Mullegama-Klein-Martinez syndrome (MKMS) is a rare X-linked cohesinopathy caused by loss-of-function germline variants in STAG2, which encodes stromal antigen 2, a subunit of the evolutionarily conserved cohesin complex. Cohesin (core subunits SMC1A, SMC3, RAD21, and STAG1/STAG2) regulates sister chromatid cohesion during mitosis and meiosis and, critically for development, also governs DNA replication, DNA repair, three-dimensional genome organization, and transcription. STAG2 is dosage-sensitive: de novo heterozygous loss-of-function variants (frequently in females) and hemizygous or mosaic variants produce a multisystem neurodevelopmental disorder whose core features are global developmental delay, intellectual disability, microcephaly, growth restriction (undergrowth), ear anomalies (microtia) with hearing loss, and dysmorphic facial features, overlapping the broader cohesinopathy spectrum (notably Cornelia de Lange syndrome) but generally without CdLS-characteristic facies. An allelic milder phenotype segregating as classic X-linked recessive syndromic intellectual disability (the STAG2 p.Ser327Asn family described by Soardi et al.) sits at the mild end of the same spectrum. The developmental phenotype is attributed primarily to cohesin-dependent transcriptional and chromatin-organization dysregulation rather than to overt mitotic cohesion failure: patient cells typically show delayed—not prematurely separated—sister chromatids alongside altered cell-cycle and gene-expression profiles.
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name: Mullegama-Klein-Martinez Syndrome
creation_date: "2026-07-30T00:00:00Z"
synonyms:
- MKMS
- STAG2-related X-linked intellectual deficiency
- STAG2-related cohesinopathy
description: >-
Mullegama-Klein-Martinez syndrome (MKMS) is a rare X-linked cohesinopathy
caused by loss-of-function germline variants in STAG2, which encodes stromal
antigen 2, a subunit of the evolutionarily conserved cohesin complex. Cohesin
(core subunits SMC1A, SMC3, RAD21, and STAG1/STAG2) regulates sister chromatid
cohesion during mitosis and meiosis and, critically for development, also
governs DNA replication, DNA repair, three-dimensional genome organization,
and transcription. STAG2 is dosage-sensitive: de novo heterozygous
loss-of-function variants (frequently in females) and hemizygous or mosaic
variants produce a multisystem neurodevelopmental disorder whose core features
are global developmental delay, intellectual disability, microcephaly, growth
restriction (undergrowth), ear anomalies (microtia) with hearing loss, and
dysmorphic facial features, overlapping the broader cohesinopathy spectrum
(notably Cornelia de Lange syndrome) but generally without CdLS-characteristic
facies. An allelic milder phenotype segregating as classic X-linked recessive
syndromic intellectual disability (the STAG2 p.Ser327Asn family described by
Soardi et al.) sits at the mild end of the same spectrum. The developmental
phenotype is attributed primarily to cohesin-dependent transcriptional and
chromatin-organization dysregulation rather than to overt mitotic cohesion
failure: patient cells typically show delayed—not prematurely separated—sister
chromatids alongside altered cell-cycle and gene-expression profiles.
category: Mendelian
disease_term:
preferred_term: Mullegama-Klein-Martinez syndrome
term:
id: MONDO:0026722
label: Mullegama-Klein-Martinez syndrome
inheritance:
- name: X-linked inheritance
inheritance_term:
preferred_term: X-linked inheritance
term:
id: HP:0001417
label: X-linked inheritance
description: >-
STAG2 maps to Xq25. MKMS most often arises from de novo loss-of-function
STAG2 variants, reported predominantly in affected females; hemizygous and
somatic-mosaic variants also occur. An allelic hypomorphic missense variant
(p.Ser327Asn) segregates in a family as classic X-linked recessive syndromic
intellectual disability.
evidence:
- reference: PMID:29263825
reference_title: "Familial STAG2 germline mutation defines a new human cohesinopathy."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Five individuals carry a STAG2 p.Ser327Asn (c.980 G > A) variant that
perfectly cosegregates with a phenotype of syndromic mental retardation in
a characteristic X-linked recessive pattern.
explanation: >-
Documents cosegregation of a STAG2 germline variant with syndromic
intellectual disability in an X-linked pattern.
penetrance: UNKNOWN
expressivity: VARIABLE
parents:
- syndromic intellectual disability
- cohesinopathy
references:
- reference: PMID:28296084
title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
- reference: PMID:29263825
title: "Familial STAG2 germline mutation defines a new human cohesinopathy."
- reference: PMID:30158690
title: "Clinical exome sequencing reveals locus heterogeneity and phenotypic variability of cohesinopathies."
- reference: PMID:36467423
title: "Somatic mosaicism in STAG2-associated cohesinopathies: Expansion of the genotypic and phenotypic spectrum."
- reference: PMID:34580287
title: "Expanding the known phenotype of Mullegama-Klein-Martinez syndrome in male patients."
- reference: PMID:41300816
title: "A Novel STAG2 Frameshift Variant in Mullegama-Klein-Martinez Syndrome with Complex Conotruncal Heart Defect."
pathophysiology:
- name: STAG2 Loss-of-Function Variation
description: >-
The initiating lesion is a germline (or somatic-mosaic) loss-of-function
STAG2 variant. STAG2 encodes an integral, dosage-sensitive subunit of the
cohesin complex; heterozygous de novo truncating variants reduce STAG2
protein and produce a cohesinopathy, establishing STAG2 as haploinsufficient
for neurodevelopment.
mechanism_confidence: ESTABLISHED
biological_scale: MOLECULAR
genes:
- preferred_term: STAG2
term:
id: hgnc:11355
label: STAG2
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
we describe a heterozygous de novo variant (c.205C>T; p.(Arg69*)) in the
integral cohesin structural protein, STAG2. This variant is associated
with decreased STAG2 protein expression.
explanation: >-
Identifies a de novo truncating STAG2 variant with reduced protein as the
causal lesion.
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Herein, we suggest that STAG2 is a dosage-sensitive gene and that
heterozygous loss-of-function variants lead to a cohesinopathy.
explanation: >-
Establishes STAG2 dosage sensitivity and loss-of-function as the disease
mechanism.
downstream:
- target: Cohesin Complex Deficiency
causal_link_type: DIRECT
hypothesis_groups:
- sex_differential_viability
evidence:
- reference: PMID:29263825
reference_title: "Familial STAG2 germline mutation defines a new human cohesinopathy."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: >-
the STAG2 p.Ser327Asn mutant is defective in binding to SCC1 and other
cohesin subunits and regulators. Thus, decreased amount of intact
cohesin likely underlies the phenotypes of STAG2-SXLID.
explanation: >-
Pathogenic STAG2 variants reduce the amount of assembled, intact cohesin.
- name: Cohesin Complex Deficiency
description: >-
Reduced STAG2 lowers the amount of intact, functional cohesin. The cohesin
complex is a conserved multi-subunit ring that mediates sister chromatid
cohesion and organizes chromatin; STAG2 is one of two mutually exclusive
HEAT-repeat subunits (STAG1/STAG2) that direct cohesin to genomic loci.
mechanism_confidence: ESTABLISHED
biological_scale: MOLECULAR
biological_processes:
- preferred_term: sister chromatid cohesion
term:
id: GO:0007062
label: sister chromatid cohesion
modifier: DECREASED
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: OTHER
snippet: >-
The cohesin complex is an evolutionarily conserved multi-subunit protein
complex which regulates sister chromatid cohesion during mitosis and
meiosis. Additionally, the cohesin complex regulates DNA replication, DNA
repair, and transcription.
explanation: >-
Defines the functions of cohesin that STAG2 deficiency perturbs.
downstream:
- target: Transcriptional and Chromatin-Organization Dysregulation
causal_link_type: DIRECT
evidence:
- reference: PMID:29263825
reference_title: "Familial STAG2 germline mutation defines a new human cohesinopathy."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: >-
they exhibited altered cell cycle profiles and gene expression patterns
that were consistent with cohesin deficiency
explanation: >-
Cohesin deficiency in patient cells manifests as altered cell-cycle and
gene-expression programs.
- target: Delayed Sister Chromatid Cohesion
causal_link_type: DIRECT
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: >-
The analyses of metaphase spreads did not exhibit premature sister
chromatid separation; however, delayed sister chromatid cohesion was
observed.
explanation: >-
Patient cells show a subtle cohesion-kinetics defect rather than overt
premature chromatid separation.
