DGAT1 Deficiency

Mendelian MONDO:0014375 Pathograph 5 Show in embeddings browser

DGAT1 deficiency is a rare autosomal recessive congenital diarrhea and enteropathy (CODE) caused by biallelic loss-of-function variants in DGAT1, which encodes diacylglycerol O-acyltransferase 1 - the enzyme catalysing the final, committed step of triglyceride synthesis in the enterocyte. It defines a mechanistic class of its own within the CODEs: neither a defect of a brush-border digestive step nor of an electrolyte transporter, but a failure of intracellular lipid handling *after* dietary fat has been absorbed across the apical membrane. Without DGAT1, absorbed diacylglycerol and fatty acyl-CoA cannot be esterified into triglyceride for packaging into lipid droplets and chylomicrons, so DGAT substrates accumulate within the intestinal mucosa and the enterocyte is injured. The clinical picture combines watery diarrhea and vomiting, beginning within the first two weeks of life in the classical presentation, with a distinctive protein-losing enteropathy - hypoalbuminemia, hypogammaglobulinemia and elevated fecal alpha-1-antitrypsin - and frequently hypertriglyceridemia. Endoscopy typically shows no specific mucosal abnormality, so the diagnosis rests on genetic testing rather than histology. The disorder is fat-dependent rather than fat-independent, which makes it treatable: a low-fat amino acid formula rapidly relieves symptoms, whereas hydrolysed and high-medium-chain-triglyceride formulas do not, and tolerance for dietary fat improves with age. Onset is not invariably neonatal - a delayed-onset presentation beginning at eight months of age has been reported. The disorder is of wider pharmacological interest because DGAT1 inhibitors have been developed for metabolic disease, and the human null phenotype is a direct read on the consequence of blocking that enzyme.

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👪

Inheritance

Autosomal recessive inheritance HP:0000007

DGAT1 deficiency is autosomal recessive. Affected individuals are homozygous or compound heterozygous for loss-of-function DGAT1 alleles; the originally described family was of Ashkenazi Jewish descent with two of three children affected and both parents unaffected carriers.

Autosomal recessive inheritance

Show evidence (1 reference)

PMID:23114594 SUPPORT Human Clinical

"Here, we report a family of Ashkenazi Jewish descent, with 2 out of 3 children affected by CDD. Both affected children presented 3 days after birth with severe, intractable diarrhea."

Describes the sibling-recurrence pattern in a single family with unaffected parents, consistent with recessive inheritance.

Discussions and Knowledge Gaps

By what mechanism does accumulation of unesterified DGAT substrates in the intestinal mucosa produce diarrhea and protein loss, given that the mucosa is endoscopically and histologically unremarkable?

KNOWLEDGE GAP dgat1_substrate_injury_mechanism

The gene-to-phenotype link in DGAT1 deficiency is secure, but the step between them is not. The gene-discovery authors state explicitly that "the precise cause of diarrhea is unknown" and offer substrate buildup as a speculation. The gap is mechanistically important because the mucosa looks normal on endoscopy and routine histology, so whatever injures it does not produce the villous atrophy or structural lesion seen in the enterocyte-structural enteropathies, yet it is sufficient to cause protein-losing enteropathy. The causal edge from the central effector to the enteropathy node is therefore typed INDIRECT_UNKNOWN_INTERMEDIATES rather than DIRECT. Resolving it also bears on the safety of pharmacological DGAT1 inhibition in adults, since the human null phenotype is the closest available read on complete target blockade.

Proposed experiments

Lipidomic profiling of DGAT1-deficient patient intestinal biopsies

dgat1_mucosal_lipidomics

Quantify diacylglycerol, fatty acyl-CoA and related species in mucosal biopsies from DGAT1-deficient patients on and off fat restriction, to test whether the proposed substrate accumulation occurs and whether its magnitude tracks symptom severity.

Patient-derived enteroid barrier and lipid-challenge assay

dgat1_enteroid_lipid_challenge

Challenge DGAT1-deficient patient-derived intestinal enteroids with graded fatty acid loads and measure transepithelial electrical resistance, macromolecular flux and cell death, to test whether lipid loading is sufficient to produce barrier failure of the kind that would cause protein-losing enteropathy without overt histological change.

⚙

Pathophysiology

Loss of Diacylglycerol O-Acyltransferase 1 Activity

The initiating lesion. DGAT1 catalyses the final and committed step of triglyceride synthesis, esterifying diacylglycerol with fatty acyl-coenzyme A to form triacylglycerol. In the enterocyte this step is what converts the products of luminal fat digestion, once absorbed, into storable and exportable triglyceride. Biallelic loss-of-function variants abolish the enzyme entirely. Critically, this lesion sits downstream of luminal digestion and apical uptake, so it is not a brush-border defect: fat is absorbed into the cell normally and then cannot be processed.

