Hereditary Spastic Paraplegia 5A

1. Disease Information

1.1. Concise Overview and Disease Definition

Hereditary spastic paraplegia type 5A (SPG5A) is one of the autosomal recessive hereditary spastic paraplegias (AR-HSPs), a genetically heterogeneous group of Mendelian neurodegenerative diseases primarily affecting the corticospinal tracts and resulting in progressive lower limb spasticity and weakness.[2][5][19] Clinically, SPG5A can present as a “pure” HSP phenotype, in which the core features are symmetric spastic paraparesis, hyperreflexia, ankle clonus, and often urinary urgency or bladder dysfunction, without other major neurologic deficits.[1][2][5][6][10] In contrast, a subset of patients show a “complex” or “complicated” phenotype, where pyramidal signs are accompanied by cerebellar ataxia, nystagmus, distal or generalized muscle atrophy, optic atrophy, cognitive impairment, and sometimes peripheral neuropathy or extrapyramidal signs.[1][2][3][5][6][10][12] Magnetic resonance imaging may reveal spinal cord atrophy and multifocal cerebral white matter T2 hyperintensities, sometimes mimicking demyelinating disease, as well as cerebellar atrophy in more complicated cases.[2][5][6][10][14][15] The disease typically begins in childhood or adolescence but may manifest from early childhood into the fifth decade, and it tends to progress slowly over decades, with many patients retaining ambulatory ability for long periods but experiencing substantial impact on mobility and quality of life.[2][5][6][10][12]

At the molecular level, SPG5A is caused by homozygous or compound heterozygous mutations in CYP7B1 on chromosome 8q12.3, encoding 25-hydroxycholesterol 7α-hydroxylase (oxysterol 7α-hydroxylase), which catalyzes the first step of a key extrahepatic cholesterol catabolic pathway and is central to oxysterol and neurosteroid metabolism.[1][2][3][5][8][16] Loss of CYP7B1 function leads to marked accumulation of 27-hydroxycholesterol and 25-hydroxycholesterol in plasma and cerebrospinal fluid, which are thought to exert neurotoxic effects on long corticospinal axons, thereby driving the characteristic axonopathy and spastic paraplegia.[5][12][13] In addition, CYP7B1 mutations can cause congenital bile acid synthesis defect type 3 (CBAS3), a severe neonatal cholestatic liver disease, indicating that CYP7B1 has context-dependent roles in hepatic bile acid metabolism versus brain neurosteroid and oxysterol handling.[11][8][16] The clinical and mechanistic distinction between SPG5A and CBAS3 is critical for diagnosis and counseling, even though they share the same gene.

1.2. Key Identifiers and Ontology Mapping

SPG5A is recognized in major rare disease and ontology frameworks with multiple identifiers that facilitate cross-database mapping. In OMIM, the phenotype “Spastic paraplegia 5A, autosomal recessive” is entry 270800, and CYP7B1 as the causal gene has entry 603711.[1] Orphanet lists the disorder as “Autosomal recessive spastic paraplegia type 5A” with Orpha number 100986, describing it as a form of hereditary spastic paraplegia with pure and complex phenotypes and highly variable age at onset.[10] The National Organization for Rare Disorders (NORD) provides a disease summary under “hereditary spastic paraplegia 5A” and notes its autosomal recessive inheritance and phenotypic heterogeneity.[6] ICD-10 classifies SPG5A under code G11.4 (“Hereditary spastic paraplegia”) and ICD-11 under 8B44.01, a specific category for autosomal recessive spastic paraplegia type 5A.[10] In MeSH, the condition is indexed under a broader term for “Hereditary Spastic Paraplegia,” while UMLS includes a concept C1849115 that corresponds to “Spastic paraplegia 5A, autosomal recessive” and is cross-linked to OMIM 270800 and Orpha 100986.[10] The user-provided MONDO identifier MONDO:0010047 corresponds to hereditary spastic paraplegia type 5A, aligning with OMIM and Orphanet definitions.

For gene-level ontologies, CYP7B1 is recognized in HGNC and Ensembl as “cytochrome P450 family 7 subfamily B member 1,” with Ensembl gene ID ENSG00000172817 and multiple transcripts including ENST00000310193.[4][8][16] In gene–disease resources such as PanelApp (Genomics England), CYP7B1 is annotated as causative for “Hereditary spastic paraplegia, childhood onset” and “Hereditary spastic paraplegia, adult onset,” with biallelic autosomal inheritance and phenotype “Spastic paraplegia 5A, autosomal recessive, 270800.”[4] In the MONDO ontology, SPG5A is classified as a Mendelian neurodegenerative disorder of the central nervous system, under the broader class “hereditary spastic paraplegia,” which itself is nested within “hereditary motor neuron disease” and “spastic paraplegia disorders.”

Suggested ontology mappings for the core disease entity include MONDO:0010047 for hereditary spastic paraplegia type 5A, OMIM:270800 for the phenotype, Orpha:100986, ICD-10:G11.4, ICD-11:8B44.01, UMLS:C1849115, and MeSH:C564811 (hereditary spastic paraplegia general category).[1][10] For the gene, HGNC:18601 (CYP7B1), OMIM:603711, and NCBI Gene ID 9421 are appropriate, with GO annotation for molecular function “oxidoreductase activity, acting on paired donors, with incorporation or reduction of molecular oxygen” (GO:0016705) and “25-hydroxycholesterol 7-alpha-hydroxylase activity” (GO:0008395).

1.3. Synonyms and Alternative Names

SPG5A has multiple synonyms that reflect its genetic, clinical, and biochemical context. Orphanet lists several synonymous disease names: “SPG5A,” “autosomal recessive spastic paraplegia type 5A,” “hereditary spastic paraplegia type 5A,” “pure or complex autosomal recessive spastic paraplegia caused by mutation in CYP7B1,” “spastic paraplegia 5A,” and “spastic paraplegia type 5B, recessive.”[10] NORD and Orphanet also emphasize the phrase “CYP7B1 pure or complex autosomal recessive spastic paraplegia” to highlight that both phenotypic extremes share the same gene.[6][10] OMIM denotes the phenotype as “Spastic paraplegia 5A, autosomal recessive” and associates the locus with CYP7B1.[1] In the context of gene nomenclature, CYP7B1 has aliases including “SPG5A,” “CBAS3,” and “CP7B,” reflecting its association with both spastic paraplegia and congenital bile acid synthesis defect type 3.[8][11][16]

Clinically, the condition is often referred to by clinicians as “SPG5” or “SPG5A” rather than by the full name, given the convention of numbering hereditary spastic paraplegia loci and genes (SPG1, SPG2, SPG3, etc.).[2][3][5][19] The “A” suffix distinguishes the autosomal recessive type 5A form from earlier locus nomenclature; historically, some literature referenced “SPG5” prior to the precise gene identification, but contemporary usage equates “SPG5” with CYP7B1-related HSP. The gene-level acronym “SPG5A” as a previous symbol for CYP7B1 is captured in various gene databases, reinforcing the close linkage between gene and phenotype.[8][16]

1.4. Source Types: Individual Patient Data vs Aggregated Disease-Level Resources

Most current information on SPG5A derives from aggregated disease-level resources—primarily case series and cohort studies synthesized in OMIM, Orphanet, NORD, and specialist reviews—rather than large-scale electronic health record–based epidemiologic studies, due to the extreme rarity of the condition.[1][2][3][5][6][10][12][19] Early mapping of the SPG5 locus to 8q12.3 and identification of CYP7B1 mutations came from linkage studies and family-based sequencing in consanguineous or multiplex families, such as the work of Tsaousidou and colleagues who identified homozygous CYP7B1 mutations in affected individuals from five autosomal recessive SPG5A families.[1][3][7] Subsequent studies expanded the mutational spectrum and described genotype–phenotype correlations using series of dozens of families, but overall sample sizes remain modest compared with more prevalent diseases.[3][7][9][12]

Natural history and biomarker data are synthesized most comprehensively in the Brain 2017 study by Schüle et al. (PMID:29126212), which assessed clinical progression, imaging, biomarkers, and therapeutic trial outcomes in a relatively large SPG5 cohort.[2][19] That work and related analyses provide aggregated longitudinal data on disease onset, progression rates, and functional outcomes, but they are still based on selected patients recruited through specialist centers rather than unselected populations.[2][19] Orphanet and NORD summaries integrate such cohort data with case reports to provide structured overviews of clinical features, imaging, inheritance, and prevalence.[6][10] ClinVar, PanelApp, OMIM, and gene-specific databases like GeneBe collate variant-level information from diagnostic laboratories and research studies, yielding aggregated insights into the mutational landscape, variant classifications, and allele frequencies.[1][4][8][11][18]

To date, there is no evidence that SPG5A has been systematically characterized using electronic health records or large administrative databases, and given its prevalence below one per million, such efforts would face major case ascertainment challenges.[10] As a result, disease information is largely derived from aggregated disease-level resources, with individual patient data appearing in primary case series and case reports that feed into these summaries.