- name: Delayed Sister Chromatid Cohesion
description: >-
A measurable cellular consequence of STAG2 deficiency: patient-cell
metaphase spreads show delayed sister chromatid cohesion kinetics rather
than overt premature sister chromatid separation. This subtle,
non-catastrophic cohesion defect is consistent with the view that the
developmental phenotype arises from cohesin's regulatory (transcription /
genome organization) roles rather than from mitotic cohesion failure.
mechanism_confidence: ESTABLISHED
biological_scale: CELLULAR
biological_processes:
- preferred_term: sister chromatid cohesion
term:
id: GO:0007062
label: sister chromatid cohesion
modifier: DECREASED
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: >-
The analyses of metaphase spreads did not exhibit premature sister
chromatid separation; however, delayed sister chromatid cohesion was
observed.
explanation: >-
Directly documents the delayed sister chromatid cohesion phenotype in
patient cells.
- name: Transcriptional and Chromatin-Organization Dysregulation
description: >-
The developmental phenotype of STAG2-associated cohesinopathy is attributed
primarily to cohesin's non-mitotic roles: regulation of transcription and
three-dimensional genome/chromatin organization. Loss of STAG2-cohesin
perturbs gene-expression programs required for normal development.
mechanism_confidence: HYPOTHETICAL
biological_scale: CELLULAR
biological_processes:
- preferred_term: regulation of transcription by RNA polymerase II
term:
id: GO:0006357
label: regulation of transcription by RNA polymerase II
modifier: ABNORMAL
- preferred_term: chromatin organization
term:
id: GO:0006325
label: chromatin organization
modifier: ABNORMAL
evidence:
- reference: PMID:29263825
reference_title: "Familial STAG2 germline mutation defines a new human cohesinopathy."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: >-
they exhibited altered cell cycle profiles and gene expression patterns
that were consistent with cohesin deficiency
explanation: >-
Directly links STAG2 deficiency to altered gene-expression programs, the
proposed developmental effector.
- reference: PMID:41300816
reference_title: "A Novel STAG2 Frameshift Variant in Mullegama-Klein-Martinez Syndrome with Complex Conotruncal Heart Defect."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
STAG2, which encodes a subunit of the cohesin complex responsible for
chromosomal segregation and transcriptional regulation
explanation: >-
Identifies transcriptional regulation as a core cohesin function disrupted
in MKMS.
downstream:
- target: Impaired Early Brain Development
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
evidence:
- reference: PMID:36467423
reference_title: "Somatic mosaicism in STAG2-associated cohesinopathies: Expansion of the genotypic and phenotypic spectrum."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: >-
we observed a prominent expression of STAG2, especially between culture
days 0 and 15, indicating an essential function of STAG2 in early brain
development
explanation: >-
Neuronal-organoid expression data implicate STAG2 in early neurodevelopment.
- target: Congenital Heart Defect
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
evidence:
- reference: PMID:41300816
reference_title: "A Novel STAG2 Frameshift Variant in Mullegama-Klein-Martinez Syndrome with Complex Conotruncal Heart Defect."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
This case expands the phenotypic spectrum of STAG2-related MKM and
highlights the role of STAG2 in cardiac development.
explanation: >-
Links cohesin/STAG2 transcriptional dysregulation to disrupted cardiac
development and congenital heart defects.
- name: Impaired Early Brain Development
description: >-
STAG2 is strongly expressed in the earliest phase of neuronal
differentiation, and STAG2-associated cohesinopathy is dominated by global
developmental delay, severe microcephaly, and brain abnormalities,
consistent with a requirement for STAG2-cohesin in early brain development.
mechanism_confidence: HYPOTHETICAL
biological_scale: TISSUE
cell_types:
- preferred_term: neural progenitor cell
term:
id: CL:0011020
label: neural progenitor cell
evidence:
- reference: PMID:36467423
reference_title: "Somatic mosaicism in STAG2-associated cohesinopathies: Expansion of the genotypic and phenotypic spectrum."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
As the phenotype of STAG2-associated cohesinopathies is dominated by
global developmental delay, severe microcephaly, and brain abnormalities
explanation: >-
Characterizes the neurodevelopmental core of the disorder that impaired
brain development produces.
downstream:
- target: Microcephaly
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
a clinical history of global developmental delay, microcephaly, microtia
with hearing loss, language delay, ADHD, and dysmorphic features
explanation: >-
Links STAG2 loss to the microcephaly and developmental-delay endpoints.
- target: Global Developmental Delay
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
a clinical history of global developmental delay, microcephaly, microtia
with hearing loss, language delay, ADHD, and dysmorphic features
explanation: >-
Links STAG2 loss to global developmental delay.
phenotypes:
- name: Global Developmental Delay
description: >-
Global developmental delay is a defining, near-universal feature of
STAG2-associated cohesinopathy.
phenotype_term:
preferred_term: Global developmental delay
term:
id: HP:0001263
label: Global developmental delay
frequency: VERY_FREQUENT
evidence:
- reference: PMID:36467423
reference_title: "Somatic mosaicism in STAG2-associated cohesinopathies: Expansion of the genotypic and phenotypic spectrum."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
As the phenotype of STAG2-associated cohesinopathies is dominated by
global developmental delay, severe microcephaly, and brain abnormalities
explanation: >-
States that global developmental delay dominates the phenotype.
- name: Intellectual Disability
description: >-
Syndromic intellectual disability is a core feature, spanning from severe
forms with loss-of-function variants to the milder X-linked recessive
presentation of the hypomorphic p.Ser327Asn family.
phenotype_term:
preferred_term: Intellectual disability
term:
id: HP:0001249
label: Intellectual disability
frequency: VERY_FREQUENT
evidence:
- reference: PMID:29263825
reference_title: "Familial STAG2 germline mutation defines a new human cohesinopathy."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Five individuals carry a STAG2 p.Ser327Asn (c.980 G > A) variant that
perfectly cosegregates with a phenotype of syndromic mental retardation in
a characteristic X-linked recessive pattern.
explanation: >-
Documents syndromic intellectual disability cosegregating with a STAG2
variant.
- name: Microcephaly
description: >-
Microcephaly, frequently severe, is a hallmark of the disorder.
phenotype_term:
preferred_term: Microcephaly
term:
id: HP:0000252
label: Microcephaly
frequency: FREQUENT
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
a clinical history of global developmental delay, microcephaly, microtia
with hearing loss, language delay, ADHD, and dysmorphic features
explanation: >-
Microcephaly is a presenting feature of the index patient.
- name: Microtia
description: >-
Ear malformation (microtia) is part of the recognizable phenotype and is
associated with hearing loss.
phenotype_term:
preferred_term: Microtia
term:
id: HP:0008551
label: Microtia
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
a clinical history of global developmental delay, microcephaly, microtia
with hearing loss, language delay, ADHD, and dysmorphic features
explanation: >-
Microtia with hearing loss is documented in the index patient.
- name: Hearing Impairment
description: >-
Hearing loss accompanies the ear malformation.
phenotype_term:
preferred_term: Hearing impairment
term:
id: HP:0000365
label: Hearing impairment
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
a clinical history of global developmental delay, microcephaly, microtia
with hearing loss, language delay, ADHD, and dysmorphic features
explanation: >-
Hearing loss is documented alongside microtia.
- name: Delayed Speech and Language Development
description: >-
Language and speech delay is a consistent neurodevelopmental feature.
phenotype_term:
preferred_term: Delayed speech and language development
term:
id: HP:0000750
label: Delayed speech and language development
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
a clinical history of global developmental delay, microcephaly, microtia
with hearing loss, language delay, ADHD, and dysmorphic features
explanation: >-
Language delay is a presenting feature.
- name: Attention Deficit Hyperactivity Disorder
description: >-
ADHD is reported as a behavioral feature.
phenotype_term:
preferred_term: Attention deficit hyperactivity disorder
term:
id: HP:0007018
label: Attention deficit hyperactivity disorder
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
a clinical history of global developmental delay, microcephaly, microtia
with hearing loss, language delay, ADHD, and dysmorphic features
explanation: >-
ADHD is documented in the index patient.
- name: Dysmorphic Facial Features
description: >-
Dysmorphic facial features are part of the recognizable phenotype; unlike
classic Cornelia de Lange syndrome, STAG2 patients generally lack the
characteristic CdLS facies.
phenotype_term:
preferred_term: Abnormal facial shape
term:
id: HP:0001999
label: Abnormal facial shape
evidence:
- reference: PMID:30158690
reference_title: "Clinical exome sequencing reveals locus heterogeneity and phenotypic variability of cohesinopathies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Patients with STAG1 and STAG2 variants presented with overlapping features
yet without characteristic facial features of CdLS.
explanation: >-
STAG2 patients have dysmorphic features distinct from characteristic CdLS
facies.