Small-intestinal enterocyte CL:0000584

Enterocyte triglyceride synthesis GO:0019432 ↓ DECREASED

Diacylglycerol O-acyltransferase activity GO:0004144 ⇓ LOSS OF FUNCTION

Small intestine UBERON:0002108

Downstream

Failure of Enterocyte Triglyceride Packaging and Substrate Accumulation

Show evidence (1 reference)

PMID:39548446 SUPPORT Other

"DGAT1 is a key enzyme involved in TG synthesis. Diacylglycerol (DAG) and fatty acyl-coenzyme A are used as substrates to catalyze TGs"

States the enzymatic reaction whose loss defines this node - esterification of diacylglycerol with fatty acyl-CoA to form triglyceride. Evidence source is OTHER because this is the introductory review section of a clinical cohort report rather than a measurement made in it.

Failure of Enterocyte Triglyceride Packaging and Substrate Accumulation

The rate-limiting step, and the point at which the disorder becomes fat-dependent. Within the enterocyte, triglyceride is either stored in cytoplasmic lipid droplets or packaged into chylomicrons for export to the lymphatic system; both routes require DGAT1-synthesised triglyceride. Without it, neither can proceed, and the unesterified DGAT substrates - diacylglycerol and fatty acyl-CoA - build up in the intestinal mucosa. This substrate accumulation, rather than the absence of triglyceride as such, is the proposed proximate cause of mucosal injury, and it explains the dose dependence on dietary fat: the more fat is presented for absorption, the more substrate accumulates.

Small-intestinal enterocyte CL:0000584

Chylomicron assembly GO:0034378 ↓ DECREASED Intestinal lipid absorption GO:0098856 ⚠ ABNORMAL

Cytoplasmic lipid droplet GO:0005811

Downstream

Enteropathy with Intestinal Protein Loss

Show evidence (2 references)

PMID:23114594 SUPPORT Human Clinical

"The precise cause of diarrhea is unknown, but we speculate that it relates to abnormal fat absorption and buildup of DGAT substrates in the intestinal mucosa."

States the mucosal substrate-accumulation mechanism modelled by this node. The authors present it explicitly as a speculation, which is why the downstream causal link from this node is typed as having unknown intermediates rather than as direct.

PMID:39548446 SUPPORT Other

"In intestinal cells, TGs are stored in lipid droplets (LDs) or packaged in chylomicrons, and then transported to the lymphatic system."

Describes the two triglyceride-dependent routes - lipid droplets and chylomicrons - that fail when DGAT1 is absent.

Enteropathy with Intestinal Protein Loss

The effector node and the disorder's most distinctive feature. Mucosal injury produces an exudative (protein-losing) enteropathy in which serum proteins leak into the gut lumen, giving hypoalbuminemia, hypogammaglobulinemia and elevated stool alpha-1-antitrypsin, with peripheral edema when albumin falls far enough and susceptibility to recurrent infection when immunoglobulins do. Hypertriglyceridemia commonly accompanies it. The mucosa is nonetheless endoscopically and histologically unremarkable in most cases, which is why this disorder is missed by the biopsy-first diagnostic pathway that serves the structural enteropathies.

Small intestine UBERON:0002108

Downstream

Fat-Dependent Diarrhea and Intestinal Failure

Show evidence (1 reference)

PMID:39548446 SUPPORT Other

"are mainly manifested by chronic diarrhea, vomiting, growth retardation, hypoalbuminemia to varying degrees, and some associated with hypogammaglobulinemia and hyperlipidemia. Endoscopy usually shows no specific changes"

Lists the protein-loss features of this node and records that endoscopy is typically unremarkable, the point that distinguishes this disorder from the structural enteropathies.

Fat-Dependent Diarrhea and Intestinal Failure

The clinical output. Severe watery diarrhea with vomiting, typically starting within the first two weeks of life, causing growth retardation and, untreated, intestinal failure that can be fatal - one of the two originally described siblings died at 17 months. The defining and therapeutically decisive feature of this node is its dependence on dietary fat load: symptoms remit on a low-fat amino acid formula and not on hydrolysed or medium-chain-triglyceride formulas, and tolerance for dietary fat improves with age. The second originally described child improved markedly with most symptoms resolved by 10 to 12 months, so the natural history is not uniformly progressive.

Small intestine UBERON:0002108

Show evidence (1 reference)

PMID:23114594 SUPPORT Human Clinical

"One child died from complications at age 17 months. The second child showed marked improvement, with resolution of most symptoms at 10 to 12 months of age."

Documents both the potential lethality and the variable natural history of the clinical output, including the spontaneous improvement seen in one sibling.

⬡

Pathograph

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

●

Phenotypes

Blood 1

Hypogammaglobulinemia OCCASIONAL Decreased circulating immunoglobulin concentration HP:0004313

Show evidence (1 reference)

PMID:39548446 SUPPORT Other

"and some associated with hypogammaglobulinemia and hyperlipidemia"

The qualifier "some associated with" supports both the phenotype and its OCCASIONAL frequency band, in contrast to the hypoalbuminemia that occurs in most patients.