2. Etiology

2.1. Primary Causal Factors: Genetic Basis

The primary causal factor in SPG5A is biallelic pathogenic variation in the CYP7B1 gene, located on chromosome 8q12.3.[1][3][4][8] The disease is unequivocally Mendelian and autosomal recessive, requiring homozygous or compound heterozygous loss-of-function or damaging missense variants to manifest clinical disease.[1][3][4][7][10][11] Linkage analysis in affected families initially narrowed the SPG5 locus to 8q11.1–q21.2, and further refinement identified a 23.6 cM interval between markers D8S1833 and D8S285, in which CYP7B1 was eventually recognized as the causative gene.[1] Tsaousidou et al. (2008) and subsequent groups demonstrated homozygous CYP7B1 mutations segregating with disease in multiple families with autosomal recessive hereditary spastic paraplegia.[1][3][7]

CYP7B1 encodes oxysterol 7α-hydroxylase, a member of the cytochrome P450 superfamily, which resides in the endoplasmic reticulum and catalyzes 7α-hydroxylation of oxysterols such as 25-hydroxycholesterol and 27-hydroxycholesterol, thereby mediating a key step in their catabolism and contributing to bile acid synthesis and neurosteroid metabolism.[2][5][8][16] In the brain, CYP7B1 is expressed in neurons and glia and is thought to provide the primary means of modifying dehydroepiandrosterone (DHEA) neurosteroids and other oxysterol substrates, because cholesterol itself cannot cross the blood–brain barrier and must be locally synthesized.[5][16] In the liver, CYP7B1 participates in an alternative pathway of bile acid synthesis, converting oxysterol intermediates into bile acids.[5][8][11][16] Loss of CYP7B1 activity thus disrupts oxysterol turnover in both brain and liver, but the clinical manifestations differ across tissues and developmental stages.

In SPG5A, most evidence supports a loss-of-function mechanism, with nonsense, frameshift, splice-site, and functionally disruptive missense variants leading to near-complete absence or severe reduction of enzyme activity.[3][7][9][11][18] The disease is therefore best classified as a monogenic, autosomal recessive, loss-of-function disorder of CYP7B1, with neurodegeneration resulting from toxic accumulation of unmetabolized oxysterols and possibly altered neurosteroid homeostasis.[2][5][12][13][16] There is currently no evidence that environmental exposures, infectious agents, or non-genetic factors can cause SPG5A in the absence of a CYP7B1 mutation.

2.2. Genetic Risk Factors: Causal Variants and Susceptibility

The primary genetic risk factors for SPG5A are pathogenic CYP7B1 variants that compromise protein structure or function. Multiple studies have cataloged these variants, and while SPG5A is rare, carrier frequencies of individual mutations can be elevated in particular populations due to founder effects.[3][7][9][11][18] The earliest comprehensive mutational analysis, published in Brain 2009 by Tsaousidou et al. (PMID:19439420), sequenced CYP7B1 in 82 unrelated autosomal recessive HSP index patients and 90 sporadic pure HSP patients.[3] They identified eight mutations in CYP7B1 segregating in nine families, including three nonsense mutations (p.R63X, p.R112X, p.Y275X) and five missense mutations (p.T297A, p.R417H, p.R417C, p.F470I, p.R486C), with the last four clustering in exon 6 at the C-terminal end of the protein.[3] Residue Arg417 emerged as a mutational hotspot, with multiple independent substitutions (R417H, R417C) associated with disease.[3] The frequency of CYP7B1 mutations in their autosomal recessive HSP series was 7.3% (6 of 82 families), and among sporadic pure HSP patients it was 3.3% (3 of 90), underscoring that SPG5A is a relatively rare but significant cause of recessive and sporadic pure spastic paraplegia.[3][5][12]

Additional mutational analyses, such as the study of 63 suspected autosomal recessive HSP patients by Criscuolo et al. (PMID:26714052), identified two novel homozygous mutations in CYP7B1, one frameshift and one missense, further expanding the spectrum.[7] Exome sequencing has been instrumental in discovering new variants, exemplified by the identification of a novel homozygous frameshift mutation c.741delA (p.K247fs) in exon 3 of CYP7B1 in the first Japanese SPG5 patient, which was confirmed by Sanger sequencing and found to be absent in controls.[9] OMIM and CBAS3 reports describe additional nonsense variants, such as R388X and R112X, in patients with congenital bile acid synthesis defect type 3, evidencing that the same gene can cause distinct phenotypes depending on developmental context and residual activity.[11]

ClinVar records numerous CYP7B1 variants with varying clinical significance; for instance, the frameshift variant NM_004820.5(CYP7B1):c.67del (p.Ala23fs) is classified as pathogenic for hereditary spastic paraplegia 5A by the Paris Brain Institute, based on clinical testing in an affected individual.[18] Gene-level resources like GeneBe and PanelApp annotate CYP7B1 as strongly associated with hereditary spastic paraplegia 5A (autosomal recessive), and also with congenital bile acid synthesis defect type 3, reflecting its dual clinical impact.[8][11] Allele frequency data from population databases such as gnomAD are not detailed in the provided sources, but given the prevalence estimate of SPG5A at less than one per million, pathogenic CYP7B1 variants are necessarily rare and usually present in heterozygous carrier state in unaffected individuals.[10]

Susceptibility beyond clearly pathogenic alleles is currently poorly defined. There is no evidence that heterozygous carriers of CYP7B1 loss-of-function variants exhibit subclinical neurologic or hepatic disease, and heterozygous parents in CBAS3 families are typically unaffected, consistent with a high level of recessive penetrance.[11] Modifier genes that influence disease severity or age at onset have not yet been systematically identified in SPG5A, though general HSP literature suggests that genetic background and other pathways of axonal maintenance may play modulating roles.[19] Genome-wide association or polygenic susceptibility studies in SPG5A have not been reported, again reflecting the rarity of the condition.

2.3. Environmental and Lifestyle Risk Factors

Available evidence does not support any specific environmental or lifestyle risk factors as primary drivers of SPG5A. The disease is defined by biallelic CYP7B1 mutations and occurs in individuals irrespective of environmental exposures, given the Mendelian nature and strong genotype–phenotype correlation.[1][2][3][5][6][10][11][19] Case series and cohort studies have not identified consistent triggers such as toxins, radiation, occupational exposures, or lifestyle factors that precipitate the onset of spastic paraplegia in CYP7B1 mutation carriers.[2][3][7][9][12] Nor has any infectious agent been implicated in disease causation or progression.

However, because CYP7B1 is involved in cholesterol and bile acid metabolism, theoretical interactions with dietary cholesterol intake, lipid-lowering drugs, and metabolic status have been considered. The Brain 2017 trial tested atorvastatin, a statin that lowers circulating cholesterol, in SPG5 patients to assess whether reduction of 27-hydroxycholesterol levels could translate into clinical benefit.[2][12][13][19] While atorvastatin markedly decreased plasma 27-hydroxycholesterol, the trial did not demonstrate a robust short-term effect on motor outcomes, suggesting that modifying systemic lipid levels alone may not significantly alter established neurodegeneration, or that longer treatment windows are needed.[2][19] There is no evidence that high-cholesterol diets or specific lifestyle patterns increase risk of disease manifestation in CYP7B1 mutation carriers; conversely, the presence of pathogenic biallelic variants appears sufficient to cause disease irrespective of cholesterol intake, although systemic lipid status might modulate biomarker levels.[2][5][12][13]

Thus, environmental and lifestyle factors are best viewed as potential modulators of biochemical markers rather than primary etiologic agents. For disease knowledge bases, environmental risk factor categories are essentially “not established” or “no known significant contributors,” with the caveat that metabolic interactions remain an area of research interest.

2.4. Protective Factors

No specific genetic or environmental protective factors have been clearly identified for SPG5A in the current literature. Given the autosomal recessive inheritance and high penetrance observed in families with biallelic loss-of-function mutations, there is little evidence for alleles that protect against disease in mutation carriers.[1][3][7][11] Heterozygous carriers are asymptomatic, which reflects the normal protective effect of having one functional copy of CYP7B1, but this is part of the basic recessive mechanism rather than a distinct modifier.[11]

At the environmental level, general neurologic health measures—such as maintaining cardiovascular fitness, avoiding neurotoxins, and using supportive therapies—may mitigate symptom severity and improve quality of life, but these are non-specific and not documented as protective factors in controlled studies of SPG5A.[2][5][19] The statin trial indicates that pharmacologic lowering of 27-hydroxycholesterol is biochemically effective, but whether long-term statin therapy has a protective effect on disease progression remains uncertain, as the randomized controlled data did not show a clear clinical benefit over the relatively short follow-up period.[2][12][13][19]

In summary, protective factors in SPG5A are essentially unknown beyond the generic protective effect of carrying only a single mutant allele, and evidence for modifiable protective exposures is lacking. Disease knowledge bases should therefore classify protective factors as “currently not defined” for this condition.

2.5. Gene–Environment Interactions

Given the monogenic nature of SPG5A, gene–environment interactions are likely to be modest compared with complex traits but may still influence biochemical profiles and potentially disease course. The most tangible interaction studied is between CYP7B1 loss-of-function and pharmacologic manipulation of cholesterol and oxysterols via statin therapy. In SPG5 patients, Schöls and colleagues showed that atorvastatin treatment significantly reduced plasma 27-hydroxycholesterol and 25-hydroxycholesterol levels, which were markedly elevated at baseline due to CYP7B1 deficiency.[12][13][19] This demonstrates a clear interaction between genetic impairment of oxysterol 7α-hydroxylase and environmental (pharmacologic) modulation of upstream cholesterol metabolism, with atorvastatin reducing the availability of substrates that accumulate when CYP7B1 is inactive.[12][13] The clinical implications of this interaction, however, remain uncertain, as short-term treatment did not robustly alter motor outcomes in the randomized trial.[2][19]

In CBAS3, where CYP7B1 mutations produce severe hepatic bile acid synthesis defects, diet and medical management of cholestasis may interact with the genetic lesion to determine survival and liver outcomes.[11][8] For instance, early recognition and treatment, including potential liver transplantation, can dramatically change prognosis in affected infants.[11][8] Yet, these interactions are more about managing the downstream consequences of the genetic defect rather than modifying risk of disease onset.

There is no evidence for classic gene–environment interactions involving toxins, infections, or lifestyle factors in SPG5A; the gene-level lesion appears to be overwhelmingly determinative. Thus, gene–environment interactions are currently limited to pharmacologic modulation of biochemical pathways, which provide mechanistic insight but no definitive clinical protection.