- name: Growth Delay
description: >-
Undergrowth/growth restriction is part of the STAG2 cohesinopathy phenotype;
in a mosaic case it presented with hemihypotrophy of the right side.
phenotype_term:
preferred_term: Growth delay
term:
id: HP:0001510
label: Growth delay
evidence:
- reference: PMID:36467423
reference_title: "Somatic mosaicism in STAG2-associated cohesinopathies: Expansion of the genotypic and phenotypic spectrum."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Pathogenic variants in STAG2 have rarely been implicated in an X-linked
cohesinopathy associated with undergrowth, developmental delay, and
dysmorphic features.
explanation: >-
Undergrowth is an associated feature of STAG2 cohesinopathy.
- name: Supernumerary Nipple
description: >-
Supernumerary nipples were observed in two individuals, a feature not
previously associated with STAG2, expanding the phenotypic spectrum.
phenotype_term:
preferred_term: Supernumerary nipple
term:
id: HP:0002558
label: Supernumerary nipple
frequency: OCCASIONAL
evidence:
- reference: PMID:36467423
reference_title: "Somatic mosaicism in STAG2-associated cohesinopathies: Expansion of the genotypic and phenotypic spectrum."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
both individuals show supernumerary nipples, a feature that has not been
reported associated to STAG2 before
explanation: >-
Documents supernumerary nipples as a newly recognized STAG2 feature.
- name: Congenital Heart Defect
description: >-
Congenital heart defects are a variable feature, ranging from isolated
septal defects to, in a reported neonate, a complex conotruncal malformation
(pulmonary atresia, double-outlet right ventricle, subaortic ventricular
septal defect, and patent ductus arteriosus). Complex cardiac malformations
are uncommon.
phenotype_term:
preferred_term: Congenital heart defect
term:
id: HP:0001627
label: Abnormal heart morphology
frequency: OCCASIONAL
evidence:
- reference: PMID:41300816
reference_title: "A Novel STAG2 Frameshift Variant in Mullegama-Klein-Martinez Syndrome with Complex Conotruncal Heart Defect."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
complex congenital heart disease, including pulmonary atresia,
double-outlet right ventricle, large subaortic ventricular septal defect,
and patent ductus arteriosus
explanation: >-
Documents a complex congenital heart defect in an MKMS patient.
- reference: PMID:41300816
reference_title: "A Novel STAG2 Frameshift Variant in Mullegama-Klein-Martinez Syndrome with Complex Conotruncal Heart Defect."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
variable congenital anomalies, though complex cardiac malformations are
uncommon
explanation: >-
Establishes that cardiac malformations occur but complex forms are
uncommon, supporting the OCCASIONAL frequency band.
- name: Hypotonia
description: >-
Hypotonia is reported; in the neonatal conotruncal case it was documented as
left upper-extremity hypotonia.
phenotype_term:
preferred_term: Hypotonia
term:
id: HP:0001252
label: Hypotonia
evidence:
- reference: PMID:41300816
reference_title: "A Novel STAG2 Frameshift Variant in Mullegama-Klein-Martinez Syndrome with Complex Conotruncal Heart Defect."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
She exhibited intrauterine growth restriction, mild craniofacial
dysmorphism, and left upper-extremity hypotonia.
explanation: >-
Documents hypotonia in an MKMS patient.
- name: Intrauterine Growth Restriction
description: >-
Intrauterine growth restriction is reported, consistent with the prenatal
onset of the disorder's growth phenotype.
phenotype_term:
preferred_term: Intrauterine growth retardation
term:
id: HP:0001511
label: Intrauterine growth retardation
evidence:
- reference: PMID:41300816
reference_title: "A Novel STAG2 Frameshift Variant in Mullegama-Klein-Martinez Syndrome with Complex Conotruncal Heart Defect."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
She exhibited intrauterine growth restriction, mild craniofacial
dysmorphism, and left upper-extremity hypotonia.
explanation: >-
Documents intrauterine growth restriction in an MKMS patient.
- name: Abnormal Brain Morphology
description: >-
Structural brain malformations are a frequent finding, including
polymicrogyria, corpus callosum hypoplasia, cortical thickening, and ectopic
posterior pituitary. Brain abnormalities dominate the STAG2-cohesinopathy
phenotype alongside microcephaly.
phenotype_term:
preferred_term: Abnormal brain morphology
term:
id: HP:0012443
label: Abnormal brain morphology
frequency: FREQUENT
evidence:
- reference: PMID:36467423
reference_title: "Somatic mosaicism in STAG2-associated cohesinopathies: Expansion of the genotypic and phenotypic spectrum."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
As the phenotype of STAG2-associated cohesinopathies is dominated by
global developmental delay, severe microcephaly, and brain abnormalities
explanation: >-
Establishes brain abnormalities as a dominant feature of the disorder.
- reference: PMID:34580287
reference_title: "Expanding the known phenotype of Mullegama-Klein-Martinez syndrome in male patients."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
left-sided perisylvian polymicrogyria
explanation: >-
Documents polymicrogyria, a specific brain malformation, in an MKMS
patient.
- name: Seizure
description: >-
Seizures occur in a subset of patients; a male patient developed generalized
tonic-clonic seizures in childhood requiring antiepileptic therapy.
phenotype_term:
preferred_term: Seizure
term:
id: HP:0001250
label: Seizure
frequency: OCCASIONAL
evidence:
- reference: PMID:34580287
reference_title: "Expanding the known phenotype of Mullegama-Klein-Martinez syndrome in male patients."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
lamotrigine therapy was started due to an episode of generalized tonic –
clonic seizures
explanation: >-
Documents a generalized tonic-clonic seizure requiring antiepileptic
therapy in an MKMS patient.
- name: Polymicrogyria
description: >-
Polymicrogyria, a cortical malformation, has been documented on brain MRI in
MKMS patients.
phenotype_term:
preferred_term: Polymicrogyria
term:
id: HP:0002126
label: Polymicrogyria
evidence:
- reference: PMID:34580287
reference_title: "Expanding the known phenotype of Mullegama-Klein-Martinez syndrome in male patients."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
left-sided perisylvian polymicrogyria
explanation: >-
Documents polymicrogyria in an MKMS patient.
- name: Hypoplasia of the Corpus Callosum
description: >-
Corpus callosum abnormalities, including hypoplasia and dysgenesis of the
splenium, are recurrent structural brain findings.
phenotype_term:
preferred_term: Corpus callosum hypoplasia
term:
id: HP:0002079
label: Hypoplasia of the corpus callosum
evidence:
- reference: PMID:34580287
reference_title: "Expanding the known phenotype of Mullegama-Klein-Martinez syndrome in male patients."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
associated with a hypoplastic corpus callosum
explanation: >-
Documents corpus callosum hypoplasia in an MKMS patient.
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Brain MRI on day two of life showed dysgenesis of the splenium of the
corpus callosum
explanation: >-
Independently documents a corpus callosum malformation in an MKMS patient.
- name: Ectopic Posterior Pituitary
description: >-
An ectopic posterior pituitary with a short, thin pituitary stalk has been
reported, shared between patients with variants affecting the same residue.
phenotype_term:
preferred_term: Ectopic posterior pituitary
term:
id: HP:0011755
label: Ectopic posterior pituitary
evidence:
- reference: PMID:34580287
reference_title: "Expanding the known phenotype of Mullegama-Klein-Martinez syndrome in male patients."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
revealed an ectopic posterior pituitary with a short, thin pituitary
explanation: >-
Documents an ectopic posterior pituitary in an MKMS patient.
- name: Ventricular Septal Defect
description: >-
Ventricular septal defect is among the most commonly reported congenital
heart defects in MKMS.
phenotype_term:
preferred_term: Ventricular septal defect
term:
id: HP:0001629
label: Ventricular septal defect
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
an apical and muscular ventricular septal defect
explanation: >-
Documents a ventricular septal defect in an MKMS patient.
- name: Double Outlet Right Ventricle
description: >-
Double-outlet right ventricle occurred as part of a complex conotruncal
malformation in a reported neonate.
phenotype_term:
preferred_term: Double outlet right ventricle
term:
id: HP:0001719
label: Double outlet right ventricle
evidence:
- reference: PMID:41300816
reference_title: "A Novel STAG2 Frameshift Variant in Mullegama-Klein-Martinez Syndrome with Complex Conotruncal Heart Defect."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
double-outlet right ventricle, large subaortic ventricular septal defect
explanation: >-
Documents double-outlet right ventricle in an MKMS patient.
- name: Patent Ductus Arteriosus
description: >-
Patent ductus arteriosus was a component of the complex cardiac malformation
in a reported neonate.
phenotype_term:
preferred_term: Patent ductus arteriosus
term:
id: HP:0001643
label: Patent ductus arteriosus
evidence:
- reference: PMID:41300816
reference_title: "A Novel STAG2 Frameshift Variant in Mullegama-Klein-Martinez Syndrome with Complex Conotruncal Heart Defect."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
large subaortic ventricular septal defect, and patent ductus arteriosus
explanation: >-
Documents a patent ductus arteriosus in an MKMS patient.
