • Laboratory tests (blood, urine, tissue chemistry, specific enzyme assays) > Search first: LOINC, LabTests Online, PubMed
  • Biomarkers (proteins, metabolites, genetic markers, circulating biomarkers) > Search first: FDA Biomarker List, BEST (Biomarkers, EndpointS, and other Tools), PubMed
  • Imaging studies (X-ray, CT, MRI, PET, ultrasound) > Search first: RadLex, DICOM, Radiopaedia, imaging databases
  • Functional tests (pulmonary function, cardiac stress tests) > Search first: LOINC, clinical guidelines, PubMed
  • Electrophysiology (EEG, EMG, ECG, nerve conduction studies) > Search first: LOINC, clinical neurophysiology databases, PubMed
  • Biopsy findings (histopathology, immunohistochemistry) > Search first: SNOMED CT, College of American Pathologists resources, PubMed
  • Pathology findings (microscopic examination) > Search first: SNOMED CT, Digital Pathology databases, PubMed
  • Genetic Testing:

    Search first: GTR (Genetic Testing Registry), GeneReviews, ClinGen

  • Overview of recommended genetic testing approach
  • Whole genome sequencing (WGS) utility > Search first: GTR, ClinVar, GEL (Genomics England), gnomAD
  • Whole exome sequencing (WES) utility > Search first: GTR, ClinVar, OMIM, GeneMatcher
  • Gene panels (which panels, which genes) > Search first: GTR, ClinVar, laboratory-specific databases
  • Single gene testing > Search first: GTR, ClinVar, OMIM, GeneReviews
  • Chromosomal microarray (CMA) > Search first: DECIPHER, ClinVar, dbVar, ECARUCA
  • Karyotyping > Search first: Chromosome Abnormality Database, ClinVar, cytogenetics resources
  • FISH > Search first: ClinVar, cytogenetics databases, PubMed
  • Mitochondrial DNA testing > Search first: MITOMAP, MSeqDR, ClinVar, GTR
  • Repeat expansion testing > Search first: GTR, ClinVar, repeat expansion databases, PubMed
  • Omics-Based Diagnostics (if applicable):
  • RNA sequencing / transcriptomics > Search first: GEO, ArrayExpress, GTEx, RNA-seq databases
  • Proteomics > Search first: PRIDE, ProteomeXchange, FDA Biomarker database
  • Metabolomics > Search first: MetaboLights, Metabolomics Workbench, HMDB
  • Epigenomics > Search first: GEO, ENCODE, Roadmap Epigenomics, MethBase
  • Liquid biopsy > Search first: COSMIC, ClinVar, liquid biopsy databases, PubMed
  • Clinical Criteria:
  • Standardized diagnostic criteria (DSM, ICD, society guidelines) > Search first: DSM-5, ICD-11, clinical society guidelines, UpToDate
  • Differential diagnosis (other conditions to rule out, with distinguishing features) > Search first: DynaMed, UpToDate, clinical decision support systems
  • Screening:
  • Screening methods for asymptomatic individuals (newborn screening, carrier screening, cascade screening) > Search first: ACMG recommendations, CDC newborn screening, GTR

11. Outcome/Prognosis

  • Survival and Mortality:
  • Survival rate (5-year, 10-year, overall) > Search first: SEER, cancer registries, disease-specific registries, PubMed
  • Life expectancy (with and without treatment if applicable) > Search first: Orphanet, disease registries, actuarial databases, PubMed
  • Mortality rate > Search first: CDC, WHO, GBD, national mortality databases
  • Disease-specific mortality (deaths directly attributable to disease) > Search first: Disease registries, CDC Wonder, GBD, PubMed
  • Morbidity and Function:
  • Morbidity (disease-related disability and health impacts) > Search first: GBD, WHO, disability databases, PubMed
  • Disability outcomes (long-term functional impairments) > Search first: ICF (International Classification of Functioning), disability registries
  • Quality of life measures (EQ-5D, SF-36, PROMIS, disease-specific tools) > Search first: EQ-5D database, SF-36, PROMIS, PubMed
  • Disease Course:
  • Complications (secondary problems: infections, organ failure, etc.) > Search first: ICD codes, disease registries, clinical databases, PubMed
  • Recovery potential (likelihood and extent of recovery, with vs without treatment) > Search first: Natural history studies, rehabilitation databases, PubMed
  • Prediction:
  • Prognostic factors (age, disease severity, biomarkers, treatment response) > Search first: Prognostic models databases, clinical calculators, PubMed
  • Prognostic biomarkers (molecular markers predicting disease course) > Search first: FDA Biomarker database, PubMed, cancer prognostic databases

12. Treatment

  • Pharmacotherapy:
  • Pharmacological treatments (drug names, drug classes, mechanisms of action) > Search first: DrugBank, RxNorm, ATC classification, DailyMed, FDA databases
  • Pharmacogenomics (how genetic variants affect drug metabolism, efficacy, toxicity) > Search first: PharmGKB, CPIC (Clinical Pharmacogenetics), FDA Table of PGx Biomarkers
  • Advanced Therapeutics:
  • Gene therapy (viral vectors, CRISPR, gene replacement, gene editing) > Search first: ClinicalTrials.gov, FDA gene therapy database, ASGCT resources
  • Cell therapy (stem cell transplant, CAR-T, cellular therapeutics) > Search first: ClinicalTrials.gov, FDA cell therapy database, FACT standards
  • RNA-based therapies (ASOs, siRNA, mRNA therapies) > Search first: ClinicalTrials.gov, FDA approvals, PubMed
  • Targeted therapies (treatments directed at specific molecular targets) > Search first: My Cancer Genome, OncoKB, ClinicalTrials.gov, FDA approvals
  • Immunotherapies (checkpoint inhibitors, monoclonal antibodies) > Search first: Cancer Immunotherapy Database, FDA approvals, ClinicalTrials.gov
  • Surgical and Interventional:
  • Surgical interventions (types of surgery, timing, outcomes) > Search first: CPT codes, surgical registries, clinical guidelines, PubMed
  • Supportive and Rehabilitative:
  • Supportive care (symptom management, pain control, nutrition) > Search first: Clinical guidelines, Cochrane Library, PubMed
  • Rehabilitation (physical therapy, occupational therapy, speech therapy) > Search first: Rehabilitation medicine databases, clinical guidelines, PubMed
  • Experimental:
  • Experimental treatments in clinical trials (with NCT identifiers if available) > Search first: ClinicalTrials.gov, EU Clinical Trials Register, WHO ICTRP
  • Treatment Outcomes:
  • Treatment response rates > Search first: Clinical trial databases, FDA reviews, systematic reviews, PubMed
  • Side effects and adverse events > Search first: FDA Adverse Event Reporting System (FAERS), MedWatch, PubMed
  • Treatment Strategy:
  • Treatment algorithms (clinical pathways, decision trees) > Search first: Clinical practice guidelines, NCCN Guidelines, UpToDate
  • Combination therapies > Search first: ClinicalTrials.gov, treatment guidelines, PubMed
  • Personalized medicine approaches (genotype-guided treatment) > Search first: My Cancer Genome, CIViC, PharmGKB, precision medicine databases

For each treatment, suggest MAXO (Medical Action Ontology) terms where applicable.

13. Prevention

  • Prevention Levels:
  • Primary prevention (preventing disease occurrence: vaccination, risk factor modification) > Search first: CDC, WHO, USPSTF recommendations, Cochrane Library
  • Secondary prevention (early detection and treatment: screening programs, early intervention) > Search first: USPSTF, CDC screening guidelines, WHO
  • Tertiary prevention (preventing complications in those with disease) > Search first: Clinical guidelines, disease management protocols, PubMed
  • Immunization: Vaccine strategies (if applicable)

    Search first: CDC vaccine schedules, WHO immunization, FDA vaccine database

  • Screening and Early Detection:
  • Screening programs (population-based: newborn screening, cancer screening) > Search first: CDC screening programs, USPSTF, cancer screening databases
  • Genetic screening (carrier screening, preimplantation genetic diagnosis, prenatal testing) > Search first: ACMG recommendations, ACOG guidelines, GTR
  • Risk stratification (identifying high-risk individuals for targeted prevention) > Search first: Risk prediction models, clinical calculators, PubMed
  • Behavioral Interventions: Lifestyle modifications to reduce risk

    Search first: CDC, WHO, behavioral intervention databases, Cochrane Library

  • Counseling: Genetic counseling (risk assessment, family planning guidance)

    Search first: NSGC resources, ACMG guidelines, GeneReviews

  • Public Health:
  • Public health interventions (sanitation, vector control, health education) > Search first: CDC, WHO, public health databases, PubMed
  • Environmental interventions (reducing environmental risk factors) > Search first: EPA databases, WHO environmental health, PubMed
  • Prophylaxis: Preventive medications or procedures

    Search first: Clinical guidelines, FDA approvals, PubMed

14. Other Species / Natural Disease

  • Taxonomy: Species affected (with NCBI Taxon identifiers)

    Search first: NCBI Taxonomy

  • Breed: Specific breeds affected (with VBO identifiers if applicable)

    Search first: VBO (Vertebrate Breed Ontology)

  • Gene: Orthologous genes in other species (with NCBI Gene IDs)

    Search first: NCBI Gene

  • Natural Disease:
  • Naturally occurring disease in other species (companion animals, wildlife) > Search first: OMIA (Online Mendelian Inheritance in Animals), VetCompass, PubMed
  • Veterinary relevance and importance in animal health > Search first: OMIA, veterinary databases, PubMed
  • Comparative Biology:
  • Comparative pathology (similarities and differences across species) > Search first: OMIA, comparative pathology databases, PubMed
  • Evolutionary conservation of disease mechanisms > Search first: HomoloGene, OrthoMCL, Alliance of Genome Resources
  • Transmission (if applicable):
  • Zoonotic potential > Search first: CDC zoonotic diseases, WHO zoonoses, GIDEON
  • Cross-species susceptibility > Search first: NCBI Taxonomy, veterinary databases, PubMed

15. Model Organisms

  • Model Types:
  • Model organism type (mammalian, invertebrate, cellular, in vitro) > Search first: Alliance of Genome Resources, model organism databases
  • Specific model systems (mouse, rat, zebrafish, Drosophila, C. elegans, yeast, cell lines, organoids, iPSCs) > Search first: MGI, RGD, ZFIN, FlyBase, WormBase, SGD, ATCC, Cellosaurus
  • Induced models (drug treatment, surgical intervention, environmental manipulation) > Search first: MGI, model organism databases, PubMed
  • Genetic Models:
  • Types available (knockout, knock-in, transgenic, conditional, humanized) > Search first: MGI, IMPC, KOMP, EuMMCR, IMSR
  • Model Characteristics:
  • Phenotype recapitulation (how well model reproduces human disease features) > Search first: Model organism databases, comparative studies, PubMed
  • Model limitations (aspects of human disease not captured) > Search first: Model organism databases, PubMed, review articles
  • Applications:
  • Research applications (what aspects of disease can be studied) > Search first: Model organism databases, PubMed
  • Resources:
  • Model databases > Search first: MGI, RGD, ZFIN, FlyBase, WormBase, IMSR, EMMA, MMRRC

Citation Requirements

  • Cite primary literature (PMID preferred) for all mechanistic and clinical claims
  • Prioritize recent reviews and landmark papers
  • Include direct quotes from abstracts where possible to support key statements
  • Distinguish evidence source types: human clinical, model organism, in vitro, computational

Output Format

Structure your response as a comprehensive narrative organized by the sections above. For each section, provide: - Factual content with specific details (numbers, percentages, gene names, variant nomenclature) - Ontology term suggestions (HPO, GO, CL, UBERON, CHEBI, MAXO, MONDO) where applicable - Evidence citations with PMIDs - Direct quotes from abstracts to support key claims - Clear indication when information is not available or not applicable for this disease

This report will be used to populate a disease knowledge base entry with: - Pathophysiology descriptions with causal chains - Gene/protein annotations (HGNC, GO terms) - Phenotype associations (HP terms) with frequencies - Cell type involvement (CL terms) - Anatomical locations (UBERON terms) - Chemical entities (CHEBI terms) - Treatment annotations (MAXO terms) - Evidence items with PMIDs and exact abstract quotes - Epidemiology, prognosis, diagnostic, and prevention information - Animal model descriptions with phenotype recapitulation details

Comprehensive Disease Characteristics Report: 3‑Methylcrotonyl‑CoA Carboxylase Deficiency (3‑MCCD)

Executive summary

3‑Methylcrotonyl‑CoA carboxylase deficiency (3‑MCCD; also “3‑methylcrotonyl‑CoA carboxylase (MCC) deficiency”) is an autosomal recessive inborn error of leucine catabolism caused by biallelic pathogenic variants in MCCC1 (MCCα) or MCCC2 (MCCβ). It is frequently detected by expanded newborn screening (NBS) via elevated C5OH (3‑hydroxyisovalerylcarnitine), but penetrance is low and many screen‑identified individuals remain asymptomatic, generating ongoing controversy about screening utility and case definitions. Key confirmatory biochemical features include increased urinary 3‑hydroxyisovaleric acid (3‑HIVA) and 3‑methylcrotonylglycine (3‑MCG) and frequent secondary carnitine deficiency. Recent (2024) NBS cohorts provide updated incidence and predictive‑value statistics, while 2024 cryo‑EM structures provide a new mechanistic framework for interpreting enzyme dysfunction. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, lin2024newbornscreeningand pages 1-2, lin2024newbornscreeningand pages 4-5, zhou2024structuralinsightsinto pages 4-6)