3. Phenotypes

3.1. Core Motor Phenotype: Spastic Paraparesis

The defining phenotype of SPG5A is a progressive spastic paraparesis of the lower limbs, which corresponds to the HPO term “Spastic paraplegia” (HP:0001251) and is characterized by bilateral lower limb weakness, hypertonia, hyperreflexia, and pyramidal signs.[2][5][6][10][19] Clinically, patients typically present with gait disturbance due to stiffness and weakness, often described as a spastic gait, with difficulty in walking long distances and frequent tripping.[2][5][6][10] Examination reveals increased tone, particularly in the hip adductors and knee flexors, brisk deep tendon reflexes, sustained ankle clonus, and extensor plantar responses, consistent with upper motor neuron dysfunction in corticospinal tracts.[2][5][19]

Age of onset of spastic paraparesis in SPG5A is highly variable, ranging from early childhood to adolescence and sometimes adulthood, with most series reporting onset in the first two decades of life.[2][3][5][6][10][12] Orphanet notes that age of onset can span from early childhood to adulthood, and MedLink emphasizes that SPG5A onset ranges from early childhood to the fifth decade.[5][10] Symptom severity is also variable; some patients remain mildly affected for decades, able to walk without assistive devices, whereas others develop moderate to severe disability requiring walking aids or wheelchairs.[2][5][19] The progression is generally slowly progressive rather than episodic or relapsing, and there is no evidence for spontaneous remission.[2][5][19]

The impact of spastic paraparesis on quality of life is substantial, affecting mobility, independence, employment, and social participation. Domains such as mobility, self-care, and usual activities in EQ-5D and SF-36 frameworks are typically impaired in moderate to severe cases, even though pain is not a primary feature of SPG5A itself.[2][5][19] The chronic nature of spasticity and weakness necessitates ongoing physiotherapy, occupational therapy, and sometimes orthotic support, and may contribute to secondary musculoskeletal complications such as joint contractures and back pain.[5][19] The HPO term “Gait disturbance” (HP:0001288) and “Hyperreflexia” (HP:0001347) are also relevant for capturing the core motor phenotype.

3.2. Bladder Dysfunction and Autonomic Features

Bladder dysfunction, particularly urinary urgency and incontinence, is a common associated phenotype in SPG5A, reflecting involvement of descending pathways that modulate micturition or spinal cord circuitry.[2][5][6][10] Orphanet specifies bladder dysfunction as part of the pure phenotype, noting that patients with pure SPG5A often show slowly progressive spastic paraplegia of the lower extremities with bladder dysfunction and pes cavus.[10] NORD similarly emphasizes bladder dysfunction as a characteristic feature in the pure form.[6] Clinically, patients may report frequent urges to urinate, nocturia, and difficulty with bladder control, sometimes requiring urologic evaluation and pharmacologic management.[2][5]

Age of onset of bladder symptoms typically coincides with or follows the onset of gait disturbance, and severity is variable, with some patients experiencing mild urgency and others more disabling incontinence.[2][5][10] Symptom progression tends to parallel motor decline, though bladder dysfunction may be more responsive to symptomatic treatment. The HPO terms “Urinary urgency” (HP:0000025) and “Neurogenic bladder” (HP:0000013) capture this phenotype.

Quality of life impact is significant, as bladder dysfunction affects personal comfort, social functioning, and mental health, especially due to embarrassment and the need for planned toilet access. In SF-36 and EQ-5D frameworks, this would be reflected in decreased scores in social functioning and mental health domains. However, bladder dysfunction does not usually drive mortality.

3.3. Skeletal and Foot Deformities: Pes Cavus

Pes cavus, a high-arched foot deformity, is frequently reported in SPG5A and other HSPs, likely as a consequence of chronic spasticity and muscle imbalance.[2][5][6][10] Orphanet explicitly lists pes cavus as part of the pure phenotype of SPG5A, along with spastic paraparesis and bladder dysfunction.[10] Pes cavus corresponds to HPO term “Pes cavus” (HP:0001761) and is often evident on physical examination and can be documented with orthopedic imaging.

Age of onset for pes cavus is often in childhood or adolescence, emerging gradually as the spasticity affects foot musculature. Severity varies from mild arch elevation to pronounced deformity with claw toes and associated foot pain or calluses. The deformity tends to be stable or slowly progressive and may exacerbate gait difficulty by compromising foot biomechanics.[2][5] Quality of life impact includes difficulty with shoe fitting, increased risk of falls, and foot pain, though these are secondary issues in many SPG5A patients.

3.4. Complex Neurologic Features: Cerebellar Signs, Nystagmus, Muscle Atrophy, Cognitive Impairment

In addition to the pure form, SPG5A can present with a complex phenotype that includes cerebellar signs such as ataxia, dysmetria, and intention tremor; nystagmus; distal or generalized muscle atrophy; and cognitive impairment.[1][2][3][5][6][10][12] Orphanet describes this complex presentation as having additional manifestations including cerebellar signs, nystagmus, distal or generalized muscle atrophy, and cognitive impairment, and notes that cerebellar and spinal cord atrophy may be observed on MRI.[10] MedLink echoes that SPG5A can result in a complicated form of HSP with ataxia, optic atrophy, and white matter lesions on MRI.[5] The Brain 2009 mutational study noted that hereditary spastic paraplegia was pure in seven SPG5 families but complex in two, demonstrating that the same CYP7B1 mutations can yield different phenotypic severity or involvement patterns.[3]

Cerebellar ataxia corresponds to HPO “Ataxia” (HP:0001251, though that term is also used for spastic paraplegia; more specific term “Cerebellar ataxia,” HP:0002148, is appropriate), and nystagmus corresponds to HP:0000639. These manifestations often present later than the core pyramidal signs, but can emerge in adolescence or adulthood depending on the individual.[2][5] Muscle atrophy (HP:0003202) may be distal or generalized and can reflect both pyramidal tract dysfunction and possible peripheral nerve involvement. Cognitive impairment, ranging from mild executive dysfunction to more global deficits, corresponds to HPO “Cognitive impairment” (HP:0100543) and is relatively uncommon but has been reported.[2][5][10]

Severity of complex features is variable. In the Brain 2017 cohort, most SPG5 patients had pure phenotypes, but a subset exhibited cerebellar signs and white matter changes.[2][19] The presence of complex features is associated with greater disability and lower quality of life, including impaired coordination, visual symptoms from nystagmus, decreased muscle strength and mass, and cognitive challenges in daily living. These manifestations may also complicate differential diagnosis with other neurodegenerative or demyelinating conditions, emphasizing the need for genetic testing.

3.5. Optic Atrophy and Visual Phenotypes

Optic atrophy has been described in SPG5A as part of the complex phenotype, though it is not universal.[2][3][5][12] MedLink notes that SPG5A can result in a complicated form of HSP with optic atrophy and white matter lesions on MRI.[5] Optic atrophy corresponds to HPO “Optic atrophy” (HP:0000648) and entails pallor of the optic disc with associated visual field defects or decreased visual acuity. Age of onset is variable; optic atrophy may appear after years of spasticity, suggesting a slower degenerative process affecting long axons of the visual system.

The frequency of optic atrophy among SPG5A patients is not precisely quantified in current series but appears to be relatively low compared with the core pyramidal phenotype, perhaps affecting a minority of cases. In those affected, visual impairment can significantly impact quality of life, adding disability beyond motor limitations. It also provides clues to a broader neuroaxonal vulnerability in CYP7B1 deficiency, consistent with the gene’s role in oxysterol handling in the central nervous system.[2][5][19]

3.6. Neuroimaging Phenotypes: White Matter Lesions and Spinal Cord Atrophy

Neuroimaging in SPG5A shows characteristic but variable features. Conventional MRI can reveal thoracic and cervical spinal cord atrophy, cerebellar atrophy, and multifocal T2-weighted white matter hyperintensities in the cerebral hemispheres.[2][5][10][14][15] Orphanet reports that white matter hyperintensity and cerebellar and spinal cord atrophy may be noted on brain MRI in some patients.[10] MedLink similarly indicates that SPG5A can show white matter lesions on MRI, and that spinal cord atrophy is common among HSP subtypes.[5] In broader HSP imaging reviews, SPG5 is specifically mentioned among autosomal recessive HSPs with white matter T2 hyperintensities, along with SPG21 and SPG35.[14][15] These lesions can resemble those seen in multiple sclerosis or other leukoencephalopathies, posing diagnostic challenges.[14][15]

Advanced MRI techniques, including diffusion tensor imaging (DTI) and volumetric analyses, have demonstrated microstructural white matter abnormalities in HSPs that are not visible on routine scans.[14][15] While most of these studies focus on SPG4 and other common subtypes, the pattern of widespread white matter damage with relatively preserved cortical grey matter in pure HSP forms likely applies to SPG5A as well.[15] In complicated forms, grey matter volumetric reduction in basal ganglia and cerebral cortex, particularly precentral and paracentral gyri, has been reported in HSP in general and might be present in SPG5A when cerebellar and cognitive features occur.[15]

Spinal cord atrophy corresponds to HPO “Spinal cord atrophy” (HP:0006865), and white matter T2 hyperintensities correspond to “Abnormality of cerebral white matter” (HP:0002500) or more specific “White matter lesions” (HP:0002499). These imaging phenotypes are important both for diagnosis and for mechanistic understanding, as they reflect chronic axonal degeneration and myelin changes in long tracts. Quality of life impact is indirectly mediated through clinical symptoms, but imaging markers are candidate biomarkers for tracking disease progression and treatment response.[14][15][19]

3.7. Hepatic Phenotypes in CBAS3: Jaundice, Cholestasis, Hepatomegaly

While SPG5A is neurologic, the same CYP7B1 mutations can cause congenital bile acid synthesis defect type 3 (CBAS3), a distinct phenotype characterized by prolonged jaundice after birth, hepatomegaly, conjugated hyperbilirubinemia, elevations in abnormal bile acids, and progressive intrahepatic cholestasis with liver fibrosis.[11][8] OMIM describes CBAS3 as an autosomal recessive disorder caused by homozygous CYP7B1 mutations, such as R388X and R112X, leading to severe disruption of bile acid synthesis.[11] These hepatic phenotypes correspond to HPO terms “Neonatal jaundice” (HP:0006579), “Hepatomegaly” (HP:0002240), “Cholestasis” (HP:0001396), and “Hyperbilirubinemia” (HP:0002904). Age of onset is neonatal, and the disease can be severe and life-threatening, often requiring early diagnosis and potentially liver transplantation.[11][8]

Although CBAS3 is distinct from SPG5A and should not be conflated, recognizing that CYP7B1 mutations can lead to liver disease is important for comprehensive gene–phenotype mapping and for understanding the tissue-specific consequences of enzyme deficiency. Quality of life impact is profound in CBAS3, with risk of liver failure, growth delay, and early mortality without treatment.[11][8] For knowledge bases, CBAS3 should be captured as a separate MONDO entity linked to CYP7B1, with distinct phenotypic profiles and ontological mappings.

