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Classifications

ICIMD (Inherited Metabolic Disorders)

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Mappings

MONDO

MONDO:0010469 epsilon-trimethyllysine hydroxylase deficiency

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Inheritance

1

X-linked recessive HP:0001419

TMLHE maps to the X chromosome; affected individuals are hemizygous males, with carrier mothers transmitting the deletion or point mutation. The male-biased ascertainment of the disorder follows directly from its X-linked hemizygous genetics.

X-linked recessive inheritance

Show evidence (2 references)

PMID:22566635 SUPPORT Human Clinical

"TMLHE maps to the X chromosome and encodes the first enzyme in carnitine biosynthesis, 6-N-trimethyllysine dioxygenase."

Establishes X-linked inheritance and the enzymatic role of TMLHE.

PMID:23092983 SUPPORT Human Clinical

"was identified in two brothers with autism and ID"

Segregation in two affected brothers is consistent with X-linked recessive transmission.

◈

Mechanistic Hypotheses

1

Deficient endogenous carnitine biosynthesis produces a brain-selective carnitine deficit that raises autism risk in males

brain_carnitine_deficiency_autism EMERGING

Evidence balance 1 support

Because most body carnitine is dietary and plasma carnitine is often normal in TMLHE-deficient males, the proposed route from the biosynthetic block to neurodevelopmental risk is a brain-selective carnitine deficit: endogenous synthesis is argued to matter disproportionately in the central nervous system, where dietary carnitine must cross the blood-brain barrier. The hypothesis further invokes a male bias from X-linked carnitine-transport genes and posits modifiable environmental co-factors (infant diet, minor illness, microbiome, drugs) that would explain the low penetrance. It is explicitly framed by its proponents as a hypothesis to be tested, and the supporting human data are association-level rather than mechanistic. Causal edges belonging to this model opt in via hypothesis_groups: [brain_carnitine_deficiency_autism].

Pathograph links

Reduced Endogenous Carnitine Supply -> Low-Penetrance Neurodevelopmental Susceptibility A brain-selective carnitine deficit is hypothesized to raise neurodevelopmental risk directly; this edge belongs to the brain-carnitine-deficiency model rather than to established causal knowledge.

Show evidence (1 reference)

PMID:28703319 SUPPORT DIRECT Other

"We hypothesize that nonsyndromic or "essential" autism involves extreme male bias in infants who are genetically normal, but they develop deficiency of carnitine and perhaps other nutrients in the brain causing autism"

States the hypothesis this group models, in the proponents' own words. Graded OTHER because the source is an explicitly hypothesis-generating essay rather than a report of primary data; the hypothesis status is carried by this entry's mechanistic_hypotheses status, not by the supports value.

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Discussions and Knowledge Gaps

2

Does the Tmlhe knockout mouse, which shows >90% carnitine reduction without any ASD-like behavioral, cognitive, or motor phenotype, refute the neural-stem-cell mechanism proposed for human TMLHE deficiency, or does it reflect a genuine species difference in the dependence of brain development on endogenous carnitine synthesis?

HUMAN MODEL MISMATCH OPEN tmlhe_mouse_ko_no_asd_phenotype

Attached to

pathophysiology#Impaired Long-Chain Fatty Acid Oxidation in Neural Progenitors pathophysiology#Shift from Neural Stem Cell Self-Renewal to Differentiating Division pathophysiology#Low-Penetrance Neurodevelopmental Susceptibility

The mechanistic chain in this entry rests substantially on mouse embryonic neocortex data showing that neural-stem-cell-autonomous TMLHE insufficiency depletes the progenitor pool. A constitutive Tmlhe knockout mouse with extremely low carnitine and gamma-butyrobetaine nevertheless showed no social, cognitive, repetitive-behavior, or motor abnormality, and normal life expectancy — and did not phenocopy primary carnitine deficiency either. The two mouse results are not formally contradictory (acute NSC-autonomous knockdown during corticogenesis versus a constitutive germline knockout with lifelong compensation), but the mismatch is material: it is part of why the human association is best read as a low-penetrance risk relationship, and it means model-organism evidence cannot by itself carry the human neurodevelopmental claim. This is a translational-validity question (evidence exists in the model; its bearing on human disease is uncertain), not an absence of evidence.

Proposed experiments

Developmentally timed conditional Tmlhe inactivation in neural progenitors

exp_tmlhe_conditional_progenitor_knockout

Conditionally inactivate Tmlhe restricted to neural progenitors during corticogenesis, with progenitor-pool size, division-mode lineage tracing, and cortical layering readouts, to test whether the constitutive knockout's normality reflects developmental compensation rather than absence of a progenitor requirement.

Direct brain (not plasma) carnitine measurement in TMLHE deficiency

exp_tmlhe_brain_carnitine_measurement

Measure carnitine and gamma-butyrobetaine directly in brain tissue of Tmlhe knockout mice and, where obtainable, in human TMLHE-deficient CSF or tissue, to test the brain-selective-deficit premise that the whole mechanistic model depends on.

Human TMLHE-null iPSC cortical organoid self-renewal assay

exp_tmlhe_human_ipsc_cortical_organoid

Generate isogenic human iPSC cortical organoids carrying TMLHE loss-of-function alleles and assay neural stem cell self-renewal versus symmetric differentiating division, to test whether the mouse NSC phenotype is reproduced in a human system.

Show evidence (4 references)

PMID:37553674 REFUTE Model Organism

"we did not observe any significant social, cognitive, or repetitive-behavior changes associated with ASD in the knockout mice; muscle strength and coordination were also not affected"

The constitutive Tmlhe knockout fails to reproduce an ASD-like phenotype, refuting a simple deterministic carnitine-deficiency-to-autism model.

PMID:37553674 REFUTE Model Organism

"inactivation of TMLD does not induce a phenotype similar to previously described primary carnitine deficiency"

Shows that in mice a >90% carnitine reduction is tolerated, distinguishing the biosynthetic block from carnitine-transport disease.

PMID:34728372 SUPPORT DIRECT Model Organism

"TMLHE gene deletion in male mice lowered acylcarnitine concentrations in blood and cardiac tissues by up to 85% and decreased fatty acid oxidation by 30% but did not affect muscle and heart function in mice."

An independent knockout cohort confirms substantial biochemical effect with preserved organ function, reinforcing that the biochemical deficit is not straightforwardly pathogenic. The quoted sentence asserts both halves of that gap.

+ 1 more reference

What determines which 2-4% of TMLHE-deficient males develop autism, when roughly 1 in 366 control males carry the same enzyme deficiency and remain healthy?

KNOWLEDGE GAP OPEN tmlhe_penetrance_modifiers

Attached to

pathophysiology#Low-Penetrance Neurodevelopmental Susceptibility

The penetrance gap is the defining unresolved feature of this entry and the reason it is curated as a susceptibility relationship rather than a Mendelian disease. Candidate modifiers proposed in the literature — dietary carnitine intake in infancy, X-linked carnitine transport across the blood-brain barrier, minor illness, microbiome, and drug exposures — are hypotheses, not established modifiers, and no modifier has been demonstrated to stratify affected from unaffected deficient males. Without this, TMLHE genotype has essentially no predictive value for an individual.

Proposed experiments

Prospective outcome cohort of screening-identified TMLHE-deficient males

exp_tmlhe_prospective_deficient_male_cohort

Follow an unselected cohort of TMLHE-deficient males identified by population screening, recording dietary carnitine intake, intercurrent illness, and neurodevelopmental outcome, to identify factors that stratify the 2-4% who become affected from the majority who do not.

Carnitine supplementation trial in infant siblings of TMLHE-deficient probands

exp_tmlhe_sibling_carnitine_prevention_trial

Randomized carnitine supplementation in unaffected infant siblings of TMLHE-deficient autism probands — the trial design explicitly proposed by the brain-carnitine-deficiency hypothesis — to test whether the neurodevelopmental risk is modifiable.

Show evidence (2 references)

PMID:22566635 SUPPORT Human Clinical

"although with low penetrance (2-4%)"

Quantifies the penetrance gap that this knowledge gap concerns.

PMID:28703319 SUPPORT DIRECT Other

"A mixed, common gene variant-environment hypothesis is proposed with diet, minor illnesses, microbiome, and drugs as possible risk modifiers."