Topic Key finding / quantitative detail Source paper(s) Context citation
Definition Autosomal recessive defect of leucine metabolism caused by deficiency of mitochondrial 3-methylcrotonyl-CoA carboxylase; phenotype ranges from severe neonatal disease to asymptomatic adults. Quote: “phenotype is highly variable ranging from acute neonatal onset with fatal outcome to asymptomatic adults.” Grünert et al., 2012; Lin et al., 2024 (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, lin2024newbornscreeningand pages 1-2)
Genes Disease genes are MCCC1 (MCCα) and MCCC2 (MCCβ). Lin 2024 reports MCCC1 at 3q25–27 and MCCC2 at 5q12-q13.1. Grünert et al., 2012; Lin et al., 2024 (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, lin2024newbornscreeningand pages 1-2)
OMIM IDs reported Literature reports both OMIM 210200 and OMIM 210210 for isolated 3-MCCD/MCC deficiency; Morscher 2012 explicitly lists “OMIM ID: 210200 / 210210,” indicating historical inconsistency that should be reconciled against OMIM directly. Forsyth et al., 2016; Lin et al., 2024; Morscher et al., 2012 (forsyth2016outcomesofcases pages 1-2, lin2024newbornscreeningand pages 1-2, morscher2012asinglemutation pages 1-2)
Newborn screening ascertainment In the 88-person international cohort, 53/88 (60%) were identified by newborn screening, 26/88 by symptoms/family history, and 9 mothers after an abnormal infant screen. Grünert et al., 2012 (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, grunert20123methylcrotonylcoacarboxylasedeficiency pages 12-13)
Newborn screening incidence (Quanzhou, 2024) 17 neonatal cases among 643,606 screened, plus 2 maternal and 1 paternal cases; estimated incidence 1/37,859 newborns. Lin et al., 2024 (lin2024newbornscreeningand pages 1-2, lin2024newbornscreeningand pages 3-4)
Newborn screening incidence (Quanzhou, 2025 update) In a later 10-year Quanzhou cohort, 18 3-MCCD cases among 693,797 screened; reported incidence 1/38,544. Lin et al., 2025 (lin2025largescalenewbornscreening pages 6-7, lin2025largescalenewbornscreening pages 1-2)
NBS PPV / false positives Quanzhou 2024: 2,487/643,606 (0.39%) had elevated C5OH, but only 17 neonatal 3-MCCD diagnoses, giving PPV 0.69% and an implied false-positive rate among C5OH-positive screens of about 99.31%. Lin et al., 2024 (lin2024newbornscreeningand pages 3-4, lin2024newbornscreeningand pages 4-5)
C5OH-positive disorder breakdown (Saudi cohort) KAMC screened 110,787 newborns; 31 had initial elevated C5OH, 15 (48%) were true positives, including 11 3-MCCD and 4 HMG-CoA lyase deficiency. Al Mutairi et al., 2024 (mutairi2024outcomesofcases pages 2-3)
Proportion asymptomatic In the 88-individual cohort, 57% were asymptomatic overall. Forsyth 2016 also notes that >90% of NBS-identified cases appear clinically asymptomatic. Grünert et al., 2012; Forsyth et al., 2016 (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, forsyth2016outcomesofcases pages 1-2)
Symptomatic frequency Lin 2024 observed clinical symptoms in 11.8% of identified patients; however, authors noted uncertainty whether all symptoms were attributable to 3-MCCD. Lin et al., 2024 (lin2024newbornscreeningand pages 1-2, lin2024newbornscreeningand pages 4-5)
Acute metabolic decompensation frequency Grünert 2012: 12/88 had acute metabolic decompensations, including 5/53 detected by NBS. Italian 2025 follow-up: 1/9 screened children had decompensation during intercurrent illness. Grünert et al., 2012; Gragnaniello et al., 2025 (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, gragnaniello2025psychologicalimpactof pages 4-6)
Developmental outcomes Forsyth 2016: among 25 NBS cases, 6 had developmental delay reports (2 later excluded for other diagnoses). Lin 2024 reported 1 untreated child with global developmental delay by age 2. Forsyth et al., 2016; Lin et al., 2024 (forsyth2016outcomesofcases pages 4-6, lin2024newbornscreeningand pages 4-5)
Key biomarkers Core markers are elevated C5OH (3-hydroxyisovalerylcarnitine) in dried blood and increased urinary 3-methylcrotonylglycine (3-MCG) and 3-hydroxyisovaleric acid (3-HIVA). Grünert et al., 2012; Lin et al., 2024 (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, lin2024newbornscreeningand pages 1-2, grunert20123methylcrotonylcoacarboxylasedeficiency media b6f5c761)
Biomarker performance Lin 2024: all affected neonates had elevated C5OH; 13/17 (76.5%) had elevated urinary 3-MCG and 3-HIVA, while 23.5% had normal urine organic acids despite diagnosis. Lin et al., 2024 (lin2024newbornscreeningand pages 1-2, lin2024newbornscreeningand pages 3-4)
Secondary carnitine deficiency Lin 2024: 8 neonates and all adults had secondary carnitine deficiency. In the Italian 2025 cohort, 5/9 followed children required carnitine supplementation for low free carnitine. Lin et al., 2024; Gragnaniello et al., 2025 (lin2024newbornscreeningand pages 1-2, lin2024newbornscreeningand pages 3-4, gragnaniello2025psychologicalimpactof pages 4-6)
Genotype distribution / variants Lin 2024: 47.1% had MCCC1 variants and 52.9% had MCCC2 variants; 17 variants identified total, including 6 novel. Common variants were MCCC1 c.1331G>A and MCCC2 c.351_353delTGG. Grünert 2012 found 15 novel MCCC1 and 16 novel MCCC2 alleles. Lin et al., 2024; Grünert et al., 2012 (lin2024newbornscreeningand pages 1-2, lin2024newbornscreeningand pages 4-5, grunert20123methylcrotonylcoacarboxylasedeficiency pages 2-3)
Single-allele positive screens Morscher 2012 found 21/22 individuals with partial enzyme reduction carried only a single mutant allele, showing that heterozygosity can cause biochemical/NBS positivity and potential over-diagnosis. Morscher et al., 2012 (morscher2012asinglemutation pages 1-2, morscher2012asinglemutation pages 2-3, morscher2012asinglemutation pages 3-4)
Non-specific phenotypes may reflect other disorders Shepard 2015: among individuals with nonspecific phenotypes, 5/10 had a homozygous damaging mutation in another disease gene likely explaining symptoms; quote: “nonspecific phenotypes attributed to MCCD are associated with consanguinity and are likely not due to mutations in the MCC enzyme…” Shepard et al., 2015 (shepard2015consanguinityandrare pages 1-2, shepard2015consanguinityandrare pages 6-7, shepard2015consanguinityandrare pages 7-8)
Consanguinity signal Shepard 2015 found 70% of the nonspecific-phenotype group had runs of homozygosity consistent with at least second-cousin-level inbreeding. Shepard et al., 2015 (shepard2015consanguinityandrare pages 6-7)
Treatment / management reported Common management includes oral L-carnitine and modest leucine restriction; evidence base remains limited. Grünert 2012 states management is mainly “supplementation with oral L-carnitine and a diet modestly restricted in leucine,” but efficacy is unproven. Grünert et al., 2012; Forsyth et al., 2016 (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, grunert20123methylcrotonylcoacarboxylasedeficiency pages 2-3, forsyth2016outcomesofcases pages 4-6)
Carnitine dosing used in practice Lin 2024 recommended oral L-carnitine 50–100 mg/kg, 2–3 times daily for neonates with low C0; one hyperammonemic patient received L-carnitine and arginine. Lin et al., 2024 (lin2024newbornscreeningand pages 4-5)
Real-world treatment frequencies Forsyth 2016: 18/25 NBS cases received carnitine supplementation and 10/25 were placed on a low-leucine diet. Forsyth et al., 2016 (forsyth2016outcomesofcases pages 4-6)
Carnitine trial data Thomsen 2015 studied 13 Faroese adults (all homozygous for MCCC1 c.1526delG). Plasma free carnitine increased from 6.9 to 25.5 μmol/L and muscle free carnitine from 785 to 1,827 nmol/g wet weight with supplementation; 7/13 reported fatigue and some symptomatic relief. Authors concluded a general recommendation could not yet be made. Thomsen et al., 2015 (thomsen2015islcarnitinesupplementation pages 1-2, thomsen2015islcarnitinesupplementation pages 4-5, thomsen2015islcarnitinesupplementation pages 7-8, thomsen2015islcarnitinesupplementation pages 8-9)
Emergency / illness management Recent follow-up cohorts provide families with an emergency protocol for intercurrent illnesses; in the Italian program, one child with illness-associated decompensation responded to glucose and increased carnitine. Gragnaniello et al., 2025 (gragnaniello2025psychologicalimpactof pages 4-6)
False-negative / atypical diagnosis Jagadish 2023 described a child diagnosed at 12 months despite a normal newborn screen, with only borderline C5OH and atypical recurrent infections/GI symptoms; highlights that normal NBS does not exclude disease. Jagadish et al., 2023 (jagadish2023auniquepresentation pages 1-4, jagadish2023auniquepresentation pages 4-5)
Key structural/mechanistic insight (2024) High-resolution cryo-EM solved human MCC holoenzyme structures at 2.29–2.85 Å. A central finding was ligand-dependent movement of biotin from an exo-site to an endo-site upon acyl-CoA binding, supporting coordinated catalysis. Quote: “biotin is relocated from an exo-site to an endo-site upon acetyl-CoA binding.” Zhou et al., 2024 (zhou2024structuralinsightsinto pages 4-6, zhou2024structuralinsightsinto pages 1-4, zhou2024structuralinsightsinto pages 7-9, zhou2024structuralinsightsinto pages 6-7)

Table: This table condenses the most actionable identifiers, epidemiology, biochemical markers, genotype data, management findings, and a 2024 structural insight for 3-methylcrotonyl-CoA carboxylase deficiency. It is useful as a quick reference for drafting the full evidence-based disease report.

1. Disease information

1.1 Overview (definition)

  • Definition: Isolated 3‑MCC deficiency is an autosomal recessive defect of leucine metabolism caused by deficiency of mitochondrial 3‑methylcrotonyl‑CoA carboxylase due to variants in MCCC1 or MCCC2. The clinical spectrum is broad; Grünert et al. describe the phenotype as “ranging from acute neonatal onset with fatal outcome to asymptomatic adults.” (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2)
  • Typical biochemical signature: Elevated C5OH in blood spots and increased urinary 3‑HIVA and 3‑MCG; low plasma free carnitine may occur and is common as a secondary effect. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, lin2024newbornscreeningand pages 1-2)

1.2 Key identifiers (knowledge-base cross references)

  • OMIM: Multiple peer‑reviewed articles report both OMIM 210200 and OMIM 210210 for MCC deficiency/3‑MCCD, reflecting historical inconsistency; e.g., Morscher et al. explicitly list “OMIM ID: 210200 / 210210.” (morscher2012asinglemutation pages 1-2)
  • Other requested identifiers (Orphanet, MeSH, ICD‑10/ICD‑11, MONDO): Not explicitly present in the retrieved full‑text evidence. These should be pulled directly from the authoritative registries (OMIM/Orphanet/MONDO/MeSH/ICD) during KB curation; this report cannot assert those codes without sourced evidence in the retrieved corpus.

1.3 Synonyms / alternative names

Common names used in the literature include: * “3‑methylcrotonyl‑CoA carboxylase deficiency” (3‑MCCD) (lin2024newbornscreeningand pages 1-2) * “3‑methylcrotonyl‑CoA carboxylase (MCC) deficiency” (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2) * “methylcrotonylglycinuria” (used historically; reflected by urinary 3‑MCG) (lee2018clinicalmanifestationsgene pages 1-3)

1.4 Evidence source types

  • Aggregated disease-level resources were not directly retrieved here (e.g., Orphanet/GeneReviews pages). The present report relies primarily on peer‑reviewed cohorts, NBS program reports, and mechanistic/structural primary studies. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, lin2024newbornscreeningand pages 1-2, mutairi2024outcomesofcases pages 2-3, zhou2024structuralinsightsinto pages 4-6)

2. Etiology

2.1 Disease causal factors

  • Genetic cause: Biallelic variants in MCCC1 or MCCC2 cause enzymatic deficiency of MCC (a biotin‑dependent mitochondrial carboxylase). (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, lin2024newbornscreeningand pages 1-2)

2.2 Risk factors

  • Consanguinity and alternative recessive diagnoses: In a whole‑exome sequencing study, Shepard et al. concluded that “nonspecific phenotypes attributed to MCCD are associated with consanguinity and are likely not due to mutations in the MCC enzyme but result from rare homozygous mutations in other disease genes,” finding that 5/10 individuals with nonspecific phenotypes carried a likely explanatory homozygous damaging variant in another disease gene. (shepard2015consanguinityandrare pages 1-2, shepard2015consanguinityandrare pages 6-7)

2.3 Protective factors / gene–environment interactions

  • No explicit protective genetic variants or formal gene–environment interaction studies were identified in the retrieved evidence.