4. Genetic and Molecular Information

4.1. CYP7B1 Gene: Structure, Expression, and Function

CYP7B1 (cytochrome P450 family 7 subfamily B member 1) is a protein-coding gene located at 8q12.3, spanning approximately 210,999 bp on the GRCh38 reference genome from positions 64,587,763 to 64,798,737.[1][8][16] It comprises six coding exons, with a major transcript ENST00000310193 encoding the full-length protein.[8] CYP7B1 encodes an endoplasmic reticulum membrane protein belonging to the cytochrome P450 superfamily, characterized by a heme-binding domain and typical P450 structural motifs, and is annotated in UniProt as a 25-hydroxycholesterol 7α-hydroxylase.[16]

Functionally, CYP7B1 catalyzes the 7α-hydroxylation of oxysterols such as 25-hydroxycholesterol and 27-hydroxycholesterol, generating 7α,25-dihydroxycholesterol (7α,25-OHC) and related metabolites, and thereby mediating the first reaction in the cholesterol catabolic pathway of extrahepatic tissues.[8][16] In humans, this pathway contributes to the conversion of cholesterol to bile acids in extrahepatic contexts and is particularly important in tissues such as brain and liver.[5][8][16] The enzyme is also involved in the metabolism of steroid hormones, including DHEA and its sulfate (DHEA-S), acting on them as neurosteroids that modulate neuronal excitability and synaptic function.[5][16] Because cholesterol cannot cross the blood–brain barrier and is synthesized locally in the brain, CYP7B1 provides a primary means of modifying and inactivating neurosteroid-derived oxysterols in the central nervous system.[5][16]

Expression data (not detailed in the provided sources but inferred from gene function) indicate that CYP7B1 is expressed in liver and brain, including hippocampus and cortex, and possibly in other steroidogenic tissues.[5][16] Gene Ontology annotations capture its molecular function as “oxidoreductase activity, acting on paired donors, with incorporation or reduction of molecular oxygen” (GO:0016705), “25-hydroxycholesterol 7-alpha-hydroxylase activity” (GO:0008395), and biological processes such as “bile acid biosynthetic process” (GO:0006699) and “cholesterol catabolic process” (GO:0006707). Subcellular localization corresponds to GO:0005789 (endoplasmic reticulum membrane).

4.2. Pathogenic Variant Classes: Loss-of-Function and Missense

Pathogenic variants in CYP7B1 associated with SPG5A encompass multiple classes, including nonsense, frameshift, missense, and splice-site changes. The Brain 2009 study identified three nonsense mutations (p.R63X, p.R112X, p.Y275X) that truncate the protein early, likely abolishing enzymatic activity.[3] Frameshift mutations, such as c.741delA (p.K247fs) found in a Japanese patient, shift the reading frame and introduce premature stop codons, equally expected to cause loss of function, either through nonsense-mediated mRNA decay or production of unstable truncated proteins.[9] ClinVar’s c.67del (p.Ala23fs) variant exemplifies another frameshift with pathogenic classification in SPG5A.[18]

Missense variants cluster at functionally important residues, particularly in exon 6 near the C-terminal region, which may include substrate-binding or catalytic domains.[3][7][9] Tsaousidou et al. reported missense changes p.T297A, p.R417H, p.R417C, p.F470I, and p.R486C as pathogenic, with residue R417 emerging as a hotspot.[3] These variants likely alter protein folding, heme incorporation, or substrate binding, resulting in reduced or absent 7α-hydroxylase activity. Functional studies (not fully detailed in the provided sources) would be needed to classify each variant’s impact on enzyme kinetics, but the phenotypic association with SPG5A supports a loss-of-function interpretation.[3][5][7][9]

Splice-site variants have also been reported, though not explicitly in the provided search results. In general, such variants can cause exon skipping, intron retention, or frameshifts, leading to truncated or misfolded proteins. OMIM’s CBAS3 entry describes nonsense mutations like R388X and R112X in hepatic disease, which again exemplify loss-of-function.[11] Overall, the pathogenic variant spectrum is dominated by alleles that severely compromise enzyme function, consistent with autosomal recessive disease.

Variant classification per ACMG/AMP guidelines is documented in ClinVar and gene-specific databases. For example, c.67del (p.Ala23fs) is classified as pathogenic for SPG5A, based on clinical and genetic evidence.[18] Many other variants are likely classified as pathogenic or likely pathogenic, though some rare missense changes may be considered variants of uncertain significance (VUS) pending functional evaluation.[8][18] Allele frequencies in gnomAD and other population databases are low, consistent with the rarity of SPG5A; truncated alleles may appear as heterozygous variants in healthy individuals, but the biallelic state is required for disease.[10]

4.3. Somatic versus Germline Origin and Mosaicism

All known pathogenic CYP7B1 variants in SPG5A and CBAS3 are germline, inherited in recessive fashion. There is no evidence of somatic mutations in CYP7B1 driving disease in adults, and somatic mosaicism has not been reported in SPG5A.[1][3][7][11][18] Parents of affected children with CBAS3 are typically heterozygous carriers without signs of liver disease, reinforcing the germline recessive model.[11] Likewise, families with SPG5A show autosomal recessive inheritance with homozygous or compound heterozygous variants in affected individuals and heterozygous carriers among parents and siblings.[3][7][9]

Somatic variants in CYP7B1 might be theoretically relevant in cancer or other hepatic diseases, but this lies outside the scope of SPG5A and is not described in the provided sources. For disease knowledge bases focusing on HSP and CBAS3, CYP7B1 variants should be annotated as germline with recessive inheritance and high penetrance.

4.4. Modifier Genes and Epigenetic Information

Modifier genes that alter SPG5A severity, onset age, or phenotypic spectrum have not yet been identified with robust evidence. Given the heterogeneity of HSP and the involvement of multiple pathways in axonal maintenance, it is plausible that variants in other genes such as PNPLA6 (SPG39) or C19orf12 (SPG43) could influence overall neurologic vulnerability, but the mutational analysis study of CYP7B1, PNPLA6, and C19orf12 in 63 HSP patients did not find overlapping mutations that would suggest combined gene effects.[7] That study detected novel CYP7B1 mutations in SPG5A but no pathogenic variants in PNPLA6 or C19orf12 in the same cohort, highlighting distinct etiologic categories.[7]

Epigenetic changes in CYP7B1 or related pathways have not been specifically described in SPG5A. There is no evidence from ENCODE or Roadmap Epigenomics in the provided sources, and methylation or chromatin alterations affecting CYP7B1 expression are not known to cause HSP. Given the monogenic loss-of-function model, epigenetic contributions are likely secondary at best. For knowledge bases, modifier genes and epigenetic factors should be annotated as “not currently established” in SPG5A.

4.5. Chromosomal Abnormalities

SPG5A is not associated with large-scale chromosomal abnormalities such as aneuploidy, translocations, or inversions. The disease locus resides in a small region of 8q12.3, and mutations are at the gene level rather than the chromosomal level.[1][3][7][8] Linkage studies that mapped SPG5 to 8q11.1–q21.2 did not uncover structural rearrangements but rather pointed to a gene-level defect.[1] DECIPHER and other structural variant databases, though not directly referenced, would likely show no recurrent large-scale abnormalities overlapping CYP7B1 in SPG5A patients.

5. Environmental Information

5.1. Environmental Factors: Toxins, Radiation, Pollution

As noted earlier, there are no specific environmental toxins, radiation exposures, or pollutants known to cause or significantly contribute to SPG5A. The condition is defined by inherited biallelic CYP7B1 mutations and has been described in families across multiple countries without consistent environmental patterns.[1][2][3][5][7][9][10][12][19] Case reports and cohort studies do not highlight occupational or environmental exposures as risk factors. Therefore, environmental categories in comparative toxicogenomics databases would likely show CYP7B1 as responsive to general stressors but not as a direct target of specific environmental etiologic factors in SPG5A.

5.2. Lifestyle Factors: Smoking, Diet, Exercise, Alcohol

Lifestyle factors such as smoking, diet, physical activity, and alcohol consumption have not been systematically studied in SPG5A cohorts, and the available literature does not implicate them as causal or major modulators of disease. Patients with SPG5A may benefit from general neurologic care recommendations, including regular exercise and avoidance of neurotoxins, but these are not disease-specific.[2][5][19] Diet that modifies cholesterol levels could, in theory, interact with oxysterol metabolism, yet the atorvastatin trial suggests that pharmacologic reductions in cholesterol and oxysterols have limited short-term clinical impact.[2][12][13][19] Thus, no lifestyle factors can be considered established risk or protective factors.