Enumerates the candidate modifiers this knowledge gap names. They are proposed rather than validated, which is what makes this a gap; that is the discussion's claim and the quote states it.

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Pathophysiology

6

TMLHE Loss of Function

Loss-of-function variants in TMLHE — most commonly a recurrent deletion of exon 2, but also nonsense and missense substitutions — abolish or reduce the activity of 6-N-trimethyllysine dioxygenase (TMLD), the enzyme that catalyzes the first step of carnitine biosynthesis.

TMLHE hgnc:18308

trimethyllysine dioxygenase activity GO:0050353 ↓ DECREASED

Downstream

Block at the First Step of Carnitine Biosynthesis Loss of TMLD activity blocks the committed first hydroxylation step of the carnitine biosynthetic pathway.

Show evidence (2 references)

PMID:21865298 SUPPORT Human Clinical

"in a male proband, an exonic deletion of the TMLHE (trimethyllysine hydroxylase epsilon) that encodes the first enzyme in the biosynthesis of carnitine"

First report of an exonic TMLHE deletion, identifying the lesion.

PMID:23092983 SUPPORT In Vitro

"Functional analyses confirmed that the mutations were associated with a loss-of-function"

Functional assays confirm that the identified TMLHE variants are loss-of-function.

Block at the First Step of Carnitine Biosynthesis

The enzymatic block produces the diagnostic biochemical signature of the disorder: the substrate 6-N-trimethyllysine accumulates while the immediate product 3-hydroxy-6-N-trimethyllysine and the downstream intermediate gamma-butyrobetaine fall, measurable as a markedly reduced BB/TML ratio in plasma and urine.

carnitine biosynthetic process GO:0045329 ↓ DECREASED

Downstream

Reduced Endogenous Carnitine Supply Loss of flux through the biosynthetic pathway removes the endogenous contribution to whole-body and tissue carnitine pools.

Show evidence (1 reference)

PMID:22566635 SUPPORT Human Clinical

"Deletion of exon 2 of TMLHE causes enzyme deficiency, resulting in increased substrate concentration (6-N-trimethyllysine) and decreased product levels (3-hydroxy-6-N-trimethyllysine and γ-butyrobetaine) in plasma and urine."

Defines the substrate-accumulation/product-depletion signature of the biosynthetic block.

Reduced Endogenous Carnitine Supply

Endogenous synthesis is only one of two carnitine sources; most body carnitine derives from the diet, so plasma free carnitine in TMLHE-deficient males is frequently normal or only mildly reduced and the biochemical diagnosis rests on the TML/BB profile rather than on carnitine itself. The endogenous deficit is therefore proposed to matter chiefly where dietary supply or transport is limiting — most notably across the blood-brain barrier — and is unmasked clinically only in a small minority of deficient individuals. In at least one reported child, TMLHE deficiency co-occurred with frank carnitine deficiency.

Deficient endogenous carnitine biosynthesis produces a brain-selective carnitine deficit that raises autism risk in males

Downstream

Impaired Long-Chain Fatty Acid Oxidation in Neural Progenitors Reduced carnitine availability limits carnitine-dependent mitochondrial import of long-chain fatty acids in neural progenitors.

Low-Penetrance Neurodevelopmental Susceptibility A brain-selective carnitine deficit is hypothesized to raise neurodevelopmental risk directly; this edge belongs to the brain-carnitine-deficiency model rather than to established causal knowledge.

Show evidence (2 references)

PMID:25943046 SUPPORT Human Clinical

"was identified to have a mutation in the TMLHE gene, which encodes the first enzyme in the carnitine biosynthesis pathway, and concurrent carnitine deficiency"

Documents concurrent carnitine deficiency in a TMLHE-deficient child.

PMID:28703319 SUPPORT Other

"Although 1 in 350 males (estimated ~460,000 males in USA) have TMLHE deficiency, only about 3% of these males develop autism, with most of the remainder becoming healthy adults"

Establishes that the endogenous-synthesis deficit is compensated in the great majority of deficient males.

Impaired Long-Chain Fatty Acid Oxidation in Neural Progenitors

Carnitine is required for the carnitine-shuttle import of long-chain fatty acids into mitochondria. Neural-stem-cell-autonomous reduction of TMLHE activity — like reduction of CPT1A or of lipid-droplet fatty acid mobilization — lowers flux through long-chain fatty acid beta-oxidation in the embryonic neocortex, positioning TMLHE within a broader class of fatty-acid-oxidation inborn errors associated with neurodevelopmental disorders.

neural stem cell CL:0000047

fatty acid beta-oxidation GO:0006635 ↓ DECREASED long-chain fatty acid metabolic process GO:0001676 ↓ DECREASED

Downstream

Shift from Neural Stem Cell Self-Renewal to Differentiating Division Reduced fatty-acid-oxidation flux changes the division mode of neural stem cells.

Show evidence (1 reference)

PMID:26832401 SUPPORT Model Organism

"neural stem cell (NSC)-autonomous insufficiencies in the activity of TMLHE (an autism risk factor that supports long-chain FAO by catalyzing carnitine biosynthesis), of CPT1A (an enzyme required for long-chain FAO transport into mitochondria), or of fatty acid mobilization from lipid droplets..."

Shows in the mouse embryonic neocortex that TMLHE insufficiency acts through long-chain fatty acid oxidation to deplete the neural stem cell pool.

Shift from Neural Stem Cell Self-Renewal to Differentiating Division

Lineage tracing in the mouse embryonic neocortex showed that reduced fatty-acid-oxidation flux drives neural stem cells toward symmetric differentiating divisions at the expense of self-renewing division modes, depleting the progenitor pool and altering the neural-stem-cell-to-intermediate-progenitor transition. This is the proposed cellular mechanism connecting fatty-acid-oxidation inborn errors to neurodevelopmental outcome.

neural stem cell CL:0000047

stem cell population maintenance GO:0019827 ↓ DECREASED neural precursor cell proliferation GO:0061351 ⚠ ABNORMAL

Downstream

Low-Penetrance Neurodevelopmental Susceptibility Progenitor-pool depletion during corticogenesis is the proposed developmental substrate for the increased neurodevelopmental risk.

Show evidence (1 reference)

PMID:26832401 SUPPORT Model Organism

"Lineage tracing experiments demonstrated that reduced flux through the FAO pathway potentiated NSC symmetric differentiating divisions at the expense of self-renewing stem cell division modes."

Identifies the division-mode shift as the cellular consequence of reduced FAO flux.

Low-Penetrance Neurodevelopmental Susceptibility

The clinical output of TMLHE deficiency is an increase in neurodevelopmental risk rather than a deterministic phenotype. Meta-analysis across simplex and multiplex autism cohorts supported TMLHE deficiency as an autism risk factor with an estimated penetrance of only 2-4%; most hemizygous males are healthy, and TMLHE deficiency accounts for well under 1% of autism in males. The unidentified modifiers (genetic background, dietary carnitine intake, carnitine transport across the blood-brain barrier, environmental exposures) that determine which deficient males become affected are the central open question of this entry.

Show evidence (2 references)

PMID:22566635 SUPPORT Human Clinical

"suggesting that TMLHE deficiency is a risk factor for autism (metaanalysis Z-score = 2.90 and P = 0.0037), although with low penetrance (2-4%)"

Establishes the risk-factor relationship and quantifies the low penetrance.

PMID:28703319 SUPPORT Other

"furthermore, less than 1% of autistic males have TMLHE deficiency"

Bounds the attributable fraction of autism in males explained by TMLHE deficiency.

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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

3

Autism Neurological HP:0000717

Show evidence (2 references)

PMID:22566635 SUPPORT Human Clinical

"suggesting that TMLHE deficiency is a risk factor for autism (metaanalysis Z-score = 2.90 and P = 0.0037), although with low penetrance (2-4%)"

Establishes autism as the risk-associated phenotype, with explicit low penetrance.

PMID:39845198 SUPPORT Human Clinical

"Lack of TMLH enzyme activity is associated with developmental delay and autistic behaviours described as X-linked recessive autism, type 6 (OMIM#300872)."