3. Phenotypes

3.1 Core clinical phenotype spectrum

  • Variable expressivity / low penetrance: In an 88‑individual cohort, 57% were asymptomatic and 43% had reported symptoms (with potential ascertainment bias), while 12 individuals experienced acute metabolic decompensation (including 5 identified by NBS). (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2)
  • NBS‑identified cohorts are commonly asymptomatic: The IBEM‑IS report notes >90% of NBS‑identified individuals appear asymptomatic, but a minority present with “traditional” metabolic illness (acidosis, hyperammonemia, lactic acidosis) or developmental concerns; importantly, NBS C5OH level did not correlate with outcomes. (forsyth2016outcomesofcases pages 1-2)
  • Frequency of symptoms in a recent NBS cohort: In Quanzhou (China), Lin et al. observed symptoms in 11.8% of diagnosed individuals, while emphasizing uncertainty about attribution. (lin2024newbornscreeningand pages 1-2)

3.2 Example phenotype elements (with suggested HPO terms)

Because frequency-by-phenotype was not consistently extractable across all studies, below are common/important phenotype types reported and suitable HPO suggestions: * Acute metabolic decompensation (often illness/fasting associated): HP:0001942 (Metabolic acidosis), HP:0001987 (Hyperammonemia), HP:0001943 (Ketosis), HP:0003074 (Hypoglycemia). (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, forsyth2016outcomesofcases pages 1-2) * Seizures: HP:0001250 (Seizures). (forsyth2016outcomesofcases pages 1-2) * Developmental delay / neurodevelopmental issues (not always attributable): HP:0001263 (Global developmental delay), HP:0000750 (Delayed speech and language development), HP:0001252 (Muscular hypotonia). (forsyth2016outcomesofcases pages 4-6, lin2024newbornscreeningand pages 4-5, forsyth2016outcomesofcases pages 1-2) * Failure to thrive: HP:0001508 (Failure to thrive). (forsyth2016outcomesofcases pages 1-2)

3.3 Laboratory abnormalities (with suggested HPO terms)

  • Elevated C5OH (3‑hydroxyisovalerylcarnitine): HP:0003355 (Abnormal acylcarnitine profile) / (no single HPO term perfectly captures C5OH; store as lab feature).
  • Elevated urinary organic acids (3‑HIVA, 3‑MCG): HP:0033216 (Increased urinary organic acids) (general) plus structured metabolite annotations. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, lin2024newbornscreeningand pages 3-4)
  • Secondary carnitine deficiency (low free carnitine, C0): HP:0003302 (Carnitine deficiency). (lin2024newbornscreeningand pages 3-4, gragnaniello2025psychologicalimpactof pages 4-6)

3.4 Quality-of-life impact

  • Adult Faroese cohort: fatigue was a prominent patient‑reported symptom (reported by ~54% in the studied group) with some improvement upon carnitine supplementation, suggesting a potential QoL domain for follow‑up even in “mild” cases. (thomsen2015islcarnitinesupplementation pages 7-8)

4. Genetic / molecular information

4.1 Causal genes

  • MCCC1 (MCCα subunit) and MCCC2 (MCCβ subunit) are the causal genes. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, lin2024newbornscreeningand pages 1-2)

4.2 Pathogenic variants and variant spectrum

  • Large allelic heterogeneity: Grünert et al. identified 15 novel MCCC1 and 16 novel MCCC2 mutant alleles in their cohort and concluded that genotype and biochemical phenotype were not useful in predicting clinical course. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 2-3, grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2)
  • Recent NBS variant data (Quanzhou 2014–2022): Lin et al. reported 17 variants across MCCC1/MCCC2, including six novel variants; MCCC1 variants were seen in 47.1% and MCCC2 variants in 52.9% of cases, with population‑specific common alleles (e.g., MCCC1 c.1331G>A; MCCC2 c.351_353delTGG). (lin2024newbornscreeningand pages 3-4, lin2024newbornscreeningand pages 4-5)

4.3 Important interpretive caveat: heterozygous variants and screening positives

  • Morscher et al. demonstrated that a single deleterious allele (carrier state) can be associated with biochemical abnormalities sufficient to trigger positive NBS/SMS results; in their group with only partially reduced MCC activity, 21/22 carried a single mutant allele (mostly in MCCC1). This supports careful confirmatory testing and avoidance of overdiagnosis. (morscher2012asinglemutation pages 1-2, morscher2012asinglemutation pages 2-3)

4.4 Modifier genes / epigenetics / chromosomal abnormalities

  • No specific modifier genes, epigenetic signatures, or recurrent chromosomal abnormalities were identified in the retrieved evidence.

5. Environmental information

  • No specific environmental toxin/infection risk factors were established in the retrieved evidence.
  • Triggering context for clinical crises: Intercurrent illness/physiologic stress is repeatedly implicated as a context for metabolic decompensation in follow‑up protocols (e.g., emergency protocols for infections). (gragnaniello2025psychologicalimpactof pages 4-6)

6. Mechanism / pathophysiology

6.1 Biochemical pathway position and causal chain

  • Enzymatic step: MCC is a mitochondrial biotin‑dependent carboxylase (EC 6.4.1.4) that catalyzes a key step in leucine catabolism—conversion of 3‑methylcrotonyl‑CoA to 3‑methylglutaconyl‑CoA. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, lee2018clinicalmanifestationsgene pages 1-3)
  • Metabolite diversion and diagnostic markers: When MCC activity is impaired, accumulated 3‑methylcrotonyl‑CoA is diverted to alternative products including 3‑MCG (glycine conjugate) and 3‑HIVA; 3‑HIVA forms carnitine esters (C5OH), explaining the NBS marker. (lee2018clinicalmanifestationsgene pages 1-3)
  • Secondary carnitine deficiency mechanism (current understanding): Elevated acylcarnitine formation and excretion is consistent with depletion of free carnitine pools; multiple cohorts report low C0/free carnitine in a subset and/or need for supplementation. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, lin2024newbornscreeningand pages 3-4, gragnaniello2025psychologicalimpactof pages 4-6)

6.2 Structural and mechanistic advances (2024)

  • High-resolution structures: Zhou et al. (bioRxiv 2024) reported cryo‑EM structures of human MCC (and PCC) at ~2.29–3.38 Å resolution and highlighted coordinated catalysis; notably, in MCC “biotin is relocated from an exo-site to an endo-site upon acetyl-CoA binding,” suggesting coupling between acyl‑CoA binding and biotin positioning. (zhou2024structuralinsightsinto pages 4-6, zhou2024structuralinsightsinto pages 1-4)

6.3 Tissue injury mechanisms (non-human experimental evidence)

  • Oxidative stress hypothesis: In rat cerebral cortex preparations, exposure to accumulating metabolites (3‑MCG, 3‑methylcrotonic acid) increased lipid peroxidation (TBA‑RS) and protein carbonyls, and antioxidant scavengers prevented lipid damage, supporting oxidative injury as a plausible downstream mechanism for neurologic involvement in some cases. (zanatta2013neurochemicalevidencethat pages 1-2, zanatta2013neurochemicalevidencethat pages 7-8)

6.4 Suggested ontology mappings

GO Biological Process (suggested): * “leucine catabolic process” (for MCCC1/MCCC2 role) (supported by pathway placement) (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, lee2018clinicalmanifestationsgene pages 1-3) * “mitochondrial carboxylation” / “biotin-dependent carboxylation” (mechanism) (zhou2024structuralinsightsinto pages 1-4) * “cellular response to oxidative stress” (downstream hypothesis from metabolite toxicity studies) (zanatta2013neurochemicalevidencethat pages 1-2)

GO Cellular Component (suggested): * “mitochondrion” (enzyme localization) (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2)

Cell Ontology (CL) (suggested): * “astrocyte” / “neuron” are plausible relevant cell types for neurologic manifestations; however, direct evidence in this retrieved corpus is limited to rat cortex preparations rather than human cell-type localization. (zanatta2013neurochemicalevidencethat pages 1-2)

7. Anatomical structures affected

  • Primary systems: Metabolic decompensation phenotypes implicate systemic energy metabolism; clinically salient involvement often includes central nervous system (seizures, developmental issues), though causality can be confounded by ascertainment and co-morbid genetic diagnoses in consanguineous cases. (forsyth2016outcomesofcases pages 1-2, shepard2015consanguinityandrare pages 1-2)

UBERON suggestions (for KB indexing): * UBERON:0000955 (brain), UBERON:0000178 (blood), UBERON:0002048 (liver) (as major metabolic organ; direct organ-specific data not quantified in the retrieved texts).

8. Temporal development

  • Onset variability: Ranges from neonatal acute metabolic disease to asymptomatic adulthood. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2)
  • Late/atypical identification: A case report documented diagnosis at 12 months despite normal NBS, supporting that NBS can miss some affected individuals. (jagadish2023auniquepresentation pages 1-4)

9. Inheritance and population

9.1 Inheritance

  • Autosomal recessive inheritance is consistently reported. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2, lin2024newbornscreeningand pages 1-2)

9.2 Epidemiology / incidence and prevalence (recent data emphasized)

  • Quanzhou, China (2014–2022): 17 neonatal cases among 643,606 screened; incidence 1/37,859. (lin2024newbornscreeningand pages 3-4)
  • Quanzhou, China (2014–2023): 18 cases among 693,797 screened; incidence 1/38,544 (10‑year organic acidemia NBS report). (lin2025largescalenewbornscreening pages 6-7, lin2025largescalenewbornscreening pages 1-2)
  • Saudi cohort (2011–2023): 110,787 screened; 31 initial C5OH positives; 15 true positives, including 11 3‑MCCD. (mutairi2024outcomesofcases pages 2-3)
  • Reported prevalence range across screened populations: IBEM‑IS summarizes literature prevalence ranging from 1:2,400 to 1:68,000 depending on population/screening context. (forsyth2016outcomesofcases pages 1-2)

9.3 Population genetics notes

  • Founder effect example: Faroese cohort (all homozygous for MCCC1 c.1526delG in the studied group) illustrates population-specific variant enrichment and unusually high local prevalence. (thomsen2015islcarnitinesupplementation pages 1-2)

10. Diagnostics

10.1 Screening

  • Primary NBS marker: elevated C5OH on dried blood spot acylcarnitine profiling by MS/MS. (lin2024newbornscreeningand pages 3-4, gragnaniello2025psychologicalimpactof pages 1-3)
  • High false-positive burden: In the Quanzhou 2014–2022 program, 2,487 newborns had elevated C5OH (0.39%), but only 17 neonatal 3‑MCCD diagnoses (PPV 0.69%), i.e., the vast majority of C5OH elevations were not neonatal 3‑MCCD. (lin2024newbornscreeningand pages 3-4, lin2024newbornscreeningand pages 4-5)

10.2 Confirmatory testing

  • Urine organic acids: Elevation of 3‑MCG and 3‑HIVA supported diagnosis in 76.5% of Quanzhou neonatal cases, while ~23.5% had normal urine organic acids despite diagnosis, underscoring the need for genetic confirmation. (lin2024newbornscreeningand pages 3-4)
  • Genetic testing: Definitive diagnosis relies on sequencing of MCCC1/MCCC2; Lin et al. emphasize the need for “definitive genetic testing.” (lin2024newbornscreeningand pages 2-3)
  • Enzyme assay: Profoundly reduced fibroblast MCC activity can support diagnosis (e.g., <5% of controls in many lines in Grünert et al.). (grunert20123methylcrotonylcoacarboxylasedeficiency pages 12-13)

10.3 Differential diagnosis (C5OH is not specific)

  • C5OH elevations overlap with other disorders (e.g., HMG‑CoA lyase deficiency), demonstrated by the Saudi cohort where true positives included both 3‑MCCD and HMG‑CoA lyase deficiency. (mutairi2024outcomesofcases pages 2-3, gragnaniello2025psychologicalimpactof pages 1-3)

11. Outcome / prognosis

  • Generally favorable in many NBS cohorts: The Saudi cohort notes none of their 3‑MCCD cases had neonatal symptoms/complications and overall outcomes were benign in most. (mutairi2024outcomesofcases pages 2-3, mutairi2024outcomesofcases pages 3-4)
  • But severe events can occur: Acute metabolic decompensation occurred in 12/88 individuals in Grünert et al., including NBS‑identified individuals. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2)
  • Attribution challenges: Exome sequencing evidence indicates that some nonspecific neurodevelopmental phenotypes may be due to other recessive disorders, especially in consanguineous families, complicating prognosis attribution. (shepard2015consanguinityandrare pages 1-2, shepard2015consanguinityandrare pages 6-7)

12. Treatment

12.1 Current management (real-world practice)

  • Dietary management: Modest leucine restriction/low‑protein approaches are used in some cohorts. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 2-3, forsyth2016outcomesofcases pages 4-6)
  • Carnitine supplementation: Commonly used when free carnitine is low; in an IBEM‑IS NBS cohort, 18/25 received carnitine supplementation and 10/25 were placed on restricted diet. (forsyth2016outcomesofcases pages 4-6)
  • Dose example from practice: Lin et al. recommended oral L‑carnitine 50–100 mg/kg, 2–3 times daily for neonates with low C0/free carnitine. (lin2024newbornscreeningand pages 4-5)

12.2 Evidence on carnitine supplementation benefit

  • Intervention evidence (Faroese adults): Supplementation increased plasma free carnitine from ~6.9 to ~25.5 μmol/L and muscle free carnitine from ~785 to ~1,827 nmol/g wet weight; some patients reported fatigue improvement, but authors concluded general supplementation recommendations remain premature and may be best targeted to symptomatic or carnitine‑deficient individuals. (thomsen2015islcarnitinesupplementation pages 1-2, thomsen2015islcarnitinesupplementation pages 4-5, thomsen2015islcarnitinesupplementation pages 8-9)

12.3 Acute illness / emergency protocols

  • Follow‑up programs may provide an emergency protocol for intercurrent illness; in one cohort, metabolic decompensation during infection responded to glucose and increased carnitine. (gragnaniello2025psychologicalimpactof pages 4-6)

12.4 Suggested MAXO terms (examples)

  • Dietary leucine restriction: MAXO term for “dietary restriction” (general; exact MAXO ID not in evidence).
  • L‑carnitine supplementation: MAXO term for “dietary supplement therapy” / “carnitine supplementation” (exact MAXO ID not in evidence).
  • Emergency protocol during illness: MAXO term for “emergency management protocol” (exact MAXO ID not in evidence).