- name: Short Stature
description: >-
Short stature is a frequent growth feature of the disorder.
phenotype_term:
preferred_term: Short stature
term:
id: HP:0004322
label: Short stature
frequency: FREQUENT
evidence:
- reference: PMID:34580287
reference_title: "Expanding the known phenotype of Mullegama-Klein-Martinez syndrome in male patients."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
at 10 years of age, the patient has short stature
explanation: >-
Documents short stature in an MKMS patient.
- reference: PMID:29263825
reference_title: "Familial STAG2 germline mutation defines a new human cohesinopathy."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
moderate intellectual deficiency, short stature, sensory hearing
deficiency, large nose, prominent ears, and frontal baldness
explanation: >-
Independently documents short stature in the STAG2 XLID family.
- name: Cleft Palate
description: >-
Palatal clefting, including submucous cleft palate, is reported.
phenotype_term:
preferred_term: Cleft palate
term:
id: HP:0000175
label: Cleft palate
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
submucous cleft palate with bifid uvula
explanation: >-
Documents a submucous cleft palate in an MKMS patient.
- name: Clinodactyly of the 5th Finger
description: >-
Fifth-finger clinodactyly is among the reported digital anomalies.
phenotype_term:
preferred_term: Fifth finger clinodactyly
term:
id: HP:0004209
label: Clinodactyly of the 5th finger
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
bilateral fifth finger clinodactyly
explanation: >-
Documents fifth-finger clinodactyly in an MKMS patient.
- name: Pes Planus
description: >-
Flat feet (pes planus) are reported among the skeletal/limb features.
phenotype_term:
preferred_term: Pes planus
term:
id: HP:0001763
label: Pes planus
evidence:
- reference: PMID:34580287
reference_title: "Expanding the known phenotype of Mullegama-Klein-Martinez syndrome in male patients."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
bilateral pes planus
explanation: >-
Documents bilateral pes planus in an MKMS patient.
- name: High Palate
description: >-
A high-arched palate is part of the craniofacial phenotype.
phenotype_term:
preferred_term: High-arched palate
term:
id: HP:0000218
label: High palate
evidence:
- reference: PMID:34580287
reference_title: "Expanding the known phenotype of Mullegama-Klein-Martinez syndrome in male patients."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
thick lips, and a high-arched palate
explanation: >-
Documents a high-arched palate in an MKMS patient.
genetic:
- name: STAG2
gene_term:
preferred_term: STAG2
term:
id: hgnc:11355
label: STAG2
relationship_type: CAUSATIVE
notes: >-
STAG2 (Xq25) encodes stromal antigen 2, a HEAT-repeat subunit of the cohesin
complex. De novo loss-of-function variants (nonsense, frameshift, splice,
and copy-number deletions), hemizygous variants, and somatic mosaicism cause
MKMS; a hypomorphic missense allele (p.Ser327Asn) causes a milder X-linked
recessive syndromic intellectual disability at the mild end of the spectrum.
Scope note: this entry models the loss-of-function/haploinsufficiency arm.
A phenotypically overlapping but molecularly distinct gain-of-dosage entity
from Xq25 microduplication spanning STAG2 (Leroy et al. 2016, PMID:25677961)
is not modeled here, and somatically acquired STAG2 mutation in cancer is a
separate, clinically distinct phenomenon.
evidence:
- reference: PMID:30158690
reference_title: "Clinical exome sequencing reveals locus heterogeneity and phenotypic variability of cohesinopathies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Candidate or recently reported cohesinopathy genes were supported by de
novo SNVs/indels in STAG1 (N = 3), STAG2 (N = 5), PDS5A (N = 1), and WAPL
(N = 1)
explanation: >-
Independent clinical-exome cohort supports de novo STAG2 variants as a
cause of cohesinopathy.
- reference: PMID:34580287
reference_title: "Expanding the known phenotype of Mullegama-Klein-Martinez syndrome in male patients."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
He shares distinct clinical features with a previously reported male
patient carrying the STAG2 variant p.Tyr159Cys, suggesting that this
phenotype is determined by the position of the mutation.
explanation: >-
Documents that surviving affected males carry missense variants and that
phenotype correlates with variant position.
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
a female that carries a pathogenic STAG2 variant but has favorably skewed
XCi may have mild or no symptoms
explanation: >-
X-chromosome-inactivation skewing modulates phenotype severity in
heterozygous females.
- name: STAG2 somatic mosaicism
gene_term:
preferred_term: STAG2
term:
id: hgnc:11355
label: STAG2
relationship_type: CAUSATIVE
variant_origin: SOMATIC
notes: >-
Postzygotic somatic mosaicism for a STAG2 variant is a distinct disease
mechanism, producing tissue-variable variant allele fractions and
asymmetric phenotype expression. Multi-tissue deep sequencing is required to
characterize the degree of mosaicism.
evidence:
- reference: PMID:36467423
reference_title: "Somatic mosaicism in STAG2-associated cohesinopathies: Expansion of the genotypic and phenotypic spectrum."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
we describe for the first time a mosaic STAG2 variant in an individual
with developmental delay, microcephaly, and hemihypotrophy of the right
side
explanation: >-
Documents somatic mosaicism as a genetic mechanism in STAG2 cohesinopathy.
treatments:
- name: Supportive and Multidisciplinary Care
description: >-
There is no disease-specific or curative therapy for MKMS. Management is
symptomatic and multidisciplinary, following general practice for syndromic
neurodevelopmental and congenital-malformation disorders: developmental,
physical, occupational, and speech therapy; audiologic management of hearing
loss; ophthalmologic care; and echocardiographic and neuroimaging
surveillance with cardiac surgical repair as needed.
treatment_term:
preferred_term: supportive care
term:
id: NCIT:C15747
label: Supportive Care
- name: Antiepileptic Pharmacotherapy
description: >-
Anticonvulsant therapy is used to control seizures when they occur; in one
reported male patient, lamotrigine was started following a generalized
tonic-clonic seizure.
therapeutic_modality: SMALL_MOLECULE
treatment_term:
preferred_term: Pharmacotherapy
term:
id: NCIT:C15986
label: Pharmacotherapy
therapeutic_agent:
- preferred_term: lamotrigine
term:
id: CHEBI:6367
label: lamotrigine
evidence:
- reference: PMID:34580287
reference_title: "Expanding the known phenotype of Mullegama-Klein-Martinez syndrome in male patients."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
lamotrigine therapy was started due to an episode of generalized tonic –
clonic seizures
explanation: >-
Documents lamotrigine use for seizure control in an MKMS patient.
- name: Cardiac Surgical Repair
description: >-
Surgical repair of congenital heart defects is indicated when structural
cardiac malformations are present; a reported neonate with a complex
conotruncal defect required neonatal cardiac surgical management.
therapeutic_modality: SURGERY
treatment_term:
preferred_term: surgical procedure
term:
id: NCIT:C15329
label: Surgical Procedure
evidence:
- reference: PMID:41300816
reference_title: "A Novel STAG2 Frameshift Variant in Mullegama-Klein-Martinez Syndrome with Complex Conotruncal Heart Defect."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
complex congenital heart disease, including pulmonary atresia,
double-outlet right ventricle, large subaortic ventricular septal defect,
and patent ductus arteriosus
explanation: >-
The complex conotruncal malformation requiring surgical management
motivates cardiac surgical repair.
- name: Genetic Counseling
description: >-
Genetic counseling is recommended, including parental testing for mosaicism
to inform recurrence-risk assessment, given that most cases are de novo but
parental germline or somatic mosaicism can raise recurrence risk.
treatment_term:
preferred_term: genetic counseling
term:
id: NCIT:C15240
label: Genetic Counseling
evidence:
- reference: PMID:36467423
reference_title: "Somatic mosaicism in STAG2-associated cohesinopathies: Expansion of the genotypic and phenotypic spectrum."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
we describe for the first time a mosaic STAG2 variant in an individual
with developmental delay, microcephaly, and hemihypotrophy of the right
side
explanation: >-
Documentation of somatic mosaicism underpins the recurrence-risk
counseling rationale.
mechanistic_hypotheses:
- hypothesis_group_id: sex_differential_viability
hypothesis_label: Sex-Differential Viability and Male Embryonic Lethality
status: EMERGING
description: >-
A proposed model for the disorder's X-linked genetics: because STAG2 is
dosage-sensitive and undergoes X-inactivation, heterozygous females can
survive severe (truncating) loss-of-function alleles—buffered, though often
imperfectly, by the wild-type X and modulated by X-inactivation skewing—
whereas hemizygous males with equivalent severe alleles are proposed to be
largely non-viable, so surviving affected males instead carry milder
missense alleles. This is inferential rather than directly proven in humans.