Digestive 3

Chronic Watery Diarrhea OBLIGATE HP:0002014

Temporal: CHRONIC

Show evidence (1 reference)

PMID:23114594 SUPPORT Human Clinical

"Both affected children presented 3 days after birth with severe, intractable diarrhea."

Documents the severe early-onset intractable diarrhea that defines the disorder.

Protein-Losing Enteropathy HP:0002243

Show evidence (1 reference)

PMID:33261563 SUPPORT Human Clinical

"The patient had vomiting, hypoalbuminemia, hypertriglyceridemia, and failure to thrive at early infancy."

Documents the hypoalbuminemia and hypertriglyceridemia of the protein-losing enteropathy phenotype in an independently reported patient.

Vomiting HP:0002013

Show evidence (1 reference)

PMID:39548446 SUPPORT Other

"A low-fat amino acid FSMP formula and diet can quickly relieve diarrhea, vomiting, and other symptoms"

Documents vomiting as a treated symptom of the disorder and its response to fat restriction.

Metabolism 2

Hypoalbuminemia VERY_FREQUENT HP:0003073

Show evidence (1 reference)

PMID:39548446 SUPPORT Other

"chronic diarrhea, vomiting, growth retardation, hypoalbuminemia to varying degrees"

Lists hypoalbuminemia among the principal manifestations, supporting the VERY_FREQUENT band and the "varying degrees" qualifier.

Hypertriglyceridemia OCCASIONAL HP:0002155

Show evidence (1 reference)

PMID:33261563 SUPPORT Human Clinical

"The patient had vomiting, hypoalbuminemia, hypertriglyceridemia, and failure to thrive at early infancy."

Documents hypertriglyceridemia in a reported patient with DGAT1 variants.

Growth 1

Failure to Thrive HP:0001508

Show evidence (1 reference)

PMID:39548446 SUPPORT Other

"Due to poor weight gain and gastrointestinal symptoms after birth, infants were treated by our clinical nutritionist."

Documents poor weight gain as the presenting problem across the reported cohort.

🧬

Genetic Associations

DGAT1 loss-of-function variants (Causative)

Gene: DGAT1 hgnc:2843 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (2 references)

PMID:23114594 SUPPORT Human Clinical

"Using exome sequencing, we identified a rare splice site mutation in the DGAT1 gene and found that both affected children were homozygous carriers."

The gene-discovery report identifying the causative homozygous DGAT1 splice-site variant.

PMID:23114594 SUPPORT In Vitro

"Molecular analysis of the mutant allele indicated a total loss of function, with no detectable DGAT1 protein or activity produced."

Establishes the variant as a complete loss of function at the protein and enzyme-activity level. Evidence source is IN_VITRO because protein abundance and enzyme activity were assayed molecularly rather than observed clinically.

💊

Medical Actions

Low-Fat Amino Acid Formula

Action: dietary intervention Ontology label: Dietary Intervention NCIT:C15447

Platform: Behavioral / lifestyle

The treatment of choice, and a clean demonstration that the disorder is fat-load-dependent. A low-fat amino-acid-based formula for special medical purposes rapidly relieves diarrhea and vomiting and improves nutritional status and growth. The comparison that makes this specific is that neither hydrolysed formula nor high-medium-chain-triglyceride formula relieved the diarrhea in the same infants - so the benefit comes from lowering the fat load presented for absorption, not from altering protein hydrolysis or fatty acid chain length. Tolerance for dietary fat improves with age, allowing gradual liberalisation.

Mechanism Target:

INHIBITS Failure of Enterocyte Triglyceride Packaging and Substrate Accumulation — Reducing dietary fat lowers the diacylglycerol and fatty acyl-CoA load presented to the enterocyte, limiting the accumulation of unesterified DGAT substrates in the mucosa without restoring enzyme activity.

Show evidence (2 references)

PMID:39548446 SUPPORT Human Clinical

"Neither hydrolyzed nor high-medium chain triglyceride (MCT) formula significantly alleviated diarrheal symptoms; however, a low-fat amino acid diet rapidly relieved symptoms and significantly improved nutritional status, with infants showing better tolerance to dietary fat content with age."

The controlled comparison establishing that the therapeutic variable is fat load specifically - hydrolysed and MCT formulas failed where the low-fat formula succeeded - which is why the treatment is modelled as acting on the substrate-accumulation node.

PMID:39548446 SUPPORT Human Clinical

"Low-fat amino acid FSMP formula and diets should be regarded as the first choice for the treatment of diarrhea in infants with DGAT1 deficiency."

States the authors' recommendation that this be first-line therapy.

