Siddiqi Syndrome

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Inheritance

1

Autosomal recessive inheritance HP:0000007

Homozygous in the consanguineous founding family; compound heterozygous genotypes have since been reported, so consanguinity is the usual route rather than a requirement.

Autosomal recessive inheritance

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Discussions and Knowledge Gaps

2

Given that FITM2 loss demonstrably causes ER stress and organ injury in mouse liver and lipodystrophy in mouse adipose tissue, why is the human disease a deafness-dystonia syndrome with neither hepatic nor adipose involvement?

KNOWLEDGE GAP siddiqi_tissue_selectivity

Attached to

pathophysiology#Injury to Post-Mitotic Metabolically Active Cells phenotypes#Absence of Lipodystrophy

This gap was originally written as "is ER stress the mediator at all", which the cited evidence no longer leaves open. Hepatocyte-specific knockout mice show the full chain in vivo: elevated acyl-CoA, ER stress, liver injury. The mechanism is not the question. What remains open is selectivity, and the mouse data make it sharper rather than softer. Tissue-specific knockouts produce liver injury and lipodystrophy - two phenotypes the human disease does not have - while the human phenotype falls on cochlea, neurons and skin, which no cited knockout has targeted. So the tissues that fail in mouse and in human are close to disjoint. Two readings are consistent with that and are not distinguished by anything cited here: the human tissue pattern reflects a threshold effect, with complete loss everywhere sparing tissues that a tissue-specific knockout destroys because compensation is possible when only one organ is affected; or FITM2 has a cochlear and neuronal requirement that the mouse work has simply never probed. The absent lipodystrophy is the strongest constraint on any answer, because it is the one tissue where mouse and human have been directly compared and they disagree.

Is the FITM2 phenotype a dosage continuum, with near-complete loss giving Siddiqi syndrome and partial loss giving hereditary spastic paraplegia?

KNOWLEDGE GAP siddiqi_allelic_dosage_series

Attached to

pathophysiology#Loss of FITM2 Acyl-CoA Diphosphatase Activity genetic#FITM2

Compound heterozygotes carrying one null and one G100R hypomorphic allele, with FIT2 protein at 20% of wild type, have hereditary spastic paraplegia rather than Siddiqi syndrome. That is a different disease from the same gene at a different residual activity, and it suggests the two are points on one dosage curve rather than separate entities. The entry does not assert that continuum, because the alternative - that G100R has a qualitative property beyond reduced abundance - is not excluded by the cited work, and because two families is a thin basis for a genotype-phenotype rule. It is recorded as an open question with a concrete discriminator: whether other hypomorphic alleles at comparable residual activity also give spastic paraplegia rather than deafness and dystonia. This bears directly on scope. If it is a continuum, the FITM2 spastic paraplegia phenotype belongs in this knowledge base as a related entry rather than as a subtype here, and the boundary between them is a quantitative one.

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Pathophysiology

4

Loss of FITM2 Acyl-CoA Diphosphatase Activity

Biallelic loss-of-function variants remove the ER-lumenal fatty acyl-CoA diphosphatase activity of FITM2. This function was unknown when the disease was described in 2017 and was identified in 2020, which is why the earliest mechanistic accounts of this syndrome describe a lipid-droplet protein rather than an enzyme.

FITM2 hgnc:16135

coenzyme A diphosphatase activity GO:0010945 ↓ DECREASED

Downstream

Unbuffered Fatty Acyl-CoA and Failed Lipid Droplet Partitioning

Show evidence (2 references)

PMID:32915949 SUPPORT In Vitro

"the molecular function of the evolutionarily conserved ER protein FIT2 as a fatty acyl-coenzyme A (CoA) diphosphatase that hydrolyzes fatty acyl-CoA to yield acyl 4'-phosphopantetheine"

Identifies the enzymatic activity this node claims is lost. FIT2 is the alternative name for FITM2 used throughout that literature.

PMID:28067622 SUPPORT Human Clinical

"a homozygous nonsense mutation, c.4G>T (p.Glu2*), in FITM2 was identified"

The founding allele. A nonsense change at codon 2 is about as close to a null as a point mutation reaches, which is what makes the entry's loss-of-function framing safe.

Unbuffered Fatty Acyl-CoA and Failed Lipid Droplet Partitioning

Without the diphosphatase, fatty acyl-CoA is not cleared in the ER lumen and triglyceride is not partitioned normally into cytosolic lipid droplets.

lipid droplet organization GO:0034389 ↓ DECREASED

Downstream

ER Structural Disruption and ER Stress

Show evidence (2 references)

PMID:28067622 SUPPORT Other

"FITM2 and its paralog FITM1 constitute an evolutionary conserved protein family involved in partitioning of triglycerides into cellular lipid droplets"

States the lipid-droplet function whose failure this node represents. Graded OTHER because the sentence is background rather than a result of the study reporting it.

PMID:30020828 SUPPORT Model Organism

"Mice with adipose tissue (AT)-specific FIT2 knockout exhibited lipodystrophic features, with reduced AT mass, insulin resistance, and greater inflammation in AT when fed a high-fat diet"

Demonstrates that removing FIT2 does disrupt triglyceride partitioning in vivo, in the tissue built to store it. The node is evidenced by the mouse rather than by the patients precisely because the patients show no lipodystrophy - the dissociation argued in the siddiqi_tissue_selectivity discussion. Placed on the mechanism rather than on the phenotype record so that a query over phenotype terms is not told that model-organism evidence supports lipodystrophy in this disease.

ER Structural Disruption and ER Stress

Maintaining optimal fatty acyl-CoA levels is what keeps the ER intact. With the diphosphatase gone, ER structure is not maintained and the cell is not protected against ER stress.

response to endoplasmic reticulum stress GO:0034976 ↑ INCREASED

Downstream

Injury to Post-Mitotic Metabolically Active Cells

Show evidence (4 references)

PMID:32915949 SUPPORT In Vitro

"is required in yeast and mammalian cells for maintaining ER structure, protecting against ER stress, and enabling normal lipid storage in lipid droplets"

Names all three consequences of losing the activity, in cells rather than in patients - which is why this node and the one above are graded IN_VITRO.

PMID:32915949 SUPPORT In Vitro

"highlight the maintenance of optimal fatty acyl-CoA levels as key to ER homeostasis"

States the causal direction the previous node feeds into: the acyl-CoA level is the variable, ER homeostasis the outcome.

PMID:36805337 SUPPORT Model Organism

"hepatocyte-specific Fitm2 knockout (FIT2-LKO) mice fed a chow diet exhibited elevated acyl-CoA levels, ER stress, and signs of liver injury"

In vivo mammalian evidence that the whole chain of this node operates in a real tissue - acyl-CoA rises, ER stress follows, the organ is injured. It is a mouse liver rather than a human cochlea, which is what the knowledge gap is now about; before this citation the node rested on cell culture alone.

+ 1 more reference

Injury to Post-Mitotic Metabolically Active Cells

The tissues that fail are the ones that cannot dilute or replace damaged ER by dividing, and that carry high metabolic flux: cochlear hair cells, central and peripheral neurons, and epidermal keratinocytes. This node is an interpretation and is graded to say so. No cited source demonstrates ER stress in patient cochlea, neurons or skin. It is the account that makes the tissue distribution intelligible, and the alternative - that these tissues are affected for an unrelated FITM2-dependent reason - is not excluded by anything cited here. The knowledge gap below records that.

sensory hair cell CL:0000855 keratinocyte CL:0000312

Animal models

Drosophila Fitm (CG10671) RNAi knockdown Fruit fly recapitulates Knockdown reproduces the locomotor, auditory and sensory components of the human syndrome, and was used by the founding study as its independent evidence of causality.

Downstream

Sensorineural Hearing Impairment

Dystonia

Developmental Regression

Ichthyosis

Sensory Neuropathy

Intellectual Disability

Global Developmental Delay

Growth Delay

Show evidence (1 reference)

PMID:28067622 SUPPORT INDIRECT Human Clinical

"a novel deafness-dystonia syndrome with motor regression, ichthyosis-like features and signs of sensory neuropathy"

Establishes which tissues are clinically affected, which is what the node's edges assert. Graded INDIRECT because it does not establish the mechanism the node's description offers for why those tissues.

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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

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Phenotypes

9

Ear 1

Sensorineural Hearing Impairment HP:0000407

Course: PROGRESSIVE

Show evidence (1 reference)

PMID:28067622 SUPPORT Human Clinical

"a novel deafness-dystonia syndrome with motor regression"

Names deafness as one of the two defining features.

Integument 1

Ichthyosis HP:0008064

Show evidence (1 reference)

PMID:28067622 SUPPORT Human Clinical

"ichthyosis-like features and signs of sensory neuropathy"

Graded PARTIAL because the source says "ichthyosis-like" and this entry binds the unhedged term. The hedge is the authors' and is worth preserving rather than promoting to a diagnosis.

Musculoskeletal 1

Absence of Lipodystrophy HP:0009125

Show evidence (1 reference)

PMID:28067622 REFUTE Human Clinical

"Despite the role of FITM2 in neutral lipid storage and metabolism, no indications for lipodystrophy were observed in the affected individuals"

Graded REFUTE against the presence of lipodystrophy. The authors frame it as a contrast with the expectation, which is why it is worth a phenotype record rather than a line of prose.

Nervous System 5

Dystonia HP:0001332

Show evidence (1 reference)

PMID:28067622 SUPPORT Human Clinical

"a novel deafness-dystonia syndrome with motor regression"

Names dystonia as the other defining feature.

Developmental Regression HP:0002376

Show evidence (1 reference)

PMID:41113320 SUPPORT Human Clinical

"generalized dystonia, motor regression, growth failure, and ichthyosis"

Names motor regression among the core features of the syndrome, in a sentence describing the disorder rather than in the title of the paper that named it.

Global Developmental Delay HP:0001263

Show evidence (1 reference)

PMID:41113320 SUPPORT Human Clinical

"combination of sensorineural deafness, progressive dystonia, global developmental delay, ichthyosis, and normal imaging"

Names global developmental delay as one of the features that together identified the syndrome in these siblings.

Intellectual Disability HP:0001249

Show evidence (1 reference)

PMID:41113320 SUPPORT Human Clinical

"often accompanied by additional manifestations such as intellectual disability and peripheral neuropathy"

Names intellectual disability among the manifestations accompanying the core features. The source hedges with "often", which is why no frequency band is recorded.

Sensory Neuropathy HP:0000763

Show evidence (1 reference)

PMID:28067622 SUPPORT Human Clinical

"ichthyosis-like features and signs of sensory neuropathy"

Graded PARTIAL for the same reason as ichthyosis - the source reports "signs of" sensory neuropathy rather than a confirmed diagnosis.

Growth 1

Growth Delay HP:0001510

Show evidence (1 reference)

PMID:41113320 SUPPORT Human Clinical

"Both presented with hallmark features, including early-onset sensorineural hearing loss, severe generalized dystonia, growth failure, and ichthyosis of the lower limbs"

Names growth failure directly. This replaces an earlier citation to the ascertainment sentence, which said nothing about growth - a PARTIAL grade does not repair a snippet that is about something else.