Confirms the autism/developmental-delay presentation in a molecularly confirmed adult case.

Intellectual disability Neurological HP:0001249

Show evidence (2 references)

PMID:39845198 SUPPORT Human Clinical

"Moderate intellectual disability along with obsessive and aggressive behaviour in the context of autism spectrum disorders was established as well as symptoms from the catatonic spectrum."

Documents moderate intellectual disability in a confirmed TMLHE-deficient adult male.

PMID:23092983 SUPPORT Human Clinical

"was identified in two brothers with autism and ID"

Intellectual disability accompanied autism in two brothers with a TMLHE nonsense mutation.

Developmental regression Neurological HP:0002376

Show evidence (1 reference)

PMID:25943046 SUPPORT Human Clinical

"A 4-year-old male with autism and two episodes of neurodevelopmental regression was identified to have a mutation in the TMLHE gene"

Documents neurodevelopmental regression in a TMLHE-deficient child.

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Genetic Associations

1

TMLHE loss-of-function variant (Low-penetrance risk factor for autism; deterministic for the biochemical enzyme deficiency)

Gene: TMLHE hgnc:18308 relationship_type: RISK_FACTOR

Show evidence (2 references)

PMID:22566635 SUPPORT Human Clinical

"Deletion of exon 2 of TMLHE causes enzyme deficiency"

The recurrent exon 2 deletion causes the enzyme deficiency.

PMID:23092983 SUPPORT Human Clinical

"By screening the TMLHE coding sequence in 501 male patients with ASD, we identified two additional missense substitutions not found in controls and not reported in databases."

Documents the allelic spectrum beyond the recurrent exon 2 deletion.

💊

Medical Actions

1

Levocarnitine supplementation

Action: nutritional supplementation Ontology label: Nutritional Support NCIT:C15433

Agent: levocarnitine CHEBI:16347

Oral carnitine (levocarnitine) supplementation bypasses the biosynthetic block by supplying carnitine exogenously. Evidence in TMLHE deficiency is limited to a single reported child in whom regression stopped and developmental gains resumed after supplementation was started; there is no controlled trial, and the natural history of the great majority of untreated deficient males is benign. Treat as a reasonable, low-risk intervention in the specific setting of documented carnitine deficiency with regression, not as an established therapy for TMLHE deficiency in general.

Mechanism Target:

BYPASSES Reduced Endogenous Carnitine Supply — Exogenous carnitine restores the pool that the blocked biosynthetic pathway fails to supply, without correcting the enzymatic lesion.

Target Phenotypes: Developmental regression HP:0002376

Show evidence (2 references)

PMID:25943046 SUPPORT Human Clinical

"Following carnitine supplementation, the patient's regression ended, and the boy started gaining developmental milestones."

Single-case evidence that carnitine supplementation halted regression.

PMID:25943046 SUPPORT DIRECT Human Clinical

"Further work to better define the role of disorders of carnitine biosynthesis in autism spectrum disorder is warranted."

The authors themselves qualify the single-case result. The claim this supports is the cautious framing of this treatment, which the quoted sentence asserts directly.

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Biochemical Markers

2

6-N-trimethyllysine (TML) (INCREASED)

Context: Accumulation of the TMLD substrate 6-N-trimethyllysine in plasma and urine is the proximal biochemical consequence of the enzyme block and the primary positive diagnostic analyte.

Show evidence (2 references)

PMID:22566635 SUPPORT Human Clinical

"resulting in increased substrate concentration (6-N-trimethyllysine) and decreased product levels (3-hydroxy-6-N-trimethyllysine and γ-butyrobetaine) in plasma and urine"

Substrate accumulation in plasma and urine is the diagnostic signature.

PMID:23092983 SUPPORT Human Clinical

"led to an increase in trimethyllysine, the precursor of carnitine biosynthesis, in the plasma of patients"

Independent confirmation of plasma trimethyllysine elevation in patients.

Gamma-butyrobetaine (BB) and the BB/TML ratio (DECREASED)

Context: Gamma-butyrobetaine, the immediate precursor of carnitine and a downstream product of the blocked step, is reduced. The ratio of gamma-butyrobetaine to trimethyllysine (BB/TML) is the discriminating index used to confirm pathogenicity of a candidate TMLHE variant, and is more informative than either analyte alone.

Show evidence (1 reference)

PMID:39845198 SUPPORT Human Clinical

"increased concentration of trimethyllysine and decreased concentration of γ-butyrobetaine were found resulting in a significantly decreased BB/TML ratio, confirming the pathogenicity of this variant"

The decreased BB/TML ratio was used to confirm variant pathogenicity.

🔬

Diagnosis

1

Biochemical and molecular diagnosis

Diagnosis rests on the plasma/urine carnitine-biosynthesis intermediate profile — elevated trimethyllysine with reduced gamma-butyrobetaine, giving a low BB/TML ratio — confirmed by identification of a TMLHE loss-of-function allele on sequencing or exon-level copy-number analysis. Plasma free carnitine may be normal and is not a reliable screening analyte for this disorder.

Show evidence (1 reference)

PMID:39845198 SUPPORT Human Clinical

"With WES, a novel variant in the TMHLE gene was identified and using NGMS, increased concentration of trimethyllysine and decreased concentration of γ-butyrobetaine were found"

Illustrates the combined molecular plus metabolomic diagnostic route.

📊

Prevalence

2

Control males (North American research cohorts screened for the TMLHE exon 2 deletion)

Point Prevalence 273.0 per 100,000 >1 in 1,000

24 of 8,787 control males (1 in 366) carried the TMLHE exon 2 deletion. This is a hemizygous-male frequency, not a population-wide rate, and is unusually high for an inborn error of metabolism — most deficient males are healthy adults.

Show evidence (1 reference)

PMID:22566635 SUPPORT Human Clinical

"TMLHE deficiency is common in control males (24 in 8,787 or 1 in 366) and was not significantly increased in frequency in probands from simplex autism families (9 in 2,904 or 1 in 323)."

Quantifies the frequency of TMLHE deficiency in control males and in simplex autism probands.

Probands from male-male multiplex autism families

Point Prevalence 769.0 per 100,000 >1 in 1,000

7 of 909 probands from male-male multiplex autism families (1 in 130), a 2.82-fold enrichment over control males.

Show evidence (1 reference)

PMID:22566635 SUPPORT Human Clinical

"it was 2.82-fold more frequent in probands from male-male multiplex autism families compared with controls (7 in 909 or 1 in 130; P = 0.023)"

Quantifies enrichment of TMLHE deficiency in male-male multiplex autism families.

🔀

Differential Diagnoses

3

Conditions with similar clinical presentations that must be differentiated from TMLHE Deficiency:

Primary Carnitine Deficiency MONDO:0008919

Overlapping Features SLC22A5/OCTN2 carnitine transport defect. Shares reduced carnitine availability but is distinguished by markedly low plasma free carnitine (the first-tier newborn-screening analyte), a normal TML/BB profile, and a systemic phenotype of hypoketotic hypoglycaemia, cardiomyopathy, and myopathy that TMLHE deficiency does not produce.

Distinguishing Features

Markedly low plasma free carnitine, rather than a discriminating low BB/TML ratio, favors primary carnitine deficiency.
Systemic hypoketotic hypoglycemia, cardiomyopathy, and myopathy favor SLC22A5 deficiency.

Carnitine Palmitoyltransferase II Deficiency MONDO:0015515

Overlapping Features Carnitine-cycle defect blocking mitochondrial import of long-chain acyl groups. Presents with rhabdomyolysis or infantile hepatocardiomuscular disease and an abnormal long-chain acylcarnitine profile, none of which is a feature of TMLHE deficiency.

Distinguishing Features

Rhabdomyolysis or infantile hepatocardiomuscular disease favors CPT II deficiency.
A long-chain acylcarnitine abnormality distinguishes CPT II deficiency from the TML/BB biosynthesis profile.

Autism Spectrum Disorder MONDO:0005258

Overlapping Features Idiopathic/nonsyndromic ASD. Because TMLHE deficiency has 2-4% penetrance and explains under 1% of autism in males, the biochemical profile — not the ASD diagnosis — is what establishes the metabolic diagnosis in an autistic male.