🔬

Diagnosis

DGAT1 sequencing

Genetic testing, typically whole-exome sequencing, is the primary diagnostic modality. This is not merely convenient but necessary: endoscopy and biopsy are usually unremarkable in DGAT1 deficiency, so the biopsy-first pathway used for the structural congenital enteropathies does not make the diagnosis.

Show evidence (2 references)

PMID:39548446 SUPPORT Human Clinical

"Infants with DGAT1 deficiency can be diagnosed by genetic testing."

States genetic testing as the diagnostic route for this disorder.

PMID:39548446 SUPPORT Human Clinical

"Genetic testing confirmed a compound heterozygous mutation in DGAT1."

Documents genetic testing establishing the diagnosis in the reported cohort.

{ }

Source YAML

click to show

name: DGAT1 Deficiency
creation_date: "2026-08-22T00:00:00Z"
category: Mendelian
synonyms:
- diarrhea 7, with exudative enteropathy
- congenital diarrhea 7
- DGAT1-related protein-losing enteropathy
- congenital chronic diarrhea with protein-losing enteropathy
description: >-
  DGAT1 deficiency is a rare autosomal recessive congenital diarrhea and
  enteropathy (CODE) caused by biallelic loss-of-function variants in DGAT1,
  which encodes diacylglycerol O-acyltransferase 1 - the enzyme catalysing the
  final, committed step of triglyceride synthesis in the enterocyte. It defines
  a mechanistic class of its own within the CODEs: neither a defect of a
  brush-border digestive step nor of an electrolyte transporter, but a failure of
  intracellular lipid handling *after* dietary fat has been absorbed across the
  apical membrane. Without DGAT1, absorbed diacylglycerol and fatty acyl-CoA
  cannot be esterified into triglyceride for packaging into lipid droplets and
  chylomicrons, so DGAT substrates accumulate within the intestinal mucosa and
  the enterocyte is injured. The clinical picture combines watery diarrhea and
  vomiting, beginning within the first two weeks of life in the classical
  presentation, with a distinctive protein-losing enteropathy - hypoalbuminemia,
  hypogammaglobulinemia and elevated fecal alpha-1-antitrypsin - and frequently
  hypertriglyceridemia. Endoscopy typically shows no specific mucosal
  abnormality, so the diagnosis rests on genetic testing rather than histology.
  The disorder is fat-dependent rather than fat-independent, which makes it
  treatable: a low-fat amino acid formula rapidly relieves symptoms, whereas
  hydrolysed and high-medium-chain-triglyceride formulas do not, and tolerance
  for dietary fat improves with age. Onset is not invariably neonatal - a
  delayed-onset presentation beginning at eight months of age has been reported.
  The disorder is of wider pharmacological interest because DGAT1 inhibitors
  have been developed for metabolic disease, and the human null phenotype is a
  direct read on the consequence of blocking that enzyme.
disease_term:
  preferred_term: DGAT1 deficiency
  term:
    id: MONDO:0014375
    label: congenital diarrhea 7 with exudative enteropathy
inheritance:
- name: Autosomal recessive inheritance
  description: >-
    DGAT1 deficiency is autosomal recessive. Affected individuals are homozygous
    or compound heterozygous for loss-of-function DGAT1 alleles; the originally
    described family was of Ashkenazi Jewish descent with two of three children
    affected and both parents unaffected carriers.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:23114594
    reference_title: "DGAT1 mutation is linked to a congenital diarrheal disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here, we report a family of Ashkenazi Jewish descent, with 2 out of 3
      children affected by CDD. Both affected children presented 3 days after
      birth with severe, intractable diarrhea.
    explanation: >-
      Describes the sibling-recurrence pattern in a single family with
      unaffected parents, consistent with recessive inheritance.
genetic:
- name: DGAT1 loss-of-function variants
  gene_term:
    preferred_term: DGAT1
    term:
      id: hgnc:2843
      label: DGAT1
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  features: >-
    Biallelic DGAT1 variants abolish diacylglycerol O-acyltransferase 1 activity.
    The index family carried a homozygous rare splice-site variant shown to be a
    total loss of function, with no detectable DGAT1 protein or activity.
    Compound heterozygous missense and other variants have since been reported,
    including in Chinese cohorts, and are associated with a broader phenotypic
    range including delayed onset.
  evidence:
  - reference: PMID:23114594
    reference_title: "DGAT1 mutation is linked to a congenital diarrheal disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Using exome sequencing, we identified a rare splice site mutation in the
      DGAT1 gene and found that both affected children were homozygous carriers.
    explanation: >-
      The gene-discovery report identifying the causative homozygous DGAT1
      splice-site variant.
  - reference: PMID:23114594
    reference_title: "DGAT1 mutation is linked to a congenital diarrheal disorder."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Molecular analysis of the mutant allele indicated a total loss of function,
      with no detectable DGAT1 protein or activity produced.
    explanation: >-
      Establishes the variant as a complete loss of function at the protein and