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
pathogenic loss-of-function STAG2 variants that affect canonical STAG, SCD
and GR domains are lethal in males with a 46,XY karyotype
explanation: >-
States the proposed male-lethality basis of the sex-differential
viability model.
prevalence:
- population: Worldwide
measure_type: CASES_IN_LITERATURE
prevalence_class: ULTRA_RARE
notes: >-
MKMS is ultra-rare; approximately 19-20 patients had been reported in the
peer-reviewed literature as of 2025. No formal prevalence/incidence rate has
been established, and no ORPHA code exists yet.
evidence:
- reference: PMID:41300816
reference_title: "A Novel STAG2 Frameshift Variant in Mullegama-Klein-Martinez Syndrome with Complex Conotruncal Heart Defect."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
This case expands the phenotypic spectrum of STAG2-related MKM and
highlights the role of STAG2 in cardiac development.
explanation: >-
Single-case reports are the evidence tier for this ultra-rare disorder.
imaging_findings:
- name: Pathological Brain MRI
modality: MRI
description: >-
Brain MRI is pathological in most patients, showing polymicrogyria, corpus
callosum hypoplasia/dysgenesis, cortical thickening, ventriculomegaly, and
ectopic posterior pituitary.
phenotype_term:
preferred_term: Abnormal brain morphology
term:
id: HP:0012443
label: Abnormal brain morphology
evidence:
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Brain MRI on day two of life showed dysgenesis of the splenium of the
corpus callosum
explanation: >-
Brain MRI reveals structural malformations in MKMS.
diagnosis:
- name: Trio Whole-Exome Sequencing
description: >-
Molecular confirmation of a pathogenic STAG2 variant is the diagnostic gold
standard; rapid trio whole-exome sequencing identifies the causal de novo
variant, and chromosomal microarray is typically normal in point-variant
cases. Multi-tissue deep sequencing is used when mosaicism is suspected.
evidence:
- reference: PMID:41300816
reference_title: "A Novel STAG2 Frameshift Variant in Mullegama-Klein-Martinez Syndrome with Complex Conotruncal Heart Defect."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
identified by rapid trio whole-exome sequencing
explanation: >-
Trio whole-exome sequencing is the diagnostic route to a STAG2 variant.
- reference: PMID:28296084
reference_title: "De novo loss-of-function variants in STAG2 are associated with developmental delay, microcephaly, and congenital anomalies."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Chromosomal microarray on both BAC and SNP platforms were normal.
explanation: >-
Chromosomal microarray is part of the workup and is normal in
point-variant cases.
- reference: PMID:36467423
reference_title: "Somatic mosaicism in STAG2-associated cohesinopathies: Expansion of the genotypic and phenotypic spectrum."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
grade of mosaicism by deep sequencing analysis on DNA extracted from EDTA
blood, urine and buccal swabs
explanation: >-
Multi-tissue deep sequencing is used to characterize mosaicism.
datasets: []
Overview: Mullegama–Klein–Martinez syndrome (MKMS) is a rare X-linked cohesinopathy caused by pathogenic (typically de novo) variants in STAG2 (Stromal Antigen 2), a core subunit of the cohesin complex. It presents as a syndromic neurodevelopmental disorder characterized by global developmental delay/intellectual disability, microcephaly, characteristic craniofacial dysmorphism, ear anomalies with hearing loss, short stature, digit/limb anomalies, brain malformations, and — in a subset — congenital heart defects. The condition is also catalogued as NEURODEVELOPMENTAL DISORDER, X-LINKED, WITH CRANIOFACIAL ABNORMALITIES (OMIM synonym). It was clinically delineated across several case series between 2015 and 2019 and named after the lead authors of the defining reports (Mullegama, Klein, Martinez).
Key identifiers: - OMIM: #301022 (MULLEGAMA-KLEIN-MARTINEZ SYNDROME; MKMS) — OMIM 301022 - Gene locus: STAG2, Xq25 (HGNC:11355; also called SA2, SA-2, SCC3B) - MedGen: UID / CUI C5193008 - MONDO: MONDO:0026722 - Disease Ontology: DOID:0111845 - Orphanet: No dedicated ORPHA number was identified as of this search; an open community tracker issue (OD4RD/Main-Help-Desk #492) explicitly requests creation of a new ORPHAcode for this condition, indicating Orphanet coverage is still pending/incomplete. - ICD-10/ICD-11: No syndrome-specific code identified; would fall under general codes for congenital malformation syndromes with intellectual disability (e.g., ICD-10 Q87.8). - Gene symbol synonyms in databases: HPE13, MKMS, NEDXCF (Neurodevelopmental disorder, X-linked, with craniofacial abnormalities)
Synonyms: "STAG2-related disorder," "STAG2 cohesinopathy," "X-linked cohesinopathy due to STAG2 deficiency," "NEDXCF."
Evidence base: Information is derived almost entirely from aggregated case-report/case-series literature (individual published patients, not large registries or EHR cohorts). As of the most recent (2025) case report, approximately 19–20 patients have been reported in the literature in total, making this an ultra-rare, case-report-level evidence base rather than a population-level epidemiological resource.
Sources: OMIM #301022; OMIM Clinical Synopsis; NCBI GTR C5193008; MalaCards
Primary cause — genetic, X-linked, dosage-sensitive: MKMS is caused by heterozygous (in females) or hemizygous (in males) deleterious variants in STAG2, located at Xq25. STAG2 encodes a cohesin-complex subunit. Mullegama et al. (2017, PMID:28296084) established the dosage-sensitivity model: "we suggest that STAG2 is a dosage-sensitive gene and that heterozygous loss-of-function variants lead to a cohesinopathy." The gene is under strong evolutionary constraint against loss-of-function (pLI ≈ 1, o/e ≈ 0.02), consistent with dosage sensitivity.
Notably, both loss-of-function (deletion, duplication, and dosage-altering variants) can cause overlapping neurodevelopmental phenotypes: - Loss-of-function (point mutations, truncating variants, deletions): the classic MKMS/de novo STAG2 cohesinopathy (Mullegama et al. 2017). - Increased dosage (Xq25 microduplication encompassing only STAG2): Leroy et al. 2016 (PMID:25677961) reported six patients from two families with Xq25 duplications refined to a 173-kb single-gene (STAG2) critical region, with "delayed milestones, speech disturbance, intellectual disability, abnormal behaviours and a characteristic facial dysmorphism," concluding that "increased STAG2 gene copy number and dysregulation of its downstream target genes may be responsible for the specific clinical findings of this syndrome" — establishing this as "a novel cohesinopathy." This is a related but molecularly distinct entity (gain of dosage vs. loss of function) and both ends of the dosage spectrum perturb cohesin stoichiometry.
Risk factors: - Genetic: Nearly all reported cases are de novo; no known population-level susceptibility loci or modifier genes have been established. Variant type and position appear to modulate phenotype (see Genetic section). - Environmental: None established — this is a monogenic disorder with no known environmental, infectious, or lifestyle contributors. - Sex as a risk-modifying factor: Because STAG2 is X-linked and dosage-sensitive, sex profoundly affects viability and phenotype (see Population section) — females (two X alleles, subject to X-inactivation) tolerate more severe/truncating variants and survive; males (single X allele/hemizygous) are proposed to be largely non-viable with severe loss-of-function alleles, explaining an ascertainment bias toward affected females and toward missense variants in affected males.
Protective factors: None specifically documented. Favorable/skewed X-chromosome inactivation (XCI) toward the wild-type allele in a carrier female could theoretically attenuate phenotype, though the literature (Mullegama et al. 2017; Aoi et al. 2020) instead documents the opposite — skewed XCI favoring the mutant allele in most reported affected females (see Mechanism section).
Gene–environment interactions: None reported; this is considered a purely monogenic/chromosomal-dosage disorder.
Sources: Mullegama et al. 2017, AJMG-A, PMID:28296084; Leroy et al. 2016, Clin Genet, PMID:25677961; PMC8476567
Phenotype data are compiled principally from the "Expanding the known phenotype of Mullegama–Klein–Martinez syndrome in male patients" case series (PMC8476567, patient 19 of the aggregated literature cohort of 19 published cases at that time) and the 2025 conotruncal-defect case report (MDPI Genes, PMC12652599).
Long eyelashes, hirsutism, cutis marmorata, hypoplastic nails, congenital diaphragmatic hernia, pulmonary hypoplasia, GERD, abnormal echocardiogram findings.
Severity/progression: Phenotype severity is highly variable ("highly variable phenotypes" — PMC8476567) and appears influenced by variant type/position, sex, and (in rare cases) somatic mosaicism level across tissues. Developmental delay and craniofacial features are present from infancy/early childhood (congenital onset); brain and cardiac malformations are present prenatally/at birth; seizures can emerge later in childhood (e.g., age 7 in one case). No systematic natural-history/QOL instrument data (EQ-5D, SF-36) were identified — QOL impact is inferred qualitatively from developmental/motor limitations (e.g., one 10-year-old patient "cannot walk unaided").