Distinguishing Features

The elevated TML and reduced gamma-butyrobetaine/BB-to-TML profile establishes TMLHE deficiency; autism alone does not.

{ }

Source YAML

click to show

name: TMLHE Deficiency
creation_date: "2026-07-26T00:00:00Z"
description: >-
  TMLHE deficiency (epsilon-trimethyllysine hydroxylase deficiency) is an X-linked
  inborn error of carnitine biosynthesis caused by loss-of-function variants —
  most commonly a recurrent deletion of exon 2 — in TMLHE, which encodes
  6-N-trimethyllysine dioxygenase (TMLD), the first enzyme of the
  four-step endogenous carnitine biosynthesis pathway. Enzyme deficiency produces a
  characteristic biochemical signature: accumulation of the substrate
  6-N-trimethyllysine (TML) with reduced levels of the downstream products
  3-hydroxy-6-N-trimethyllysine and gamma-butyrobetaine (BB), giving a markedly
  decreased BB/TML ratio in plasma and urine.

  TMLHE deficiency is unusual among inborn errors of metabolism in two respects.
  First, it is common — roughly 1 in 366 control males — because most body carnitine
  is obtained from the diet rather than synthesized de novo, so hemizygous loss of
  the biosynthetic pathway is largely compensated and the great majority of affected
  males are healthy. Second, its disease status is therefore a low-penetrance risk
  relationship rather than a deterministic Mendelian one: TMLHE deficiency is
  enriched in probands from male-male multiplex autism families (an estimated 2-4%
  penetrance for autism), and individual case reports document autism with
  intellectual disability and regressive episodes in deficient males, but most
  deficient males never develop a neurodevelopmental phenotype. MONDO reflects this
  by classifying the entity in its `predisposition` / `omim_susceptibility` subsets
  under "autism, susceptibility to" rather than as a fully penetrant metabolic
  disease.

  The proposed mechanism links the biosynthetic block to neurodevelopment through
  long-chain fatty acid oxidation: carnitine is required to shuttle long-chain fatty
  acids into mitochondria, and neural-stem-cell-autonomous reduction of TMLHE
  activity in the mouse embryonic neocortex shifts progenitors from self-renewing to
  symmetric differentiating divisions, depleting the neural stem cell pool. Whether
  this translates to human disease remains open: a constitutive Tmlhe knockout mouse
  with >90% carnitine reduction showed no ASD-like behavioral or motor phenotype,
  which is curated here as an explicit human/model mismatch.

