Microcephaly, Short Stature, and Impaired Glucose Metabolism (MSSGM)

name: Microcephaly, Short Stature, and Impaired Glucose Metabolism
display_name: Microcephaly, Short Stature, and Impaired Glucose Metabolism (MSSGM)
creation_date: "2026-08-20T07:00:00Z"
description: >-
  A union of two autosomal recessive syndromes that are clinically almost
  indistinguishable — congenital microcephaly with intellectual disability,
  severe proportionate short stature, and autoantibody-negative young-onset
  diabetes — but are caused by different genes acting on different steps of
  protein synthesis. MSSGM1 (TRMT10A) removes the m1G9 tRNA modification, so
  specific tRNAs are destabilized and fragmented and translation fails in a
  codon-specific way. MSSGM2 (PPP1R15B/CReP) disables the constitutive
  eIF2-alpha phosphatase, so basal phospho-eIF2-alpha rises and cap-dependent
  initiation is chronically attenuated. Both lesions land hardest on the same
  two tissues — the developing brain and the pancreatic beta cell — which is
  why one clinical description covers both, and why Orphanet and OMIM disagree
  about whether this is one disease or two.
grouping_basis:
- SHARED_MECHANISM
- SHARED_PHENOTYPE
- CLINICAL_CONVENTION
grouping_rationale: >-
  This grouping exists because the nosology is genuinely split. OMIM and MONDO
  split the concept by gene into MSSGM1 (OMIM:616033 / MONDO:0000208, TRMT10A)
  and MSSGM2 (OMIM:616817 / MONDO:0014785, PPP1R15B), joined under the
  phenotypic series MONDO:0800450. Orphanet lumps them into a single disorder,
  ORPHA:391408 "primary microcephaly-mild intellectual disability-young-onset
  diabetes syndrome" (MONDO:0018320), whose own cross-references record it as
  BROADER than both OMIM entries, and whose Orphadata gene list carries both
  TRMT10A and PPP1R15B. Neither view is wrong: the clinical picture is one
  syndrome, the mechanisms are two.