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Genetic Associations

1

FITM2

Gene: FITM2 hgnc:16135 relationship_type: CAUSATIVE variant_origin: GERMLINE

Autosomal recessive inheritance

Show evidence (3 references)

PMID:28067622 SUPPORT Human Clinical

"a homozygous nonsense mutation, c.4G>T (p.Glu2*), in FITM2 was identified"

The founding allele and its consequence.

PMID:39974099 SUPPORT Human Clinical

"In humans, homozygous null mutations in FITM2 are associated with a syndrome characterized by deafness and dystonia"

Confirms the genotype class for this entry's disease from outside the founding report, and does so in a paper describing a different FITM2 phenotype - which is what makes it useful for the boundary rather than only for the diagnosis.

PMID:39974099 SUPPORT Human Clinical

"the G100R allele is hypomorphic, with FIT2 protein levels reduced to 20% of wild type, leading to proportionately decreased enzyme activity"

Quantifies the residual activity that produces spastic paraplegia instead of this disease. Graded PARTIAL because it characterises a genotype that causes a different disorder - it bounds this entry's genotype class from outside rather than evidencing it.

Variants (2)

FITM2 alleles reported across the described families Pathogenic

Mixed - nonsense, frameshift and missense

The reported allele catalog, kept as one record because the families are individually single cases and the genotype-phenotype claim below is made across them rather than within any one. Pakistani founding family: homozygous c.4G>T (p.Glu2*). United States: compound heterozygous c.39dupC and c.652C>T. Germany: homozygous c.694G>A (p.Gly232Arg). China: homozygous c.611_612dupTG (p.Met205fs). Russia: homozygous c.452A>G (p.Asp151Gly). Iran: homozygous c.114dupC (p.Lys39Glnfs*113). This record is also the source for the compound-heterozygote claim made in the notes above and in the inheritance block, which was previously uncited.

Show evidence (2 references)

PMID:41113320 SUPPORT Human Clinical

"The American case involved compound heterozygous loss‐of‐function variants (c.39dupC and c.652C>T). A German patient had a homozygous missense variant (c.694G>A, p.Gly232Arg), while the Chinese patient harbored a homozygous frameshift variant (c.611_612dupTG, p.Met205fs)."

The allele catalog across the previously reported families, including the compound heterozygous genotype.

PMID:41113320 SUPPORT Human Clinical

"A novel homozygous frameshift variant was identified in the FITM2 gene: c.114dupC (p.Lys39Glnfs*113)"

The Iranian allele reported by this paper.

Genotype-phenotype gradient across truncating and missense alleles

Observation across allele classes

A severity gradient is proposed rather than demonstrated: truncating alleles with more severe disease, missense alleles milder. The authors state the limitation themselves, and with one or two families per allele class it is a pattern in a case-report series rather than a correlation. It is recorded because it is the same axis as the G100R hypomorph in the notes above - residual FIT2 activity - approached from within this disease instead of from the spastic paraplegia end.

Show evidence (1 reference)

PMID:41113320 SUPPORT Human Clinical

"In contrast, patients harboring missense variants (e.g., p.Asp151Gly and p.Gly232Arg) may exhibit relatively milder phenotypes, although data remain limited"

Graded PARTIAL on the authors' own hedge. "May exhibit" and "data remain limited" are theirs, and are preserved rather than promoted into an asserted correlation.

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External Assertions

1

OMIM Siddiqi syndrome record

OMIM disease record OMIM:618635

The OMIM record for the disease. Recorded here because DiseaseMappings carries no OMIM slot.

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Medical Actions

2

Supportive Management

Action: supportive care Ontology label: Supportive Care NCIT:C15747

There is no disease-modifying therapy. Reported management is supportive and multidisciplinary: nutritional rehabilitation for the growth failure, physiotherapy for the dystonia and contractures, dermatologic care for the ichthyosis, and audiologic follow-up.

Show evidence (1 reference)

PMID:41113320 SUPPORT Human Clinical

"Management for both siblings remained supportive, focusing on nutritional rehabilitation, physiotherapy, dermatologic care, and audiologic evaluation."

The management actually given, in the only report that states one.

Cochlear Implantation

Action: cochlear implantation Ontology label: Surgical Procedure NCIT:C15329

Named by the same authors as an early intervention that may improve function, alongside pharmacologic and rehabilitative support for the dystonia. It is proposed rather than reported: neither sibling in that report is described as having received an implant, and no outcome is given. The entry records it as a suggested intervention, not an established one.

Show evidence (1 reference)

PMID:41113320 SUPPORT Human Clinical

"cochlear implantation, pharmacologic and rehabilitative support for dystonia, and nutritional management"

Graded PARTIAL because the authors write that such interventions "may improve functional outcomes", with no patient in the report having received one. That hedge is theirs and is preserved.

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Diagnosis

2

Molecular Diagnosis by Exome Sequencing

Diagnosis is molecular. There is no biochemical marker and no imaging finding that identifies the disease - MRI is characteristically normal, which is itself part of the diagnostic argument because it excludes the leukodystrophies and neurodegeneration with brain iron accumulation that the dystonia would otherwise suggest. Exome sequencing was first-tier in the most recently reported family, with Sanger confirmation of segregation.

whole exome sequencing NCIT:C101295

Show evidence (2 references)

PMID:41113320 SUPPORT Human Clinical

"Sanger sequencing confirmed homozygosity in both affected siblings and heterozygosity in both parents, consistent with autosomal recessive inheritance"

The confirmatory step after exome sequencing, and the evidence that the genotype segregates with the phenotype in the family.

PMID:41113320 SUPPORT Human Clinical

"Brain MRI was normal, showing no structural or white matter abnormalities"

A normal MRI in a child with progressive dystonia is a discriminating negative, not an absence of information.

Auditory Brainstem Response Testing

The hearing loss is severe, bilateral, sensorineural and present in infancy, before behavioural audiometry is informative, so it is established by evoked-response testing.

auditory brainstem response test NCIT:C184949

Show evidence (1 reference)

PMID:41113320 SUPPORT Human Clinical

"ABR testing confirmed severe bilateral sensorineural hearing loss"

The test used to establish the auditory phenotype in both siblings.

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Prevalence

1

Published cases worldwide

Cases In Literature Ultra Rare

Defined in 2017 in a single consanguineous Pakistani family, with a small number of additional families reported since. No point prevalence has been estimated. Recorded as a case count rather than a rate because there is no denominator.

Show evidence (1 reference)

PMID:28067622 SUPPORT Human Clinical

"A consanguineous family from Pakistan was ascertained to have a novel deafness-dystonia syndrome"

The founding report, establishing that the disease entered the literature as a single family.

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Differential Diagnoses

4

Conditions with similar clinical presentations that must be differentiated from Siddiqi Syndrome:

Dyskinetic cerebral palsy

Overlapping Features The first consideration when perinatal complications are present, and the one the diagnosis actually turns on: cerebral palsy is static, and this disease is progressive. In the Iranian family the older sibling had a complicated delivery and the younger did not, yet both have the same progressive course - which is what argued against a static encephalopathy.

Distinguishing Features

Progressive rather than static course
Normal brain MRI
Recurrence in a sibling with an uneventful perinatal history

Neurodegeneration with brain iron accumulation

Overlapping Features Considered on the early-onset dystonia. Excluded on imaging.

Distinguishing Features

Basal ganglia iron deposition on MRI, absent here

Show evidence (1 reference)

PMID:41113320 SUPPORT Human Clinical

"NBIA was considered given the presence of early"

Records that the differential was actively considered rather than overlooked. The exclusion is on the absence of basal ganglia iron.

Syndromic ichthyoses

Overlapping Features Refsum disease, Sjogren-Larsson syndrome and Chanarin-Dorfman syndrome all pair ichthyosis with neurological disease, which is the combination that makes this entry's disease recognisable in the first place.

Distinguishing Features

No retinitis pigmentosa
No spasticity or leukoencephalopathy
No hepatic involvement or specific biochemical marker

Leukodystrophies and mitochondrial disorders

Overlapping Features Metachromatic leukodystrophy and Pelizaeus-Merzbacher disease are considered on the hypotonia and motor regression; metabolic disorders on the developmental failure. All are excluded by normal imaging and normal biochemistry.

Distinguishing Features

No white matter abnormality
Normal metabolic screen

Show evidence (1 reference)

PMID:41113320 SUPPORT Human Clinical

"Leukodystrophies, mitochondrial disorders, and NBIA were excluded based on imaging, biochemical screening, and clinical presentation"

States the exclusions and the basis for each.

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Animal Models

1

Drosophila Fitm (CG10671) RNAi knockdown

The fly has one Fitm ortholog where mammals have two paralogs, which makes a single knockdown a cleaner test of the pathway than it would be in mouse.

Species

Fruit fly

Genotype

RNAi knockdown of CG10671, the single Fitm ortholog

Publication

PMID:28067622

Pathograph links

Injury to Post-Mitotic Metabolically Active Cells recapitulates fidelity: moderate Knockdown reproduces the locomotor, auditory and sensory components of the human syndrome, and was used by the founding study as its independent evidence of causality.

Limitations: Fidelity is MODERATE rather than HIGH for reasons worth naming. It is a knockdown rather than a null, so residual activity is likely. Fly auditory and sensory organs are not homologous to the mammalian cochlea or dorsal root ganglion, so "hearing loss" and "disturbed sensory function" are analogous rather than equivalent readouts. And the model was not used to test the ER-stress mechanism this entry proposes - it establishes that the gene matters, not why.

Show evidence (1 reference)

PMID:28067622 SUPPORT Model Organism

"Characteristics of the syndrome, including progressive locomotor impairment, hearing loss and disturbed sensory functions, were recapitulated in Drosophila"

Names the three recapitulated features that justify the RECAPITULATES relationship.

{ }

Source YAML

click to show

name: Siddiqi Syndrome
creation_date: "2026-08-28T19:45:00Z"
category: Mendelian
description: >-
  Siddiqi syndrome is an autosomal recessive deafness-dystonia syndrome caused
  by biallelic loss-of-function variants in FITM2. FITM2 is an
  endoplasmic-reticulum membrane protein whose molecular function was resolved
  only in 2020: it is a fatty acyl-CoA diphosphatase, hydrolysing fatty acyl-CoA
  in the ER lumen, and that activity is what allows triglyceride to be
  partitioned into cytosolic lipid droplets. Losing it leaves fatty acyl-CoA
  unbuffered, disrupts ER structure and provokes ER stress.

  The clinical picture is progressive sensorineural hearing loss with generalized
  dystonia, developmental delay followed by motor regression, and growth failure,
  with ichthyosis of the lower limbs and signs of sensory neuropathy in some
  patients. The tissues that fail are metabolically active and post-mitotic -
  cochlear hair cells, neurons, keratinocytes - which is the pattern an ER
  lipid-handling defect would be expected to produce.