  This entry is curated as a distinct disorder — rather than folded into the
  carnitine-transport (SLC22A5) or carnitine-cycle (CPT1A/CPT2/SLC25A20) entries —
  because the lesion is in carnitine *biosynthesis* rather than uptake or
  mitochondrial transport, and because the clinical consequence is a low-penetrance
  neurodevelopmental risk rather than the fasting-intolerance/cardiomyopathy
  phenotype of those disorders.
synonyms:
- Epsilon-trimethyllysine hydroxylase deficiency
- TMLHED
- 6-N-trimethyllysine dioxygenase deficiency
- TMLD deficiency
- Autism, susceptibility to, X-linked 6
- AUTSX6
category: Mendelian
disease_term:
  preferred_term: epsilon-trimethyllysine hydroxylase deficiency
  term:
    id: MONDO:0010469
    label: epsilon-trimethyllysine hydroxylase deficiency
classifications:
  icimd_category:
  - classification_value: carnitine_metabolism
    notes: >-
      TMLHE deficiency blocks the first step of endogenous carnitine
      biosynthesis and belongs to the ICIMD disorders-of-carnitine-metabolism
      category (WP-010).
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0010469
      label: epsilon-trimethyllysine hydroxylase deficiency
    mapping_predicate: skos:exactMatch
    mapping_source: MONDO
inheritance:
- name: X-linked recessive
  description: >-
    TMLHE maps to the X chromosome; affected individuals are hemizygous males,
    with carrier mothers transmitting the deletion or point mutation. The
    male-biased ascertainment of the disorder follows directly from its X-linked
    hemizygous genetics.
  inheritance_term:
    preferred_term: X-linked recessive inheritance
    term:
      id: HP:0001419
      label: X-linked recessive inheritance
  evidence:
  - reference: PMID:22566635
    reference_title: "A common X-linked inborn error of carnitine biosynthesis may be a risk factor for nondysmorphic autism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "TMLHE maps to the X chromosome and encodes the first enzyme in carnitine biosynthesis, 6-N-trimethyllysine dioxygenase."
    explanation: Establishes X-linked inheritance and the enzymatic role of TMLHE.
  - reference: PMID:23092983
    reference_title: "Analysis of the chromosome X exome in patients with autism spectrum disorders identified novel candidate genes, including TMLHE."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "was identified in two brothers with autism and ID"
    explanation: Segregation in two affected brothers is consistent with X-linked recessive transmission.
prevalence:
- population: Control males (North American research cohorts screened for the TMLHE exon 2 deletion)
  measure_type: POINT_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 273.0
  notes: >-
    24 of 8,787 control males (1 in 366) carried the TMLHE exon 2 deletion. This is
    a hemizygous-male frequency, not a population-wide rate, and is unusually high
    for an inborn error of metabolism — most deficient males are healthy adults.
  evidence:
  - reference: PMID:22566635
    reference_title: "A common X-linked inborn error of carnitine biosynthesis may be a risk factor for nondysmorphic autism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "TMLHE deficiency is common in control males (24 in 8,787 or 1 in 366) and was not significantly increased in frequency in probands from simplex autism families (9 in 2,904 or 1 in 323)."
    explanation: Quantifies the frequency of TMLHE deficiency in control males and in simplex autism probands.
- population: Probands from male-male multiplex autism families
  measure_type: POINT_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 769.0
  notes: >-
    7 of 909 probands from male-male multiplex autism families (1 in 130), a
    2.82-fold enrichment over control males.
  evidence:
  - reference: PMID:22566635
    reference_title: "A common X-linked inborn error of carnitine biosynthesis may be a risk factor for nondysmorphic autism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "it was 2.82-fold more frequent in probands from male-male multiplex autism families compared with controls (7 in 909 or 1 in 130; P = 0.023)"
    explanation: Quantifies enrichment of TMLHE deficiency in male-male multiplex autism families.
mechanistic_hypotheses:
- hypothesis_group_id: brain_carnitine_deficiency_autism
  hypothesis_label: >-
    Deficient endogenous carnitine biosynthesis produces a brain-selective carnitine
    deficit that raises autism risk in males
  status: EMERGING
  description: >-
    Because most body carnitine is dietary and plasma carnitine is often normal in
    TMLHE-deficient males, the proposed route from the biosynthetic block to
    neurodevelopmental risk is a brain-selective carnitine deficit: endogenous
    synthesis is argued to matter disproportionately in the central nervous system,
    where dietary carnitine must cross the blood-brain barrier. The hypothesis
    further invokes a male bias from X-linked carnitine-transport genes and posits
    modifiable environmental co-factors (infant diet, minor illness, microbiome,
    drugs) that would explain the low penetrance. It is explicitly framed by its
    proponents as a hypothesis to be tested, and the supporting human data are
    association-level rather than mechanistic. Causal edges belonging to this model
    opt in via hypothesis_groups: [brain_carnitine_deficiency_autism].
  evidence:
  - reference: PMID:28703319
    reference_title: "Brain carnitine deficiency causes nonsyndromic autism with an extreme male bias: A hypothesis."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: OTHER
    snippet: "We hypothesize that nonsyndromic or \"essential\" autism involves extreme male bias in infants who are genetically normal, but they develop deficiency of carnitine and perhaps other nutrients in the brain causing autism"
    explanation: >-
      States the hypothesis this group models, in the proponents' own words.
      Graded OTHER because the source is an explicitly hypothesis-generating
      essay rather than a report of primary data; the hypothesis status is
      carried by this entry's mechanistic_hypotheses status, not by the
      supports value.
pathophysiology:
- name: TMLHE Loss of Function
  biological_scale: MOLECULAR
  description: >-
    Loss-of-function variants in TMLHE — most commonly a recurrent deletion of exon
    2, but also nonsense and missense substitutions — abolish or reduce the activity
    of 6-N-trimethyllysine dioxygenase (TMLD), the enzyme that catalyzes the first
    step of carnitine biosynthesis.
  genes:
  - preferred_term: TMLHE
    term:
      id: hgnc:18308
      label: TMLHE
  molecular_functions:
  - preferred_term: trimethyllysine dioxygenase activity
    term:
      id: GO:0050353
      label: trimethyllysine dioxygenase activity
    modifier: DECREASED
  evidence:
  - reference: PMID:21865298
    reference_title: "Use of array CGH to detect exonic copy number variants throughout the genome in autism families detects a novel deletion in TMLHE."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "in a male proband, an exonic deletion of the TMLHE (trimethyllysine hydroxylase epsilon) that encodes the first enzyme in the biosynthesis of carnitine"
    explanation: First report of an exonic TMLHE deletion, identifying the lesion.
  - reference: PMID:23092983
    reference_title: "Analysis of the chromosome X exome in patients with autism spectrum disorders identified novel candidate genes, including TMLHE."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Functional analyses confirmed that the mutations were associated with a loss-of-function"
    explanation: Functional assays confirm that the identified TMLHE variants are loss-of-function.
  downstream:
  - target: Block at the First Step of Carnitine Biosynthesis
    description: >-
      Loss of TMLD activity blocks the committed first hydroxylation step of the
      carnitine biosynthetic pathway.
- name: Block at the First Step of Carnitine Biosynthesis
  biological_scale: MOLECULAR
  description: >-
    The enzymatic block produces the diagnostic biochemical signature of the
    disorder: the substrate 6-N-trimethyllysine accumulates while the immediate
    product 3-hydroxy-6-N-trimethyllysine and the downstream intermediate
    gamma-butyrobetaine fall, measurable as a markedly reduced BB/TML ratio in plasma
    and urine.
  biological_processes:
  - preferred_term: carnitine biosynthetic process
    term:
      id: GO:0045329
      label: carnitine biosynthetic process
    modifier: DECREASED
  chemical_entities:
  - preferred_term: N-6-Trimethyllysine
    term:
      id: CHEBI:165870
      label: N-6-Trimethyllysine
    modifier: INCREASED
  evidence:
  - reference: PMID:22566635
    reference_title: "A common X-linked inborn error of carnitine biosynthesis may be a risk factor for nondysmorphic autism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Deletion of exon 2 of TMLHE causes enzyme deficiency, resulting in increased substrate concentration (6-N-trimethyllysine) and decreased product levels (3-hydroxy-6-N-trimethyllysine and γ-butyrobetaine) in plasma and urine."
    explanation: Defines the substrate-accumulation/product-depletion signature of the biosynthetic block.
  downstream:
  - target: Reduced Endogenous Carnitine Supply
    description: >-
      Loss of flux through the biosynthetic pathway removes the endogenous
      contribution to whole-body and tissue carnitine pools.
- name: Reduced Endogenous Carnitine Supply
  biological_scale: ORGANISM
  description: >-
    Endogenous synthesis is only one of two carnitine sources; most body carnitine
    derives from the diet, so plasma free carnitine in TMLHE-deficient males is
    frequently normal or only mildly reduced and the biochemical diagnosis rests on
    the TML/BB profile rather than on carnitine itself. The endogenous deficit is
    therefore proposed to matter chiefly where dietary supply or transport is
    limiting — most notably across the blood-brain barrier — and is unmasked
    clinically only in a small minority of deficient individuals. In at least one
    reported child, TMLHE deficiency co-occurred with frank carnitine deficiency.
  chemical_entities:
  - preferred_term: carnitine
    term:
      id: CHEBI:17126
      label: carnitine
    modifier: DECREASED
  evidence:
  - reference: PMID:25943046
    reference_title: "Improvement of regressive autism symptoms in a child with TMLHE deficiency following carnitine supplementation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "was identified to have a mutation in the TMLHE gene, which encodes the first enzyme in the carnitine biosynthesis pathway, and concurrent carnitine deficiency"
    explanation: Documents concurrent carnitine deficiency in a TMLHE-deficient child.
  - reference: PMID:28703319
    reference_title: "Brain carnitine deficiency causes nonsyndromic autism with an extreme male bias: A hypothesis."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Although 1 in 350 males (estimated ~460,000 males in USA) have TMLHE deficiency, only about 3% of these males develop autism, with most of the remainder becoming healthy adults"
    explanation: >-
      Establishes that the endogenous-synthesis deficit is compensated in the great
      majority of deficient males.
  downstream:
  - target: Impaired Long-Chain Fatty Acid Oxidation in Neural Progenitors
    description: >-
      Reduced carnitine availability limits carnitine-dependent mitochondrial import
      of long-chain fatty acids in neural progenitors.
  - target: Low-Penetrance Neurodevelopmental Susceptibility
    description: >-
      A brain-selective carnitine deficit is hypothesized to raise neurodevelopmental
      risk directly; this edge belongs to the brain-carnitine-deficiency model rather
      than to established causal knowledge.
    hypothesis_groups:
    - brain_carnitine_deficiency_autism
- name: Impaired Long-Chain Fatty Acid Oxidation in Neural Progenitors
  biological_scale: CELLULAR
  description: >-
    Carnitine is required for the carnitine-shuttle import of long-chain fatty acids
    into mitochondria. Neural-stem-cell-autonomous reduction of TMLHE activity — like
    reduction of CPT1A or of lipid-droplet fatty acid mobilization — lowers flux
    through long-chain fatty acid beta-oxidation in the embryonic neocortex,
    positioning TMLHE within a broader class of fatty-acid-oxidation inborn errors
    associated with neurodevelopmental disorders.
  cell_types:
  - preferred_term: neural stem cell
    term:
      id: CL:0000047
      label: neural stem cell
  biological_processes:
  - preferred_term: fatty acid beta-oxidation
    term:
      id: GO:0006635
      label: fatty acid beta-oxidation
    modifier: DECREASED
  - preferred_term: long-chain fatty acid metabolic process
    term:
      id: GO:0001676
      label: long-chain fatty acid metabolic process
    modifier: DECREASED
  evidence:
  - reference: PMID:26832401
    reference_title: "Inborn Errors of Long-Chain Fatty Acid β-Oxidation Link Neural Stem Cell Self-Renewal to Autism."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "neural stem cell (NSC)-autonomous insufficiencies in the activity of TMLHE (an autism risk factor that supports long-chain FAO by catalyzing carnitine biosynthesis), of CPT1A (an enzyme required for long-chain FAO transport into mitochondria), or of fatty acid mobilization from lipid droplets reduced NSC pools in the mouse embryonic neocortex"