  dismech keeps the two as separate Disease entries because the differentiating
  facts are mechanistic and consequential — the deficient gene product, the
  step of protein synthesis affected, and the treatment-relevant metabolic
  detail (MSSGM1 has documented peripheral insulin resistance responsive to
  metformin and hypoglycaemia preceding diabetes; MSSGM2 has infantile
  cirrhosis in one family and a mechanistic mirror-image relationship to
  Wolcott-Rallison syndrome). Blending them would produce an incoherent
  mechanism graph in which two mutually exclusive molecular lesions sit on the
  same chain. The grouping records the union that Orphanet asserts, so the
  Orphanet-level concept is represented without either Disease entry having to
  claim it.
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0018320
      label: primary microcephaly-mild intellectual disability-young-onset diabetes syndrome
    mapping_predicate: skos:exactMatch
    mapping_source: Orphanet
    mapping_justification: >-
      The Orphanet concept ORPHA:391408, which Orphadata maps to MONDO:0018320
      as an exact mapping. Its scope is exactly the union curated here:
      Orphadata lists TRMT10A and PPP1R15B as its two assessed disease-causing
      genes, and cross-references the ORPHAcode as broader than both
      OMIM:616033 and OMIM:616817. exactMatch is asserted at the level of this
      grouping, not at the level of either member Disease entry, where it would
      overstate scope.
    consistency:
    - reference: Orphanet
      consistent: CONSISTENT
      notes: >-
        Both genes Orphadata assigns to ORPHA:391408 have a corresponding
        member Disease entry in this grouping, and no third gene is assigned.
  - term:
      id: MONDO:0800450
      label: microcephaly, short stature, and impaired glucose metabolism
    mapping_predicate: skos:exactMatch
    mapping_source: MONDO
    mapping_justification: >-
      The OMIM phenotypic series grouping (OMIMPS:616033). MONDO:0000208 and
      MONDO:0014785 are its only two is_a children, so its extension is
      identical to this grouping's membership. Recorded as a second exactMatch
      because MONDO maintains the OMIM-PS grouping and the Orphanet grouping as
      distinct unmerged terms while both denote the same union; that is an
      upstream modelling artefact, not a difference in scope.
    consistency:
    - reference: MONDO
      consistent: CONSISTENT
      notes: >-
        Both is_a children of MONDO:0800450 are listed as members; there are no
        other children.
membership_criteria:
- description: >-
    A disorder belongs to MSSGM if it is an autosomal recessive syndrome whose
    causal gene is TRMT10A or PPP1R15B and whose phenotype combines congenital
    microcephaly with short stature.
  criteria_semantics: NECESSARY_AND_SUFFICIENT
  logic:
    operator: AND
    operands:
    - operator: OR
      description: >-
        Caused by biallelic variants in one of the two MSSGM genes. This is the
        defining criterion; the phenotype alone is shared with many syndromic
        microcephalies and cannot define the grouping.
      operands:
      - criterion_predicate: HAS_GENE
        description: Biallelic TRMT10A variants (MSSGM1).
        gene:
          preferred_term: TRMT10A
          term:
            id: hgnc:28403
            label: TRMT10A
      - criterion_predicate: HAS_GENE
        description: Biallelic PPP1R15B variants (MSSGM2).
        gene:
          preferred_term: PPP1R15B
          term:
            id: hgnc:14951
            label: PPP1R15B
    - criterion_predicate: HAS_PHENOTYPE
      description: >-
        Congenital microcephaly. Cited at HP:0000252 rather than at
        HP:0011451 (Primary microcephaly) because that is the claim the
        grouping actually makes: both members have congenital microcephaly, and
        whether it is formally "primary" is a per-entry refinement. Criteria are
        evaluated over the ontology closure, so a member annotated with the more
        specific HP:0011451 satisfies this leaf.
      phenotype_term:
        preferred_term: Microcephaly
        term:
          id: HP:0000252
          label: Microcephaly
    - criterion_predicate: HAS_PHENOTYPE
      description: Short stature.
      phenotype_term:
        preferred_term: Short stature
        term:
          id: HP:0004322
          label: Short stature
    - criterion_predicate: HAS_INHERITANCE
      description: Autosomal recessive inheritance (HP:0000007).
- description: >-
    Every member manifests impaired glucose metabolism at some point in life —
    the third element of the syndrome name. This is stated as a separate
    NECESSARY block rather than folded into the defining criterion because it is
    age-dependent: molecularly confirmed individuals in both entries have been
    reported without dysglycaemia at the time of assessment, so requiring it for
    membership would exclude correctly diagnosed patients and, in MSSGM1's case,
    a whole reported phenotype.
  criteria_semantics: NECESSARY
  logic:
    criterion_predicate: HAS_PHENOTYPE
    description: Diabetes mellitus or another disorder of glucose homeostasis.
    phenotype_term:
      preferred_term: Diabetes mellitus
      term:
        id: HP:0000819
        label: Diabetes mellitus
members:
- member: Microcephaly, Short Stature, and Impaired Glucose Metabolism 1
  member_type: DISEASE
  display_name: MSSGM1 (TRMT10A)
  disease_term:
    preferred_term: microcephaly, short stature, and impaired glucose metabolism 1
    term:
      id: MONDO:0000208
      label: microcephaly, short stature, and impaired glucose metabolism 1
  differentiating_mechanisms:
  - description: >-
      Biallelic loss of TRMT10A, the nucleolar tRNA (guanosine(9)-N1)
      methyltransferase. Loss of m1G9 depletes tRNA-Gln and initiator methionine
      tRNA and generates cytotoxic 5'-tRNA-Gln fragments that kill beta cells;
      in brain it causes codon-specific ribosome slowdown and translational
      derepression of ATF4. The lesion is in tRNA maturation, upstream of the
      ribosome.
    gene:
      preferred_term: TRMT10A
      term:
        id: hgnc:28403
        label: TRMT10A
    biological_processes:
    - preferred_term: tRNA methylation
      term:
        id: GO:0030488
        label: tRNA methylation
      modifier: DECREASED
  - description: >-
      Clinically differentiating: hypoglycaemia, often hyperinsulinaemic,
      commonly precedes diabetes by years; peripheral insulin resistance is
      documented and metformin-responsive in a subset; epilepsy is prominent.
    phenotype_term:
      preferred_term: Hyperinsulinemic hypoglycemia
      term:
        id: HP:0000825
        label: Hyperinsulinemic hypoglycemia
  evidence:
  - reference: PMID:24204302
    reference_title: "tRNA methyltransferase homolog gene TRMT10A mutation in young onset diabetes and primary microcephaly in humans."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We describe a new syndrome of young onset diabetes, short stature and microcephaly with intellectual disability in a large consanguineous family with three affected children."
    explanation: >-
      Establishes the MSSGM1 clinical description that qualifies it for this
      grouping.