      enzyme-activity level. Evidence source is IN_VITRO because protein
      abundance and enzyme activity were assayed molecularly rather than observed
      clinically.
pathophysiology:
- name: Loss of Diacylglycerol O-Acyltransferase 1 Activity
  description: >-
    The initiating lesion. DGAT1 catalyses the final and committed step of
    triglyceride synthesis, esterifying diacylglycerol with fatty acyl-coenzyme A
    to form triacylglycerol. In the enterocyte this step is what converts the
    products of luminal fat digestion, once absorbed, into storable and
    exportable triglyceride. Biallelic loss-of-function variants abolish the
    enzyme entirely. Critically, this lesion sits downstream of luminal digestion
    and apical uptake, so it is not a brush-border defect: fat is absorbed into
    the cell normally and then cannot be processed.
  role: trigger
  biological_scale: MOLECULAR
  cell_types:
  - preferred_term: Small-intestinal enterocyte
    term:
      id: CL:0000584
      label: enterocyte
  molecular_functions:
  - preferred_term: Diacylglycerol O-acyltransferase activity
    term:
      id: GO:0004144
      label: diacylglycerol O-acyltransferase activity
    modifier: LOSS_OF_FUNCTION
  biological_processes:
  - preferred_term: Enterocyte triglyceride synthesis
    term:
      id: GO:0019432
      label: triglyceride biosynthetic process
    modifier: DECREASED
  locations:
  - preferred_term: Small intestine
    term:
      id: UBERON:0002108
      label: small intestine
  evidence:
  - reference: PMID:39548446
    reference_title: "A low-fat amino acid diet reverses intestinal failure and shows good growth trends in five infants with diacylglycerol transferase 1 (DGAT1) deficiency: a prospective cohort study."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      DGAT1 is a key enzyme involved in TG synthesis. Diacylglycerol (DAG) and
      fatty acyl-coenzyme A are used as substrates to catalyze TGs
    explanation: >-
      States the enzymatic reaction whose loss defines this node - esterification
      of diacylglycerol with fatty acyl-CoA to form triglyceride. Evidence source
      is OTHER because this is the introductory review section of a clinical
      cohort report rather than a measurement made in it.
  downstream:
  - target: Failure of Enterocyte Triglyceride Packaging and Substrate Accumulation
    causal_link_type: DIRECT
- name: Failure of Enterocyte Triglyceride Packaging and Substrate Accumulation
  description: >-
    The rate-limiting step, and the point at which the disorder becomes
    fat-dependent. Within the enterocyte, triglyceride is either stored in
    cytoplasmic lipid droplets or packaged into chylomicrons for export to the
    lymphatic system; both routes require DGAT1-synthesised triglyceride. Without
    it, neither can proceed, and the unesterified DGAT substrates - diacylglycerol
    and fatty acyl-CoA - build up in the intestinal mucosa. This substrate
    accumulation, rather than the absence of triglyceride as such, is the
    proposed proximate cause of mucosal injury, and it explains the dose
    dependence on dietary fat: the more fat is presented for absorption, the more
    substrate accumulates.
  role: central_effector
  biological_scale: CELLULAR
  cell_types:
  - preferred_term: Small-intestinal enterocyte
    term:
      id: CL:0000584
      label: enterocyte
  biological_processes:
  - preferred_term: Chylomicron assembly
    term:
      id: GO:0034378
      label: chylomicron assembly
    modifier: DECREASED
  - preferred_term: Intestinal lipid absorption
    term:
      id: GO:0098856
      label: intestinal lipid absorption
    modifier: ABNORMAL
  cellular_components:
  - preferred_term: Cytoplasmic lipid droplet
    term:
      id: GO:0005811
      label: lipid droplet
  evidence:
  - reference: PMID:23114594
    reference_title: "DGAT1 mutation is linked to a congenital diarrheal disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The precise cause of diarrhea is unknown, but we speculate that it relates
      to abnormal fat absorption and buildup of DGAT substrates in the intestinal
      mucosa.
    explanation: >-
      States the mucosal substrate-accumulation mechanism modelled by this node.
      The authors present it explicitly as a speculation, which is why the
      downstream causal link from this node is typed as having unknown
      intermediates rather than as direct.
  - reference: PMID:39548446
    reference_title: "A low-fat amino acid diet reverses intestinal failure and shows good growth trends in five infants with diacylglycerol transferase 1 (DGAT1) deficiency: a prospective cohort study."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      In intestinal cells, TGs are stored in lipid droplets (LDs) or packaged in
      chylomicrons, and then transported to the lymphatic system.
    explanation: >-
      Describes the two triglyceride-dependent routes - lipid droplets and
      chylomicrons - that fail when DGAT1 is absent.
  downstream:
  - target: Enteropathy with Intestinal Protein Loss
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- name: Enteropathy with Intestinal Protein Loss
  description: >-
    The effector node and the disorder's most distinctive feature. Mucosal injury
    produces an exudative (protein-losing) enteropathy in which serum proteins
    leak into the gut lumen, giving hypoalbuminemia, hypogammaglobulinemia and