  What the disease is not is as informative as what it is. FITM2 partitions
  triglyceride into lipid droplets, so the naive prediction from the gene is a
  lipodystrophy. The founding report looked and found none.
parents:
  - Inborn Error of Metabolism
synonyms:
  - deafness-dystonia syndrome, FITM2-related
  - FITM2-related deafness-dystonia syndrome
  - SIDDIQI
disease_term:
  preferred_term: Siddiqi syndrome
  term:
    id: MONDO:0032842
    label: Siddiqi syndrome
notes: >-
  Evidence-base caveat. The syndrome was defined in 2017 in one consanguineous
  Pakistani family, with a small number of further families reported since. There
  is no cohort and no natural-history series, so no phenotype in this entry
  carries a frequency value. The absence of a denominator is the reason.

  Gene identifier correction, and this is the second time in this batch. The
  deep-research report committed alongside gives the causal gene as HGNC:14929
  in three places, including its header and its summary table. HGNC:14929 is
  SIRT1 - a sirtuin deacetylase with no relation to lipid-droplet biology.
  FITM2 is hgnc:16135, which is what this entry uses.

  The detail that makes this worth recording rather than silently fixing: the
  three other identifiers on the same line of the report - OMIM 612029, NCBI
  Gene 128486, UniProt Q8N6M3 - are all correct for FITM2. A wrong HGNC ID sat
  between correct identifiers for the right gene, which is exactly the
  configuration in which a reader stops checking. The companion SPG81 entry in
  this same batch carries the same class of error with a different gene, so a
  curator should treat HGNC identifiers in these reports as unverified regardless
  of how well-formed the surrounding record looks.

  Treatments. No disease-modifying therapy exists. The treatments section
  records the supportive management actually given in the Iranian family, plus
  cochlear implantation, which those authors propose rather than report - graded
  and described to keep the two apart. An earlier revision of this entry omitted
  the section on the grounds that no cited source stated a protocol; that was
  true of the two references the entry then had and stopped being true once
  PMID:41113320 was fetched.
external_assertions:
  - name: OMIM Siddiqi syndrome record
    source: OMIM
    assertion_type: disease_record
    external_id: OMIM:618635
    url: https://omim.org/entry/618635
    description: >-
      The OMIM record for the disease. Recorded here because DiseaseMappings
      carries no OMIM slot.
inheritance:
  - name: Autosomal recessive inheritance
    inheritance_term:
      preferred_term: Autosomal recessive inheritance
      term:
        id: HP:0000007
        label: Autosomal recessive inheritance
    description: >-
      Homozygous in the consanguineous founding family; compound heterozygous
      genotypes have since been reported, so consanguinity is the usual route
      rather than a requirement.
pathophysiology:
  - name: Loss of FITM2 Acyl-CoA Diphosphatase Activity
    biological_scale: MOLECULAR
    description: >-
      Biallelic loss-of-function variants remove the ER-lumenal fatty acyl-CoA
      diphosphatase activity of FITM2. This function was unknown when the disease
      was described in 2017 and was identified in 2020, which is why the earliest
      mechanistic accounts of this syndrome describe a lipid-droplet protein
      rather than an enzyme.
    genes:
      - preferred_term: FITM2
        term:
          id: hgnc:16135
          label: FITM2
    molecular_functions:
      - preferred_term: coenzyme A diphosphatase activity
        term:
          id: GO:0010945
          label: coenzyme A diphosphatase activity
        modifier: DECREASED
    downstream:
      - target: Unbuffered Fatty Acyl-CoA and Failed Lipid Droplet Partitioning
    evidence:
      - reference: PMID:32915949
        reference_title: "FIT2 is an acyl-coenzyme A diphosphatase crucial for endoplasmic reticulum homeostasis."
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: >-
          the molecular function of the evolutionarily conserved ER protein FIT2
          as a fatty acyl-coenzyme A (CoA) diphosphatase that hydrolyzes fatty
          acyl-CoA to yield acyl 4'-phosphopantetheine
        explanation: >-
          Identifies the enzymatic activity this node claims is lost. FIT2 is
          the alternative name for FITM2 used throughout that literature.
      - reference: PMID:28067622
        reference_title: "A homozygous FITM2 mutation causes a deafness-dystonia syndrome with motor regression and signs of ichthyosis and sensory neuropathy."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          a homozygous nonsense mutation, c.4G>T (p.Glu2*), in FITM2 was
          identified
        explanation: >-
          The founding allele. A nonsense change at codon 2 is about as close to
          a null as a point mutation reaches, which is what makes the entry's
          loss-of-function framing safe.
  - name: Unbuffered Fatty Acyl-CoA and Failed Lipid Droplet Partitioning
    biological_scale: CELLULAR
    description: >-
      Without the diphosphatase, fatty acyl-CoA is not cleared in the ER lumen
      and triglyceride is not partitioned normally into cytosolic lipid droplets.
    biological_processes:
      - preferred_term: lipid droplet organization
        term:
          id: GO:0034389
          label: lipid droplet organization
        modifier: DECREASED
    downstream:
      - target: ER Structural Disruption and ER Stress
    evidence:
      - reference: PMID:28067622
        reference_title: "A homozygous FITM2 mutation causes a deafness-dystonia syndrome with motor regression and signs of ichthyosis and sensory neuropathy."
        supports: SUPPORT
        evidence_source: OTHER
        snippet: >-
          FITM2 and its paralog FITM1 constitute an evolutionary conserved
          protein family involved in partitioning of triglycerides into cellular
          lipid droplets
        explanation: >-
          States the lipid-droplet function whose failure this node represents.
          Graded OTHER because the sentence is background rather than a result
          of the study reporting it.
      - reference: PMID:30020828
        reference_title: "Fat storage-inducing transmembrane protein 2 (FIT2) is less abundant in type 2 diabetes, and regulates triglyceride accumulation and insulin sensitivity in adipocytes."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          Mice with adipose tissue (AT)-specific FIT2 knockout exhibited
          lipodystrophic features, with reduced AT mass, insulin resistance, and
          greater inflammation in AT when fed a high-fat diet
        explanation: >-
          Demonstrates that removing FIT2 does disrupt triglyceride partitioning
          in vivo, in the tissue built to store it. The node is evidenced by the
          mouse rather than by the patients precisely because the patients show
          no lipodystrophy - the dissociation argued in the
          siddiqi_tissue_selectivity discussion. Placed on the mechanism rather
          than on the phenotype record so that a query over phenotype terms is
          not told that model-organism evidence supports lipodystrophy in this
          disease.
  - name: ER Structural Disruption and ER Stress
    biological_scale: CELLULAR
    description: >-
      Maintaining optimal fatty acyl-CoA levels is what keeps the ER intact.
      With the diphosphatase gone, ER structure is not maintained and the cell
      is not protected against ER stress.
    biological_processes:
      - preferred_term: response to endoplasmic reticulum stress
        term:
          id: GO:0034976
          label: response to endoplasmic reticulum stress
        modifier: INCREASED
    downstream:
      - target: Injury to Post-Mitotic Metabolically Active Cells
    evidence:
      - reference: PMID:32915949
        reference_title: "FIT2 is an acyl-coenzyme A diphosphatase crucial for endoplasmic reticulum homeostasis."
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: >-
          is required in yeast and mammalian cells for maintaining ER structure,
          protecting against ER stress, and enabling normal lipid storage in
          lipid droplets
        explanation: >-
          Names all three consequences of losing the activity, in cells rather
          than in patients - which is why this node and the one above are graded
          IN_VITRO.
      - reference: PMID:32915949
        reference_title: "FIT2 is an acyl-coenzyme A diphosphatase crucial for endoplasmic reticulum homeostasis."
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: >-
          highlight the maintenance of optimal fatty acyl-CoA levels as key to
          ER homeostasis
        explanation: >-
          States the causal direction the previous node feeds into: the acyl-CoA
          level is the variable, ER homeostasis the outcome.
      - reference: PMID:36805337
        reference_title: "Fitm2 is required for ER homeostasis and normal function of murine liver."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          hepatocyte-specific Fitm2 knockout (FIT2-LKO) mice fed a chow diet
          exhibited elevated acyl-CoA levels, ER stress, and signs of liver
          injury
        explanation: >-
          In vivo mammalian evidence that the whole chain of this node operates
          in a real tissue - acyl-CoA rises, ER stress follows, the organ is
          injured. It is a mouse liver rather than a human cochlea, which is
          what the knowledge gap is now about; before this citation the node
          rested on cell culture alone.
      - reference: PMID:36805337
        reference_title: "Fitm2 is required for ER homeostasis and normal function of murine liver."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          Our findings support the model that FIT2 acts as an acyl-CoA
          diphosphatase in vivo
        explanation: >-
          Confirms the enzymatic function in an animal rather than only in a
          test tube, which matters because the 2020 identification was
          biochemical.
  - name: Injury to Post-Mitotic Metabolically Active Cells
    biological_scale: TISSUE
    description: >-
      The tissues that fail are the ones that cannot dilute or replace damaged
      ER by dividing, and that carry high metabolic flux: cochlear hair cells,
      central and peripheral neurons, and epidermal keratinocytes.