5.3. Infectious Agents

No infectious agents have been associated with SPG5A. The disease does not follow epidemic patterns, does not cluster with infections, and is clearly linked to inherited mutations. For knowledge bases, infectious etiologies should be marked as “not applicable” for SPG5A.

6. Mechanism / Pathophysiology

6.1. Ordered Causal Chain from Mutation to Clinical Manifestation

Step 1: Biallelic loss-of-function mutations in CYP7B1 lead to deficient oxysterol 7α-hydroxylase activity in liver and brain, resulting in impaired 7α-hydroxylation of substrates such as 25-hydroxycholesterol and 27-hydroxycholesterol.[1][2][3][5][8][11][13][16]

Step 2: Deficient oxysterol 7α-hydroxylase activity leads to accumulation of unmetabolized oxysterols, particularly 27-hydroxycholesterol and 25-hydroxycholesterol, in plasma and cerebrospinal fluid, with levels increased six- to nine-fold for 27-hydroxycholesterol in plasma and thirty- to fifty-fold in CSF, and approximately one hundred-fold for 25-hydroxycholesterol in plasma, as demonstrated in SPG5 patients.[12][13]

Step 3: Accumulated 27-hydroxycholesterol and related oxysterols result in chronic exposure of central nervous system cells, especially long corticospinal motor neurons and associated oligodendrocytes, to neurotoxic sterols, which experimentally and inferentially leads to axonal damage via mechanisms such as oxidative stress, membrane perturbation, and disruption of cholesterol homeostasis.[5][12][13][16][19]

Step 4: Chronic oxysterol-induced axonal damage leads to progressive degeneration (axonopathy) of corticospinal tracts, with loss of myelinated long axons in spinal cord and brain white matter, manifesting as spastic paraparesis and pyramidal signs in the lower limbs.[2][5][14][15][19]

Step 5: In some individuals, oxysterol accumulation and neurosteroid imbalance also lead to damage in additional neural systems, including cerebellar pathways, visual pathways (optic nerve), and cognitive networks, resulting in cerebellar ataxia, optic atrophy, and cognitive impairment in the complex phenotype.[2][3][5][10][12][15]

Step 6: In the liver, especially in neonates, CYP7B1 deficiency leads to impaired bile acid synthesis via alternative pathways, resulting in accumulation of atypical bile acids and cholestasis; this manifests clinically as CBAS3 with jaundice, hepatomegaly, hyperbilirubinemia, and progressive intrahepatic cholestasis.[8][11]

Step 7: Over years, corticospinal axonopathy and associated white matter changes lead to spinal cord atrophy and multifocal cerebral white matter lesions visible on MRI, which reflect structural correlates of the chronic neurodegenerative process.[2][5][10][14][15][19]

Step 8: The progressive neurodegeneration and structural changes result in slowly worsening spasticity, weakness, gait disturbance, bladder dysfunction, and, in some cases, additional neurologic deficits, producing the characteristic lifelong course of SPG5A.[2][5][10][19]

Where specific mechanistic links (such as oxidative stress pathways or exact cellular toxicity mechanisms of 27-hydroxycholesterol) are not fully demonstrated in human tissue, they are inferred from biochemical data, model organisms, and in vitro studies.

6.2. Molecular Pathways: Oxysterol and Cholesterol Metabolism

The central molecular pathway implicated in SPG5A is oxysterol and cholesterol metabolism, within both bile acid synthesis and neurosteroid catabolism. CYP7B1 catalyzes 7α-hydroxylation of oxysterols like 25-hydroxycholesterol and 27-hydroxycholesterol, producing metabolites such as 7α,25-dihydroxycholesterol.[8][16] This reaction sits at the intersection of cholesterol catabolism and bile acid biosynthesis, as oxysterols are intermediates that can be converted into bile acids via alternative pathways, particularly in extrahepatic tissues.[5][8][11][16]

In the central nervous system, oxysterols and neurosteroids such as DHEA and DHEA-S modulate synaptic function, neuronal excitability, and neuroprotection. CYP7B1-mediated hydroxylation represents a catabolic route that likely inactivates or modifies these molecules.[5][16] Loss of CYP7B1 in SPG5A disrupts this pathway, leading to accumulation of 27-hydroxycholesterol and 25-hydroxycholesterol. Schöls and colleagues showed that SPG5 patients have dramatically elevated levels of 27-hydroxycholesterol (six- to nine-fold in plasma, thirty- to fifty-fold in CSF) and 25-hydroxycholesterol (approximately one hundred-fold in plasma), whereas serum bile acids remain normal, suggesting that classical bile acid synthesis is intact.[12][13] This indicates that the primary biochemical defect is in oxysterol catabolism rather than global bile acid production.

Oxysterols such as 27-hydroxycholesterol can cross the blood–brain barrier and act on nuclear receptors like liver X receptor (LXR), modulating lipid homeostasis, inflammation, and transcription of genes involved in cholesterol transport and metabolism. Excessive 27-hydroxycholesterol has been implicated in atherosclerosis and neurodegeneration in other contexts, and in SPG5A the extreme elevation likely perturbs these pathways, contributing to neuronal and glial dysfunction.[12][13][16] KEGG and Reactome pathways related to cholesterol metabolism, bile acid biosynthesis, and oxysterol signaling (e.g., “Primary bile acid biosynthesis” and “Cholesterol metabolism”) are the primary loci for CYP7B1’s molecular role.

6.3. Cellular Processes: Axonal Degeneration, Myelin Changes, and Neurosteroid Imbalance

At the cellular level, SPG5A is characterized by chronic axonal degeneration of long corticospinal tracts, with relative preservation of neuronal cell bodies in early stages. This process can be classified as a distal axonopathy, where long axons are particularly vulnerable to metabolic stress and toxic insults.[2][5][14][15] Elevated oxysterols may induce oxidative stress, disrupt membrane integrity, and interfere with axonal transport. Over time, these insults lead to mitochondrial dysfunction, cytoskeletal damage, and eventual axonal die-back, culminating in thinning of the spinal cord and loss of white matter integrity.[14][15][19] GO biological process terms such as “axon degeneration” (GO:0030425), “regulation of neuron projection development” (GO:0010975), and “response to oxidative stress” (GO:0006979) are relevant to SPG5A pathophysiology.

Oligodendrocytes and myelin sheaths surrounding long axons are also affected, as evidenced by white matter T2 hyperintensities and microstructural abnormalities on diffusion imaging.[14][15] These changes suggest demyelination or dysmyelination secondary to axonal degeneration or direct sterol toxicity to oligodendrocytes. Cellular processes including “myelination” (GO:0042552) and “maintenance of myelin sheath” (GO:0032286) are likely disrupted.

In the brain’s steroidogenic microenvironment, neurosteroid imbalance may further modulate neuronal function. CYP7B1 can hydroxylate DHEA and DHEA-S, and its absence may alter levels of these neuroactive steroids, affecting synaptic plasticity, GABAergic and glutamatergic signaling, and neuroprotection.[5][16] While direct data are limited, neurosteroid-related GO terms such as “response to steroid hormone” (GO:0048545) and “regulation of synaptic transmission” (GO:0050804) may be involved.

6.4. Protein Dysfunction: Loss of Oxysterol 7α-Hydroxylase Activity

Pathogenic CYP7B1 variants result in altered protein structure and function, predominantly through loss of enzymatic activity. Nonsense and frameshift mutations truncate the protein, removing key domains required for heme binding and catalytic activity, leading to complete loss-of-function.[3][9][11][18] Missense mutations, particularly those at conserved residues like R417 and F470, likely disrupt proper folding or active-site architecture, reducing enzyme activity. These changes can be classified under GO molecular function “loss of oxidoreductase activity” with consequences for downstream metabolic processes.

Structural modeling (not fully detailed in the provided sources) would predict that the C-terminal region of CYP7B1 houses essential motifs for substrate binding and catalysis, explaining why exon 6 variants are particularly pathogenic.[3] Misfolding may also lead to endoplasmic reticulum stress, though this has not been directly demonstrated in SPG5A. In any case, the primary protein dysfunction is loss of oxysterol 7α-hydroxylase activity, with a functional consequence of accumulating substrates and decreased production of downstream metabolites.

6.5. Metabolic Changes: Oxysterols and Bile Acids

The most striking metabolic changes in SPG5A involve oxysterols, as described above. Patients show markedly elevated 27-hydroxycholesterol and 25-hydroxycholesterol in plasma and CSF, measured by mass spectrometry.[12][13] Importantly, serum bile acids are normal, indicating that primary bile acid synthesis via CYP7A1 and related enzymes is intact and that the defect is specific to CYP7B1-dependent pathways.[13] This distinguishes SPG5A from CBAS3, where bile acid profiles are abnormal and cholestasis is prominent.[11][8]

Chemically, 27-hydroxycholesterol and 25-hydroxycholesterol correspond to CHEBI entities for oxysterols, such as CHEBI:27771 (27-hydroxycholesterol). These molecules act as signaling oxysterols, binding nuclear receptors like LXR and influencing lipid and inflammatory gene expression. Excessive levels could dysregulate LXR signaling, contributing to neuroinflammation or metabolic stress. In SPG5A, the dramatic accumulation in CSF emphasizes the importance of CYP7B1 in central oxysterol clearance, and treatment strategies such as statins aim to lower these levels.[12][13][19]

Metabolic profiling in SPG5A (beyond oxysterols) has not been extensively published, but HMDB and MetaboLights could, in principle, capture associated changes in lipidome if studies were conducted. For bile acid synthesis, CBAS3 provides evidence that CYP7B1 is necessary for alternative bile acid pathways; lacking this activity leads to atypical bile acid accumulation, cholestasis, and liver injury.[11][8] Thus, in knowledge bases, metabolic changes should highlight oxysterol accumulation in SPG5A and bile acid abnormalities in CBAS3.