    elevated stool alpha-1-antitrypsin, with peripheral edema when albumin falls
    far enough and susceptibility to recurrent infection when immunoglobulins do.
    Hypertriglyceridemia commonly accompanies it. The mucosa is nonetheless
    endoscopically and histologically unremarkable in most cases, which is why
    this disorder is missed by the biopsy-first diagnostic pathway that serves
    the structural enteropathies.
  role: effector
  biological_scale: TISSUE
  locations:
  - preferred_term: Small intestine
    term:
      id: UBERON:0002108
      label: small intestine
  evidence:
  - reference: PMID:39548446
    reference_title: "A low-fat amino acid diet reverses intestinal failure and shows good growth trends in five infants with diacylglycerol transferase 1 (DGAT1) deficiency: a prospective cohort study."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      are mainly manifested by chronic diarrhea, vomiting, growth retardation,
      hypoalbuminemia to varying degrees, and some associated with
      hypogammaglobulinemia and hyperlipidemia. Endoscopy usually shows no
      specific changes
    explanation: >-
      Lists the protein-loss features of this node and records that endoscopy is
      typically unremarkable, the point that distinguishes this disorder from
      the structural enteropathies.
  downstream:
  - target: Fat-Dependent Diarrhea and Intestinal Failure
    causal_link_type: DIRECT
- name: Fat-Dependent Diarrhea and Intestinal Failure
  description: >-
    The clinical output. Severe watery diarrhea with vomiting, typically starting
    within the first two weeks of life, causing growth retardation and, untreated,
    intestinal failure that can be fatal - one of the two originally described
    siblings died at 17 months. The defining and therapeutically decisive feature
    of this node is its dependence on dietary fat load: symptoms remit on a
    low-fat amino acid formula and not on hydrolysed or medium-chain-triglyceride
    formulas, and tolerance for dietary fat improves with age. The second
    originally described child improved markedly with most symptoms resolved by
    10 to 12 months, so the natural history is not uniformly progressive.
  role: consequence
  biological_scale: ORGANISM
  locations:
  - preferred_term: Small intestine
    term:
      id: UBERON:0002108
      label: small intestine
  evidence:
  - reference: PMID:23114594
    reference_title: "DGAT1 mutation is linked to a congenital diarrheal disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      One child died from complications at age 17 months. The second child showed
      marked improvement, with resolution of most symptoms at 10 to 12 months of
      age.
    explanation: >-
      Documents both the potential lethality and the variable natural history of
      the clinical output, including the spontaneous improvement seen in one
      sibling.
phenotypes:
- name: Chronic Watery Diarrhea
  category: Gastrointestinal
  description: >-
    Severe intractable watery diarrhea, classically beginning within the first
    two weeks of life, though a delayed-onset form beginning at eight months has
    been reported. Severity tracks dietary fat intake.
  phenotype_term:
    preferred_term: Chronic watery diarrhea
    term:
      id: HP:0002014
      label: Diarrhea
    temporality: CHRONIC
  frequency: OBLIGATE
  evidence:
  - reference: PMID:23114594
    reference_title: "DGAT1 mutation is linked to a congenital diarrheal disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Both affected children presented 3 days after birth with severe,
      intractable diarrhea.
    explanation: >-
      Documents the severe early-onset intractable diarrhea that defines the
      disorder.
- name: Hypoalbuminemia
  category: Metabolic
  description: >-
    Low serum albumin from exudative protein-losing enteropathy, of varying
    degree; the accompanying elevated stool alpha-1-antitrypsin confirms
    intestinal rather than renal or hepatic protein loss.
  phenotype_term:
    preferred_term: Hypoalbuminemia
    term:
      id: HP:0003073
      label: Hypoalbuminemia
  frequency: VERY_FREQUENT
  evidence:
  - reference: PMID:39548446
    reference_title: "A low-fat amino acid diet reverses intestinal failure and shows good growth trends in five infants with diacylglycerol transferase 1 (DGAT1) deficiency: a prospective cohort study."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      chronic diarrhea, vomiting, growth retardation, hypoalbuminemia to varying
      degrees
    explanation: >-
      Lists hypoalbuminemia among the principal manifestations, supporting the
      VERY_FREQUENT band and the "varying degrees" qualifier.
- name: Protein-Losing Enteropathy
  category: Gastrointestinal
  description: >-
    Exudative enteropathy with loss of serum proteins into the gut lumen,
    evidenced by elevated fecal alpha-1-antitrypsin. It is the feature that names
    the MONDO concept for this disorder.
  phenotype_term:
    preferred_term: Protein-losing enteropathy
    term:
      id: HP:0002243
      label: Protein-losing enteropathy
  evidence:
  - reference: PMID:33261563
    reference_title: "DGAT1 mutations leading to delayed chronic diarrhoea: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The patient had vomiting, hypoalbuminemia, hypertriglyceridemia, and
      failure to thrive at early infancy.
    explanation: >-