      This node is an interpretation and is graded to say so. No cited source
      demonstrates ER stress in patient cochlea, neurons or skin. It is the
      account that makes the tissue distribution intelligible, and the
      alternative - that these tissues are affected for an unrelated
      FITM2-dependent reason - is not excluded by anything cited here. The
      knowledge gap below records that.
    cell_types:
      - preferred_term: sensory hair cell
        term:
          id: CL:0000855
          label: sensory hair cell
      - preferred_term: keratinocyte
        term:
          id: CL:0000312
          label: keratinocyte
    downstream:
      - target: Sensorineural Hearing Impairment
      - target: Dystonia
      - target: Developmental Regression
      - target: Ichthyosis
      - target: Sensory Neuropathy
      - target: Intellectual Disability
      - target: Global Developmental Delay
      - target: Growth Delay
    evidence:
      - reference: PMID:28067622
        reference_title: "A homozygous FITM2 mutation causes a deafness-dystonia syndrome with motor regression and signs of ichthyosis and sensory neuropathy."
        supports: SUPPORT
        directness: INDIRECT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          a novel deafness-dystonia syndrome with motor regression,
          ichthyosis-like features and signs of sensory neuropathy
        explanation: >-
          Establishes which tissues are clinically affected, which is what the
          node's edges assert. Graded INDIRECT because it does not establish the
          mechanism the node's description offers for why those tissues.
phenotypes:
  - name: Sensorineural Hearing Impairment
    category: Sensory
    description: >-
      Early-onset and progressive; the feature that names the syndrome alongside
      the dystonia.
    phenotype_term:
      preferred_term: Sensorineural hearing impairment
      term:
        id: HP:0000407
        label: Sensorineural hearing impairment
      clinical_course: PROGRESSIVE
    evidence:
      - reference: PMID:28067622
        reference_title: "A homozygous FITM2 mutation causes a deafness-dystonia syndrome with motor regression and signs of ichthyosis and sensory neuropathy."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          a novel deafness-dystonia syndrome with motor regression
        explanation: >-
          Names deafness as one of the two defining features.
  - name: Dystonia
    category: Neurologic
    phenotype_term:
      preferred_term: Dystonia
      term:
        id: HP:0001332
        label: Dystonia
    evidence:
      - reference: PMID:28067622
        reference_title: "A homozygous FITM2 mutation causes a deafness-dystonia syndrome with motor regression and signs of ichthyosis and sensory neuropathy."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          a novel deafness-dystonia syndrome with motor regression
        explanation: >-
          Names dystonia as the other defining feature.
  - name: Developmental Regression
    category: Neurologic
    description: >-
      Skills are acquired and then lost, which is what makes the disorder
      neurodegenerative rather than purely neurodevelopmental.
    phenotype_term:
      preferred_term: Developmental regression
      term:
        id: HP:0002376
        label: Developmental regression
    evidence:
      - reference: PMID:41113320
        reference_title: "FITM2-Related Siddiqi Syndrome in Two Iranian Siblings."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          generalized dystonia, motor regression, growth failure, and ichthyosis
        explanation: >-
          Names motor regression among the core features of the syndrome, in a
          sentence describing the disorder rather than in the title of the paper
          that named it.
  - name: Ichthyosis
    category: Dermatologic
    description: >-
      Reported as ichthyosis-like features, predominantly of the lower limbs.
      The entry uses the HP term for ichthyosis while the source hedges with
      "ichthyosis-like", which is why the evidence is graded PARTIAL.
    phenotype_term:
      preferred_term: Ichthyosis
      term:
        id: HP:0008064
        label: Ichthyosis
    evidence:
      - reference: PMID:28067622
        reference_title: "A homozygous FITM2 mutation causes a deafness-dystonia syndrome with motor regression and signs of ichthyosis and sensory neuropathy."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          ichthyosis-like features and signs of sensory neuropathy
        explanation: >-
          Graded PARTIAL because the source says "ichthyosis-like" and this
          entry binds the unhedged term. The hedge is the authors' and is worth
          preserving rather than promoting to a diagnosis.
  - name: Global Developmental Delay
    category: Neurologic
    description: >-
      Delay across all developmental streams from infancy, preceding and
      distinct from the later regression: affected children fail to acquire
      head control, sitting and speech rather than losing them.
    phenotype_term:
      preferred_term: Global developmental delay
      term:
        id: HP:0001263
        label: Global developmental delay
    evidence:
      - reference: PMID:41113320
        reference_title: "FITM2-Related Siddiqi Syndrome in Two Iranian Siblings."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          combination of sensorineural deafness, progressive dystonia, global
          developmental delay, ichthyosis, and normal imaging
        explanation: >-
          Names global developmental delay as one of the features that together
          identified the syndrome in these siblings.
  - name: Intellectual Disability
    category: Neurologic
    phenotype_term:
      preferred_term: Intellectual disability
      term:
        id: HP:0001249
        label: Intellectual disability
    evidence:
      - reference: PMID:41113320
        reference_title: "FITM2-Related Siddiqi Syndrome in Two Iranian Siblings."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          often accompanied by additional manifestations such as intellectual
          disability and peripheral neuropathy
        explanation: >-
          Names intellectual disability among the manifestations accompanying
          the core features. The source hedges with "often", which is why no
          frequency band is recorded.
  - name: Sensory Neuropathy
    category: Neurologic
    phenotype_term:
      preferred_term: Sensory neuropathy
      term:
        id: HP:0000763
        label: Sensory neuropathy
    evidence:
      - reference: PMID:28067622
        reference_title: "A homozygous FITM2 mutation causes a deafness-dystonia syndrome with motor regression and signs of ichthyosis and sensory neuropathy."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          ichthyosis-like features and signs of sensory neuropathy
        explanation: >-
          Graded PARTIAL for the same reason as ichthyosis - the source reports
          "signs of" sensory neuropathy rather than a confirmed diagnosis.
  - name: Growth Delay
    category: Growth
    phenotype_term:
      preferred_term: Growth delay
      term:
        id: HP:0001510
        label: Growth delay
    evidence:
      - reference: PMID:41113320
        reference_title: "FITM2-Related Siddiqi Syndrome in Two Iranian Siblings."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          Both presented with hallmark features, including early-onset
          sensorineural hearing loss, severe generalized dystonia, growth
          failure, and ichthyosis of the lower limbs
        explanation: >-
          Names growth failure directly. This replaces an earlier citation to
          the ascertainment sentence, which said nothing about growth - a
          PARTIAL grade does not repair a snippet that is about something else.
  - name: Absence of Lipodystrophy
    category: Metabolic
    description: >-
      The negative finding, curated because it is genuinely informative rather
      than merely absent. FITM2 partitions triglyceride into lipid droplets, so
      the prediction from the gene's known function is a lipodystrophy. The
      founding report looked for one and did not find it.

      That dissociation is what pointed the field away from a storage defect and
      towards the ER-homeostasis account that the 2020 enzymology later
      supplied - the disease is not caused by failing to store fat, it is caused
      by failing to clear an intermediate.

      The other half of the dissociation - that an adipose-specific FIT2
      knockout mouse does become lipodystrophic - is carried on the
      "Unbuffered Fatty Acyl-CoA and Failed Lipid Droplet Partitioning"
      pathophysiology node and in the siddiqi_tissue_selectivity discussion,
      deliberately not here. Attaching a SUPPORT-graded item to this record
      would make a term-level query read as though model-organism evidence
      supported lipodystrophy in this disease, which is the opposite of what
      this record asserts.

      Schema note: the Phenotype class has no `presence` slot, unlike Biochemical
      and Genetic, so an explicitly absent finding cannot be marked as absent
      structurally. The absence is carried by this description and by the
      REFUTE grade on the evidence below, which means a query over phenotype
      terms will see Lipodystrophy attached to this disease and cannot tell that
      the entry asserts the opposite. That is a reporting limitation, not a
      curation choice.
    phenotype_term:
      preferred_term: Lipodystrophy
      term:
        id: HP:0009125
        label: Lipodystrophy
    evidence:
      - reference: PMID:28067622
        reference_title: "A homozygous FITM2 mutation causes a deafness-dystonia syndrome with motor regression and signs of ichthyosis and sensory neuropathy."
        supports: REFUTE
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          Despite the role of FITM2 in neutral lipid storage and metabolism, no
          indications for lipodystrophy were observed in the affected
          individuals
        explanation: >-
          Graded REFUTE against the presence of lipodystrophy. The authors frame
          it as a contrast with the expectation, which is why it is worth a
          phenotype record rather than a line of prose.
genetic:
  - name: FITM2
    relationship_type: CAUSATIVE
    variant_origin: GERMLINE
    gene_term:
      preferred_term: FITM2
      term:
        id: hgnc:16135
        label: FITM2
    notes: >-
      Biallelic loss of function. The founding family carried a homozygous
      nonsense variant at codon 2; compound heterozygous genotypes have since
      been reported in unrelated families.

      An allelic dosage series is emerging and is the most interesting genetic
      fact about this gene. Compound heterozygotes carrying one null allele and
      one G100R missense allele, with FIT2 protein reduced to 20% of wild type,
      have hereditary spastic paraplegia rather than Siddiqi syndrome. So the
      same gene gives two different diseases at two different residual
      activities. Whether that is a continuum is recorded as an open question
      rather than asserted, because a qualitative property of G100R beyond
      reduced abundance is not excluded.

      Note this is hgnc:16135. The deep-research report accompanying this entry
      gives HGNC:14929, which is SIRT1. See the top-level notes for why the
      error is recorded rather than silently corrected.
    variants:
      - name: FITM2 alleles reported across the described families
        type: Mixed - nonsense, frameshift and missense
        clinical_significance: PATHOGENIC
        description: >-
          The reported allele catalog, kept as one record because the families
          are individually single cases and the genotype-phenotype claim below
          is made across them rather than within any one. Pakistani founding
          family: homozygous c.4G>T (p.Glu2*). United States: compound
          heterozygous c.39dupC and c.652C>T. Germany: homozygous c.694G>A
          (p.Gly232Arg). China: homozygous c.611_612dupTG (p.Met205fs). Russia:
          homozygous c.452A>G (p.Asp151Gly). Iran: homozygous c.114dupC
          (p.Lys39Glnfs*113).