6.6. Immune System Involvement and Inflammation

Direct immune involvement in SPG5A has not been described. There is no evidence of autoimmunity, immunodeficiency, or chronic systemic inflammation as primary mechanisms. However, oxysterols such as 27-hydroxycholesterol can modulate immune cell function and inflammatory pathways via LXR and other nuclear receptors.[12][13][16] In other contexts, 27-hydroxycholesterol has been linked to pro-inflammatory macrophage activation and atherosclerosis. It is reasonable to infer that elevated oxysterols in SPG5A may influence microglial activation and neuroinflammatory milieus, but direct evidence from human brain or CSF inflammatory markers is lacking.

In HSP more broadly, neuroinflammation is not a dominant feature, unlike in multiple sclerosis or other demyelinating diseases, and white matter lesions in SPG5A are not primarily inflammatory.[14][15] Thus, immune system involvement should be considered secondary or speculative, mediated through oxysterol signaling in central immune cells rather than overt autoimmunity or inflammation.

6.7. Tissue Damage Mechanisms: Axonopathy, White Matter Loss, and Liver Fibrosis

In the nervous system, tissue damage in SPG5A is dominated by axonopathy of corticospinal tracts and associated white matter loss. The long descending axons from primary motor cortex to spinal cord are particularly vulnerable to metabolic insults, and chronic oxysterol accumulation likely damages axonal membranes and cytoskeleton, leading to Wallerian-like degeneration.[2][5][14][15][19] Spinal cord atrophy observed on MRI reflects loss of axonal mass, and white matter hyperintensities represent areas of demyelination or gliosis.[14][15] Mechanisms may involve oxidative stress, excitotoxicity, and impaired axonal transport, though these have not been explicitly demonstrated in SPG5A.

In CBAS3, liver fibrosis and intrahepatic cholestasis represent tissue damage mechanisms in hepatocytes and bile ducts. Impaired bile acid synthesis leads to accumulation of toxic intermediates, causing hepatocellular injury, inflammation, and fibrotic remodeling.[11][8] Over time, this can progress to cirrhosis and liver failure if untreated. GO terms such as “cholestasis” (GO:0006693 in bile acid context) and “fibrosis” (GO:0005882, though more general) are relevant.

6.8. Epigenetic Changes and Molecular Profiling

Epigenetic changes in SPG5A have not been described. Transcriptomic, proteomic, metabolomic, and lipidomic profiling in SPG5A is limited to targeted oxysterol measurements and perhaps clinical biomarker analyses.[2][12][13][19] There are no reports of large-scale omics studies in SPG5A, reflecting its rarity and the technical challenges of collecting tissue. Consequently, multi-omics integration, single-cell analysis, spatial transcriptomics, and functional genomics screens (CRISPR, RNAi) have not been applied specifically to SPG5A, though they may be relevant in future research.

6.9. Cell Types Involved: Upper Motor Neurons, Oligodendrocytes, Hepatocytes

The primary cell types involved in SPG5A include upper motor neurons in motor cortex, corticospinal tract axons, and their associated oligodendrocytes and astrocytes. Upper motor neurons correspond to CL terms such as “corticospinal motor neuron” (not explicitly coded but conceptually within CL:0000107 for neuron type) and “pyramidal neuron” (CL:0002392). Oligodendrocytes correspond to CL:0000129 and astrocytes to CL:0000127. In the liver, hepatocytes (CL:0000182) are key cells affected in CBAS3.

CYP7B1 is expressed in both neurons and hepatocytes, and its deficiency in these cell types underlies the tissue-specific manifestations—spastic paraplegia versus cholestatic liver disease.[5][8][11][16] For knowledge bases, GO cell component terms such as “axon” (GO:0030424), “myelin sheath” (GO:0043218), and “endoplasmic reticulum” (GO:0005783) are relevant, as CYP7B1 functions in ER membranes in these cells.

7. Anatomical Structures Affected

7.1. Organ-Level Involvement: Central Nervous System and Liver

The primary organ system affected in SPG5A is the central nervous system, specifically the corticospinal tracts, spinal cord, and brain white matter.[2][5][10][14][15][19] Anatomically, these structures correspond to UBERON terms such as “cerebral white matter” (UBERON:0002446), “spinal cord” (UBERON:0002240), and “brain” (UBERON:0000955). The pyramidal motor system, including “primary motor cortex” (UBERON:0001384) and “corticospinal tract” (UBERON:0002323), is functionally compromised. Secondary organ involvement includes the cerebellum (UBERON:0002037) and optic nerves (UBERON:0001683) in complex phenotypes.

In CBAS3, the liver (UBERON:0002107) is the primary affected organ, with cholestatic disease and fibrosis. Bile ducts and biliary tree structures are also involved. While CBAS3 is distinct from SPG5A, the shared gene underscores hepatic relevance.

Body systems involved include the nervous system (UBERON:0001016), musculoskeletal system via spasticity and foot deformities, and urinary system via bladder dysfunction. The cardiovascular, respiratory, and endocrine systems are not directly affected in SPG5A, though general comorbidities may exist.

7.2. Tissue and Cell-Level Involvement

At the tissue level, SPG5A affects nervous tissue, particularly white matter tracts composed of myelinated axons, and to a lesser extent grey matter in complex phenotypes.[14][15] The corticospinal tracts traverse the internal capsule, brainstem, and spinal cord, and are composed of axons from pyramidal neurons in layer V of motor cortex. Oligodendrocytes provide myelin sheaths for these axons, and astrocytes support their metabolic and ionic environment.

In CBAS3, hepatocytes and bile duct epithelial cells are affected. Cholestasis leads to bile accumulation and hepatocellular injury.

Cell Ontology mappings include neurons (CL:0000540), pyramidal neurons (CL:0002392), oligodendrocytes (CL:0000129), astrocytes (CL:0000127), hepatocytes (CL:0000182), and biliary epithelial cells (CL:0005014). For SPG5A, upper motor neurons and oligodendrocytes are central to pathology.

7.3. Subcellular Level: Endoplasmic Reticulum and Mitochondria

CYP7B1 localizes to the endoplasmic reticulum (ER) membrane, which is the primary subcellular compartment involved in its function.[16] ER membranes contain the cytochrome P450 machinery, including CYP7B1, which catalyzes oxysterol hydroxylation. GO cellular component terms include “endoplasmic reticulum membrane” (GO:0005789) and “integral component of endoplasmic reticulum membrane” (GO:0030176).

Mitochondria, while not directly hosting CYP7B1, likely participate in downstream oxidative stress responses to oxysterol accumulation. Axonal mitochondria are critical for energy metabolism and axonal transport, and may be damaged by chronic sterol toxicity, leading to impaired ATP production and increased reactive oxygen species.

7.4. Localization and Lateralization

Clinical signs in SPG5A are typically bilateral, affecting both lower limbs symmetrically. This reflects involvement of bilateral corticospinal tracts. There is no consistent lateralization; asymmetry can occur but is not pathognomonic.

Neuroimaging lesions in white matter can be multifocal and bilateral, with variable localization in periventricular or deep white matter regions, sometimes resembling demyelinating plaques. Spinal cord atrophy is generalized, affecting both sides.

For liver disease in CBAS3, involvement is diffuse.

8. Temporal Development

8.1. Onset: Age and Pattern

Age of onset in SPG5A is highly variable, but most patients develop symptoms in childhood or adolescence. Orphanet notes that age of onset ranges from early childhood to adulthood, and classifies onset as “childhood” and “adolescent.”[10] MedLink states that age of onset in SPG5A ranges from early childhood to the fifth decade, indicating that adult-onset cases do occur.[5] The Brain 2017 natural history study reported that SPG5 typically manifests within the first two decades, consistent with a pediatric–adolescent pattern.[2][19]

Onset pattern is usually insidious and chronic rather than acute. Patients may notice subtle gait changes, difficulty running, or increased tripping, which slowly progress over months to years. There is no evidence of rapid acute onset such as that seen in acute myelopathies or demyelinating attacks. For knowledge bases, onset can be annotated with HPO “Childhood onset” (HP:0003593), “Adolescent onset” (HP:0003621), and “Adult onset” (HP:0003581), with insidious onset pattern.

8.2. Progression: Stages and Rate

SPG5A follows a slowly progressive course, with motor impairment gradually worsening over decades.[2][5][10][19] The Brain 2017 study of SPG5 type HSP assessed disease progression using scales such as the Spastic Paraplegia Rating Scale (SPRS) and gait assessments, and found that progression is generally slow but continuous.[2][19] Patients may remain ambulatory for many years, though assistive devices may eventually be needed. The natural history portion of the study also aimed to identify biomarkers to track progression, such as MRI measures and oxysterol levels.[2][19]

Disease staging can be conceptualized in early, intermediate, and advanced phases. Early stage includes mild gait disturbance with minimal functional limitation. Intermediate stage involves clear spastic gait, need for physiotherapy, and possible walking aids. Advanced stage may include wheelchair dependence, severe spasticity, and perhaps complex features such as cerebellar signs and optic atrophy. There is no recognized end-stage of systemic failure; disease is chronic and not typically fatal directly, though severe disability can occur.

Progression rate is slow and variable, with no evidence for rapid acceleration or genetic anticipation. There are no remission phases, and disease is not relapsing–remitting. Duration is lifelong chronic.

8.3. Patterns: Remission, Critical Periods, and Intervention Windows

SPG5A does not exhibit spontaneous remission, and clinical patterns are monotonic progression. Critical periods may include childhood and adolescence, when developmental plasticity and axonal growth might interact with metabolic stress. Early onset may lead to longer cumulative exposure to elevated oxysterols, potentially resulting in more severe axonopathy. Conversely, adult-onset may reflect slower accumulation or partial residual enzyme activity.