      Documents the hypoalbuminemia and hypertriglyceridemia of the
      protein-losing enteropathy phenotype in an independently reported patient.
- name: Hypogammaglobulinemia
  category: Immunological
  description: >-
    Reduced circulating immunoglobulins from intestinal loss, present in a subset
    of patients and conferring susceptibility to recurrent infection.
  phenotype_term:
    preferred_term: Hypogammaglobulinemia
    term:
      id: HP:0004313
      label: Decreased circulating immunoglobulin concentration
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:39548446
    reference_title: "A low-fat amino acid diet reverses intestinal failure and shows good growth trends in five infants with diacylglycerol transferase 1 (DGAT1) deficiency: a prospective cohort study."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      and some associated with hypogammaglobulinemia and hyperlipidemia
    explanation: >-
      The qualifier "some associated with" supports both the phenotype and its
      OCCASIONAL frequency band, in contrast to the hypoalbuminemia that occurs
      in most patients.
- name: Hypertriglyceridemia
  category: Metabolic
  description: >-
    Elevated circulating triglycerides, reported in a subset of patients. Its
    coexistence with a triglyceride-synthesis defect is mechanistically
    counterintuitive and not fully explained.
  phenotype_term:
    preferred_term: Hypertriglyceridemia
    term:
      id: HP:0002155
      label: Hypertriglyceridemia
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:33261563
    reference_title: "DGAT1 mutations leading to delayed chronic diarrhoea: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The patient had vomiting, hypoalbuminemia, hypertriglyceridemia, and
      failure to thrive at early infancy.
    explanation: >-
      Documents hypertriglyceridemia in a reported patient with DGAT1 variants.
- name: Vomiting
  category: Gastrointestinal
  description: >-
    Vomiting accompanies the diarrhea and, like it, responds to reduction of
    dietary fat load.
  phenotype_term:
    preferred_term: Vomiting
    term:
      id: HP:0002013
      label: Vomiting
  evidence:
  - reference: PMID:39548446
    reference_title: "A low-fat amino acid diet reverses intestinal failure and shows good growth trends in five infants with diacylglycerol transferase 1 (DGAT1) deficiency: a prospective cohort study."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      A low-fat amino acid FSMP formula and diet can quickly relieve diarrhea,
      vomiting, and other symptoms
    explanation: >-
      Documents vomiting as a treated symptom of the disorder and its response to
      fat restriction.
- name: Failure to Thrive
  category: Growth
  description: >-
    Growth retardation and poor weight gain from combined fat malabsorption,
    protein loss and chronic diarrhea; growth catches up on appropriate
    nutritional therapy.
  phenotype_term:
    preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  evidence:
  - reference: PMID:39548446
    reference_title: "A low-fat amino acid diet reverses intestinal failure and shows good growth trends in five infants with diacylglycerol transferase 1 (DGAT1) deficiency: a prospective cohort study."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Due to poor weight gain and gastrointestinal symptoms after birth, infants
      were treated by our clinical nutritionist.
    explanation: >-
      Documents poor weight gain as the presenting problem across the reported
      cohort.
diagnosis:
- name: DGAT1 sequencing
  description: >-
    Genetic testing, typically whole-exome sequencing, is the primary diagnostic
    modality. This is not merely convenient but necessary: endoscopy and biopsy
    are usually unremarkable in DGAT1 deficiency, so the biopsy-first pathway
    used for the structural congenital enteropathies does not make the diagnosis.
  evidence:
  - reference: PMID:39548446
    reference_title: "A low-fat amino acid diet reverses intestinal failure and shows good growth trends in five infants with diacylglycerol transferase 1 (DGAT1) deficiency: a prospective cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Infants with DGAT1 deficiency can be diagnosed by genetic testing.
    explanation: >-
      States genetic testing as the diagnostic route for this disorder.
  - reference: PMID:39548446
    reference_title: "A low-fat amino acid diet reverses intestinal failure and shows good growth trends in five infants with diacylglycerol transferase 1 (DGAT1) deficiency: a prospective cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Genetic testing confirmed a compound heterozygous mutation in DGAT1.
    explanation: >-
      Documents genetic testing establishing the diagnosis in the reported
      cohort.
treatments:
- name: Low-Fat Amino Acid Formula
  description: >-
    The treatment of choice, and a clean demonstration that the disorder is
    fat-load-dependent. A low-fat amino-acid-based formula for special medical
    purposes rapidly relieves diarrhea and vomiting and improves nutritional
    status and growth. The comparison that makes this specific is that neither
    hydrolysed formula nor high-medium-chain-triglyceride formula relieved the
    diarrhea in the same infants - so the benefit comes from lowering the fat
    load presented for absorption, not from altering protein hydrolysis or fatty