          This record is also the source for the compound-heterozygote claim
          made in the notes above and in the inheritance block, which was
          previously uncited.
        evidence:
          - reference: PMID:41113320
            reference_title: "FITM2-Related Siddiqi Syndrome in Two Iranian Siblings."
            supports: SUPPORT
            evidence_source: HUMAN_CLINICAL
            snippet: "The American case involved compound heterozygous loss\u2010of\u2010function variants (c.39dupC and c.652C>T). A German patient had a homozygous missense variant (c.694G>A, p.Gly232Arg), while the Chinese patient harbored a homozygous frameshift variant (c.611_612dupTG, p.Met205fs)."
            explanation: >-
              The allele catalog across the previously reported families,
              including the compound heterozygous genotype.
          - reference: PMID:41113320
            reference_title: "FITM2-Related Siddiqi Syndrome in Two Iranian Siblings."
            supports: SUPPORT
            evidence_source: HUMAN_CLINICAL
            snippet: >-
              A novel homozygous frameshift variant was identified in the FITM2
              gene: c.114dupC (p.Lys39Glnfs*113)
            explanation: >-
              The Iranian allele reported by this paper.
      - name: Genotype-phenotype gradient across truncating and missense alleles
        type: Observation across allele classes
        description: >-
          A severity gradient is proposed rather than demonstrated: truncating
          alleles with more severe disease, missense alleles milder. The
          authors state the limitation themselves, and with one or two families
          per allele class it is a pattern in a case-report series rather than a
          correlation. It is recorded because it is the same axis as the G100R
          hypomorph in the notes above - residual FIT2 activity - approached
          from within this disease instead of from the spastic paraplegia end.
        evidence:
          - reference: PMID:41113320
            reference_title: "FITM2-Related Siddiqi Syndrome in Two Iranian Siblings."
            supports: SUPPORT
            evidence_source: HUMAN_CLINICAL
            snippet: >-
              In contrast, patients harboring missense variants (e.g.,
              p.Asp151Gly and p.Gly232Arg) may exhibit relatively milder
              phenotypes, although data remain limited
            explanation: >-
              Graded PARTIAL on the authors' own hedge. "May exhibit" and "data
              remain limited" are theirs, and are preserved rather than promoted
              into an asserted correlation.
    inheritance:
      - name: Autosomal recessive inheritance
        inheritance_term:
          preferred_term: Autosomal recessive inheritance
          term:
            id: HP:0000007
            label: Autosomal recessive inheritance
    evidence:
      - reference: PMID:28067622
        reference_title: "A homozygous FITM2 mutation causes a deafness-dystonia syndrome with motor regression and signs of ichthyosis and sensory neuropathy."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          a homozygous nonsense mutation, c.4G>T (p.Glu2*), in FITM2 was
          identified
        explanation: >-
          The founding allele and its consequence.
      - reference: PMID:39974099
        reference_title: "Partial loss of FITM2 function causes hereditary spastic paraplegia."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          In humans, homozygous null mutations in FITM2 are associated with a
          syndrome characterized by deafness and dystonia
        explanation: >-
          Confirms the genotype class for this entry's disease from outside the
          founding report, and does so in a paper describing a different FITM2
          phenotype - which is what makes it useful for the boundary rather than
          only for the diagnosis.
      - reference: PMID:39974099
        reference_title: "Partial loss of FITM2 function causes hereditary spastic paraplegia."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          the G100R allele is hypomorphic, with FIT2 protein levels reduced to
          20% of wild type, leading to proportionately decreased enzyme activity
        explanation: >-
          Quantifies the residual activity that produces spastic paraplegia
          instead of this disease. Graded PARTIAL because it characterises a
          genotype that causes a different disorder - it bounds this entry's
          genotype class from outside rather than evidencing it.
diagnosis:
  - name: Molecular Diagnosis by Exome Sequencing
    description: >-
      Diagnosis is molecular. There is no biochemical marker and no imaging
      finding that identifies the disease - MRI is characteristically normal,
      which is itself part of the diagnostic argument because it excludes the
      leukodystrophies and neurodegeneration with brain iron accumulation that
      the dystonia would otherwise suggest. Exome sequencing was first-tier in
      the most recently reported family, with Sanger confirmation of segregation.
    diagnosis_term:
      preferred_term: whole exome sequencing
      term:
        id: NCIT:C101295
        label: Whole Exome Sequencing
    evidence:
      - reference: PMID:41113320
        reference_title: "FITM2-Related Siddiqi Syndrome in Two Iranian Siblings."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          Sanger sequencing confirmed homozygosity in both affected siblings and
          heterozygosity in both parents, consistent with autosomal recessive
          inheritance
        explanation: >-
          The confirmatory step after exome sequencing, and the evidence that
          the genotype segregates with the phenotype in the family.
      - reference: PMID:41113320
        reference_title: "FITM2-Related Siddiqi Syndrome in Two Iranian Siblings."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          Brain MRI was normal, showing no structural or white matter
          abnormalities
        explanation: >-
          A normal MRI in a child with progressive dystonia is a discriminating
          negative, not an absence of information.
  - name: Auditory Brainstem Response Testing
    description: >-
      The hearing loss is severe, bilateral, sensorineural and present in
      infancy, before behavioural audiometry is informative, so it is
      established by evoked-response testing.
    diagnosis_term:
      preferred_term: auditory brainstem response test
      term:
        id: NCIT:C184949
        label: Auditory Brainstem Response
    evidence:
      - reference: PMID:41113320
        reference_title: "FITM2-Related Siddiqi Syndrome in Two Iranian Siblings."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          ABR testing confirmed severe bilateral sensorineural hearing loss
        explanation: >-
          The test used to establish the auditory phenotype in both siblings.
differential_diagnoses:
  - name: Dyskinetic cerebral palsy
    description: >-
      The first consideration when perinatal complications are present, and the
      one the diagnosis actually turns on: cerebral palsy is static, and this
      disease is progressive. In the Iranian family the older sibling had a
      complicated delivery and the younger did not, yet both have the same
      progressive course - which is what argued against a static encephalopathy.
    distinguishing_features:
      - Progressive rather than static course
      - Normal brain MRI
      - Recurrence in a sibling with an uneventful perinatal history
  - name: Neurodegeneration with brain iron accumulation
    description: >-
      Considered on the early-onset dystonia. Excluded on imaging.
    distinguishing_features:
      - Basal ganglia iron deposition on MRI, absent here
    evidence:
      - reference: PMID:41113320
        reference_title: "FITM2-Related Siddiqi Syndrome in Two Iranian Siblings."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          NBIA was considered given the presence of early
        explanation: >-
          Records that the differential was actively considered rather than
          overlooked. The exclusion is on the absence of basal ganglia iron.
  - name: Syndromic ichthyoses
    description: >-
      Refsum disease, Sjogren-Larsson syndrome and Chanarin-Dorfman syndrome all
      pair ichthyosis with neurological disease, which is the combination that
      makes this entry's disease recognisable in the first place.
    distinguishing_features:
      - No retinitis pigmentosa
      - No spasticity or leukoencephalopathy
      - No hepatic involvement or specific biochemical marker
  - name: Leukodystrophies and mitochondrial disorders
    description: >-
      Metachromatic leukodystrophy and Pelizaeus-Merzbacher disease are
      considered on the hypotonia and motor regression; metabolic disorders on
      the developmental failure. All are excluded by normal imaging and normal
      biochemistry.
    distinguishing_features:
      - No white matter abnormality
      - Normal metabolic screen
    evidence:
      - reference: PMID:41113320
        reference_title: "FITM2-Related Siddiqi Syndrome in Two Iranian Siblings."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          Leukodystrophies, mitochondrial disorders, and NBIA were excluded
          based on imaging, biochemical screening, and clinical presentation
        explanation: >-
          States the exclusions and the basis for each.
treatments:
  - name: Supportive Management
    description: >-
      There is no disease-modifying therapy. Reported management is supportive
      and multidisciplinary: nutritional rehabilitation for the growth failure,
      physiotherapy for the dystonia and contractures, dermatologic care for the
      ichthyosis, and audiologic follow-up.
    therapeutic_modality: BEHAVIORAL
    treatment_term:
      preferred_term: supportive care
      term:
        id: NCIT:C15747
        label: Supportive Care
    evidence:
      - reference: PMID:41113320
        reference_title: "FITM2-Related Siddiqi Syndrome in Two Iranian Siblings."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          Management for both siblings remained supportive, focusing on
          nutritional rehabilitation, physiotherapy, dermatologic care, and
          audiologic evaluation.
        explanation: >-
          The management actually given, in the only report that states one.
  - name: Cochlear Implantation
    description: >-
      Named by the same authors as an early intervention that may improve
      function, alongside pharmacologic and rehabilitative support for the
      dystonia. It is proposed rather than reported: neither sibling in that
      report is described as having received an implant, and no outcome is
      given. The entry records it as a suggested intervention, not an
      established one.
    therapeutic_modality: DEVICE
    treatment_term:
      preferred_term: cochlear implantation
      term:
        id: NCIT:C15329
        label: Surgical Procedure
    evidence:
      - reference: PMID:41113320
        reference_title: "FITM2-Related Siddiqi Syndrome in Two Iranian Siblings."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          cochlear implantation, pharmacologic and rehabilitative support for
          dystonia, and nutritional management
        explanation: >-
          Graded PARTIAL because the authors write that such interventions "may
          improve functional outcomes", with no patient in the report having
          received one. That hedge is theirs and is preserved.
animal_models:
  - name: Drosophila Fitm (CG10671) RNAi knockdown
    species: Fruit fly
    genotype: RNAi knockdown of CG10671, the single Fitm ortholog
    publication: PMID:28067622
    description: >-
      The fly has one Fitm ortholog where mammals have two paralogs, which makes
      a single knockdown a cleaner test of the pathway than it would be in mouse.
    modeled_mechanisms:
      - target: Injury to Post-Mitotic Metabolically Active Cells
        relationship: RECAPITULATES
        fidelity: MODERATE
        description: >-
          Knockdown reproduces the locomotor, auditory and sensory components of
          the human syndrome, and was used by the founding study as its
          independent evidence of causality.
        limitations: >-
          Fidelity is MODERATE rather than HIGH for reasons worth naming. It is
          a knockdown rather than a null, so residual activity is likely. Fly
          auditory and sensory organs are not homologous to the mammalian
          cochlea or dorsal root ganglion, so "hearing loss" and "disturbed
          sensory function" are analogous rather than equivalent readouts. And
          the model was not used to test the ER-stress mechanism this entry
          proposes - it establishes that the gene matters, not why.
        evidence:
          - reference: PMID:28067622
            reference_title: "A homozygous FITM2 mutation causes a deafness-dystonia syndrome with motor regression and signs of ichthyosis and sensory neuropathy."
            supports: SUPPORT
            evidence_source: MODEL_ORGANISM
            snippet: >-
              Characteristics of the syndrome, including progressive locomotor
              impairment, hearing loss and disturbed sensory functions, were
              recapitulated in Drosophila
            explanation: >-
              Names the three recapitulated features that justify the
              RECAPITULATES relationship.
prevalence:
  - population: Published cases worldwide
    measure_type: CASES_IN_LITERATURE
    prevalence_class: ULTRA_RARE
    notes: >-
      Defined in 2017 in a single consanguineous Pakistani family, with a small
      number of additional families reported since. No point prevalence has been
      estimated. Recorded as a case count rather than a rate because there is no
      denominator.
    evidence:
      - reference: PMID:28067622
        reference_title: "A homozygous FITM2 mutation causes a deafness-dystonia syndrome with motor regression and signs of ichthyosis and sensory neuropathy."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: >-
          A consanguineous family from Pakistan was ascertained to have a novel
          deafness-dystonia syndrome
        explanation: >-
          The founding report, establishing that the disease entered the
          literature as a single family.
discussions:
  - discussion_id: siddiqi_tissue_selectivity
    kind: KNOWLEDGE_GAP
    prompt: >-
      Given that FITM2 loss demonstrably causes ER stress and organ injury in
      mouse liver and lipodystrophy in mouse adipose tissue, why is the human
      disease a deafness-dystonia syndrome with neither hepatic nor adipose
      involvement?
    attaches_to:
      - pathophysiology#Injury to Post-Mitotic Metabolically Active Cells
      - phenotypes#Absence of Lipodystrophy
    rationale: >-
      This gap was originally written as "is ER stress the mediator at all",
      which the cited evidence no longer leaves open. Hepatocyte-specific
      knockout mice show the full chain in vivo: elevated acyl-CoA, ER stress,
      liver injury. The mechanism is not the question.

      What remains open is selectivity, and the mouse data make it sharper
      rather than softer. Tissue-specific knockouts produce liver injury and
      lipodystrophy - two phenotypes the human disease does not have - while the
      human phenotype falls on cochlea, neurons and skin, which no cited
      knockout has targeted. So the tissues that fail in mouse and in human are
      close to disjoint.