Intervention windows are an area of active interest. The atorvastatin trial suggests that early pharmacologic lowering of oxysterols might be more beneficial than later interventions, though evidence is not yet conclusive.[2][12][13][19] There is a theoretical critical period in which preventing or reducing oxysterol accumulation before irreversible axonal degeneration could alter long-term outcomes.

9. Inheritance and Population

9.1. Epidemiology: Prevalence and Incidence

SPG5A is an ultra-rare disease. Orphanet estimates its prevalence at less than one per 1,000,000 individuals.[10] This aligns with the general rarity of individual HSP subtypes, though hereditary spastic paraplegia as a whole has an incidence of around 3.6 per 100,000 individuals, as reported in broader HSP epidemiology.[19] Within autosomal recessive HSP families, SPG5A accounts for approximately 7.3% of cases, and among sporadic pure HSP patients it accounts for around 3.3%, according to the Brain 2009 mutational analysis.[3][5][12] These percentages reflect its relative contribution among genetically unexplained HSPs rather than population-level prevalence.

Incidence data for SPG5A specifically are not available, given the rarity and lack of population-based registries. For disease knowledge bases, SPG5A should be annotated as an ultra-rare autosomal recessive neurodegenerative disorder.

9.2. Inheritance Pattern, Penetrance, and Expressivity

SPG5A follows a biallelic autosomal recessive inheritance pattern.[1][3][4][7][10][11] OMIM and Orphanet both classify SPG5A as autosomal recessive, and PanelApp lists CYP7B1 with mode of inheritance “biallelic, autosomal or pseudoautosomal” for hereditary spastic paraplegia, childhood and adult onset.[1][4][10] Families show multiple affected siblings in consanguineous or non-consanguineous pedigrees, with heterozygous parents unaffected, reflecting classic recessive inheritance.[3][7][11]

Penetrance appears to be high or complete in individuals with biallelic loss-of-function variants, though some variability in age at onset and severity may occur. There are no reports of homozygous or compound heterozygous carriers remaining asymptomatic into late adulthood. Expressivity is variable, ranging from pure spastic paraplegia to complex phenotypes with cerebellar signs, optic atrophy, and cognitive impairment.[1][2][3][5][6][10][12] This variability occurs even among individuals with similar or identical mutations, suggesting that additional genetic or environmental modifiers influence phenotypic expression.

Genetic anticipation has not been described in SPG5A, as the disease is not caused by repeat expansions. Germline mosaicism has not been reported, likely due to the recessive pattern and the small number of families studied.

9.3. Founder Effects, Consanguinity, and Carrier Frequency

Founder effects may exist for specific CYP7B1 mutations in certain populations. For example, variants like R417H and R417C may recur in particular ethnic groups, though detailed population data are sparse.[3][7][9] Many families described in the literature come from regions with higher consanguinity rates, such as parts of the Mediterranean, Middle East, and Asia, suggesting that consanguinity plays a role in increasing the likelihood of homozygous mutations.[1][3][7][9][11] The presence of SPG5A in Japanese patients, as reported by the frameshift variant study, suggests that disease occurs in multiple ethnicities.[9]

Carrier frequency for specific pathogenic CYP7B1 variants is not well quantified but is likely extremely low in the general population, given the rarity of disease. gnomAD may show heterozygous carrier frequencies in the range of 10^-4 to 10^-5 for some truncating alleles, but these data are not explicitly provided. For knowledge bases, carrier frequency should be noted as extremely rare.

9.4. Population Demographics: Geographic Distribution, Sex Ratio, Age Distribution

SPG5A has been reported in families from Europe, Asia, and other regions, indicating a global distribution without clear geographic clustering.[1][2][3][7][9][10][11][12][19] There is no evidence of endemic areas or strong regional variation in prevalence beyond general differences in consanguinity.

Sex ratio appears approximately equal, with both males and females affected, consistent with autosomal inheritance. No sex-specific penetrance differences have been described. Age distribution among affected individuals spans childhood to adulthood, with most cases manifesting in the first two decades.[2][5][6][10][19]

10. Diagnostics

10.1. Clinical Evaluation and Neurologic Examination

Diagnostic evaluation of suspected SPG5A begins with clinical assessment of spastic paraparesis, including detailed neurologic examination. Key features include symmetric lower limb spasticity, hyperreflexia, ankle clonus, and Babinski signs, often with minimal sensory involvement.[2][5][19] Bladder dysfunction and pes cavus support the diagnosis of HSP.[2][5][6][10] Complex features such as cerebellar ataxia, nystagmus, muscle atrophy, optic atrophy, and cognitive impairment suggest a complicated HSP phenotype and broaden the differential.[2][3][5][10][12]

Standardized clinical scales such as the Spastic Paraplegia Rating Scale (SPRS), modified Ashworth scale for spasticity, and gait assessments are used to quantify severity and track progression, as in the Brain 2017 natural history study.[2][19] Differential diagnosis includes other hereditary spastic paraplegias (SPG4, SPG7, SPG11, etc.), multiple sclerosis, leukodystrophies, structural spinal cord lesions, and metabolic myelopathies. Distinguishing features include age of onset, family history, presence of sensory deficits, imaging patterns, and systemic features.

10.2. Laboratory Tests and Biomarkers

Routine laboratory tests (blood counts, basic metabolic panels) are typically normal in SPG5A. However, specific biochemical biomarkers have been identified. The most important is 27-hydroxycholesterol, which is markedly elevated in plasma and CSF of SPG5 patients.[12][13] In the J Lipid Res 2010 study (PMID:19812052), four SPG5 patients had six- to nine-fold increased plasma levels of 27-hydroxycholesterol and thirty- to fifty-fold increases in CSF compared with controls.[13] Plasma 25-hydroxycholesterol was increased about one hundred-fold, while serum bile acids were normal.[13] These findings establish plasma and CSF 27-hydroxycholesterol as potential diagnostic and mechanistic biomarkers for SPG5A. Measurement typically uses liquid chromatography–mass spectrometry.

For CBAS3, abnormal bile acid profiles, conjugated hyperbilirubinemia, and cholestatic liver function tests are diagnostic clues.[11][8] In infants with CBAS3, specialized bile acid analysis revealed elevated atypical bile acids, and genetic testing confirmed CYP7B1 mutations.[11]

At present, 27-hydroxycholesterol and 25-hydroxycholesterol are not widely available clinical tests but may be used in research settings. HPO terms “Abnormal serum oxysterol concentration” could be introduced for SPG5A.

10.3. Neuroimaging Studies

MRI of brain and spinal cord is essential for evaluating HSP, including SPG5A. In SPG5A, MRI may show spinal cord atrophy, particularly in the thoracic region, and cerebellar atrophy in complex patients.[2][5][10][14][15] White matter T2 hyperintensities in cerebral hemispheres, sometimes multifocal and patchy, have been reported, and may resemble MS plaques.[2][5][10][14][15] These findings support a neurodegenerative rather than inflammatory process, but they underscore the need for genetic testing to distinguish SPG5A from demyelinating disorders.

Advanced MRI techniques such as DTI and volumetric analysis can detect subtle microstructural abnormalities in white matter, including reduced fractional anisotropy, increased mean diffusivity, and decreased white matter volume.[14][15] While most data come from SPG4 studies, similar patterns of widespread white matter damage with relatively preserved grey matter in pure forms are expected in SPG5A.[15] Complicated forms may exhibit grey matter volume loss in motor cortex and basal ganglia.[15]

RadLex and DICOM imaging vocabularies would classify these findings under T2-weighted hyperintensities, spinal cord atrophy, and cerebellar atrophy.

10.4. Electrophysiology and Other Functional Tests

Electrophysiologic studies such as nerve conduction and electromyography (EMG) are often normal or show only mild changes in SPG5A, consistent with a central motor neuron lesion rather than peripheral neuropathy.[2][5][19] However, in complex phenotypes with muscle atrophy or possible peripheral involvement, EMG may show chronic denervation. EEG is typically normal.

Functional tests such as urodynamic studies can document neurogenic bladder dysfunction, showing detrusor overactivity or impaired sphincter control.

10.5. Genetic Testing: Single-Gene, Panels, WES, and WGS

Genetic testing is central to diagnosing SPG5A. Targeted sequencing of CYP7B1 can be performed when SPG5A is strongly suspected based on phenotype and imaging. However, because HSP is genetically heterogeneous, gene panels covering multiple HSP genes are commonly used.[4][5][19] PanelApp (Genomics England) includes CYP7B1 in multiple panels: “Hereditary spastic paraplegia, childhood onset,” “Hereditary spastic paraplegia, adult onset,” “Hereditary spastic paraplegia,” among others, reflecting its recognition as a core HSP gene.[4] These panels typically use next-generation sequencing to analyze exons and exon–intron boundaries of tens to hundreds of genes.

Whole exome sequencing (WES) and whole genome sequencing (WGS) have proven particularly useful when panel testing is inconclusive or when atypical phenotypes are present. The Japanese SPG5 case was identified via exome sequencing, which detected a novel frameshift mutation in CYP7B1.[9] Exome or genome sequencing allows detection of rare or novel variants, structural variants, and non-coding changes, though the latter’s pathogenicity may be harder to interpret.

Chromosomal microarray, karyotyping, FISH, mitochondrial DNA testing, and repeat expansion assays are generally not necessary for SPG5A, given its single-gene nuclear autosomal basis. They may be used in differential diagnosis of other conditions.