    acid chain length. Tolerance for dietary fat improves with age, allowing
    gradual liberalisation.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: dietary intervention
    term:
      id: NCIT:C15447
      label: Dietary Intervention
  target_mechanisms:
  - target: Failure of Enterocyte Triglyceride Packaging and Substrate Accumulation
    treatment_effect: INHIBITS
    description: >-
      Reducing dietary fat lowers the diacylglycerol and fatty acyl-CoA load
      presented to the enterocyte, limiting the accumulation of unesterified DGAT
      substrates in the mucosa without restoring enzyme activity.
    evidence:
    - reference: PMID:39548446
      reference_title: "A low-fat amino acid diet reverses intestinal failure and shows good growth trends in five infants with diacylglycerol transferase 1 (DGAT1) deficiency: a prospective cohort study."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Neither hydrolyzed nor high-medium chain triglyceride (MCT) formula
        significantly alleviated diarrheal symptoms; however, a low-fat amino
        acid diet rapidly relieved symptoms and significantly improved
        nutritional status, with infants showing better tolerance to dietary fat
        content with age.
      explanation: >-
        The controlled comparison establishing that the therapeutic variable is
        fat load specifically - hydrolysed and MCT formulas failed where the
        low-fat formula succeeded - which is why the treatment is modelled as
        acting on the substrate-accumulation node.
    - reference: PMID:39548446
      reference_title: "A low-fat amino acid diet reverses intestinal failure and shows good growth trends in five infants with diacylglycerol transferase 1 (DGAT1) deficiency: a prospective cohort study."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Low-fat amino acid FSMP formula and diets should be regarded as the first
        choice for the treatment of diarrhea in infants with DGAT1 deficiency.
      explanation: >-
        States the authors' recommendation that this be first-line therapy.
discussions:
- discussion_id: dgat1_substrate_injury_mechanism
  kind: KNOWLEDGE_GAP
  prompt: >-
    By what mechanism does accumulation of unesterified DGAT substrates in the
    intestinal mucosa produce diarrhea and protein loss, given that the mucosa is
    endoscopically and histologically unremarkable?
  rationale: >-
    The gene-to-phenotype link in DGAT1 deficiency is secure, but the step between
    them is not. The gene-discovery authors state explicitly that "the precise
    cause of diarrhea is unknown" and offer substrate buildup as a speculation.
    The gap is mechanistically important because the mucosa looks normal on
    endoscopy and routine histology, so whatever injures it does not produce the
    villous atrophy or structural lesion seen in the enterocyte-structural
    enteropathies, yet it is sufficient to cause protein-losing enteropathy. The
    causal edge from the central effector to the enteropathy node is therefore
    typed INDIRECT_UNKNOWN_INTERMEDIATES rather than DIRECT. Resolving it also
    bears on the safety of pharmacological DGAT1 inhibition in adults, since the
    human null phenotype is the closest available read on complete target
    blockade.
  proposed_experiments:
  - experiment_id: dgat1_mucosal_lipidomics
    name: Lipidomic profiling of DGAT1-deficient patient intestinal biopsies
    description: >-
      Quantify diacylglycerol, fatty acyl-CoA and related species in mucosal
      biopsies from DGAT1-deficient patients on and off fat restriction, to test
      whether the proposed substrate accumulation occurs and whether its
      magnitude tracks symptom severity.
  - experiment_id: dgat1_enteroid_lipid_challenge
    name: Patient-derived enteroid barrier and lipid-challenge assay
    description: >-
      Challenge DGAT1-deficient patient-derived intestinal enteroids with graded
      fatty acid loads and measure transepithelial electrical resistance,
      macromolecular flux and cell death, to test whether lipid loading is
      sufficient to produce barrier failure of the kind that would cause
      protein-losing enteropathy without overt histological change.
notes: >-
  DGAT1 deficiency occupies a distinct mechanistic position among the congenital
  diarrheas and enteropathies and deliberately does NOT conform to the
  diet_induced_osmotic_diarrhea module, despite being dietarily triggered and
  dietarily treatable. That module models failure of a substrate-specific step at
  the apical brush border, where the offending nutrient is never absorbed and is
  retained in the lumen as an osmotic solute; here the fat IS absorbed and the
  lesion is intracellular, after uptake, so the diarrhea is not driven by a
  retained luminal osmole and the enteropathy involves protein loss rather than
  osmotic water flux. The superficial resemblance - a dietary constituent whose
  removal abolishes symptoms - is a good example of why the module's conformance
  criterion is the location and nature of the failed step rather than the
  responsiveness to diet. It is likewise distinct from abetalipoproteinemia and
  chylomicron retention disease, which impair chylomicron assembly and export
  downstream of normal triglyceride synthesis rather than blocking triglyceride
  synthesis itself.

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