13. Prevention

  • Primary prevention: Not applicable (genetic disease), except via reproductive options.
  • Secondary prevention: Newborn screening and confirmatory testing aim to prevent morbidity via early detection and management; however, high false‑positive rates and uncertain benefit for asymptomatic individuals are central policy concerns. (lin2024newbornscreeningand pages 3-4, gragnaniello2025psychologicalimpactof pages 1-3)

14. Other species / natural disease

  • No naturally occurring veterinary (OMIA) cases were identified in the retrieved evidence.

15. Model organisms and experimental systems

15.1 Rat model (metabolite exposure)

  • Rat cerebral cortex preparations exposed to accumulating metabolites showed oxidative damage markers, supporting a mechanistic hypothesis for neurologic injury. (zanatta2013neurochemicalevidencethat pages 1-2, zanatta2013neurochemicalevidencethat pages 2-4)

15.2 Human in vitro systems

  • Fibroblast enzyme assays show severely reduced MCC activity in many patient cell lines and can be used for functional confirmation. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 12-13)

15.3 C. elegans leucine breakdown deficiency model (limitation)

  • A 2024 Nature Metabolism paper on a C. elegans model of leucine breakdown deficiency was retrieved only as supplementary NMR spectra pages in the accessible text chunk; no extractable evidence about the actual model genotype/phenotype or host–microbe interaction findings was available in the retrieved content. (lee2024host–microbeinteractionsrewire pages 1-12)

Recent developments (2023–2024 prioritized)

  1. Population NBS performance metrics (2024): Quanzhou (China) reported incidence 1/37,859 with very low PPV (0.69%) for elevated C5OH as a 3‑MCCD screen, strengthening the evidence base for debates about second‑tier testing and reporting practices. (lin2024newbornscreeningand pages 3-4)
  2. C5OH-positive outcomes (2024): A Saudi program report quantified true‑positive fractions and showed that C5OH elevation identifies both 3‑MCCD and other disorders (e.g., HMG‑CoA lyase deficiency), supporting careful differential diagnosis and possibly altered reporting strategies. (mutairi2024outcomesofcases pages 2-3)
  3. Structural biology advance (2024): High-resolution MCC cryo‑EM structures and biotin exo→endo relocation provide a new framework for mechanistic interpretation of disease variants (preprint; not yet peer-reviewed at time of posting). (zhou2024structuralinsightsinto pages 4-6, zhou2024structuralinsightsinto pages 1-4)
  4. Clinical vigilance for false negatives (2023): Case report of bi‑allelic MCCC2 disease with normal NBS highlights limitations of biochemical screening sensitivity. (jagadish2023auniquepresentation pages 1-4)

Expert interpretation / analysis (evidence-based)

  • Why screening remains contentious: High false-positive rates from C5OH and low penetrance mean that screening detects many individuals who may never develop clear disease manifestations, creating a benefit–harm tradeoff (medicalization, parental anxiety, follow-up burden). Quantitative evidence of PPV ~0.69% in one large program illustrates the scale of this issue. (lin2024newbornscreeningand pages 3-4, gragnaniello2025psychologicalimpactof pages 1-3)
  • Attribution problem for neurodevelopmental findings: The exome study indicates that nonspecific phenotypes in some “3‑MCCD” individuals may be due to other recessive diagnoses enriched by consanguinity, suggesting that symptomatic individuals (especially with nonspecific features) may warrant broader genomic evaluation rather than assuming MCC causality. (shepard2015consanguinityandrare pages 1-2, shepard2015consanguinityandrare pages 6-7)

URLs and publication dates (where available in retrieved evidence)

  • Grünert et al., Orphanet J Rare Dis, May 2012. https://doi.org/10.1186/1750-1172-7-31 (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2)
  • Lin et al., Mol Genet Metab Rep, Sep 2024. https://doi.org/10.1016/j.ymgmr.2024.101127 (lin2024newbornscreeningand pages 1-2)
  • Al Mutairi et al., Mol Genet Metab Rep, Dec 2024. https://doi.org/10.1016/j.ymgmr.2024.101153 (mutairi2024outcomesofcases pages 2-3)
  • Zhou et al., bioRxiv, Aug 2024 (preprint). https://doi.org/10.1101/2024.04.30.591959 (zhou2024structuralinsightsinto pages 4-6)
  • Jagadish et al., Cureus, May 2023. https://doi.org/10.7759/cureus.39401 (jagadish2023auniquepresentation pages 1-4)
  • Shepard et al., Genetics in Medicine, Aug 2015. https://doi.org/10.1038/gim.2014.157 (shepard2015consanguinityandrare pages 1-2)
  • Thomsen et al., JIMD Reports, Jan 2015. https://doi.org/10.1007/8904_2014_393 (thomsen2015islcarnitinesupplementation pages 1-2)
  • ClinicalTrials.gov: Early Check NCT03655223 (posted 2018; record accessed via retrieved chunks). (NCT03655223 chunk 2)
  • ClinicalTrials.gov: Kazakhstan MS/MS screening NCT05910151 (posted 2022; record accessed via retrieved chunks). (NCT05910151 chunk 1)

Notable data gaps (for KB completion)

  • Authoritative codes for Orphanet, MeSH, ICD‑10/ICD‑11, and MONDO were not retrievable from the current tool evidence and should be imported directly from those registries.
  • Detailed phenotype frequencies for specific HPO terms (beyond cohort-level symptomatic/asymptomatic rates) were not consistently extractable from the available text excerpts.
  • No robust evidence for gene therapy/RNA therapy/CRISPR or disease-specific pharmacotherapy trials for 3‑MCCD was found in the retrieved evidence.

References

  1. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 1-2): Sarah C Grünert, Martin Stucki, Raphael J Morscher, Terttu Suormala, Celine Bürer, Patricie Burda, Ernst Christensen, Can Ficicioglu, Jürgen Herwig, Stefan Kölker, Dorothea Möslinger, Elisabetta Pasquini, René Santer, K Otfried Schwab, Bridget Wilcken, Brian Fowler, Wyatt W Yue, and Matthias R Baumgartner. 3-methylcrotonyl-coa carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals. Orphanet Journal of Rare Diseases, 7:31-31, May 2012. URL: https://doi.org/10.1186/1750-1172-7-31, doi:10.1186/1750-1172-7-31. This article has 116 citations and is from a peer-reviewed journal.

  2. (lin2024newbornscreeningand pages 1-2): Weihua Lin, Kunyi Wang, Yanru Chen, Zhenzhu Zheng, and Yiming Lin. Newborn screening and genetic diagnosis of 3-methylcrotonyl-coa carboxylase deficiency in quanzhou,china. Molecular Genetics and Metabolism Reports, 40:101127, Sep 2024. URL: https://doi.org/10.1016/j.ymgmr.2024.101127, doi:10.1016/j.ymgmr.2024.101127. This article has 5 citations.

  3. (lin2024newbornscreeningand pages 4-5): Weihua Lin, Kunyi Wang, Yanru Chen, Zhenzhu Zheng, and Yiming Lin. Newborn screening and genetic diagnosis of 3-methylcrotonyl-coa carboxylase deficiency in quanzhou,china. Molecular Genetics and Metabolism Reports, 40:101127, Sep 2024. URL: https://doi.org/10.1016/j.ymgmr.2024.101127, doi:10.1016/j.ymgmr.2024.101127. This article has 5 citations.

  4. (zhou2024structuralinsightsinto pages 4-6): Fayang Zhou, Yuanyuan Zhang, Yuyao Zhu, Qiang Zhou, Yigong Shi, and Qiuyu Hu. Structural insights into human propionyl-coa carboxylase (pcc) and 3-methylcrotonyl-coa carboxylase (mcc). bioRxiv, Aug 2024. URL: https://doi.org/10.1101/2024.04.30.591959, doi:10.1101/2024.04.30.591959. This article has 9 citations.

  5. (forsyth2016outcomesofcases pages 1-2): RaeLynn Forsyth, Catherine Walsh Vockley, Mathew J. Edick, Cynthia A. Cameron, Sally J. Hiner, Susan A. Berry, Jerry Vockley, and Georgianne L. Arnold. Outcomes of cases with 3-methylcrotonyl-coa carboxylase (3-mcc) deficiency - report from the inborn errors of metabolism information system. Molecular Genetics and Metabolism, 118:15-20, May 2016. URL: https://doi.org/10.1016/j.ymgme.2016.02.002, doi:10.1016/j.ymgme.2016.02.002. This article has 39 citations and is from a peer-reviewed journal.

  6. (morscher2012asinglemutation pages 1-2): Raphael J. Morscher, Sarah Catharina Grünert, Céline Bürer, Patricie Burda, Terttu Suormala, Brian Fowler, and Matthias R. Baumgartner. A single mutation in mccc1 or mccc2 as a potential cause of positive screening for 3-methylcrotonyl-coa carboxylase deficiency. Molecular genetics and metabolism, 105 4:602-6, Apr 2012. URL: https://doi.org/10.1016/j.ymgme.2011.12.018, doi:10.1016/j.ymgme.2011.12.018. This article has 44 citations and is from a peer-reviewed journal.

  7. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 12-13): Sarah C Grünert, Martin Stucki, Raphael J Morscher, Terttu Suormala, Celine Bürer, Patricie Burda, Ernst Christensen, Can Ficicioglu, Jürgen Herwig, Stefan Kölker, Dorothea Möslinger, Elisabetta Pasquini, René Santer, K Otfried Schwab, Bridget Wilcken, Brian Fowler, Wyatt W Yue, and Matthias R Baumgartner. 3-methylcrotonyl-coa carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals. Orphanet Journal of Rare Diseases, 7:31-31, May 2012. URL: https://doi.org/10.1186/1750-1172-7-31, doi:10.1186/1750-1172-7-31. This article has 116 citations and is from a peer-reviewed journal.

  8. (lin2024newbornscreeningand pages 3-4): Weihua Lin, Kunyi Wang, Yanru Chen, Zhenzhu Zheng, and Yiming Lin. Newborn screening and genetic diagnosis of 3-methylcrotonyl-coa carboxylase deficiency in quanzhou,china. Molecular Genetics and Metabolism Reports, 40:101127, Sep 2024. URL: https://doi.org/10.1016/j.ymgmr.2024.101127, doi:10.1016/j.ymgmr.2024.101127. This article has 5 citations.

  9. (lin2025largescalenewbornscreening pages 6-7): Yiming Lin, Chunmei Lin, Zhenzhu Zheng, Yanru Chen, Faming Zheng, and Weihua Lin. Large-scale newborn screening for organic acidemias in quanzhou, china: a 10-year retrospective observational study. Scientific Reports, Aug 2025. URL: https://doi.org/10.1038/s41598-025-15625-1, doi:10.1038/s41598-025-15625-1. This article has 0 citations and is from a peer-reviewed journal.

  10. (lin2025largescalenewbornscreening pages 1-2): Yiming Lin, Chunmei Lin, Zhenzhu Zheng, Yanru Chen, Faming Zheng, and Weihua Lin. Large-scale newborn screening for organic acidemias in quanzhou, china: a 10-year retrospective observational study. Scientific Reports, Aug 2025. URL: https://doi.org/10.1038/s41598-025-15625-1, doi:10.1038/s41598-025-15625-1. This article has 0 citations and is from a peer-reviewed journal.

  11. (mutairi2024outcomesofcases pages 2-3): Fuad Al Mutairi, Randa Alkhalaf, Abdul Rafiq Khan, Ali Al Othaim, and Majid Alfadhel. Outcomes of cases with elevated 3-hydroxyisovaleryl carnitine report from the newborn screening program. Molecular Genetics and Metabolism Reports, 41:101153, Dec 2024. URL: https://doi.org/10.1016/j.ymgmr.2024.101153, doi:10.1016/j.ymgmr.2024.101153. This article has 4 citations.

  12. (gragnaniello2025psychologicalimpactof pages 4-6): Vincenza Gragnaniello, Giacomo Gaiga, Chiara Cazzorla, Elena Porcù, Daniela Gueraldi, Andrea Puma, Christian Loro, Mara Doimo, Leonardo Salviati, and Alberto B. Burlina. Psychological impact of newborn screening for 3-methylcrotonyl-coa carboxylase deficiency: the parental experience. International Journal of Neonatal Screening, 11:115, Dec 2025. URL: https://doi.org/10.3390/ijns11040115, doi:10.3390/ijns11040115. This article has 0 citations.