Sources: PMC8476567 (Expanding known phenotype in males); MDPI Genes 2025, PMC12652599; Frontiers 2022 mosaicism paper, PMC9710855; NCBI GTR
Causal gene: STAG2 (HGNC:11355; Gene ID: 10735; located Xq25). Transcripts referenced in the literature: NM_001042749.2, NM_001042750.2, and NM_006603 (canonical). OMIM gene entry: *STAG2, 300826.
Variant spectrum reported in MKMS (aggregated ~19–20 published cases): - Truncating variants (nonsense, frameshift): predominant in females — 13/15 female cases in the 2021 aggregation carried truncating variants (e.g., c.205C>T; p.(Arg69) — the original 2017 index case; c.2972_2975dup, p.His992Glnfs11 — 2025 conotruncal case, which truncates the final 237 amino acids and removes ~20% of the C-terminal domain). - Missense variants: the only variant class reported in males (3–4 total male patients as of 2021), clustering within functional domains — the STAG domain and the C-terminal Stromalin Conservative Domain (SCD)/SA_C domain (e.g., p.Tyr159His and p.Tyr159Cys at the identical residue in two unrelated patients with a shared distinctive phenotype including ectopic posterior pituitary; K1009N in a 4-year-old boy, Mullegama et al. 2019). - Splice-site variants: 1 reported female case; also seen in the mosaic cohort (predicted-but-unconfirmed aberrant splicing by RNA-seq). - Copy-number/dosage variants: Xq25 microduplications spanning only STAG2 (Leroy et al. 2016) causing a phenotypically overlapping but molecularly distinct gain-of-dosage cohesinopathy. - Mosaic/postzygotic variants: first reported in 2022 (PMC9710855), variant allele fractions 29–42% across tissues, associated with asymmetric phenotype expression.
Variant classification (ACMG/AMP): Reported MKMS variants in ClinVar are generally classified Pathogenic/Likely Pathogenic; examples curated: NM_001042750.2:c.1811G>A (p.Arg604Gln) [RCV000761368]; c.1894T>A (p.Cys632Ser) [RCV001823050]; c.1279G>A (p.Ala427Thr) [RCV003444413]; c.445A>G (p.Thr149Ala) [RCV004789936]. VUS and likely-benign STAG2 variants are also catalogued separately in ClinVar Miner for unrelated (non-MKMS) phenotype contexts, underscoring the need for careful variant-disease correlation.
Allele frequency: Novel MKMS-causing variants (e.g., p.Tyr159His, c.2972_2975dup) are consistently absent from gnomAD and ClinVar prior to publication, consistent with strong purifying selection against germline STAG2 loss-of-function (gnomAD pLI ≈ 1, o/e ≈ 0.02 — among the most LOF-intolerant genes in the genome).
Somatic vs. germline: MKMS variants are germline (constitutional), typically de novo, with one documented instance each of confirmed low-level parental (maternal/sibling) carrier status without phenotype (2021 case — asymptomatic heterozygous mother and sister) and postzygotic somatic mosaicism in the proband (2022 case). Important distinction: STAG2 is also one of the most frequently somatically mutated cohesin genes in human cancer (bladder cancer, Ewing sarcoma, myeloid malignancies) — this is a mechanistically related but clinically and pathophysiologically distinct entity from germline MKMS and should not be conflated in curation.
Functional consequence: Predominantly loss of function (haploinsufficiency in females via truncation/nonsense-mediated decay or protein truncation removing the RAD21-interaction SA_C domain; complete/near-complete loss in hemizygous males restricted to milder missense alleles compatible with survival) — plus a distinct gain-of-dosage mechanism for Xq25 duplication cases. No dominant-negative mechanism has been proposed; STAG2 is not itself an enzyme (it is an HEAT-repeat scaffolding/adaptor subunit), so no catalytic gain-of-function is expected.
Modifier genes: None formally established; compensatory upregulation of the paralogous cohesin subunits STAG1 and ectopic STAG3 has been documented as a cellular response to STAG2 loss (forming "chimeric cohesin complexes"), which may modulate phenotype severity but is not a classical inherited modifier.
Epigenetic information: X-chromosome inactivation (XCI) skewing is the central "epigenetic" determinant of phenotype in affected females. Molecular analyses of patient fibroblasts show highly skewed XCI favoring the mutant allele, resulting in loss of STAG2 expression in most tested cells — i.e., the "wrong" X is preferentially silenced, exposing the mutant allele's effects despite heterozygosity. This is the key epigenetic/mechanistic explanation for why females survive severe truncating variants (mosaic expression across cell populations) while a fully hemizygous truncating male would not.
Chromosomal abnormalities: Xq25 microduplications (see Etiology) are the chromosomal-scale correlate of this locus; no aneuploidy or translocation syndromes are otherwise associated.
Suggested ontology terms: Gene — hgnc:11355 (STAG2); related GO molecular function — GO:0032116 (SMC loading complex) is not quite right; more precisely GO:0008278 (cohesin complex, cellular component) and GO:0007062 (sister chromatid cohesion, biological process); protein family — Pfam STAG domain, InterPro stromalin conservative domain (SCD).
Sources: Mullegama et al. 2017, PMID:28296084; PMC8476567; MDPI Genes 2025; Frontiers 2022, PMC9710855; ClinVar records cited above; ClinGen dosage curation, STAG2/HGNC:11355
No environmental, lifestyle, or infectious contributory factors have been identified or proposed in the literature — MKMS is a purely monogenic disorder. Not applicable for toxin/occupational-exposure, dietary, or pathogen-mediated etiology.
Causal chain (molecular → cellular → tissue → clinical):
Protein dysfunction type: Predominantly loss of function via truncation/domain disruption (SA_C/RAD21-interaction domain); the Xq25 duplication mechanism instead represents dosage gain. No evidence for a dominant-negative or aggregation-based mechanism; STAG2 is an HEAT-repeat-containing non-enzymatic scaffold protein (UniProt Q8N3U4), so classical "misfolding/enzyme deficiency" framing does not apply — this is a genome-architecture/regulatory disorder rather than a classical metabolic one.
Immune system involvement: Not implicated; no autoimmune or immunodeficiency phenotype described.
Tissue damage mechanisms: Not a degenerative/tissue-injury disorder in the classical sense (no oxidative stress, ischemia, or fibrosis mechanism); pathology is developmental/morphogenetic (structural malformation from disrupted transcriptional programs during embryogenesis), not post-natal tissue destruction.
Molecular profiling: No large-scale transcriptomic, proteomic, or single-cell datasets specific to MKMS patient tissue were identified in this search (this is consistent with the ultra-rare, case-report-level evidence base). Related mechanistic insight comes from model systems (mouse Stag2-null embryos, zebrafish stag1/stag2 morphants — see Model Organisms) rather than human multi-omics.
Suggested ontology terms: - GO (biological process): GO:0007062 (sister chromatid cohesion), GO:0006325 (chromatin organization), GO:0006338 (chromatin remodeling), GO:0007507 (heart development), GO:0021987 (cerebral cortex development) - GO (cellular component): GO:0008278 (cohesin complex), GO:0000785 (chromatin) - CL (cell types): CL:0000047 (neural stem cell) / CL:0002608 (cortical neuron precursor) for neurodevelopmental phenotypes; CL:0008034 (cardiac neural crest cell) for the cardiac/conotruncal mechanism - UBERON: UBERON:0000955 (brain), UBERON:0000948 (heart), UBERON:0001690 (ear), UBERON:0002616 (skeletal system)
Sources: Mullegama et al. 2017, PMID:28296084; MDPI Genes 2025, PMC12652599; PMC8476567; PMC9710855; PMC12765388 — STAG2-truncating variants mosaic inactivation/compensatory remodeling
Organ level: - Primary: Brain/CNS (microcephaly, polymicrogyria, corpus callosum hypoplasia, ectopic posterior pituitary), craniofacial skeleton, ear (microtia, hearing apparatus), heart (septal defects to complex conotruncal malformations), skeletal system/limbs (hands, feet, digits) - Secondary/complication-associated: Eyes (strabismus, retinal/uveal scarring), gastrointestinal (GERD, diaphragmatic hernia in some female cases), respiratory (pulmonary hypoplasia secondary to diaphragmatic hernia), endocrine (pituitary stalk/ectopic posterior pituitary abnormalities) - Body systems involved: Nervous system, cardiovascular system, musculoskeletal system, special senses (auditory, visual), endocrine system
Tissue/cell level: Neuroepithelium/cortical progenitors (polymicrogyria implies a migration/lamination defect), cardiac neural crest cells and secondary heart field mesoderm (conotruncal septation), otic placode-derived structures (microtia/hearing loss), chondro-osseous tissue (digit/limb skeletal anomalies).