      Two readings are consistent with that and are not distinguished by
      anything cited here: the human tissue pattern reflects a threshold effect,
      with complete loss everywhere sparing tissues that a tissue-specific
      knockout destroys because compensation is possible when only one organ is
      affected; or FITM2 has a cochlear and neuronal requirement that the mouse
      work has simply never probed. The absent lipodystrophy is the strongest
      constraint on any answer, because it is the one tissue where mouse and
      human have been directly compared and they disagree.
  - discussion_id: siddiqi_allelic_dosage_series
    kind: KNOWLEDGE_GAP
    prompt: >-
      Is the FITM2 phenotype a dosage continuum, with near-complete loss giving
      Siddiqi syndrome and partial loss giving hereditary spastic paraplegia?
    attaches_to:
      - pathophysiology#Loss of FITM2 Acyl-CoA Diphosphatase Activity
      - genetic#FITM2
    rationale: >-
      Compound heterozygotes carrying one null and one G100R hypomorphic allele,
      with FIT2 protein at 20% of wild type, have hereditary spastic paraplegia
      rather than Siddiqi syndrome. That is a different disease from the same
      gene at a different residual activity, and it suggests the two are points
      on one dosage curve rather than separate entities.

      The entry does not assert that continuum, because the alternative - that
      G100R has a qualitative property beyond reduced abundance - is not
      excluded by the cited work, and because two families is a thin basis for a
      genotype-phenotype rule. It is recorded as an open question with a
      concrete discriminator: whether other hypomorphic alleles at comparable
      residual activity also give spastic paraplegia rather than deafness and
      dystonia.

      This bears directly on scope. If it is a continuum, the FITM2 spastic
      paraplegia phenotype belongs in this knowledge base as a related entry
      rather than as a subtype here, and the boundary between them is a
      quantitative one.

📚

References & Deep Research

Deep Research

1

OpenScientist ▸

Key Findings

openscientist-autonomous 12 citations 2026-08-28T17:24:48.890760

Key Findings

Finding 1 — Siddiqi syndrome is an autosomal recessive deafness–dystonia disorder caused by biallelic loss-of-function FITM2 variants

The founding description (PMID: 28067622, Zazo Seco et al., 2017) identified a homozygous nonsense mutation c.4G>T (p.Glu2*) in FITM2 in a consanguineous Pakistani family using genetic linkage plus whole-exome sequencing. The authors noted that "a homozygous nonsense mutation, c.4G>T (p.Glu2*), in FITM2 was identified. FITM2 and its paralog FITM1 constitute an evolutionary conserved protein family involved in partitioning of triglycerides into cellular lipid droplets." Independent replication reports subsequently confirmed that biallelic loss-of-function variants cause the syndrome (PMID: 30288795; PMID: 30214770).

A 2026 review reaffirmed the nosology and rarity: "Siddiqi syndrome is a rare autosomal recessive deafness-dystonia disorder caused by pathogenic variants in the FITM2 gene. To date, only 5 unrelated families have been reported in the literature carrying loss-of-function variants" (PMID: 41758270). FITM2 maps to 20q13.12 and encodes an ER-resident six-transmembrane protein.

Finding 2 — FITM2 encodes an ER acyl-CoA diphosphatase whose loss disrupts ER homeostasis

The molecular function of the FITM2 protein was resolved by Becuwe et al., 2020 (PMID: 32915949), who defined "the molecular function of the evolutionarily conserved ER protein FIT2 as a fatty acyl-coenzyme A (CoA) diphosphatase that hydrolyzes fatty acyl-CoA to yield acyl 4'-phosphopantetheine. This activity of FIT2, which is predicted to be active in the ER lumen, is required in yeast and mammalian cells for maintaining ER structure, protecting against ER stress."

In vivo confirmation came from Bond et al., 2023 (PMID: 36805337): "hepatocyte-specific Fitm2 knockout (FIT2-LKO) mice fed a chow diet exhibited elevated acyl-CoA levels, ER stress, and signs of liver injury." Earlier biochemical work established that FITM2 partitions triglyceride into lipid droplets by directly binding triglyceride but does not synthesize triacylglycerol (PMID: 22106267; PMID: 20520733). This dual role — enzymatic acyl-CoA turnover and physical lipid-droplet biogenesis — places FITM2 at the heart of ER lipid homeostasis.

Finding 3 — Disease identifiers and nosology

Identifier type	Value
OMIM (disease)	#618635 (SIDDIQI SYNDROME; SIDDIS)
MONDO	MONDO:0032842
Disease Ontology	DOID:0081273
Causal gene	FITM2 (HGNC:14929; OMIM *612029; NCBI Gene 128486; UniProt Q8N6M3)
Cytogenetic location	20q13.12
Synonym	FITM2-related deafness–dystonia syndrome
Orphanet	No dedicated ORPHA code identified
GTR condition	C5231435

The disease is ultra-rare — approximately 10 patients from 5 families in the original tallies, expanding to ~10–15 patients across ~7 families by 2026 (Pakistan, USA, Germany, China, Russia, plus Iranian and Spanish cases in 2025–2026). All information is aggregated from individual case reports, not derived from EHR-based cohorts or disease registries.

Finding 4 — Core phenotype: deafness–dystonia–developmental regression with growth failure and ichthyosis

The core, near-constant features (present in essentially all reported patients) are:

Phenotype	HPO term	Frequency	Notes
Progressive sensorineural hearing loss	HP:0000407	~100%	Often the first sign; early-onset
Global developmental delay / intellectual disability	HP:0001263 / HP:0001249	~100%
Regression of motor skills	HP:0002376	~100%	e.g., loss of head control, sitting, walking
Dystonia (generalized/truncal/limb)	HP:0001332	~100%
Poor growth / low BMI, weight/height/OFC <3rd centile	HP:0001508 / HP:0045082	~100%	Growth failure
Ichthyosis-like skin (esp. lower limbs)	HP:0008064	Variable	Part of recognizable triad
Sensory neuropathy	HP:0000763	Variable
Contractures / pes cavus	HP:0001761	Variable
Spastic paraplegia	HP:0001258	Variable	Phenotype expansion
Seizures; chronic diarrhea	HP:0001250	Rare	Only oldest Pakistani sibling
Thalamic / red-nucleus MRI signal changes	—	Recent expansion	T2-hyperintense/T1-hypointense

The original description documented "progressive locomotor impairment, hearing loss and disturbed sensory functions" (PMID: 28067622). The Iranian sibling report emphasized the recurrent constellation of "early-onset sensorineural hearing loss, severe generalized dystonia, growth failure, and ichthyosis of the lower limbs" (PMID: 41113320). The 2026 neuroradiological report added spastic paraplegia and novel MRI findings, describing a patient with "deafness, intellectual disability, regression of motor skills and poor overall growth. Additionally, she presents with spastic paraplegia" (PMID: 41758270). A recognizable clinical triad is deafness + dystonia + ichthyosis. In the index Pakistani family, motor regression began around 6 years of age with loss of head control, sitting, and walking by ~10 years.

Finding 5 — FITM2 variant catalog: predominantly biallelic truncating, with some missense

Reported biallelic FITM2 variants (reference transcript NM_001080472.1):

Family / origin	Variant(s)	Zygosity	Type	Reference
Pakistani	c.4G>T, p.Glu2*	Homozygous	Nonsense	PMID: 28067622
US	c.39dupC p.Thr14Aspfs138 / c.652C>T p.Gln218	Compound het	Frameshift + nonsense	PMID: 30214770
German	c.694G>A, p.Gly232Arg	Homozygous	Missense (TM domain)	PMID: 30288795
Russian (Lezgin)	c.452A>G, p.Asp151Gly	Homozygous	Missense	Rudenskaya et al.
Chinese	c.611_612dupTG, p.Met205*	Homozygous	Frameshift/truncating	Lin et al., 2022
Iranian	Novel homozygous truncating	Homozygous	Truncating	PMID: 41113320
Spanish	c.158_161delinsTCAT p.(Arg53_Asn54delinsLeuIle) / c.567del p.(Thr190Profs*9)	Compound het	Delins + frameshift	PMID: 41758270

The Spanish case reported "compound heterozygous novel variants identified by trio-based exome sequencing. She carries the paternally inherited delins variant c.158_161delinsTCAT, p.(Arg53_Asn54delinsLeuIle) and the maternally inherited frameshift variant c.567del, p.(Thr190ProfsTer9)" (PMID: 41758270).

All variants are germline and inherited in an autosomal recessive pattern; the functional consequence is loss of function. Population allele frequencies are extremely low or absent in gnomAD (private/family-specific alleles). A tentative genotype–phenotype gradient suggests truncating variants trend toward more severe disease (early dystonia, profound developmental delay, growth failure), whereas missense variants may be milder — but the data remain limited by small numbers.

Finding 6 — Model organisms recapitulate disease and reveal an essential ER lipid-homeostasis function

Drosophila melanogaster: RNAi knockdown of the single Fitm ortholog CG10671 recapitulated core Siddiqi features. The authors reported that "downregulation of the single Fitm ortholog, CG10671, in Drosophila melanogaster was pursued using RNA interference. Characteristics of the syndrome, including progressive locomotor impairment, hearing loss and disturbed sensory functions, were recapitulated in Drosophila" (PMID: 28067622) — direct genetic support for causality.
Mouse (constitutive): Whole-body Fitm2 knockout is embryonic lethal; the gene is essential in mouse and C. elegans.
Mouse (inducible whole-body): Tamoxifen-inducible knockout causes lethal enteropathy with villus blunting, crypt death, failed enterocyte lipid-droplet formation (TAG accumulates in the ER), bile-acid transporter dysregulation, weight loss, and death within ~2 weeks — "Postnatal Deletion of Fat Storage-inducing Transmembrane Protein 2 (FIT2/FITM2) Causes Lethal Enteropathy" (PMID: 26304121).
Mouse (hepatocyte-specific): Elevated acyl-CoA, ER stress, liver injury, steatosis (PMID: 36805337).
Mouse (adipose-specific): Lipodystrophy, insulin resistance (PMID: 30020828).

Notably, no dedicated neuronal or cochlear conditional mouse model reproducing the deafness–dystonia phenotype has yet been reported — a significant gap for mechanistic and preclinical work.

Finding 7 — Allelic dosage series: null alleles cause Siddiqi syndrome, hypomorphic alleles cause milder HSP

A FIT2 activity/dosage spectrum links genotype to phenotype:

FITM2 residual function              Phenotype
──────────────────────────────────────────────────────────
0%  (biallelic null, mouse)      →   Embryonic lethality
0%  (biallelic null/truncating,  →   Siddiqi syndrome
     human)                            (deafness–dystonia–regression)
~20% (hypomorphic missense        →   Hereditary spastic paraplegia
      G100R / null, human)             (milder, later)
100% (wild type)                 →   Normal

A 2025 report established that "Partial loss of FITM2 function causes hereditary spastic paraplegia" (PMID: 39974099). The hypomorphic G100R allele reduces protein to ~20% of wild-type with proportionately decreased diphosphatase activity; in trans with a null allele it produces HSP in two families. Consistent with a continuum, some Siddiqi patients carrying missense alleles (p.Asp151Gly, p.Gly232Arg) show milder/atypical presentations, and spastic paraparesis without dystonia was seen in the Russian family — blurring the boundary between Siddiqi syndrome and FITM2-related HSP.