    explanation: >-
      Shows in the mouse embryonic neocortex that TMLHE insufficiency acts through
      long-chain fatty acid oxidation to deplete the neural stem cell pool.
  downstream:
  - target: Shift from Neural Stem Cell Self-Renewal to Differentiating Division
    description: >-
      Reduced fatty-acid-oxidation flux changes the division mode of neural stem
      cells.
- name: Shift from Neural Stem Cell Self-Renewal to Differentiating Division
  biological_scale: CELLULAR
  description: >-
    Lineage tracing in the mouse embryonic neocortex showed that reduced
    fatty-acid-oxidation flux drives neural stem cells toward symmetric
    differentiating divisions at the expense of self-renewing division modes,
    depleting the progenitor pool and altering the
    neural-stem-cell-to-intermediate-progenitor transition. This is the proposed
    cellular mechanism connecting
    fatty-acid-oxidation inborn errors to neurodevelopmental outcome.
  cell_types:
  - preferred_term: neural stem cell
    term:
      id: CL:0000047
      label: neural stem cell
  biological_processes:
  - preferred_term: stem cell population maintenance
    term:
      id: GO:0019827
      label: stem cell population maintenance
    modifier: DECREASED
  - preferred_term: neural precursor cell proliferation
    term:
      id: GO:0061351
      label: neural precursor cell proliferation
    modifier: ABNORMAL
  evidence:
  - reference: PMID:26832401
    reference_title: "Inborn Errors of Long-Chain Fatty Acid β-Oxidation Link Neural Stem Cell Self-Renewal to Autism."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Lineage tracing experiments demonstrated that reduced flux through the FAO pathway potentiated NSC symmetric differentiating divisions at the expense of self-renewing stem cell division modes."
    explanation: Identifies the division-mode shift as the cellular consequence of reduced FAO flux.
  downstream:
  - target: Low-Penetrance Neurodevelopmental Susceptibility
    description: >-
      Progenitor-pool depletion during corticogenesis is the proposed developmental
      substrate for the increased neurodevelopmental risk.
- name: Low-Penetrance Neurodevelopmental Susceptibility
  biological_scale: ORGANISM
  description: >-
    The clinical output of TMLHE deficiency is an increase in neurodevelopmental risk
    rather than a deterministic phenotype. Meta-analysis across simplex and multiplex
    autism cohorts supported TMLHE deficiency as an autism risk factor with an
    estimated penetrance of only 2-4%; most hemizygous males are healthy, and TMLHE
    deficiency accounts for well under 1% of autism in males. The unidentified
    modifiers (genetic background, dietary carnitine intake, carnitine transport
    across the blood-brain barrier, environmental exposures) that determine which
    deficient males become affected are the central open question of this entry.
  evidence:
  - reference: PMID:22566635
    reference_title: "A common X-linked inborn error of carnitine biosynthesis may be a risk factor for nondysmorphic autism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "suggesting that TMLHE deficiency is a risk factor for autism (metaanalysis Z-score = 2.90 and P = 0.0037), although with low penetrance (2-4%)"
    explanation: Establishes the risk-factor relationship and quantifies the low penetrance.
  - reference: PMID:28703319
    reference_title: "Brain carnitine deficiency causes nonsyndromic autism with an extreme male bias: A hypothesis."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "furthermore, less than 1% of autistic males have TMLHE deficiency"
    explanation: Bounds the attributable fraction of autism in males explained by TMLHE deficiency.
phenotypes:
- name: Autism
  category: Neurological
  description: >-
    Nondysmorphic autism spectrum disorder is the phenotype for which TMLHE
    deficiency is an established (low-penetrance) risk factor, and the presentation
    in reported affected individuals. Only about 2-4% of TMLHE-deficient males are
    affected, so this is a susceptibility association rather than an obligate
    feature.
  phenotype_term:
    preferred_term: Autism
    term:
      id: HP:0000717
      label: Autism
  evidence:
  - reference: PMID:22566635
    reference_title: "A common X-linked inborn error of carnitine biosynthesis may be a risk factor for nondysmorphic autism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "suggesting that TMLHE deficiency is a risk factor for autism (metaanalysis Z-score = 2.90 and P = 0.0037), although with low penetrance (2-4%)"
    explanation: Establishes autism as the risk-associated phenotype, with explicit low penetrance.
  - reference: PMID:39845198
    reference_title: "X-Linked Autism Type 9 Caused by a Hemizygote Pathogenic Variant in the TMLHE Gene: Etiological Diagnosis in an Adult Male with Moderate Intellectual Disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Lack of TMLH enzyme activity is associated with developmental delay and autistic behaviours described as X-linked recessive autism, type 6 (OMIM#300872)."
    explanation: Confirms the autism/developmental-delay presentation in a molecularly confirmed adult case.
- name: Intellectual disability
  category: Neurological
  description: >-
    Intellectual disability, reported as moderate in a molecularly and biochemically
    confirmed adult male, and present in two brothers carrying a TMLHE nonsense
    mutation. Reported in a small number of affected individuals; frequency across
    TMLHE-deficient males is not established, so no frequency band is asserted.
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  evidence:
  - reference: PMID:39845198
    reference_title: "X-Linked Autism Type 9 Caused by a Hemizygote Pathogenic Variant in the TMLHE Gene: Etiological Diagnosis in an Adult Male with Moderate Intellectual Disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Moderate intellectual disability along with obsessive and aggressive behaviour in the context of autism spectrum disorders was established as well as symptoms from the catatonic spectrum."
    explanation: Documents moderate intellectual disability in a confirmed TMLHE-deficient adult male.
  - reference: PMID:23092983
    reference_title: "Analysis of the chromosome X exome in patients with autism spectrum disorders identified novel candidate genes, including TMLHE."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "was identified in two brothers with autism and ID"
    explanation: Intellectual disability accompanied autism in two brothers with a TMLHE nonsense mutation.
- name: Developmental regression
  category: Neurological
  description: >-
    Episodes of neurodevelopmental regression have been reported in a TMLHE-deficient
    child with concurrent carnitine deficiency; the regression ended after carnitine
    supplementation was started. This is the phenotype that motivates biochemical
    screening of the carnitine-biosynthesis pathway in regressive autism.
  phenotype_term:
    preferred_term: Developmental regression
    term:
      id: HP:0002376
      label: Developmental regression
  evidence:
  - reference: PMID:25943046
    reference_title: "Improvement of regressive autism symptoms in a child with TMLHE deficiency following carnitine supplementation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A 4-year-old male with autism and two episodes of neurodevelopmental regression was identified to have a mutation in the TMLHE gene"
    explanation: Documents neurodevelopmental regression in a TMLHE-deficient child.
biochemical:
- name: 6-N-trimethyllysine (TML)
  presence: INCREASED
  context: >-
    Accumulation of the TMLD substrate 6-N-trimethyllysine in plasma and urine is the
    proximal biochemical consequence of the enzyme block and the primary positive
    diagnostic analyte.
  biomarker_term:
    preferred_term: N-6-Trimethyllysine
    term:
      id: CHEBI:165870
      label: N-6-Trimethyllysine
  evidence:
  - reference: PMID:22566635
    reference_title: "A common X-linked inborn error of carnitine biosynthesis may be a risk factor for nondysmorphic autism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "resulting in increased substrate concentration (6-N-trimethyllysine) and decreased product levels (3-hydroxy-6-N-trimethyllysine and γ-butyrobetaine) in plasma and urine"
    explanation: Substrate accumulation in plasma and urine is the diagnostic signature.
  - reference: PMID:23092983
    reference_title: "Analysis of the chromosome X exome in patients with autism spectrum disorders identified novel candidate genes, including TMLHE."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "led to an increase in trimethyllysine, the precursor of carnitine biosynthesis, in the plasma of patients"
    explanation: Independent confirmation of plasma trimethyllysine elevation in patients.
- name: Gamma-butyrobetaine (BB) and the BB/TML ratio
  presence: DECREASED
  context: >-
    Gamma-butyrobetaine, the immediate precursor of carnitine and a downstream product
    of the blocked step, is reduced. The ratio of gamma-butyrobetaine to
    trimethyllysine (BB/TML) is the discriminating index used to confirm pathogenicity
    of a candidate TMLHE variant, and is more informative than either analyte alone.
  biomarker_term:
    preferred_term: gamma-butyrobetaine
    term:
      id: CHEBI:16244
      label: 4-(trimethylammonio)butanoate
  evidence:
  - reference: PMID:39845198
    reference_title: "X-Linked Autism Type 9 Caused by a Hemizygote Pathogenic Variant in the TMLHE Gene: Etiological Diagnosis in an Adult Male with Moderate Intellectual Disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "increased concentration of trimethyllysine and decreased concentration of γ-butyrobetaine were found resulting in a significantly decreased BB/TML ratio, confirming the pathogenicity of this variant"
    explanation: The decreased BB/TML ratio was used to confirm variant pathogenicity.
genetic:
- name: TMLHE loss-of-function variant
  gene_term:
    preferred_term: TMLHE
    term:
      id: hgnc:18308
      label: TMLHE
  association: Low-penetrance risk factor for autism; deterministic for the biochemical enzyme deficiency
  relationship_type: RISK_FACTOR
  notes: >-
    Two distinct claims should be kept apart. (1) A hemizygous TMLHE
    loss-of-function allele deterministically produces the biochemical phenotype
    (raised trimethyllysine, low gamma-butyrobetaine, low BB/TML ratio) — this is
    established and functionally validated. (2) The same allele confers only a small
    increase in autism risk (estimated 2-4% penetrance), which is why MONDO places
    MONDO:0010469 in its `predisposition` / `omim_susceptibility` subsets under
    "autism, susceptibility to". The recurrent exon 2 deletion is the most common
    allele; nonsense and missense loss-of-function substitutions are also reported.
  evidence:
  - reference: PMID:22566635
    reference_title: "A common X-linked inborn error of carnitine biosynthesis may be a risk factor for nondysmorphic autism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Deletion of exon 2 of TMLHE causes enzyme deficiency"
    explanation: The recurrent exon 2 deletion causes the enzyme deficiency.
  - reference: PMID:23092983
    reference_title: "Analysis of the chromosome X exome in patients with autism spectrum disorders identified novel candidate genes, including TMLHE."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "By screening the TMLHE coding sequence in 501 male patients with ASD, we identified two additional missense substitutions not found in controls and not reported in databases."
    explanation: Documents the allelic spectrum beyond the recurrent exon 2 deletion.
treatments:
- name: Levocarnitine supplementation
  description: >-
    Oral carnitine (levocarnitine) supplementation bypasses the biosynthetic block by
    supplying carnitine exogenously. Evidence in TMLHE deficiency is limited to a
    single reported child in whom regression stopped and developmental gains resumed
    after supplementation was started; there is no controlled trial, and the natural
    history of the great majority of untreated deficient males is benign. Treat as a
    reasonable, low-risk intervention in the specific setting of documented carnitine
    deficiency with regression, not as an established therapy for TMLHE deficiency in
    general.
  treatment_term:
    preferred_term: nutritional supplementation
    term:
      id: NCIT:C15433
      label: Nutritional Support
    therapeutic_agent:
    - preferred_term: levocarnitine
      term:
        id: CHEBI:16347
        label: (R)-carnitine
  target_phenotypes:
  - preferred_term: Developmental regression
    term:
      id: HP:0002376
      label: Developmental regression
  target_mechanisms:
  - target: Reduced Endogenous Carnitine Supply
    treatment_effect: BYPASSES
    description: >-
      Exogenous carnitine restores the pool that the blocked biosynthetic pathway