Subcellular level: Nucleus/chromatin (cohesin complex operates at centromeres and along chromosome arms during interphase for loop extrusion — GO:0000785 chromatin, GO:0000775 chromosome centromeric region).
Localization/lateralization: Predominantly bilateral/symmetric malformations (craniofacial, cardiac), but the 2022 mosaic case demonstrated markedly asymmetric/unilateral findings (right-sided supernumerary nipple, unilateral ear dysplasia and hearing impairment, asymmetric growth) attributable to postzygotic mosaic distribution rather than a bilateral developmental field defect.
Suggested UBERON terms: UBERON:0000955 (brain), UBERON:0002021 (cerebral cortex), UBERON:0002336 (corpus callosum), UBERON:0000948 (heart), UBERON:0002078 (right ventricle) / UBERON:0002080 (heart outflow tract) for DORV/pulmonary atresia, UBERON:0001690 (external ear), UBERON:0001846 (pituitary gland)/ UBERON:0002116 (posterior pituitary).
Onset: Congenital/prenatal for structural anomalies (cardiac malformations detected at birth or in utero via IUGR; craniofacial dysmorphism present from birth); developmental delay recognized in infancy (failure to meet motor/speech milestones); some features (e.g., seizures) can have later childhood onset (documented onset at age 7 in one case).
Onset pattern: Insidious/developmental rather than acute — this is a static structural/neurodevelopmental malformation syndrome, not an episodic or acutely progressive disease.
Progression: Predominantly non-progressive/stable structural phenotype (congenital malformations do not worsen per se), but the functional/developmental trajectory (motor, speech, cognitive) shows ongoing delay through childhood; one 10-year-old proband "cannot walk unaided," indicating persistent rather than resolving motor impairment. No formal staging system exists (this is not a staged disease like cancer). Seizures, once they emerge, may require ongoing anticonvulsant management (chronic, not self-limited).
Disease course pattern: Largely stable congenital malformation plus chronic developmental disability; a subset of findings are described as resolving (e.g., patent foramen ovale that was "minimal" and spontaneously closed by 1-year follow-up in two patients).
Critical periods: The embryonic/fetal period is the critical window for the structural anomalies (cardiac septation defects arise from disrupted neural crest/secondary heart field function during cardiogenesis; cortical malformations arise during neuronal migration). Early childhood is the critical window for surveillance and intervention for developmental delay, hearing loss (early identification is critical for speech/language outcomes), and seizure monitoring.
Remission patterns: Not a remitting-relapsing disease; the closest analog is spontaneous resolution of the minimal PFO noted above, which is a known feature of many congenital PFOs generally rather than a syndrome-specific remission.
Epidemiology: MKMS is ultra-rare — approximately 19–20 patients reported in the peer-reviewed literature as of the most recent (2025) case report. No formal prevalence or incidence rate (per 100,000) has been established or published; there is no disease registry, and Orphanet coverage appears to be pending (per the open Orphanet-code request tracked at OD4RD/Main-Help-Desk #492). This places MKMS in the "ultra-rare, case-report-only" epidemiological tier (analogous to PrevalenceClassEnum.NOT_YET_DOCUMENTED in dismech schema terms).
Inheritance pattern: X-linked, most commonly described as X-linked, typically de novo (databases variably label it "X-linked recessive" or "X-linked dominant" depending on source — MalaCards/GTR describe it as X-linked recessive while acknowledging females are more severely, not less, affected, which is somewhat atypical for classic X-linked recessive inheritance and instead reflects dosage-sensitivity/XCI-driven pathophysiology rather than a simple recessive/dominant dichotomy). Virtually all reported cases are de novo, with rare exceptions of asymptomatic carrier relatives (heterozygous mother/sister documented in one 2021 case).
Penetrance/expressivity: Full penetrance is implied for the truncating variants reported in females (all are symptomatic), but expressivity is markedly variable — phenotype severity and specific feature combinations differ substantially between patients, correlating with variant type/position, sex, and (in mosaic cases) variant allele fraction/tissue distribution.
Genetic anticipation: Not applicable (not a repeat-expansion disorder).
Germline mosaicism: Not directly documented for parental transmission in MKMS specifically, but the general precedent for de novo disorders (recurrence risk can rise from <1% to as high as 50% if germline mosaicism is present in a parent) applies, and parental testing is recommended for accurate recurrence-risk counseling. Somatic (postzygotic) mosaicism in the proband has been directly documented (2022 case, variant allele fraction 29–42% across tissues).
Founder effects / consanguinity: None reported; consistent with a de novo mutational mechanism rather than a founder-population or consanguinity-driven recessive disorder.
Carrier frequency: Not established (too rare / not systematically screened; no population carrier-frequency data in gnomAD given the extreme rarity and severity of causal variants).
Sex ratio and viability model: Reported cases skew heavily female (≈15 female : 3–4 male in the 2021 aggregated series). The prevailing model: "females, who carry 2 copies of the STAG2 gene, are able to survive with deleterious de novo variants but show severe phenotypes, while males, who have only 1 copy of the gene, are unable to survive with similar [severe/truncating] variants due to early embryonic lethality" — surviving males are restricted to missense variants within structured functional domains (STAG domain/SCD), producing a generally milder or differently patterned phenotype. This mirrors mouse Stag2-knockout embryonic lethality data (see Model Organisms).
Population demographics: No specific ethnic/geographic enrichment identified; reported patients span multiple countries/ancestries (including at least one Hispanic patient in the 2025 report), consistent with a pan-ethnic de novo disorder rather than a population-specific one.
Age distribution: All reported patients are pediatric at time of ascertainment (from neonatal/infant presentation through at least age 10 in follow-up); no adult natural-history data identified.
Sources: PMC8476567; MalaCards; NCBI GTR; OD4RD Orphanet-code tracker issue #492
Genetic testing (primary diagnostic modality): - Molecular confirmation of a pathogenic STAG2 variant is the diagnostic gold standard, typically achieved via: - Whole exome sequencing (WES) — the modality used to identify the causal variant in essentially all published cases (trio WES enabling de novo variant confirmation). - Single-gene STAG2 sequencing — feasible once suspected clinically, though most cases were identified via unbiased exome/genome approaches given the nonspecific/overlapping phenotype. - Chromosomal microarray (CMA) — the diagnostic method for the Xq25-duplication cohesinopathy variant (array CGH identified the 173-kb critical duplicated region in Leroy et al. 2016). - Per NCBI GTR, 90 clinical tests are listed as available for STAG2, reflecting availability through commercial/clinical gene panels (X-linked intellectual disability panels, cohesinopathy panels) as well as standalone sequencing. - Tissue-specific testing for mosaicism: the 2022 mosaicism report recommends testing multiple tissues (blood, urine, buccal) when mosaicism is suspected (asymmetric phenotype, lower-than-expected variant allele fraction in a single tissue), since variant allele fraction varied substantially by tissue (29.65% blood vs. 40–42% buccal in the reported case).
Imaging: - Brain MRI: recommended given the high yield of pathological findings (15/18 in the aggregated cohort) — polymicrogyria, corpus callosum hypoplasia/hypoplastic presplenial portion, cortical thickening, ventriculomegaly, ectopic posterior pituitary with thin infundibulum. - Echocardiography: essential given the cardiac defect prevalence (7/19, ranging from isolated septal defects to complex conotruncal malformation) — should be performed at diagnosis regardless of presenting phenotype, given the 2025 case demonstrating that severe cardiac disease can occur even when other features (brain MRI, early neurodevelopment) are relatively preserved.
Other clinical tests: - Cytogenetic assay for sister chromatid cohesion defects — used investigationally in the original 2017 index-case report to functionally validate pathogenicity (delayed sister chromatid cohesion demonstrated on cytogenetic analysis). - EEG for seizure/epileptiform activity surveillance. - Standard audiology evaluation given high rate of hearing loss/ear anomalies.
Differential diagnosis: Other cohesinopathies (Cornelia de Lange syndrome — NIPBL, SMC1A, SMC3, HDAC8, RAD21; STAG1-related disorder), other X-linked intellectual disability syndromes, and other syndromic causes of microcephaly with congenital heart disease and craniofacial dysmorphism (e.g., 22q11.2 deletion syndrome for conotruncal defects specifically) should be considered and excluded, particularly before WES/WGS results are available.
Screening: No population-based newborn screening or carrier-screening program exists for this ultra-rare de novo disorder; family-specific carrier testing (of the mother and potentially siblings) is warranted after a proband diagnosis to assess for parental mosaicism and recurrence risk.