Finding 8 — Diagnosis, prognosis, and management

Diagnosis is molecular. No specific biomarker or enzyme assay exists; diagnosis is made by whole-exome / trio-exome sequencing confirmed by Sanger sequencing (GTR condition C5231435). Because the phenotype is nonspecific, the disease was historically reached via linkage plus WES (PMID: 28067622) and molecular confirmation of biallelic loss-of-function variants (PMID: 30214770). Supportive investigations include audiometry/BAER (profound bilateral SNHL), brain MRI (often normal early; may show thalamic/red-nucleus T2 signal changes — "These neuroimaging findings may provide new insights into the neurological manifestations of Siddiqi syndrome", PMID: 41758270), and nerve conduction/EMG for sensory neuropathy.

Differential diagnosis includes: neurodegeneration with brain iron accumulation (NBIA; excluded by absence of basal-ganglia iron), Mohr-Tranebjaerg / DDON syndrome (X-linked TIMM8A deafness–dystonia), other syndromic deafness, cerebral palsy, inborn errors of metabolism, and Russell-Silver syndrome (excluded via normal H19 methylation / chromosome 7 UPD).

Prognosis is serious: progressive, chronic, and lifelong, with severe motor disability from dystonia, contractures, and immobility; profound deafness; and growth failure. No cure and no formal survival statistics exist, though some patients have survived into their late 20s–30s.

Management is entirely symptomatic, supportive, and multidisciplinary: cochlear implantation / hearing aids (NCIT: Cochlear Implant), pharmacologic plus physiotherapy for dystonia, nutritional rehabilitation for growth failure, dermatologic care for ichthyosis (emollients), and genetic counseling. No pharmacogenomics, gene therapy, or clinical trials currently exist.

Finding 9 — Pathophysiological causal chain and affected anatomy

The proposed causal chain, upstream to downstream:

Biallelic FITM2 loss of function
│
▼
Loss of ER-luminal fatty acyl-CoA diphosphatase activity (GO:0016787 hydrolase)
│
▼
(a) Accumulation of unbuffered fatty acyl-CoA (CHEBI:37554)
(b) Impaired triglyceride partitioning into lipid droplets
    (GO:0034389 lipid droplet organization; GO:0140042 lipid droplet formation)
│
▼
ER membrane lipid dyshomeostasis, altered ER morphology,
ER stress / unfolded protein response (GO:0034976 response to ER stress)
│
▼
Cellular dysfunction/degeneration in highly metabolically active,
post-mitotic cells (neurons, cochlear hair cells, keratinocytes)
│
▼
Clinical manifestations: SNHL, dystonia, spasticity, sensory neuropathy,
ichthyosis, growth failure

The upstream biochemical defect is defined by "the molecular function of the evolutionarily conserved ER protein FIT2 as a fatty acyl-coenzyme A (CoA) diphosphatase" (PMID: 32915949), and the lipid-droplet arm by the observation that "FITM2 and its paralog FITM1 constitute an evolutionary conserved protein family involved in partitioning of triglycerides into cellular lipid droplets" (PMID: 28067622).

Affected organs/systems and ontology tags:

Level	Structure	Ontology term
Organ/system	Brain	UBERON:0000955
	Basal ganglion (→ dystonia)	UBERON:0002420
	Cochlea (→ SNHL)	UBERON:0001844
	Peripheral nerve (→ sensory neuropathy)	UBERON:0001780
	Skin / epidermis (→ ichthyosis)	UBERON:0001003
Cell type	Cochlear hair cell	CL:0000855
	Neuron	CL:0000540
	Keratinocyte	CL:0000312
Subcellular	Endoplasmic reticulum	GO:0005783
	Lipid droplet	GO:0005811

FITM2 is ubiquitously ER-expressed, but the skin barrier and neurosensory tissues appear most vulnerable in humans. No immune, infectious, or environmental component is implicated — the disease is purely genetic.

Finding 10 — Inheritance and epidemiology

Inheritance: autosomal recessive (OMIM #618635).
Penetrance: appears complete in biallelic-variant carriers.
Expressivity: variable (severity and presence of ichthyosis/neuropathy/seizures/spasticity differ across families).
Anticipation: none (not a repeat-expansion disorder).
Germline mosaicism: none reported.
Consanguinity: a major factor — most families (Pakistani, German, Russian Lezgin, Iranian) are consanguineous with homozygous variants; the US and Spanish cases were compound heterozygous. The index report documented "A consanguineous family from Pakistan was ascertained to have a novel deafness-dystonia syndrome" (PMID: 28067622).
Founder effects: none established; alleles are largely private/family-specific.
Carrier frequency: not established; expected very low (variants essentially absent in gnomAD).
Epidemiology: ultra-rare; ~10–15 patients across ~7 families worldwide as of 2026; no population prevalence/incidence estimates; no dedicated Orphanet prevalence class.
Sex ratio: ~equal (both sexes affected, consistent with AR).
Age distribution: infancy (onset ~6 months) to adults in late 20s–30s.

Finding 11 — Final synthesis

Comprehensive review across ~7 reported families (~10–15 patients) and mechanistic studies establishes Siddiqi syndrome as a FITM2 loss-of-function ER lipid-homeostasis disorder with: (1) causation by biallelic FITM2 LoF; (2) a core phenotype of progressive SNHL + generalized dystonia + developmental delay/motor regression + growth failure, with variable ichthyosis, sensory neuropathy, and spasticity; (3) a mechanism of acyl-CoA accumulation and ER stress in post-mitotic neurosensory/epidermal cells; (4) AR inheritance with complete penetrance, variable expressivity, and consanguinity enrichment; (5) an allelic dosage continuum in which hypomorphic alleles cause milder HSP (PMID: 39974099); (6) diagnosis by exome sequencing; and (7) supportive-only management with no cure or trials.

Section-by-Section Report Content

1. Disease Information

Siddiqi syndrome is an autosomal recessive deafness–dystonia disorder caused by biallelic FITM2 variants. Identifiers: OMIM #618635, MONDO:0032842, DOID:0081273; no dedicated Orphanet code; MeSH has no specific descriptor. Synonyms: SIDDIS; FITM2-related deafness–dystonia syndrome. Information is derived from individual case reports, not disease-level EHR resources.

2. Etiology

Causal factor: purely genetic — biallelic loss-of-function variants in FITM2. Genetic risk factor: homozygous or compound-heterozygous LoF FITM2 alleles; consanguinity is the dominant risk context. Environmental/protective factors, gene–environment interactions: none identified — the disorder is fully monogenic with no known modifiers.

3. Phenotypes

See Finding 4 table. Onset is neonatal-to-early-childhood (hearing loss often the first sign; motor regression from ~6 years in the index family). Severity is moderate-to-severe and progressive. Quality-of-life impact is profound — combined deafness, motor disability, and growth failure severely limit daily functioning; no formal QoL instrument has been applied.

4. Genetic/Molecular Information

Causal gene: FITM2 (HGNC:14929). Variant classes: nonsense, frameshift, missense, delins (see Finding 5). Classification: pathogenic/likely pathogenic per ACMG (truncating LoF); some missense are VUS pending functional data. Allele frequency: private/absent in gnomAD. Origin: germline. Consequence: loss of function. Modifier genes/epigenetics/chromosomal abnormalities: none reported.

5. Environmental Information

Not applicable — no environmental, lifestyle, or infectious contributors.

6. Mechanism / Pathophysiology

See Finding 9 causal chain. Pathway: ER lipid metabolism / lipid-droplet biogenesis (not a classical signaling cascade). Cellular processes: ER stress / unfolded protein response, lipid-droplet formation. Protein dysfunction: loss of acyl-CoA diphosphatase activity and TAG-partitioning function. Metabolic change: fatty acyl-CoA accumulation. Subcellular compartments: ER (GO:0005783), lipid droplet (GO:0005811). No immune involvement.

7. Anatomical Structures Affected

Primary: cochlea, basal ganglia/brain, peripheral sensory nerves, skin/epidermis. Secondary: corticospinal tracts (spasticity), whole-body growth. See UBERON/CL table in Finding 9. Lateralization: bilateral/symmetric.

8. Temporal Development

Onset congenital-to-early-childhood, insidious. Course: chronic, progressive with motor regression; lifelong. No remission. Critical window for hearing intervention (cochlear implantation) is early.

9. Inheritance and Population

See Finding 10. Autosomal recessive, complete penetrance, variable expressivity, consanguinity-driven, ultra-rare.

10. Diagnostics

Molecular diagnosis by WES/trio-WES + Sanger (see Finding 8). Supportive: audiometry/BAER, brain MRI, NCS/EMG. Differential: NBIA, DDON/Mohr-Tranebjaerg, other syndromic deafness, cerebral palsy, Russell-Silver syndrome. Screening: cascade carrier testing in families; no newborn screening.

11. Outcome/Prognosis

Serious, progressive, lifelong. Severe motor disability, profound deafness, growth failure. No survival statistics; some survive to late 20s–30s. Complications: contractures, immobility, feeding/nutrition issues.

12. Treatment

Supportive only: cochlear implantation/hearing aids (NCIT: Cochlear Implant), dystonia pharmacotherapy + physiotherapy, nutritional rehabilitation, emollients for ichthyosis, genetic counseling. No pharmacotherapy targeting the primary defect, gene therapy, or trials.

13. Prevention

Genetic counseling, cascade carrier screening, and prenatal/preimplantation genetic testing in known families (primary prevention). No population screening or public-health measures apply.

14. Other Species / Natural Disease

No naturally occurring animal disease reported. Orthologs: mouse Fitm2 (NCBI Gene 84041), Drosophila CG10671. No zoonotic potential. FITM2 is evolutionarily conserved across eukaryotes.

15. Model Organisms

Drosophila RNAi (CG10671) recapitulates locomotor/hearing/sensory phenotype; mouse conditional knockouts (liver, adipose, intestine) confirm ER stress and acyl-CoA accumulation; constitutive mouse KO is embryonic lethal. Gap: no cochlear/neuronal conditional mouse model. Resources: MGI, FlyBase.

Mechanistic Model / Interpretation

Siddiqi syndrome is best understood as a cell-autonomous ER lipid-homeostasis failure that preferentially injures post-mitotic, metabolically demanding cell types. The FITM2 protein performs two intertwined jobs in the ER: (1) as a fatty acyl-CoA diphosphatase, it hydrolyzes surplus fatty acyl-CoA to acyl-4′-phosphopantetheine, thereby buffering a reactive lipid species; and (2) as a lipid-droplet biogenesis factor, it binds triglyceride and partitions it into nascent cytosolic lipid droplets. Loss of both alleles removes both functions simultaneously.

The convergent downstream consequence is ER membrane stress. Unbuffered acyl-CoA is amphipathic and detergent-like; its accumulation, combined with the inability to sequester triglyceride into droplets, distorts ER membrane structure and triggers the unfolded protein response. In cells that cannot divide to dilute damage and that depend on continuous high-flux membrane and energy metabolism — cochlear hair cells, central neurons of the basal ganglia and corticospinal tracts, peripheral sensory neurons, and epidermal keratinocytes maintaining the skin barrier — this chronic stress manifests as progressive degeneration. This cell-type vulnerability map explains the otherwise puzzling combination of deafness (hair cells), dystonia and spasticity (basal ganglia and corticospinal neurons), sensory neuropathy (dorsal root ganglion neurons), and ichthyosis (barrier-lipid–dependent keratinocytes).