      fails to supply, without correcting the enzymatic lesion.
  evidence:
  - reference: PMID:25943046
    reference_title: "Improvement of regressive autism symptoms in a child with TMLHE deficiency following carnitine supplementation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Following carnitine supplementation, the patient's regression ended, and the boy started gaining developmental milestones."
    explanation: Single-case evidence that carnitine supplementation halted regression.
  - reference: PMID:25943046
    reference_title: "Improvement of regressive autism symptoms in a child with TMLHE deficiency following carnitine supplementation."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: "Further work to better define the role of disorders of carnitine biosynthesis in autism spectrum disorder is warranted."
    explanation: >-
      The authors themselves qualify the single-case result. The claim this
      supports is the cautious framing of this treatment, which the quoted
      sentence asserts directly.
diagnosis:
- name: Biochemical and molecular diagnosis
  description: >-
    Diagnosis rests on the plasma/urine carnitine-biosynthesis intermediate profile —
    elevated trimethyllysine with reduced gamma-butyrobetaine, giving a low BB/TML
    ratio — confirmed by identification of a TMLHE loss-of-function allele on
    sequencing or exon-level copy-number analysis. Plasma free carnitine may be
    normal and is not a reliable screening analyte for this disorder.
  evidence:
  - reference: PMID:39845198
    reference_title: "X-Linked Autism Type 9 Caused by a Hemizygote Pathogenic Variant in the TMLHE Gene: Etiological Diagnosis in an Adult Male with Moderate Intellectual Disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "With WES, a novel variant in the TMHLE gene was identified and using NGMS, increased concentration of trimethyllysine and decreased concentration of γ-butyrobetaine were found"
    explanation: Illustrates the combined molecular plus metabolomic diagnostic route.
differential_diagnoses:
- name: Primary Carnitine Deficiency
  disease_term:
    preferred_term: systemic primary carnitine deficiency disease
    term:
      id: MONDO:0008919
      label: systemic primary carnitine deficiency disease
  description: >-
    SLC22A5/OCTN2 carnitine transport defect. Shares reduced carnitine availability
    but is distinguished by markedly low plasma free carnitine (the first-tier
    newborn-screening analyte), a normal TML/BB profile, and a systemic phenotype of
    hypoketotic hypoglycaemia, cardiomyopathy, and myopathy that TMLHE deficiency does
    not produce.
  distinguishing_features:
  - >-
    Markedly low plasma free carnitine, rather than a discriminating low BB/TML
    ratio, favors primary carnitine deficiency.
  - >-
    Systemic hypoketotic hypoglycemia, cardiomyopathy, and myopathy favor SLC22A5
    deficiency.
- name: Carnitine Palmitoyltransferase II Deficiency
  disease_term:
    preferred_term: carnitine palmitoyltransferase II deficiency
    term:
      id: MONDO:0015515
      label: carnitine palmitoyltransferase II deficiency
  description: >-
    Carnitine-cycle defect blocking mitochondrial import of long-chain acyl groups.
    Presents with rhabdomyolysis or infantile hepatocardiomuscular disease and an
    abnormal long-chain acylcarnitine profile, none of which is a feature of TMLHE
    deficiency.
  distinguishing_features:
  - >-
    Rhabdomyolysis or infantile hepatocardiomuscular disease favors CPT II
    deficiency.
  - >-
    A long-chain acylcarnitine abnormality distinguishes CPT II deficiency from
    the TML/BB biosynthesis profile.
- name: Autism Spectrum Disorder
  disease_term:
    preferred_term: autism spectrum disorder
    term:
      id: MONDO:0005258
      label: autism spectrum disorder
  description: >-
    Idiopathic/nonsyndromic ASD. Because TMLHE deficiency has 2-4% penetrance and
    explains under 1% of autism in males, the biochemical profile — not the ASD
    diagnosis — is what establishes the metabolic diagnosis in an autistic male.
  distinguishing_features:
  - >-
    The elevated TML and reduced gamma-butyrobetaine/BB-to-TML profile establishes
    TMLHE deficiency; autism alone does not.
discussions:
- discussion_id: tmlhe_mouse_ko_no_asd_phenotype
  prompt: >-
    Does the Tmlhe knockout mouse, which shows >90% carnitine reduction without any
    ASD-like behavioral, cognitive, or motor phenotype, refute the neural-stem-cell
    mechanism proposed for human TMLHE deficiency, or does it reflect a genuine
    species difference in the dependence of brain development on endogenous carnitine
    synthesis?
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Impaired Long-Chain Fatty Acid Oxidation in Neural Progenitors
  - pathophysiology#Shift from Neural Stem Cell Self-Renewal to Differentiating Division
  - pathophysiology#Low-Penetrance Neurodevelopmental Susceptibility
  rationale: >-
    The mechanistic chain in this entry rests substantially on mouse embryonic
    neocortex data showing that neural-stem-cell-autonomous TMLHE insufficiency
    depletes the progenitor pool. A constitutive Tmlhe knockout mouse with extremely
    low carnitine and gamma-butyrobetaine nevertheless showed no social, cognitive,
    repetitive-behavior, or motor abnormality, and normal life expectancy — and did
    not phenocopy primary carnitine deficiency either. The two mouse results are not
    formally contradictory (acute NSC-autonomous knockdown during corticogenesis
    versus a constitutive germline knockout with lifelong compensation), but the
    mismatch is material: it is part of why the human association is best read as a
    low-penetrance risk relationship, and it means model-organism evidence cannot by
    itself carry the human neurodevelopmental claim. This is a translational-validity
    question (evidence exists in the model; its bearing on human disease is
    uncertain), not an absence of evidence.
  proposed_experiments:
  - experiment_id: exp_tmlhe_conditional_progenitor_knockout
    name: Developmentally timed conditional Tmlhe inactivation in neural progenitors
    description: >-
      Conditionally inactivate Tmlhe restricted to neural progenitors during
      corticogenesis, with progenitor-pool size, division-mode lineage tracing, and
      cortical layering readouts, to test whether the constitutive knockout's
      normality reflects developmental compensation rather than absence of a
      progenitor requirement.
  - experiment_id: exp_tmlhe_brain_carnitine_measurement
    name: Direct brain (not plasma) carnitine measurement in TMLHE deficiency
    description: >-
      Measure carnitine and gamma-butyrobetaine directly in brain tissue of Tmlhe
      knockout mice and, where obtainable, in human TMLHE-deficient CSF or tissue, to
      test the brain-selective-deficit premise that the whole mechanistic model
      depends on.
  - experiment_id: exp_tmlhe_human_ipsc_cortical_organoid
    name: Human TMLHE-null iPSC cortical organoid self-renewal assay
    description: >-
      Generate isogenic human iPSC cortical organoids carrying TMLHE loss-of-function
      alleles and assay neural stem cell self-renewal versus symmetric differentiating
      division, to test whether the mouse NSC phenotype is reproduced in a human
      system.
  evidence:
  - reference: PMID:37553674
    reference_title: "Knockout of Tmlhe in mice is not associated with autism spectrum disorder phenotypes or motor dysfunction despite low carnitine levels."
    supports: REFUTE
    evidence_source: MODEL_ORGANISM
    snippet: "we did not observe any significant social, cognitive, or repetitive-behavior changes associated with ASD in the knockout mice; muscle strength and coordination were also not affected"
    explanation: >-
      The constitutive Tmlhe knockout fails to reproduce an ASD-like phenotype,
      refuting a simple deterministic carnitine-deficiency-to-autism model.
  - reference: PMID:37553674
    reference_title: "Knockout of Tmlhe in mice is not associated with autism spectrum disorder phenotypes or motor dysfunction despite low carnitine levels."
    supports: REFUTE
    evidence_source: MODEL_ORGANISM
    snippet: "inactivation of TMLD does not induce a phenotype similar to previously described primary carnitine deficiency"
    explanation: >-
      Shows that in mice a >90% carnitine reduction is tolerated, distinguishing the
      biosynthetic block from carnitine-transport disease.
  - reference: PMID:34728372
    reference_title: "Low cardiac content of long-chain acylcarnitines in TMLHE knockout mice prevents ischaemia-reperfusion-induced mitochondrial and cardiac damage."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: MODEL_ORGANISM
    snippet: "TMLHE gene deletion in male mice lowered acylcarnitine concentrations in blood and cardiac tissues by up to 85% and decreased fatty acid oxidation by 30% but did not affect muscle and heart function in mice."
    explanation: >-
      An independent knockout cohort confirms substantial biochemical effect with
      preserved organ function, reinforcing that the biochemical deficit is not
      straightforwardly pathogenic. The quoted sentence asserts both halves of
      that gap.
  - reference: PMID:26832401
    reference_title: "Inborn Errors of Long-Chain Fatty Acid β-Oxidation Link Neural Stem Cell Self-Renewal to Autism."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "suggest NSC self renewal as a cellular mechanism underlying the association between IEMs and autism"
    explanation: The countervailing mouse evidence that motivates the mechanism this discussion questions.
- discussion_id: tmlhe_penetrance_modifiers
  prompt: >-
    What determines which 2-4% of TMLHE-deficient males develop autism, when roughly 1
    in 366 control males carry the same enzyme deficiency and remain healthy?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Low-Penetrance Neurodevelopmental Susceptibility
  rationale: >-
    The penetrance gap is the defining unresolved feature of this entry and the reason
    it is curated as a susceptibility relationship rather than a Mendelian disease.
    Candidate modifiers proposed in the literature — dietary carnitine intake in
    infancy, X-linked carnitine transport across the blood-brain barrier, minor
    illness, microbiome, and drug exposures — are hypotheses, not established
    modifiers, and no modifier has been demonstrated to stratify affected from
    unaffected deficient males. Without this, TMLHE genotype has essentially no
    predictive value for an individual.
  proposed_experiments:
  - experiment_id: exp_tmlhe_prospective_deficient_male_cohort
    name: Prospective outcome cohort of screening-identified TMLHE-deficient males
    description: >-
      Follow an unselected cohort of TMLHE-deficient males identified by population
      screening, recording dietary carnitine intake, intercurrent illness, and
      neurodevelopmental outcome, to identify factors that stratify the 2-4% who
      become affected from the majority who do not.
  - experiment_id: exp_tmlhe_sibling_carnitine_prevention_trial
    name: Carnitine supplementation trial in infant siblings of TMLHE-deficient probands
    description: >-
      Randomized carnitine supplementation in unaffected infant siblings of
      TMLHE-deficient autism probands — the trial design explicitly proposed by the
      brain-carnitine-deficiency hypothesis — to test whether the neurodevelopmental
      risk is modifiable.
  evidence:
  - reference: PMID:22566635
    reference_title: "A common X-linked inborn error of carnitine biosynthesis may be a risk factor for nondysmorphic autism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "although with low penetrance (2-4%)"
    explanation: Quantifies the penetrance gap that this knowledge gap concerns.
  - reference: PMID:28703319
    reference_title: "Brain carnitine deficiency causes nonsyndromic autism with an extreme male bias: A hypothesis."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: OTHER
    snippet: "A mixed, common gene variant-environment hypothesis is proposed with diet, minor illnesses, microbiome, and drugs as possible risk modifiers."
    explanation: >-
      Enumerates the candidate modifiers this knowledge gap names. They are
      proposed rather than validated, which is what makes this a gap; that is
      the discussion's claim and the quote states it.
notes: >-
  Nosological note. MONDO:0010469 carries the `predisposition` and
  `omim_susceptibility` subsets and is classified under MONDO:0020836 ("autism,
  susceptibility to"), i.e. MONDO models this entity primarily as an autism
  susceptibility locus. dismech curates it as a disorder entry because the enzyme
  deficiency itself is a fully penetrant, biochemically defined inborn error of
  carnitine biosynthesis with a specific diagnostic metabolite profile; the
  low-penetrance part is the neurodevelopmental consequence, which is modeled
  explicitly as a susceptibility edge plus a knowledge-gap discussion rather than as
  an asserted disease phenotype. This entry is therefore a relevant worked example
  for the general question of how dismech entries relate to MONDO disease versus
  susceptibility terms.