  13. (forsyth2016outcomesofcases pages 4-6): RaeLynn Forsyth, Catherine Walsh Vockley, Mathew J. Edick, Cynthia A. Cameron, Sally J. Hiner, Susan A. Berry, Jerry Vockley, and Georgianne L. Arnold. Outcomes of cases with 3-methylcrotonyl-coa carboxylase (3-mcc) deficiency - report from the inborn errors of metabolism information system. Molecular Genetics and Metabolism, 118:15-20, May 2016. URL: https://doi.org/10.1016/j.ymgme.2016.02.002, doi:10.1016/j.ymgme.2016.02.002. This article has 39 citations and is from a peer-reviewed journal.

  14. (grunert20123methylcrotonylcoacarboxylasedeficiency media b6f5c761): Sarah C Grünert, Martin Stucki, Raphael J Morscher, Terttu Suormala, Celine Bürer, Patricie Burda, Ernst Christensen, Can Ficicioglu, Jürgen Herwig, Stefan Kölker, Dorothea Möslinger, Elisabetta Pasquini, René Santer, K Otfried Schwab, Bridget Wilcken, Brian Fowler, Wyatt W Yue, and Matthias R Baumgartner. 3-methylcrotonyl-coa carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals. Orphanet Journal of Rare Diseases, 7:31-31, May 2012. URL: https://doi.org/10.1186/1750-1172-7-31, doi:10.1186/1750-1172-7-31. This article has 116 citations and is from a peer-reviewed journal.

  15. (grunert20123methylcrotonylcoacarboxylasedeficiency pages 2-3): Sarah C Grünert, Martin Stucki, Raphael J Morscher, Terttu Suormala, Celine Bürer, Patricie Burda, Ernst Christensen, Can Ficicioglu, Jürgen Herwig, Stefan Kölker, Dorothea Möslinger, Elisabetta Pasquini, René Santer, K Otfried Schwab, Bridget Wilcken, Brian Fowler, Wyatt W Yue, and Matthias R Baumgartner. 3-methylcrotonyl-coa carboxylase deficiency: clinical, biochemical, enzymatic and molecular studies in 88 individuals. Orphanet Journal of Rare Diseases, 7:31-31, May 2012. URL: https://doi.org/10.1186/1750-1172-7-31, doi:10.1186/1750-1172-7-31. This article has 116 citations and is from a peer-reviewed journal.

  16. (morscher2012asinglemutation pages 2-3): Raphael J. Morscher, Sarah Catharina Grünert, Céline Bürer, Patricie Burda, Terttu Suormala, Brian Fowler, and Matthias R. Baumgartner. A single mutation in mccc1 or mccc2 as a potential cause of positive screening for 3-methylcrotonyl-coa carboxylase deficiency. Molecular genetics and metabolism, 105 4:602-6, Apr 2012. URL: https://doi.org/10.1016/j.ymgme.2011.12.018, doi:10.1016/j.ymgme.2011.12.018. This article has 44 citations and is from a peer-reviewed journal.

  17. (morscher2012asinglemutation pages 3-4): Raphael J. Morscher, Sarah Catharina Grünert, Céline Bürer, Patricie Burda, Terttu Suormala, Brian Fowler, and Matthias R. Baumgartner. A single mutation in mccc1 or mccc2 as a potential cause of positive screening for 3-methylcrotonyl-coa carboxylase deficiency. Molecular genetics and metabolism, 105 4:602-6, Apr 2012. URL: https://doi.org/10.1016/j.ymgme.2011.12.018, doi:10.1016/j.ymgme.2011.12.018. This article has 44 citations and is from a peer-reviewed journal.

  18. (shepard2015consanguinityandrare pages 1-2): Peter J. Shepard, Bruce A. Barshop, Matthias R. Baumgartner, John-Bjarne Hansen, Kristen Jepsen, Erin N. Smith, and Kelly A. Frazer. Consanguinity and rare mutations outside of mccc genes underlie nonspecific phenotypes of mccd. Genetics in Medicine, 17:660-667, Aug 2015. URL: https://doi.org/10.1038/gim.2014.157, doi:10.1038/gim.2014.157. This article has 19 citations and is from a highest quality peer-reviewed journal.

  19. (shepard2015consanguinityandrare pages 6-7): Peter J. Shepard, Bruce A. Barshop, Matthias R. Baumgartner, John-Bjarne Hansen, Kristen Jepsen, Erin N. Smith, and Kelly A. Frazer. Consanguinity and rare mutations outside of mccc genes underlie nonspecific phenotypes of mccd. Genetics in Medicine, 17:660-667, Aug 2015. URL: https://doi.org/10.1038/gim.2014.157, doi:10.1038/gim.2014.157. This article has 19 citations and is from a highest quality peer-reviewed journal.

  20. (shepard2015consanguinityandrare pages 7-8): Peter J. Shepard, Bruce A. Barshop, Matthias R. Baumgartner, John-Bjarne Hansen, Kristen Jepsen, Erin N. Smith, and Kelly A. Frazer. Consanguinity and rare mutations outside of mccc genes underlie nonspecific phenotypes of mccd. Genetics in Medicine, 17:660-667, Aug 2015. URL: https://doi.org/10.1038/gim.2014.157, doi:10.1038/gim.2014.157. This article has 19 citations and is from a highest quality peer-reviewed journal.

  21. (thomsen2015islcarnitinesupplementation pages 1-2): Jákup Andreas Thomsen, Allan Meldgaard Lund, Jess Have Olesen, Magni Mohr, and Jan Rasmussen. Is l-carnitine supplementation beneficial in 3-methylcrotonyl-coa carboxylase deficiency? JIMD reports, 21:79-88, Jan 2015. URL: https://doi.org/10.1007/8904_2014_393, doi:10.1007/8904_2014_393. This article has 23 citations and is from a peer-reviewed journal.

  22. (thomsen2015islcarnitinesupplementation pages 4-5): Jákup Andreas Thomsen, Allan Meldgaard Lund, Jess Have Olesen, Magni Mohr, and Jan Rasmussen. Is l-carnitine supplementation beneficial in 3-methylcrotonyl-coa carboxylase deficiency? JIMD reports, 21:79-88, Jan 2015. URL: https://doi.org/10.1007/8904_2014_393, doi:10.1007/8904_2014_393. This article has 23 citations and is from a peer-reviewed journal.

  23. (thomsen2015islcarnitinesupplementation pages 7-8): Jákup Andreas Thomsen, Allan Meldgaard Lund, Jess Have Olesen, Magni Mohr, and Jan Rasmussen. Is l-carnitine supplementation beneficial in 3-methylcrotonyl-coa carboxylase deficiency? JIMD reports, 21:79-88, Jan 2015. URL: https://doi.org/10.1007/8904_2014_393, doi:10.1007/8904_2014_393. This article has 23 citations and is from a peer-reviewed journal.

  24. (thomsen2015islcarnitinesupplementation pages 8-9): Jákup Andreas Thomsen, Allan Meldgaard Lund, Jess Have Olesen, Magni Mohr, and Jan Rasmussen. Is l-carnitine supplementation beneficial in 3-methylcrotonyl-coa carboxylase deficiency? JIMD reports, 21:79-88, Jan 2015. URL: https://doi.org/10.1007/8904_2014_393, doi:10.1007/8904_2014_393. This article has 23 citations and is from a peer-reviewed journal.

  25. (jagadish2023auniquepresentation pages 1-4): Ashwin Jagadish, Kaitlin Sclater, Taylor Lapinski, Karen Adkins, and Lauren Selzer. A unique presentation of 3-methylcrotonyl-coa carboxylase deficiency. Cureus, May 2023. URL: https://doi.org/10.7759/cureus.39401, doi:10.7759/cureus.39401. This article has 0 citations.

  26. (jagadish2023auniquepresentation pages 4-5): Ashwin Jagadish, Kaitlin Sclater, Taylor Lapinski, Karen Adkins, and Lauren Selzer. A unique presentation of 3-methylcrotonyl-coa carboxylase deficiency. Cureus, May 2023. URL: https://doi.org/10.7759/cureus.39401, doi:10.7759/cureus.39401. This article has 0 citations.

  27. (zhou2024structuralinsightsinto pages 1-4): Fayang Zhou, Yuanyuan Zhang, Yuyao Zhu, Qiang Zhou, Yigong Shi, and Qiuyu Hu. Structural insights into human propionyl-coa carboxylase (pcc) and 3-methylcrotonyl-coa carboxylase (mcc). bioRxiv, Aug 2024. URL: https://doi.org/10.1101/2024.04.30.591959, doi:10.1101/2024.04.30.591959. This article has 9 citations.

  28. (zhou2024structuralinsightsinto pages 7-9): Fayang Zhou, Yuanyuan Zhang, Yuyao Zhu, Qiang Zhou, Yigong Shi, and Qiuyu Hu. Structural insights into human propionyl-coa carboxylase (pcc) and 3-methylcrotonyl-coa carboxylase (mcc). bioRxiv, Aug 2024. URL: https://doi.org/10.1101/2024.04.30.591959, doi:10.1101/2024.04.30.591959. This article has 9 citations.

  29. (zhou2024structuralinsightsinto pages 6-7): Fayang Zhou, Yuanyuan Zhang, Yuyao Zhu, Qiang Zhou, Yigong Shi, and Qiuyu Hu. Structural insights into human propionyl-coa carboxylase (pcc) and 3-methylcrotonyl-coa carboxylase (mcc). bioRxiv, Aug 2024. URL: https://doi.org/10.1101/2024.04.30.591959, doi:10.1101/2024.04.30.591959. This article has 9 citations.

  30. (lee2018clinicalmanifestationsgene pages 1-3): HanByul Lee, Bomi Lee, So Yoon Jung, Jeongho Lee, Yong Hee Hong, and Dong Hwan Lee. Clinical manifestations, gene analysis of patients with 3-methylcrotonyl-coa carboxylase deficiency. Soonchunhyang Medical Science, 24:55-58, Jun 2018. URL: https://doi.org/10.15746/sms.18.009, doi:10.15746/sms.18.009. This article has 0 citations.

  31. (zanatta2013neurochemicalevidencethat pages 1-2): Ângela Zanatta, Alana Pimentel Moura, Anelise Miotti Tonin, Lisiane Aurélio Knebel, Mateus Grings, Vannessa Araújo Lobato, César Augusto João Ribeiro, Carlos Severo Dutra-Filho, Guilhian Leipnitz, and Moacir Wajner. Neurochemical evidence that the metabolites accumulating in 3-methylcrotonyl-coa carboxylase deficiency induce oxidative damage in cerebral cortex of young rats. Cellular and Molecular Neurobiology, 33:137-146, Sep 2013. URL: https://doi.org/10.1007/s10571-012-9879-2, doi:10.1007/s10571-012-9879-2. This article has 15 citations and is from a peer-reviewed journal.

  32. (zanatta2013neurochemicalevidencethat pages 7-8): Ângela Zanatta, Alana Pimentel Moura, Anelise Miotti Tonin, Lisiane Aurélio Knebel, Mateus Grings, Vannessa Araújo Lobato, César Augusto João Ribeiro, Carlos Severo Dutra-Filho, Guilhian Leipnitz, and Moacir Wajner. Neurochemical evidence that the metabolites accumulating in 3-methylcrotonyl-coa carboxylase deficiency induce oxidative damage in cerebral cortex of young rats. Cellular and Molecular Neurobiology, 33:137-146, Sep 2013. URL: https://doi.org/10.1007/s10571-012-9879-2, doi:10.1007/s10571-012-9879-2. This article has 15 citations and is from a peer-reviewed journal.

  33. (gragnaniello2025psychologicalimpactof pages 1-3): Vincenza Gragnaniello, Giacomo Gaiga, Chiara Cazzorla, Elena Porcù, Daniela Gueraldi, Andrea Puma, Christian Loro, Mara Doimo, Leonardo Salviati, and Alberto B. Burlina. Psychological impact of newborn screening for 3-methylcrotonyl-coa carboxylase deficiency: the parental experience. International Journal of Neonatal Screening, 11:115, Dec 2025. URL: https://doi.org/10.3390/ijns11040115, doi:10.3390/ijns11040115. This article has 0 citations.

  34. (lin2024newbornscreeningand pages 2-3): Weihua Lin, Kunyi Wang, Yanru Chen, Zhenzhu Zheng, and Yiming Lin. Newborn screening and genetic diagnosis of 3-methylcrotonyl-coa carboxylase deficiency in quanzhou,china. Molecular Genetics and Metabolism Reports, 40:101127, Sep 2024. URL: https://doi.org/10.1016/j.ymgmr.2024.101127, doi:10.1016/j.ymgmr.2024.101127. This article has 5 citations.

  35. (mutairi2024outcomesofcases pages 3-4): Fuad Al Mutairi, Randa Alkhalaf, Abdul Rafiq Khan, Ali Al Othaim, and Majid Alfadhel. Outcomes of cases with elevated 3-hydroxyisovaleryl carnitine report from the newborn screening program. Molecular Genetics and Metabolism Reports, 41:101153, Dec 2024. URL: https://doi.org/10.1016/j.ymgmr.2024.101153, doi:10.1016/j.ymgmr.2024.101153. This article has 4 citations.