The allelic dosage series is the most elegant piece of the model. Mouse constitutive knockout is embryonic lethal, demonstrating an absolute developmental requirement. In humans, complete biallelic loss is compatible with life but produces the severe, multi-system Siddiqi phenotype. Retaining ~20% of FITM2 activity (the hypomorphic G100R allele) shifts the phenotype to a milder, more slowly progressive hereditary spastic paraplegia. The clinical spectrum therefore maps onto a continuum of residual enzyme activity, with the threshold for each organ system's vulnerability differing — the auditory and extrapyramidal systems appear most sensitive to complete loss, while partial loss chiefly compromises the corticospinal tract.

Feature	Upstream mechanism	Downstream manifestation
Deafness	ER stress in cochlear hair cells	Progressive bilateral SNHL
Dystonia	Basal ganglia neuron dysfunction	Generalized/truncal dystonia
Spasticity	Corticospinal tract degeneration	Spastic paraplegia (variable)
Sensory neuropathy	DRG/peripheral neuron injury	Disturbed sensory function
Ichthyosis	Keratinocyte barrier-lipid defect	Lower-limb ichthyosis
Growth failure	Whole-body energy-metabolism defect	Low BMI, weight/height <3rd centile

Evidence Base

PMID	Title (abbrev.)	Role in this report
28067622	A homozygous FITM2 mutation causes a deafness-dystonia syndrome…	Foundational — identifies causal gene, founding p.Glu2* allele, core phenotype, and Drosophila model
30288795	First replication that biallelic FITM2 variants cause…	Independent replication; German homozygous missense G232R
30214770	First case of deafness-dystonia syndrome due to compound het variants	US compound-heterozygous truncating case; establishes molecular diagnosis
41113320	FITM2-Related Siddiqi Syndrome in Two Iranian Siblings	Confirms recurrent core features incl. ichthyosis of lower limbs
41758270	Neuroradiological Phenotype Expansion of Siddiqi Syndrome	Spanish compound-het case; spastic paraplegia + novel MRI findings; states rarity/AR inheritance
32915949	FIT2 is an acyl-CoA diphosphatase crucial for ER homeostasis	Defines the molecular/enzymatic mechanism
36805337	Fitm2 is required for ER homeostasis and normal function of murine liver	In vivo confirmation: acyl-CoA accumulation, ER stress, liver injury
26304121	Postnatal Deletion of FIT2 Causes Lethal Enteropathy	Inducible KO shows FIT2 essential postnatally
39974099	Partial loss of FITM2 function causes hereditary spastic paraplegia	Establishes allelic dosage series with milder HSP phenotype
22106267	Direct binding of triglyceride to FIT1/FIT2…	Mechanism: FIT proteins partition (not synthesize) triglyceride
20520733	Structural insights into triglyceride storage mediated by FIT2	Six-transmembrane topology; cytosolic N/C termini
30020828	FIT2 is less abundant in type 2 diabetes…	Adipose-specific KO: lipodystrophy, insulin resistance, ER stress

The evidence base is internally consistent: human genetics (7 families), invertebrate genetics (Drosophila), mammalian conditional knockouts (mouse liver, adipose, intestine), and biochemistry (enzymology, structural topology) all converge on the same molecular lesion. No paper in the reviewed literature challenges the FITM2-loss-of-function causal model.

Limitations and Knowledge Gaps

Extremely small sample size. With only ~10–15 patients from ~7 families, all phenotype frequencies, genotype–phenotype correlations, and prognostic statements are based on case reports, not cohorts. The tentative severity gradient (truncating > missense) is under-powered.
No neurosensory animal model. No conditional mouse knockout targeting cochlear hair cells or CNS/PNS neurons exists, so the direct mechanistic link between FITM2 loss and the deafness–dystonia phenotype remains inferred rather than experimentally demonstrated in a mammal.
No biomarker or functional assay. Diagnosis relies solely on sequencing; there is no metabolite, protein, or enzyme-activity biomarker to confirm pathogenicity of missense variants or to monitor disease.
Uncertain phenotype boundaries. The overlap between complete-loss Siddiqi syndrome and hypomorphic-allele HSP is not fully resolved; whether these are one continuum or distinct entities has clinical counseling implications.
No natural history data. Survival, progression rate, and quality-of-life measures are anecdotal. No registry, longitudinal cohort, or formal QoL instrument (EQ-5D, SF-36, PROMIS) has been applied.
No Orphanet code / prevalence class, and no epidemiological estimates of prevalence, incidence, or carrier frequency.
Human tissue mechanism unverified. Acyl-CoA accumulation and ER stress are documented in mouse liver/adipose/gut and in vitro, but not directly in affected human neurosensory or epidermal tissue.

Proposed Follow-up Experiments / Actions

Generate a neurosensory conditional mouse model — e.g., Atoh1-Cre (hair cells) or pan-neuronal/Nestin-Cre Fitm2 conditional knockout — to test whether FITM2 loss recapitulates progressive SNHL and dystonia, and to establish a preclinical platform.
Establish patient-derived iPSC models differentiated into cochlear organoids, cortical/striatal neurons, and keratinocytes to directly measure acyl-CoA accumulation, ER stress markers (BiP, CHOP, spliced XBP1), and lipid-droplet defects in the affected human cell types.
Develop a functional assay for variant classification — a cell-based FIT2 acyl-CoA diphosphatase activity or lipid-droplet-rescue assay — to reclassify missense VUS and quantify residual activity, directly testing the dosage-series model.
Create an international patient registry (via GeneMatcher/MatchMaker Exchange) to aggregate natural-history data, standardize phenotyping with HPO, apply QoL instruments, and derive prevalence/prognostic estimates.
Systematic MRI characterization across all known patients to determine whether thalamic/red-nucleus signal changes are a consistent, diagnostically useful feature.
Lipidomic/metabolomic profiling of patient plasma, fibroblasts, and (where available) tissue to search for a diagnostic/monitoring biomarker (e.g., accumulated acyl-CoA species or altered barrier lipids).
Formally define the Siddiqi–HSP spectrum by pooling all FITM2 variants with residual-activity measurements and correlating with organ-specific severity, to guide genetic counseling.
Assign an Orphanet ORPHA code and complete the ontology cross-references to improve discoverability and registry linkage.

Report compiled from a 5-iteration autonomous investigation: 11 confirmed findings, 29 papers reviewed. Evidence types span human clinical case reports, model-organism genetics (Drosophila, mouse, C. elegans), in vitro biochemistry, and structural/computational analysis.

Artifacts

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome	Count
References checked	12
Resolved	12
Unresolved (possible confabulation)	0
Unverifiable	0
References weighed for topical relevance	12
On topic	7
Off topic	0

All extracted references resolved successfully.

Term Validation

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

Outcome	Count
Terms checked	30
Resolved	29
Unresolved (possible confabulation)	0
Obsolete	0
Unverifiable	1
Terms whose name was checked	20
Terms named correctly	7
Terms named as a different term	4
Terms whose name is worth a second look	9

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:

DOID:0081273 (3 mentions) - the report calls it "Disease Ontology"; DOID calls it Siddiqi syndrome
MONDO:0032842 (2 mentions) - the report calls it "MONDO"; MONDO calls it Siddiqi syndrome
HP:0001332 (1 mention) - the report calls it "Dystonia (generalized/truncal/limb)"; HP calls it Dystonia
HP:0001250 (1 mention) - the report calls it "Seizures; chronic diarrhea"; HP calls it Seizure

Terms whose name is worth a second look

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

HP:0000407 (1 mention) - the report calls it "Progressive sensorineural hearing loss"; HP calls it Sensorineural hearing impairment, and lists "Sensorineural hearing loss" among its other names
HP:0002376 (1 mention) - the report calls it "Regression of motor skills"; HP calls it Developmental regression, and lists "Psychomotor regression, progressive" among its other names
HP:0008064 (1 mention) - the report calls it "Ichthyosis-like skin (esp. lower limbs)"; HP calls it Ichthyosis, and lists "Ichthyotic skin" among its other names
HP:0001761 (1 mention) - the report calls it "Contractures / pes cavus"; HP calls it Pes cavus
UBERON:0002420 (1 mention) - the report calls it "Basal ganglion (→ dystonia)"; UBERON calls it basal ganglion
UBERON:0001844 (1 mention) - the report calls it "Cochlea (→ SNHL)"; UBERON calls it cochlea
UBERON:0001780 (1 mention) - the report calls it "Peripheral nerve (→ sensory neuropathy)"; UBERON calls it spinal nerve, and lists "spinal nerve tree" among its other names
UBERON:0001003 (1 mention) - the report calls it "Skin / epidermis (→ ichthyosis)"; UBERON calls it skin epidermis
CL:0000855 (1 mention) - the report calls it "Cochlear hair cell"; CL calls it sensory hair cell

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Inheritance

Discussions and Knowledge Gaps

Pathophysiology

Pathograph

Phenotypes

Genetic Associations

External Assertions

Medical Actions

Diagnosis

Prevalence

Differential Diagnoses

Animal Models

Source YAML

References & Deep Research

Deep Research

Key Findings

Finding 1 — Siddiqi syndrome is an autosomal recessive deafness–dystonia disorder caused by biallelic loss-of-function FITM2 variants

Finding 2 — FITM2 encodes an ER acyl-CoA diphosphatase whose loss disrupts ER homeostasis

Finding 3 — Disease identifiers and nosology

Finding 4 — Core phenotype: deafness–dystonia–developmental regression with growth failure and ichthyosis

Finding 5 — FITM2 variant catalog: predominantly biallelic truncating, with some missense

Finding 6 — Model organisms recapitulate disease and reveal an essential ER lipid-homeostasis function

Finding 7 — Allelic dosage series: null alleles cause Siddiqi syndrome, hypomorphic alleles cause milder HSP

Finding 8 — Diagnosis, prognosis, and management

Finding 9 — Pathophysiological causal chain and affected anatomy

Finding 10 — Inheritance and epidemiology

Finding 11 — Final synthesis

Section-by-Section Report Content

1. Disease Information

2. Etiology

3. Phenotypes

4. Genetic/Molecular Information

5. Environmental Information

6. Mechanism / Pathophysiology

7. Anatomical Structures Affected

8. Temporal Development

9. Inheritance and Population

10. Diagnostics

11. Outcome/Prognosis

12. Treatment

13. Prevention

14. Other Species / Natural Disease

15. Model Organisms

Mechanistic Model / Interpretation

Evidence Base

Limitations and Knowledge Gaps

Proposed Follow-up Experiments / Actions

Artifacts

Reference Validation

Term Validation

Terms the report names something else

Terms whose name is worth a second look