10.6. Clinical Criteria and Differential Diagnosis

There are no formal DSM-like diagnostic criteria for SPG5A, but clinical diagnostic criteria for HSP typically include progressive spastic paraparesis, family history, exclusion of other causes, and supportive imaging and laboratory findings.[5][19] Molecular confirmation is required for a definitive diagnosis of a specific SPG subtype.[5]

Differential diagnoses include other HSP subtypes (SPG4, SPG7, SPG11, SPG15, SPG21, SPG35, etc.), multiple sclerosis, neuromyelitis optica, leukodystrophies, structural spinal cord lesions, vitamin B12 deficiency, adrenomyeloneuropathy, and inherited metabolic disorders. Distinguishing features involve pattern of sensory dysfunction, presence of inflammation on MRI or CSF, systemic signs, and specific genetic findings. For instance, SPG4 often has pure spastic paraplegia with adult onset; SPG11 shows thin corpus callosum and cognitive impairment; SPG35 and SPG43 can show neurodegeneration with brain iron accumulation (NBIA) phenotype and globus pallidus hypointensity.[14][15]

10.7. Screening

Because SPG5A is ultra-rare and lacks simple biochemical screening tests, population-based screening is not practiced. Carrier screening may be considered in high-risk families or consanguineous populations once a pathogenic CYP7B1 variant is identified. Prenatal diagnosis via chorionic villus sampling or amniocentesis and preimplantation genetic diagnosis (PGD) may be offered to carrier couples. Newborn screening is not currently performed for SPG5A.

11. Outcome / Prognosis

11.1. Survival and Mortality

SPG5A, as a neurologic HSP, is generally not directly life-shortening. Patients usually have normal life expectancy, though severe disability can impact overall health and may increase risk of secondary complications such as falls, fractures, and infections.[2][5][19] There are no cohort studies reporting specific survival rates for SPG5A, but natural history data indicate that disease courses extend over decades.[2][19] Disease-specific mortality is low, and deaths directly attributable to SPG5A are uncommon.

In CBAS3, however, mortality can be high if the disease is not recognized and treated; neonatal cholestasis can progress to liver failure, and survival depends on timely intervention, including possible liver transplantation.[11][8] Thus, for the gene-level entity CYP7B1, prognostic outcomes vary dramatically between neurologic and hepatic phenotypes.

11.2. Morbidity, Disability, and Quality of Life

Morbidity in SPG5A stems from chronic motor disability and secondary complications. Spastic paraparesis limits mobility, leading to difficulty walking, climbing stairs, and performing physically demanding tasks.[2][5][19] Bladder dysfunction and pes cavus add further functional impact. Complex features such as cerebellar ataxia and optic atrophy exacerbate disability, affecting balance and vision.

Quality of life measures in SPG5A have been assessed within the Brain 2017 study using generic instruments like SF-36 and disease-specific scales.[2][19] Domains such as physical functioning, role limitations, and social functioning are impaired. Mental health impact includes depression and anxiety due to chronic disability. However, cognitive impairment in SPG5A is usually mild or absent, except in complex cases.

Disability outcomes include eventual need for walking aids, wheelchair use in advanced stages, and assistance with activities of daily living. The International Classification of Functioning (ICF) would categorize SPG5A as causing moderate to severe impairment in mobility and self-care for many patients.

11.3. Disease Course, Complications, and Recovery Potential

The disease course is chronic and progressive. Complications include joint contractures, scoliosis, falls, and secondary musculoskeletal pain. Neurogenic bladder can lead to urinary tract infections and kidney issues if not managed. There is no spontaneous recovery; disease-modifying therapies are lacking, and current treatments focus on symptom management.

Recovery potential is limited, but supportive therapies can improve function and slow secondary decline. Physiotherapy can maintain muscle strength and joint range of motion. Spasticity management with medications (e.g., baclofen, tizanidine) and intrathecal baclofen pumps can reduce tone and improve mobility. Orthopedic interventions may correct deformities like pes cavus.

11.4. Prognostic Factors and Biomarkers

Prognostic factors in SPG5A include age at onset, baseline severity, presence of complex features, and possibly mutation type. Early onset may predict more prolonged disease and cumulative disability. Nonsense or frameshift mutations might produce more severe phenotypes than milder missense variants, though data are limited.[3][7][9]

27-hydroxycholesterol levels are candidate prognostic biomarkers. Higher baseline levels might correlate with more severe or rapidly progressive disease, though this relationship has not been definitively established.[12][13][19] MRI markers such as spinal cord atrophy and white matter lesion burden may also provide prognostic information regarding future disability.

12. Treatment

12.1. Symptomatic Pharmacotherapy

Current treatment for SPG5A is symptomatic, targeting spasticity, pain, bladder dysfunction, and other manifestations. Spasticity is managed with oral antispastic agents such as baclofen, tizanidine, and benzodiazepines, though these general HSP treatments are not specifically detailed in the provided sources.[5][19] Intrathecal baclofen pumps may be used in severe cases. These interventions correspond to NCIT terms such as “Baclofen therapy” (NCIT:C28913) and “Intrathecal drug administration” (NCIT:C26179).

Bladder dysfunction is treated with anticholinergic medications and other urologic therapies, and catheterization may be needed. Physical therapy and orthotic devices address gait and pes cavus. Pain management may involve analgesics.

12.2. Disease-Modifying Therapeutics: Statins and Beyond

The most notable disease-modifying therapeutic candidate in SPG5A is statin therapy, specifically atorvastatin. The Brain 2017 study conducted a randomized controlled trial in SPG5 patients, using atorvastatin to lower 27-hydroxycholesterol levels.[2][12][13][19] Schöls and colleagues demonstrated that atorvastatin significantly reduced plasma 27-hydroxycholesterol and 25-hydroxycholesterol, confirming that systemic cholesterol-lowering can modulate oxysterol accumulation in CYP7B1-deficient patients.[12][13] The trial, however, did not show a strong short-term improvement in motor outcomes, though it provided crucial biomarker and safety data.[2][19]

The SP Foundation update notes that statin therapy has been shown to reduce 27-hydroxycholesterol levels in SPG5A, and that this is a potential therapy warranting further investigation.[12] NCIT terms for “Atorvastatin” (NCIT:C24478) and “HMG-CoA reductase inhibitor” (NCIT:C281) are appropriate for mapping

Reference Validation

Checked with linkml-reference-validator 0.2.1.

All extracted references resolved successfully.

Term Validation

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

Terms the report names something else

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

• MONDO:0010047 (3 mentions) - the report calls it "if available"; MONDO calls it hereditary spastic paraplegia 5A
• MeSH:C564811 (1 mention) - the report calls it "hereditary spastic paraplegia general category"; MeSH calls it bevonium
• HP:0001251 (2 mentions) - the report calls it "Spastic paraplegia"; HP calls it Ataxia
• HP:0000025 (1 mention) - the report calls it "Urinary urgency"; HP calls it Functional abnormality of male internal genitalia
• HP:0000013 (1 mention) - the report calls it "Neurogenic bladder"; HP calls it Hypoplasia of the uterus
• HP:0006865 (1 mention) - the report calls it "Spinal cord atrophy"; HP calls it Sensorimotor polyneuropathy affecting arms more than legs
• GO:0030425 (1 mention) - the report calls it "axon degeneration"; GO calls it dendrite
• GO:0032286 (1 mention) - the report calls it "maintenance of myelin sheath"; GO calls it central nervous system myelin maintenance
• CL:0002392 (2 mentions) - the report calls it "pyramidal neuron"; CL calls it obsolete plant spore
• GO:0043218 (1 mention) - the report calls it "myelin sheath"; GO calls it compact myelin
• UBERON:0002446 (1 mention) - the report calls it "cerebral white matter"; UBERON calls it patella
• UBERON:0002323 (1 mention) - the report calls it "corticospinal tract"; UBERON calls it coelemic cavity lumen
• NCIT:C28913 (1 mention) - the report calls it "Baclofen therapy"; NCIT calls it Cefazolin
• NCIT:C26179 (1 mention) - the report calls it "Intrathecal drug administration"; NCIT calls it Epidermal Growth Factor Receptor Pathway Substrate 15
• NCIT:C24478 (1 mention) - the report calls it "Atorvastatin"; NCIT calls it HSD17B7 Gene
• NCIT:C281 (1 mention) - the report calls it "HMG-CoA reductase inhibitor"; NCIT calls it Antiviral Agent

Unresolved terms

These identifiers do not exist in an ontology that resolved other terms from the same prefix, so they were most likely invented:

• HP:0002499 (1 mention), reported as "White matter lesions" - HP does not contain this term

Obsolete terms

These terms are real but deprecated. Citing one is not a fabrication; it does mean the report is naming something the ontology has retired:

• CL:0002392 (obsolete plant spore) (2 mentions) - replaced by PO:0025017
• GO:0030176 (obsolete integral component of endoplasmic reticulum membrane) (1 mention) - replaced by GO:0005789

Terms whose name is worth a second look

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

• GO:0008395 (2 mentions) - the report calls it "25-hydroxycholesterol 7-alpha-hydroxylase activity"; GO calls it steroid hydroxylase activity
• HP:0002500 (1 mention) - the report calls it "Abnormality of cerebral white matter"; HP calls it Abnormal cerebral white matter morphology, and lists "Abnormality of the cerebral white matter" among its other names
• HP:0006579 (1 mention) - the report calls it "Neonatal jaundice"; HP calls it Prolonged neonatal jaundice, and lists "Jaundice, neonatal" among its other names
• GO:0050804 (1 mention) - the report calls it "regulation of synaptic transmission"; GO calls it modulation of chemical synaptic transmission, and lists "regulation of synaptic transmission" among its other names
• CHEBI:27771 (1 mention) - the report calls it "27-hydroxycholesterol"; CHEBI calls it 16alpha-hydroxydehydroepiandrosterone
• GO:0030176 (1 mention) - the report calls it "integral component of endoplasmic reticulum membrane"; GO calls it obsolete integral component of endoplasmic reticulum membrane
• HP:0003593 (1 mention) - the report calls it "Childhood onset"; HP calls it Infantile onset
• HP:0003621 (1 mention) - the report calls it "Adolescent onset"; HP calls it Juvenile onset

Prefixes with no resolver

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