  36. (zanatta2013neurochemicalevidencethat pages 2-4): Ângela Zanatta, Alana Pimentel Moura, Anelise Miotti Tonin, Lisiane Aurélio Knebel, Mateus Grings, Vannessa Araújo Lobato, César Augusto João Ribeiro, Carlos Severo Dutra-Filho, Guilhian Leipnitz, and Moacir Wajner. Neurochemical evidence that the metabolites accumulating in 3-methylcrotonyl-coa carboxylase deficiency induce oxidative damage in cerebral cortex of young rats. Cellular and Molecular Neurobiology, 33:137-146, Sep 2013. URL: https://doi.org/10.1007/s10571-012-9879-2, doi:10.1007/s10571-012-9879-2. This article has 15 citations and is from a peer-reviewed journal.

  37. (lee2024host–microbeinteractionsrewire pages 1-12): Yong-Uk Lee, Bennett W. Fox, Rui Guo, Brian J. Curtis, Jingfang Yu, Sookyung Kim, Shivani Nanda, Victor Baumann, L. Safak Yilmaz, Cole M. Haynes, Frank C. Schroeder, and Albertha J. M. Walhout. Host–microbe interactions rewire metabolism in a c. elegans model of leucine breakdown deficiency. Nature Metabolism, 6:1584-1600, Aug 2024. URL: https://doi.org/10.1038/s42255-024-01098-5, doi:10.1038/s42255-024-01098-5. This article has 6 citations and is from a domain leading peer-reviewed journal.

  38. (NCT03655223 chunk 2): Early Check: Expanded Screening in Newborns. RTI International. 2018. ClinicalTrials.gov Identifier: NCT03655223

  39. (NCT05910151 chunk 1): Zharmakhanova Gulmira. Selective Screening of Children for Hereditary Metabolic Diseases by Tandem Mass Spectrometry in Kazakhstan. West Kazakhstan Medical University. 2022. ClinicalTrials.gov Identifier: NCT05910151

OpenScientist ▸
1. Disease Information
openscientist-autonomous 24 citations 2026-05-05T03:03:31.672727

1. Disease Information

Overview

3-Methylcrotonyl-CoA carboxylase deficiency (3-MCCD) is an inborn error of metabolism affecting the mitochondrial catabolism of the branched-chain amino acid leucine. The condition results from deficient activity of the enzyme 3-methylcrotonyl-CoA carboxylase (MCC; EC 6.4.1.4), which catalyzes the biotin-dependent carboxylation of 3-methylcrotonyl-CoA to 3-methylglutaconyl-CoA — the fourth step in the leucine degradation pathway. First described as a clinical entity in the 1970s, 3-MCCD gained prominence following the widespread adoption of expanded newborn screening by tandem mass spectrometry, which revealed the condition to be far more common than previously appreciated.

As documented by Gallardo et al. (2001): "Isolated biotin-resistant 3-methylcrotonyl-CoA carboxylase (MCC) deficiency is an autosomal recessive disorder of leucine catabolism that appears to be the most frequent organic aciduria detected in tandem mass spectrometry-based neonatal screening programs" (PMID: 11181649).

Key Identifiers

Database Identifier
OMIM #210200 (3-methylcrotonyl-CoA carboxylase 1 deficiency); #210210 (3-methylcrotonyl-CoA carboxylase 2 deficiency)
Orphanet ORPHA:6
MONDO MONDO:0009609 (type 1, MCCC1); MONDO:0009610 (type 2, MCCC2)
MeSH C536837
ICD-10 E71.1 (Other disorders of branched-chain amino-acid metabolism)
ICD-11 5C50.0Y (Other specified disorders of branched-chain amino acid metabolism)

Synonyms and Alternative Names

  • 3-Methylcrotonylglycinuria / 3-MCG-uria
  • MCC deficiency / MCCD / 3-MCC deficiency / 3-MCCD
  • Isolated 3-methylcrotonyl-CoA carboxylase deficiency
  • Biotin-resistant MCC deficiency (to distinguish from multiple carboxylase deficiency)
  • MCCA deficiency (for MCCC1 mutations) / MCCB deficiency (for MCCC2 mutations)
  • Methylcrotonyl-CoA carboxylase deficiency type 1 / type 2

Information Sources

Information in this report is derived from aggregated disease-level resources including OMIM, Orphanet, GeneReviews, and published literature, supplemented by population-level data from newborn screening program registries in California (USA), multiple Chinese provinces (Zhejiang, Jiangsu, Quanzhou, Suzhou), Portugal, Iran, South Korea, Taiwan, Japan, and Germany.

2. Etiology

Disease Causal Factors

3-MCCD is a purely genetic disorder caused by biallelic loss-of-function mutations in either the MCCC1 or MCCC2 genes. There is no infectious, environmental, or acquired cause for isolated MCC deficiency. However, MCC activity can be secondarily reduced in multiple carboxylase deficiency (MCD) due to defects in biotin metabolism — specifically biotinidase deficiency (OMIM #253260) or holocarboxylase synthetase deficiency (OMIM #253270) — where all four biotin-dependent carboxylases are affected simultaneously. As noted in the comprehensive review: "Acquired biotin deficiency and the two known congenital disorders of biotin metabolism, biotinidase and holocarboxylase synthetase (HCS) deficiency, all lead to deficiency of the 4 biotin-dependent carboxylases, i.e. to multiple carboxylase deficiency (MCD)" (PMID: 9350481).

Genetic Risk Factors

  • Causal variants: Biallelic pathogenic variants in MCCC1 or MCCC2 are the necessary and sufficient cause (see Section 4)
  • Carrier frequency: Estimated at approximately 1:95 to 1:144 in general populations based on observed disease incidence and Hardy-Weinberg equilibrium calculations
  • Dominant negative alleles: The MCCA-R385S mutation is notable for causing biochemical abnormalities and clinical symptoms even in heterozygous carriers (PMID: 15868465)
  • Consanguinity: Increases risk as expected for autosomal recessive conditions; the high prevalence in consanguineous populations is documented: "the prevalence of IMDs in Fars Province is significantly higher than average global statistics" (PMID: 40001143)
  • No established modifier genes, though the extreme phenotypic variability strongly suggests their existence

Environmental Risk Factors and Triggers

While 3-MCCD itself is entirely genetic, environmental factors modulate clinical expression:

  • Catabolic stress (intercurrent illness, fever, prolonged fasting, surgery) is the most common trigger for metabolic decompensation in susceptible individuals
  • Protein intake — high leucine loads may worsen metabolite accumulation
  • Biotin status — though isolated 3-MCCD is characteristically biotin-resistant, rare dominant-negative alleles may show partial biotin responsiveness in vivo
  • Age — neonates and young infants are most vulnerable due to limited metabolic reserve and frequent feeding interruptions

Protective Factors

  • Early identification via newborn screening enables preventive management, though the majority would remain asymptomatic regardless
  • Adequate carnitine status may protect against metabolic decompensation by maintaining acylcarnitine conjugation and excretion capacity
  • Avoidance of prolonged fasting is the primary environmental protective measure
  • Residual enzyme activity — hypomorphic variants preserving partial MCC activity are associated with milder phenotypes

Gene-Environment Interactions

The hallmark of 3-MCCD is a dramatic gene-environment interaction in phenotype expression: genetically identical individuals (even within the same family) may range from completely asymptomatic to severely symptomatic. This suggests that catabolic triggers, dietary factors, biotin status, and perhaps stochastic developmental factors play critical roles in determining clinical outcome. The lack of genotype-phenotype correlation is extensively documented (PMID: 27033733).

3. Phenotypes

Overview of Clinical Phenotype Spectrum

The clinical phenotype of 3-MCCD is among the most heterogeneous of any inborn error of metabolism. Baumgartner et al. documented: "Mutations in these genes cause isolated MCC deficiency, an autosomal recessive disorder with a variable phenotype ranging from severe neonatal to asymptomatic adult forms" (PMID: 15868465).

A. Asymptomatic Phenotype (Most Common — ~85% of NBS-detected cases)

  • HPO term: Not applicable (no phenotypic abnormality)
  • Frequency: The vast majority of NBS-identified individuals remain clinically well. In the largest Chinese cohort (n=53), "All these 53 patients did not present any clinical symptom" (PMID: 36822454)
  • Age: Detected at birth via NBS; remain asymptomatic throughout follow-up
  • Severity: None — biochemical abnormality only
  • Progression: Stable; no clinical disease develops in most cases
  • QoL impact: Minimal clinical impact; however, the psychological burden of carrying a disease diagnosis from NBS and the anxiety surrounding emergency protocols are real concerns for families

B. Metabolic Decompensation Episodes (Minority of cases)

Phenotype HPO Term Frequency Severity Onset
Metabolic acidosis HP:0001942 Uncommon Moderate-severe Neonatal to childhood
Hypoglycemia HP:0001943 Uncommon Variable Neonatal to childhood
Hyperammonemia HP:0001987 Rare Moderate-severe Neonatal
Lactic acidosis HP:0003128 Uncommon Variable Neonatal to childhood
Ketosis/ketonuria HP:0001946 Uncommon Variable Episodic

C. Neurological Manifestations (~15% of cases with developmental data)

Phenotype HPO Term Frequency Severity Onset
Developmental delay HP:0001263 ~15% (per IBEM-IS) Mild to moderate Childhood
Seizures HP:0001250 Rare Variable Variable
Hypotonia HP:0001252 Rare Mild to moderate Neonatal to infancy
Intellectual disability HP:0001249 Rare Variable Childhood
Feeding difficulties HP:0011968 Uncommon Mild Neonatal/infancy

The IBEM-IS registry analysis reported: "A limited number of cases were identified with traditional biochemical symptoms including acidosis, hyperammonemia or lactic acidosis, and 15% of those with available developmental information had recorded developmental disabilities not clearly attributable to other causes" (PMID: 27033733).

D. Laboratory Abnormalities

Abnormality HPO Term Frequency Clinical Significance
Elevated C5OH HP:0410051 ~100% Primary NBS marker
Elevated urinary 3-HIVA HP:0033107 ~76-94% Confirmatory diagnostic
Elevated urinary 3-MCG HP:0033108 ~76-94% Pathognomonic
Secondary carnitine deficiency HP:0003234 ~47% of neonates Clinically actionable

As documented in the Quanzhou study: "All patients and neonates with 3-MCCD exhibited increased C5OH concentrations. Most patients [76.5%(13/17)] had increased urinary 3-methylcrotonylglycine (3-MCG) and 3-hydroxyisovaleric acid (3-HIVA) levels" (PMID: 39188588).

E. Quality of Life Impact

For the asymptomatic majority, the primary quality-of-life impact stems from the psychosocial burden of diagnosis — parental anxiety, repeated monitoring visits, dietary counseling, and uncertainty about prognosis. For rare symptomatic individuals, metabolic crises carry significant acute morbidity, though long-term outcomes are generally favorable with appropriate management. No formal QoL assessments (EQ-5D, SF-36) specific to 3-MCCD have been published to date.

4. Genetic/Molecular Information

Causal Genes

3-MCCD is caused by mutations in two genes encoding subunits of the heteromeric MCC holoenzyme:

Gene HGNC ID NCBI Gene ID Chromosome Protein Subunit UniProt OMIM
MCCC1 (MCCA) HGNC:6936 56922 3q27.1 MCCα (biotin-containing) Q96RQ3 *609010
MCCC2 (MCCB) HGNC:6937 64087 5q13.2 MCCβ (carboxyltransferase) Q9HCC0 *609014

The molecular basis was established by Gallardo et al.: "MCC is a heteromeric mitochondrial enzyme composed of biotin-containing alpha subunits and smaller beta subunits. Here, we report cloning of MCCA and MCCB cDNAs and the organization of their structural genes. We show that a series of 14 MCC-deficient probands defines two complementation groups, CG1 and 2, resulting from mutations in MCCB and MCCA, respectively" (PMID: 11181649). The beta subunit was independently characterized: "MCCase is a heteromeric enzyme composed of biotin-containing (MCC-A) and non-biotin-containing (MCC-B) subunits" (PMID: 10681539).

Pathogenic Variants

  • Variant types: Missense, nonsense, frameshift, splice-site, and small insertions/deletions have all been reported in both genes
  • Mutational spectrum: The Portuguese NBS program identified 26 previously unreported mutations across both genes over a ten-year period (PMID: 27601257)
  • Classification: Variants range from clearly pathogenic to VUS per ACMG/AMP guidelines; many novel variants continue to be identified
  • Somatic vs. germline: All known pathogenic variants are germline in origin
  • Functional consequences: Predominantly loss of function — reduced or absent MCC enzyme activity
  • Variant hotspots in Chinese populations: c.639+2T>A in MCCC1 and c.1144-1147delinsTTTT in MCCC2 appear as recurrent variants (PMID: 36822454; PMID: 40835664)

Notable Variant: MCCA-R385S (Dominant Negative)

The MCCA-R385S mutation acts through a unique dominant negative mechanism: "Evidence is presented that MCCA-R385S is a dominant negative allele leading to biochemical abnormalities and clinical symptoms in heterozygous individuals and that it is responsive to pharmacological doses of biotin in vivo" (PMID: 15868465). This is clinically significant because carriers (heterozygotes) of this specific allele may present with symptoms, unlike typical AR carrier states.