- member: Microcephaly, Short Stature, and Impaired Glucose Metabolism 2
  member_type: DISEASE
  display_name: MSSGM2 (PPP1R15B)
  disease_term:
    preferred_term: microcephaly, short stature, and impaired glucose metabolism 2
    term:
      id: MONDO:0014785
      label: microcephaly, short stature, and impaired glucose metabolism 2
  differentiating_mechanisms:
  - description: >-
      Biallelic PPP1R15B variants damaging CReP, the constitutively expressed
      non-catalytic subunit that presents eIF2-alpha to protein phosphatase 1.
      Reduced dephosphorylation raises basal phospho-eIF2-alpha and chronically
      attenuates cap-dependent translation initiation. The lesion is at
      initiation, downstream of tRNA maturation, and is the mechanistic mirror
      image of Wolcott-Rallison syndrome, where PERK loss lowers eIF2-alpha
      phosphorylation instead.
    gene:
      preferred_term: PPP1R15B
      term:
        id: hgnc:14951
        label: PPP1R15B
    biological_processes:
    - preferred_term: regulation of translational initiation by eIF2 alpha phosphorylation
      term:
        id: GO:0010998
        label: regulation of translational initiation by eIF2 alpha phosphorylation
      modifier: INCREASED
  - description: >-
      Clinically differentiating: skeletal, dental, hair and white-matter
      involvement, and a third family with compound heterozygous variants
      presenting with infantile cirrhosis requiring liver transplantation — a
      hepatic phenotype not described in MSSGM1.
  evidence:
  - reference: PMID:26159176
    reference_title: "A Missense Mutation in PPP1R15B Causes a Syndrome Including Diabetes, Short Stature, and Microcephaly."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The R658C mutation in PPP1R15B affects a conserved amino acid within the domain important for protein phosphatase 1 (PP1) binding."
    explanation: >-
      Establishes the distinct PPP1R15B molecular lesion that separates this
      member from MSSGM1.
references:
- reference: PMID:24204302
  title: "tRNA methyltransferase homolog gene TRMT10A mutation in young onset diabetes and primary microcephaly in humans."
- reference: PMID:26159176
  title: "A Missense Mutation in PPP1R15B Causes a Syndrome Including Diabetes, Short Stature, and Microcephaly."
notes: >-
  The two members are near-indistinguishable clinically and are told apart only
  by sequencing, which is the practical reason this grouping is useful: a
  clinician recognizing the triad should test both genes, not one.

  On the double exactMatch. Asserting skos:exactMatch to two distinct MONDO
  terms is deliberate and is documented in each mapping_justification.
  MONDO:0800450 (OMIM phenotypic series) and MONDO:0018320 (Orphanet disorder)
  have identical extensions — the union of the TRMT10A and PPP1R15B diseases —
  but MONDO has not merged them. The alternative, downgrading one to
  closeMatch, would misrepresent a modelling artefact as a difference in scope.

  A candidate upstream MONDO issue: MONDO:0018320 currently carries no
  RO:0004003 causal-gene relationship and sits directly under
  MONDO:0002254 (syndromic disease), unconnected to the MONDO:0800450 series
  that covers the same two diseases. Reconciling the two grouping terms is a
  MONDO question rather than a dismech one, and is recorded here rather than
  worked around silently.