  Naming note. Sources disagree on the OMIM series label attached to OMIM 300872:
  MONDO lists the synonym "autism, susceptibility to, X-linked 6" and PMID:39845198
  refers in its abstract to "X-linked recessive autism, type 6 (OMIM#300872)" while
  its own title says "X-Linked Autism Type 9". The unambiguous identifiers are
  OMIM:300872 / MONDO:0010469 / the TMLHE gene; the numbered autism-series label
  should not be relied on.
references:
- reference: PMID:22566635
  title: "A common X-linked inborn error of carnitine biosynthesis may be a risk factor for nondysmorphic autism."
  findings: []
- reference: PMID:21865298
  title: "Use of array CGH to detect exonic copy number variants throughout the genome in autism families detects a novel deletion in TMLHE."
  findings: []
- reference: PMID:23092983
  title: "Analysis of the chromosome X exome in patients with autism spectrum disorders identified novel candidate genes, including TMLHE."
  findings: []
- reference: PMID:25943046
  title: "Improvement of regressive autism symptoms in a child with TMLHE deficiency following carnitine supplementation."
  findings: []
- reference: PMID:26832401
  title: "Inborn Errors of Long-Chain Fatty Acid β-Oxidation Link Neural Stem Cell Self-Renewal to Autism."
  findings: []
- reference: PMID:28703319
  title: "Brain carnitine deficiency causes nonsyndromic autism with an extreme male bias: A hypothesis."
  findings: []
- reference: PMID:34728372
  title: "Low cardiac content of long-chain acylcarnitines in TMLHE knockout mice prevents ischaemia-reperfusion-induced mitochondrial and cardiac damage."
  findings: []
- reference: PMID:37553674
  title: "Knockout of Tmlhe in mice is not associated with autism spectrum disorder phenotypes or motor dysfunction despite low carnitine levels."
  findings: []
- reference: PMID:39845198
  title: "X-Linked Autism Type 9 Caused by a Hemizygote Pathogenic Variant in the TMLHE Gene: Etiological Diagnosis in an Adult Male with Moderate Intellectual Disability."
  findings: []

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References & Deep Research

References

9

PMID:22566635

A common X-linked inborn error of carnitine biosynthesis may be a risk factor for nondysmorphic autism.