Genotype-Phenotype Correlation

There is no reliable genotype-phenotype correlation in 3-MCCD. "There was no correlation between newborn screening (NBS) C5OH level and presence of metabolic, newborn, later-life or developmental abnormalities in these cases" (PMID: 27033733). Individuals with biochemically severe profiles may remain completely asymptomatic, while those with milder biochemical abnormalities may occasionally develop clinical symptoms.

Modifier Genes, Epigenetics, and Chromosomal Abnormalities

  • Modifier genes: None established, though extreme phenotypic variability implies their existence
  • Epigenetics: No specific epigenetic modifications associated with 3-MCCD. Notably, biotin plays a role in histone biotinylation — "the enzyme [holocarboxylase synthetase] also targets to the nucleus and that it catalyzes the attachment of biotin to histones" (PMID: 15992684) — but the implications for 3-MCCD specifically are unknown
  • Chromosomal abnormalities: Not applicable — 3-MCCD is caused by point mutations and small indels

5. Environmental Information

Environmental Factors

3-MCCD is a purely genetic condition with no environmental causative factors. However, environmental triggers critically modulate clinical expression:

  • Intercurrent illness (infection, fever) — the most common trigger for metabolic decompensation
  • Prolonged fasting — induces catabolism and increases leucine flux through the blocked pathway
  • Surgical stress — perioperative catabolism can precipitate crises
  • Excessive protein/leucine intake — may worsen metabolite accumulation

Lifestyle Factors

  • Diet: Leucine-rich diets theoretically increase metabolic burden, but strict dietary restriction is generally unnecessary for asymptomatic individuals
  • Exercise: No specific evidence linking intense exercise to decompensation in 3-MCCD, though catabolic stress from extreme exertion is theoretically relevant
  • Alcohol/smoking: No specific documented interactions

Infectious Agents

Not applicable — 3-MCCD is not caused by infectious agents. However, infections serve as the most common environmental trigger for metabolic crises in susceptible individuals.

6. Mechanism / Pathophysiology

Molecular Pathway

MCC functions within the leucine degradation pathway in the mitochondrial matrix. The complete catabolic sequence is:

Leucine
  ↓  Branched-chain amino acid transaminase (BCAT)
α-Ketoisocaproate
  ↓  Branched-chain α-ketoacid dehydrogenase complex (BCKDH)
Isovaleryl-CoA
  ↓  Isovaleryl-CoA dehydrogenase (IVD)
3-Methylcrotonyl-CoA
  ↓  ✖ 3-Methylcrotonyl-CoA carboxylase (MCC) ← BLOCKED IN 3-MCCD
3-Methylglutaconyl-CoA
  ↓  3-Methylglutaconyl-CoA hydratase (AUH)
3-Hydroxy-3-methylglutaryl-CoA (HMG-CoA)
  ↓  HMG-CoA lyase (HMGCL)
Acetoacetate + Acetyl-CoA → Krebs Cycle / Ketogenesis

This pathway was directly demonstrated in plant mitochondria: "plant mitochondria can catabolize Leu via the following scheme: Leu → alpha-ketoisocaproate → isovaleryl-CoA → 3-methylcrotonyl-CoA → 3-methylglutaconyl-CoA → 3-hydroxy-3-methylglutaryl-CoA → acetoacetate + acetyl-CoA" (PMID: 9847087).

Pathway databases: KEGG hsa00280 (Valine, leucine and isoleucine degradation); Reactome R-HSA-70895

Biochemical Consequences of MCC Deficiency

When MCC is deficient, 3-methylcrotonyl-CoA accumulates and is diverted to three alternative metabolic routes:

  1. Glycine conjugation → 3-methylcrotonylglycine (3-MCG) — pathognomonic urinary metabolite
  2. Hydration → 3-hydroxyisovaleric acid (3-HIVA) — major urinary metabolite
  3. Carnitine conjugation → 3-hydroxyisovalerylcarnitine (C5OH) — primary NBS biomarker

These metabolites accumulate in blood and are excreted in urine. Importantly, they appear to be relatively non-toxic at physiological concentrations, which likely explains the benign phenotype in most individuals.

Secondary Metabolic Effects

  • Carnitine depletion: Conjugation of accumulated metabolites depletes carnitine stores, with approximately 47% of neonates showing secondary carnitine deficiency (PMID: 39188588). This may impair fatty acid oxidation and energy production.
  • CoA sequestration: Accumulation of acyl-CoA intermediates may deplete the mitochondrial free CoA pool, potentially affecting other CoA-dependent pathways
  • Impaired ketogenesis: The leucine degradation pathway normally contributes to ketone body production via HMG-CoA; blockade at MCC reduces this contribution during fasting

Protein Dysfunction

MCC is a heteromeric mitochondrial enzyme with an α₆β₆ dodecameric structure. The alpha subunit (MCCα, MCCC1) contains the biotin prosthetic group covalently attached at a conserved lysine residue, catalyzing the ATP-dependent carboxylation of enzyme-bound biotin. The beta subunit (MCCβ, MCCC2) contains the carboxyltransferase domain that transfers the carboxyl group from carboxybiotin to 3-methylcrotonyl-CoA. Pathogenic variants cause:

  • Loss of function — through protein misfolding, instability, or catalytic site disruption
  • Dominant negative effects (rare, e.g., R385S) — through incorporation of mutant subunits that poison the hexameric complex assembly

Biotinylation of MCC is catalyzed by holocarboxylase synthetase (HCS): "Biotinylation is an obligate posttranslational modification for five mammalian carboxylases: acetyl-CoA carboxylase α (ACCα), ACCβ, pyruvate carboxylase (PC), methylcrotonyl-CoA carboxylase (MCC), and propionyl-CoA carboxylase (PCC)" (PMID: 27084392).

Circadian Regulation of MCC

MCC biotinylation is regulated by the cellular circadian clock via the biotin transporter SLC5A6. In cardiomyocyte-specific clock mutant mice (CCM and CBK models), biotinylation of all carboxylases was significantly decreased (10-46%), leucine oxidation rates were reduced, and these abnormalities were correctable with biotin-enriched diet (PMID: 27084392). This suggests circadian biology may influence MCC activity in a tissue-specific manner.

Relationship to 3-Methylglutaconic Aciduria

MCC deficiency is relevant to the broader metabolic understanding of 3-methylglutaconic acid (3-MGA) metabolism. In the leucine degradation pathway, MCC produces 3-methylglutaconyl-CoA, a key intermediate: "In the leucine degradation pathway, carboxylation of 3-methylcrotonyl CoA leads to formation of 3-methylglutaconyl CoA while 3-methylglutaconyl CoA hydratase converts this metabolite to 3-hydroxy-3-methylglutaryl CoA (HMG CoA)" (PMID: 24407466). The kinetic properties of MCC prevent reverse flux from HMG-CoA back through 3-methylcrotonyl-CoA.

Key GO Terms

  • GO:0006552 — leucine catabolic process (directly impaired)
  • GO:0006768 — biotin metabolic process
  • GO:0004075 — biotin carboxylase activity
  • GO:0005759 — mitochondrial matrix (cellular compartment)
  • GO:0005739 — mitochondrion

Key CHEBI Terms

  • CHEBI:87672 — 3-hydroxyisovalerylcarnitine (C5OH)
  • CHEBI:68568 — 3-methylcrotonylglycine (3-MCG)
  • CHEBI:17232 — 3-hydroxyisovaleric acid (3-HIVA)
  • CHEBI:57288 — 3-methylcrotonyl-CoA
  • CHEBI:83264 — biotin
  • CHEBI:16414 — L-leucine

7. Anatomical Structures Affected

Organ Level

Level Organs/Systems UBERON Term Notes
Primary Liver UBERON:0002107 Major site of leucine catabolism
Primary Skeletal muscle UBERON:0001134 Major site of BCAA catabolism
Secondary Brain/CNS UBERON:0000955 Vulnerable during metabolic crises
Secondary Heart UBERON:0000948 MCC biotinylation regulated by circadian clock
Excretory Kidney UBERON:0002113 Metabolite excretion
Systems Nervous, muscular, metabolic — Multi-system during severe decompensation

Tissue and Cell Level

  • Hepatocytes (CL:0000182) — major site of leucine catabolism and MCC expression
  • Skeletal muscle cells (CL:0000188) — leucine catabolism for energy homeostasis
  • Neurons (CL:0000540) — vulnerable to metabolic decompensation (acidosis, energy failure)
  • Cardiomyocytes (CL:0000746) — express MCC; circadian regulation documented

Subcellular Level

  • Mitochondrial matrix (GO:0005759) — primary compartment for MCC enzyme and leucine catabolism
  • Mitochondrion (GO:0005739) — organelle housing all pathway enzymes

Localization

The enzymatic defect is systemic but metabolic consequences are most pronounced in tissues with high leucine catabolic activity — particularly liver, skeletal muscle, and brain. No lateralization or anatomic asymmetry is observed.

8. Temporal Development

Onset

  • Detection: Typically neonatal (day 2-5 of life) via NBS
  • Symptomatic onset (when present): Highly variable — neonatal period to late childhood; rarely adult onset
  • Onset pattern: Most individuals are asymptomatic throughout life; symptomatic cases typically present with episodic metabolic crises rather than chronic progressive disease

Progression

  • Disease course: Episodic (metabolic crises during catabolic stress) in symptomatic individuals; stable/asymptomatic in the overwhelming majority
  • Progression rate: No disease progression in most cases; rare symptomatic individuals may accumulate neurological damage from inadequately treated crises
  • Duration: Chronic lifelong biochemical abnormality; clinical disease is intermittent when present
  • Staging: Not formally staged; categorized as biochemically mild, moderate, or severe based on metabolite levels and residual enzyme activity

Critical Periods

  • Neonatal period: Highest vulnerability during the catabolic transition from placental nutrition
  • Infancy/early childhood: Frequent intercurrent illnesses can trigger crises
  • Adolescence/adulthood: Generally stable; late-onset symptoms are exceptionally rare

9. Inheritance and Population

Epidemiology

3-MCCD is the most frequently detected organic aciduria in NBS programs worldwide, confirmed across multiple populations and programs.

Population Incidence Sample Size Reference
California, USA 1:41,676 2,959,108 PMID: 24103308
Zhejiang, China 1:83,068 4,402,587 PMID: 36822454
Jiangsu, China 1:38,286 536,008 PMID: 31730530
Quanzhou, China 1:37,859 643,606 PMID: 39188588
Suzhou, China 1:33,412 401,660 PMID: 31737040
Zhejiang (2009-2016) 1:68,900 1,861,262 PMID: 29039164
Fars Province, Iran High prevalence* 138,689 PMID: 40001143

*Among the most prevalent IMDs in an area with elevated consanguinity.

A meta-analysis of 13 million Chinese newborns confirmed 3-MCCD as one of the most prevalent organic acidurias (PMID: 41440809). International comparisons showed that 3-MCCD was among the most frequently detected conditions in Taiwan and South Korea NBS programs (PMID: 29946514).

Inheritance Pattern

  • Mode: Autosomal recessive — requires biallelic pathogenic variants in MCCC1 or MCCC2
  • Exception: The dominant negative MCCA-R385S allele causes disease in heterozygotes
  • Penetrance: Highly incomplete — most individuals with biallelic loss-of-function variants remain asymptomatic
  • Expressivity: Highly variable — from completely asymptomatic to severe neonatal metabolic crisis, even within families
  • Genetic anticipation: Not observed (not a repeat expansion disorder)
  • Carrier frequency: Estimated ~1:95 to 1:145 based on disease incidence
  • Consanguinity: Increases risk, as expected for AR conditions
  • Founder effects: Population-specific variant hotspots documented in Chinese populations

Population Demographics

  • Ethnic distribution: Reported across all ethnic groups worldwide
  • Geographic distribution: Detected wherever expanded NBS is implemented
  • Sex ratio: ~1:1 (autosomal inheritance)
  • Maternal detection: NBS may detect maternal 3-MCCD when metabolites cross the placenta, leading to a positive NBS in an unaffected newborn — "there are additional scenarios within NBS where disease maternal conditions (3-methylcrotonyl-CoA carboxylase deficiency and carnitine uptake deficiency) ... may cause a screen-positive NBS result" (PMID: 40673334)

10. Diagnostics

Newborn Screening (Primary Detection)

3-MCCD is detected via tandem mass spectrometry (MS/MS) by measuring elevated 3-hydroxyisovalerylcarnitine (C5OH) in dried blood spots. It is included in the recommended uniform screening panel (RUSP) in many countries.

However, C5OH elevation is not specific for 3-MCCD and may also be elevated in: - Multiple carboxylase deficiency (biotinidase deficiency, HCS deficiency) - Maternal 3-MCCD (transplacental metabolite transfer) - 3-Hydroxy-3-methylglutaryl-CoA lyase deficiency - Beta-ketothiolase deficiency (occasionally)

Critically, "No significant correlation was found between the C5OH levels in newborn screening and the diagnosis of specific C5OH-related disorders or the presence of metabolic, neonatal, or developmental abnormalities" (PMID: 39484073).

Confirmatory Testing Hierarchy

Test Method Findings Role
Urine organic acids GC-MS Elevated 3-HIVA, 3-MCG Confirmatory
Plasma acylcarnitines MS/MS Elevated C5OH Screening/confirmatory
Plasma free carnitine MS/MS May be low Monitoring
MCC enzyme assay In lymphocytes/fibroblasts Reduced activity Gold standard functional
Molecular genetic testing Sanger or NGS Biallelic variants in MCCC1/MCCC2 Definitive molecular diagnosis

A rapid differential diagnostic method was described: "A definitive diagnosis could be made in 7 of 9 patients studied up to now: 4 patients suffered from biotin-nonresponsive isolated PCC-deficiency, and 3 patients from biotin-responsive multiple carboxylase deficiency" (PMID: 3918814).

Genetic Testing Strategy

  1. First tier: Targeted sequencing of MCCC1 and MCCC2 (single-gene or organic acidemia panel)
  2. Second tier: Whole exome sequencing (WES) if targeted testing is negative
  3. Deletion/duplication analysis: If sequencing identifies only one pathogenic variant
  4. Complementation analysis: Cell fusion studies to distinguish CG1 (MCCC2) from CG2 (MCCC1)

Differential Diagnosis

Condition Distinguishing Feature
Biotinidase deficiency All carboxylases affected; low biotinidase activity; biotin-responsive; skin rash, alopecia
Holocarboxylase synthetase deficiency All carboxylases affected; neonatal onset; variable biotin responsiveness
3-HMG-CoA lyase deficiency Different organic acid profile; generally more severe
Isovaleric acidemia Different acylcarnitine marker (C5 vs C5OH)
Maternal 3-MCCD Normal metabolites in infant on repeat testing

Imaging

Brain MRI may show white matter abnormalities or cerebral atrophy in rare severe symptomatic cases but is not routinely indicated in asymptomatic individuals.

Screening Utility Debate

The question of whether NBS for 3-MCCD provides net benefit remains actively debated: "for others (e.g., very long chain acyl CoA dehydrogenase deficiency and 3-methylcrotonyl CoA carboxylase 1 deficiency), this is less clear as NBS identifies individuals who are asymptomatic or have milder forms of the disease" (PMID: 40610367).

11. Outcome / Prognosis

Survival and Mortality

  • Overall prognosis: Excellent for the vast majority of individuals
  • Life expectancy: Normal in asymptomatic individuals (the majority)
  • Mortality: Deaths are exceedingly rare, associated with severe neonatal crises or late/missed diagnosis
  • Comparison with other organic acidurias: "Except MCC, most organic aciduria may lead to metabolism decompensation, complications or even death" — highlighting the distinctly benign course of 3-MCCD (PMID: 29039164)

Morbidity

  • Developmental disability: ~15% of cases with developmental data, though "not clearly attributable to other causes" (PMID: 27033733)
  • Secondary carnitine deficiency: ~47% of neonates; clinically actionable
  • Metabolic crises: Rare but potentially life-threatening if untreated

Prognostic Factors

  • No reliable prognostic biomarkers: NBS C5OH levels do not predict outcome
  • Genotype does not predict phenotype: "There was no correlation between newborn screening (NBS) C5OH level and presence of metabolic, newborn, later-life or developmental abnormalities" (PMID: 27033733)
  • Early management with emergency protocols may prevent rare crises

12. Treatment

Pharmacotherapy

There is no specific pharmacological treatment for 3-MCCD. Management is primarily supportive and preventive.

L-Carnitine Supplementation (MAXO:0001298)

  • Indication: Secondary carnitine deficiency (present in ~47% of neonates)
  • Mechanism: Replenishes depleted carnitine stores; promotes excretion of toxic acyl-CoA intermediates as acylcarnitines
  • Dosage: Typically 50-100 mg/kg/day in divided doses
  • Monitoring: Plasma free carnitine and acylcarnitine levels
  • CHEBI: CHEBI:16347 (L-carnitine)

Biotin (MAXO:0010003)

  • Generally NOT effective in isolated 3-MCCD (biotin-resistant), unlike MCD
  • Exception: MCCA-R385S dominant negative allele shows biotin responsiveness (PMID: 15868465)
  • CHEBI: CHEBI:15956 (biotin)

Dietary Management (MAXO:0000127)

  • Leucine restriction: Generally not required for asymptomatic individuals
  • Protein management: Normal protein intake typically recommended; only patients with recurrent crises may benefit from moderate leucine restriction
  • Fasting avoidance (MAXO:0000134): Key preventive measure — regular feeding schedules, especially during illness

Emergency Management (MAXO:0000088)

During acute metabolic crises: - IV dextrose (10%) to suppress catabolism - Fluid resuscitation for dehydration - Bicarbonate for severe metabolic acidosis - IV L-carnitine if oral not tolerated - Temporary protein restriction (24-48 hours) - ICU monitoring for severe cases

Advanced Therapeutics

  • Gene therapy: No current trials or approved therapies
  • Enzyme replacement therapy: Not developed
  • Transplantation: Not indicated given the predominantly benign phenotype

Monitoring Protocol

Parameter Frequency Method
Growth and development Every 3-6 months (infancy), then annually Clinical assessment
Plasma carnitine/acylcarnitines Every 6-12 months MS/MS
Urine organic acids As clinically indicated GC-MS
Developmental assessment Annual (early childhood) Standardized tools

Treatment Strategy Summary

  • Asymptomatic NBS-detected: Monitoring only; prophylactic carnitine at some centers
  • Mild/intermittent symptoms: Carnitine supplementation, dietary guidance, emergency protocol
  • Severe symptomatic: Protein restriction, carnitine, biotin trial (for responsive genotypes), emergency management

13. Prevention

Primary Prevention

  • Genetic counseling (MAXO:0000079) for families with known affected members
  • Carrier testing for at-risk relatives
  • Preimplantation genetic diagnosis (PGD) technically available for known familial variants
  • Prenatal diagnosis via CVS or amniocentesis with molecular testing

Secondary Prevention (Early Detection)

  • Newborn screening (MAXO:0000127): C5OH elevation by MS/MS on dried blood spots
  • Included in NBS panels of many countries (USA RUSP, European programs, Chinese national programs)
  • Clinical utility debated: "is routine screening necessary?" (PMID: 31730530)
  • Reverse cascade testing: NBS-positive infants serve as index cases for detecting undiagnosed maternal 3-MCCD (PMID: 40673334)

Tertiary Prevention

  • Fasting avoidance protocols and sick-day management plans
  • Emergency letters for healthcare providers
  • L-carnitine supplementation to prevent secondary carnitine deficiency
  • Regular metabolic follow-up

Genetic Counseling (MAXO:0000079)

  • Recurrence risk: 25% for siblings (AR inheritance)
  • Exception: 50% risk for MCCA-R385S dominant negative allele
  • Importance of distinguishing isolated 3-MCCD from MCD for accurate counseling
  • Need for long-term monitoring emphasized: "Adult metabolic specialists should be included in the development of NBS programs to provide data from this long-term monitoring and to contribute specific knowledge about later onset phenotypes" (PMID: 40610367)

14. Other Species / Natural Disease

Comparative Biology and Orthologous Genes

MCC is a highly conserved enzyme across eukaryotes, reflecting its essential role in leucine catabolism.

Species NCBI Taxon ID Gene(s) Notes
Homo sapiens 9606 MCCC1, MCCC2 Disease-causing genes
Mus musculus (mouse) 10090 Mccc1 (72039), Mccc2 (78038) Orthologous genes; knockout models
Rattus norvegicus (rat) 10116 Mccc1, Mccc2 Orthologs present
Danio rerio (zebrafish) 7955 mccc1, mccc2 Pathway conserved
Glycine max (soybean) 3847 MCCase Functionally characterized
Arabidopsis thaliana 3702 MCCase MCC-B subunit cloned

The leucine catabolic pathway in plants was directly demonstrated: "These findings demonstrate for the first time, to our knowledge, that the enzymes responsible for Leu catabolism are present in plant mitochondria" (PMID: 9847087).

Natural Disease in Animals

Naturally occurring MCC deficiency has not been extensively documented in companion animals or livestock (no OMIA entry). Given the predominantly benign phenotype in humans, mild forms in animals would likely go undetected.

Zoonotic/Transmission

Not applicable — 3-MCCD is a genetic/metabolic condition, not transmissible between species.

15. Model Organisms

Mouse Models

  • Mccc1 and Mccc2 knockout mice are available through IMPC and other consortia
  • Phenotypic characterization specific to 3-MCCD has been limited in published literature
  • The circadian clock mouse models (CCM and CBK) provide indirect models for tissue-specific MCC dysfunction — decreased MCC biotinylation leads to reduced leucine oxidation, correctable with biotin-enriched diet (PMID: 27084392)

Cellular Models

  • Patient fibroblasts: Most commonly used for enzyme assays and complementation analysis (PMID: 11181649)
  • Patient lymphocytes: Enable rapid diagnostic enzyme measurement (PMID: 3918814)

Plant Models

  • Arabidopsis thaliana and Glycine max provided foundational biochemistry of the MCC-B subunit and leucine catabolic pathway (PMID: 9847087; PMID: 10681539)

Model Limitations

  • The predominantly benign phenotype makes it challenging to develop clinically relevant animal models
  • Animal models may not recapitulate the environmental triggers necessary for symptom manifestation
  • Species differences in leucine catabolism rates and alternative pathways may limit translational applicability

Research Applications

  • Study of leucine catabolic pathway regulation
  • Understanding incomplete penetrance in metabolic disorders
  • Biotin metabolism and biotinylation biology
  • NBS program evaluation and clinical utility assessment
  • Genotype-phenotype dissociation mechanisms

Evidence Base Summary

PMID Key Contribution Evidence Type
11181649 Molecular basis: gene cloning, complementation groups Human genetics
10681539 MCCβ subunit characterization Biochemistry
15868465 Dominant negative MCCA-R385S Human genetics
24103308 California NBS incidence 1:41,676 Population screening
36822454 Zhejiang: all 53 cases asymptomatic Population screening
31730530 Jiangsu NBS; screening necessity questioned Population screening
27033733 IBEM-IS registry; no prognostic biomarkers Registry study
39188588 Quanzhou NBS; biomarker characterization Population screening
39484073 C5OH levels non-predictive Clinical study
27601257 Portuguese mutational spectrum (26 novel mutations) Human genetics
40610367 Adult specialist perspective on NBS Clinical perspective
40673334 Reverse cascade testing for maternal detection Methodology
27084392 Circadian clock regulation of MCC biotinylation Animal model
9847087 Leucine catabolic pathway in plant mitochondria Comparative biology
9350481 Multiple carboxylase deficiency overview Clinical review
41440809 Chinese meta-analysis of organic acidurias Meta-analysis
29946514 International NBS comparison (Asia/Germany) Population screening
40001143 Iranian NBS epidemiology Population screening
3918814 Rapid lymphocyte diagnostic assay Diagnostics
15992684 Biotin metabolism and histone biotinylation Basic science
24407466 3-Methylglutaconic aciduria metabolic biology Biochemistry

Limitations and Knowledge Gaps

  1. Genotype-phenotype correlation: No reliable correlation exists; the molecular basis for variable penetrance remains completely unknown, representing the most fundamental gap in understanding this disease.

  2. Long-term outcomes: Most NBS cohorts have limited follow-up (<10 years). The lifelong natural history of NBS-detected 3-MCCD is unknown. Adult outcomes are largely unstudied.

  3. NBS clinical utility: Whether screening for 3-MCCD provides net benefit versus harm (psychological burden, medicalization of healthy individuals) is unresolved and actively debated.

  4. Prognostic biomarkers: No biomarkers exist to identify the minority of individuals who will develop clinical symptoms — this is the most critical unmet clinical need.

  5. Mechanism of incomplete penetrance: Potential explanations (epigenetic variation, modifier genes, microbiome, stochastic factors) are entirely uninvestigated in 3-MCCD.

  6. Quality of life data: No formal QoL assessments quantify the psychosocial impact of 3-MCCD diagnosis on families.

  7. Animal model phenotyping: Limited published characterization of MCC-deficient mouse models.

  8. Maternal 3-MCCD: Prevalence and clinical significance of previously undiagnosed maternal cases need systematic study.

Proposed Follow-up Experiments / Actions

  1. Prospective longitudinal cohort study of NBS-identified 3-MCCD individuals through adulthood (20+ year follow-up) to establish definitive natural history and detect any late-onset complications.

  2. Multi-omics profiling (transcriptomics, metabolomics, epigenomics) comparing symptomatic versus asymptomatic individuals with equivalent genotypes to identify modifiers of penetrance.

  3. Functional variant characterization — standardized enzyme activity assays and structural modeling for all reported MCCC1/MCCC2 variants to enable residual activity-based risk stratification.

  4. Psychosocial impact assessment using validated instruments (PedsQL, EQ-5D) in families of NBS-identified individuals to quantify the harm/benefit balance of screening.

  5. Prognostic risk score development integrating genotype, residual enzyme activity, metabolomic profile, and carnitine status to stratify individuals at diagnosis.

  6. MCCA-R385S mechanism investigation and systematic screening for other dominant negative alleles across diverse populations.

  7. Gut microbiome characterization in 3-MCCD patients to assess whether microbial leucine metabolism modifies disease expression.

  8. International consensus guidelines on management of asymptomatic NBS-detected 3-MCCD, including recommendations on continued NBS utility.

  9. NBS cut-off optimization to reduce false-positive burden while maintaining detection of the rare symptomatic individuals who may benefit from early identification.

  10. Comprehensive mouse model phenotyping under basal and catabolic stress conditions to understand tissue-specific vulnerability and test potential interventions.

Report generated: 2026-05-05 Based on systematic review of 40+ peer-reviewed publications and established disease databases