Suggested LOINC/MAXO/ontology anchors: MAXO:0000009 (medical imaging procedure) or more specifically brain MRI/echocardiography clinical procedure terms via NCIT; genetic testing terms via NCIT (Whole Exome Sequencing, Chromosomal Microarray Analysis).
Sources: PMC8476567; Mullegama et al. 2017, PMID:28296084; Leroy et al. 2016, PMID:25677961; PMC9710855; NCBI GTR
Survival/mortality: No formal survival statistics (5-year, 10-year) exist given the extreme rarity of the condition; reported patients have survived to at least early-to-mid childhood (oldest follow-up identified: age 10). The proposed embryonic lethality of severe hemizygous STAG2 loss-of-function in males (paralleling mouse data) implies that a subset of the most severe genotype-sex combinations may not be compatible with live birth at all — this is a survivorship-bias caveat relevant to interpreting the published cohort (only survivors are ascertained).
Morbidity/function: Chronic neurodevelopmental disability is the dominant long-term morbidity — intellectual disability (14/19), motor impairment (one 10-year-old unable to walk unaided), speech/language impairment, and hearing loss (with implications for speech development if unaddressed). Congenital heart defects, when severe (as in the 2025 conotruncal case), require neonatal surgical intervention and carry attendant surgical/perioperative morbidity risk. No formal QOL instrument (EQ-5D/SF-36/PROMIS) data are published for this condition.
Complications: Seizures (requiring anticonvulsant therapy in at least one reported case), recurrent GERD and pulmonary hypoplasia in association with congenital diaphragmatic hernia (in some female patients), ophthalmologic complications (strabismus, retinal/uveal scarring).
Prognostic factors: Variant type (truncating vs. missense) and position (functional-domain missense variants in males vs. domain-agnostic truncating variants in females) correlate with phenotype pattern/severity; degree and tissue distribution of mosaicism (in mosaic cases) correlates with asymmetry and severity; X-inactivation skewing in females is a proposed (though not exhaustively studied) severity modifier.
Recovery potential: Developmental gains are possible with early intervention (physical/occupational/speech therapy), consistent with general principles for neurodevelopmental disability, though no MKMS-specific outcome studies of intervention efficacy were identified.
There is no disease-specific or curative therapy for MKMS — management is entirely symptomatic/supportive and multidisciplinary, following general practice for syndromic neurodevelopmental/congenital-malformation disorders. No pharmacogenomic, gene-therapy, or targeted-molecular therapy specific to STAG2 dosage correction has been reported in the human clinical literature (STAG2 is not a druggable enzyme, so classical small-molecule "restore function" strategies do not apply; investigational cohesin-related therapeutics in the literature relate to STAG2's role as a synthetic-lethal target in cancer, not to correcting germline dosage in MKMS).
Documented management from case reports:
- Antiepileptic pharmacotherapy: Lamotrigine was initiated in one patient following a generalized tonic-clonic seizure at age 7 (Pharmacotherapy; MAXO term: generic anticonvulsant use — no MAXO-specific "lamotrigine" term, would use treatment_term NCIT:C15986 Pharmacotherapy with therapeutic_agent CHEBI:6367 lamotrigine).
- Cardiac surgical intervention: The 2025 conotruncal-defect case required neonatal cardiac surgical management for pulmonary atresia/DORV/VSD/ASD/PDA (MAXO:0000004 surgical procedure; more specific NCIT:C15329 Surgical Procedure / cardiac surgery subtype).
- Supportive/rehabilitative care: Physical therapy, occupational therapy, and speech-language therapy are the standard of care implied by the motor and speech delay phenotype (MAXO:0000011 physical therapy; MAXO:0001351 occupational therapy; MAXO:0000930 speech therapy), though not explicitly detailed as an intervention protocol in the sourced case reports.
- Audiological management: hearing aids or other amplification/intervention would be indicated given the high rate of sensorineural hearing loss (MAXO:0009030 hearing aid usage), though not explicitly documented in a specific case.
- Genetic counseling: emphasized in the mosaicism literature as essential for accurate recurrence-risk assessment once mosaic or germline status is clarified (MAXO:0000079 genetic counseling).
Treatment algorithm: No published disease-specific clinical pathway exists; management follows a multidisciplinary, phenotype-driven approach (cardiology, neurology, genetics, audiology, developmental pediatrics, physical/occupational/speech therapy) analogous to other syndromic X-linked intellectual disability disorders.
Clinical trials: No MKMS-specific or STAG2-germline-targeted clinical trials were identified in this search (ClinicalTrials.gov not directly queried in this session, but no trial was surfaced through literature/database search).
Sources: MDPI Genes 2025, PMC12652599; PMC8476567
No primary, secondary, or tertiary prevention strategy exists for this de novo genetic disorder beyond standard reproductive genetic counseling:
No naturally occurring veterinary or wildlife disease analog of MKMS has been reported (this is not a condition documented in OMIA or veterinary literature as a spontaneous animal disease). STAG2 is a highly conserved gene across vertebrates (ortholog present in mouse Stag2, zebrafish stag2), but no spontaneous companion-animal or livestock phenotype attributable to STAG2 variants was identified in this search. No zoonotic or cross-species transmission relevance (this is a non-infectious monogenic disorder).
Mouse (Mus musculus, NCBITaxon:10090): - Constitutive Stag2-null mice: homozygous-null embryos are embryonic lethal at mid-gestation, exhibiting global developmental delay and defective heart morphogenesis, "most prominently in structures derived from secondary heart field progenitors" — directly recapitulating and mechanistically explaining the human conotruncal/outflow-tract cardiac phenotype (relevant papers: "STAG2 cohesin is essential for heart morphogenesis," bioRxiv/PMID pending formal citation retrieval; "Essential Roles of Cohesin STAG2 in Mouse Embryonic Development and Adult Tissue Homeostasis," ScienceDirect/Cell Reports). - Adult conditional loss of Stag2 is tolerated (viable), indicating an embryonic-specific essential requirement — consistent with the disease model that surviving human patients (all carrying partial-function or mosaic/heterozygous-buffered alleles) retain sufficient STAG2 function to avoid the lethal embryonic phenotype seen with complete null alleles. - This mouse model directly supports the proposed mechanism of male embryonic lethality with severe (truncating/null) human alleles, and models the cardiac neural crest/secondary heart field mechanism implicated in the human conotruncal phenotype.
Zebrafish (Danio rerio, NCBITaxon:7955): - stag1/stag2 double morphant/mutant studies show that the two paralogous cohesin STAG subunits differentially influence hematopoietic mesoderm development, providing a model for studying tissue-specific requirements of STAG2 versus its paralog STAG1 during early vertebrate development (relevant to understanding compensatory paralog dynamics also seen in human patient fibroblasts).
Model characteristics — recapitulation and limitations: - The mouse model recapitulates the cardiac/outflow-tract malformation mechanism and demonstrates the embryonic-lethality/dosage-sensitivity principle underlying the human sex-differential viability model. - Limitation: Mouse and zebrafish models to date have focused on embryonic lethality and cardiac/hematopoietic development; they do not yet directly model the postnatal neurodevelopmental phenotype (intellectual disability, speech delay, seizures) that dominates the human clinical picture, since complete loss is embryonic lethal and viable hypomorphic/conditional models recapitulating the surviving human phenotype spectrum (missense, mosaic, or dosage-duplication alleles) are less developed in the literature surveyed here. - No iPSC-derived organoid or patient-derived cellular model specific to MKMS neurodevelopmental phenotypes was identified in this search (an area for potential future model development, e.g., via the MorPhiC-style null-allele iPSC phenotyping approach referenced in the dismech project's own conventions, given STAG2 is exactly the kind of dosage-sensitive gene such platforms are designed to characterize).
Sources: search results citing "STAG2 cohesin is essential for heart morphogenesis" (bioRxiv), "Essential Roles of Cohesin STAG2 in Mouse Embryonic Development and Adult Tissue Homeostasis" (ScienceDirect/Cell Reports), and "Cohesin Components Stag1 and Stag2 Differentially Influence Haematopoietic Mesoderm Development in Zebrafish Embryos" (Frontiers in Cell and Developmental Biology, 2020)
| Category | Term | ID |
|---|---|---|
| Disease | Mullegama-Klein-Martinez syndrome | OMIM:301022 / MONDO:0026722 / DOID:0111845 |
| Gene | STAG2 | hgnc:11355 |
| Phenotype | Global developmental delay | HP:0001263 |
| Phenotype | Intellectual disability | HP:0001249 |
| Phenotype | Microcephaly | HP:0000252 |
| Phenotype | Microtia | HP:0008551 |
| Phenotype | Sensorineural hearing loss | HP:0000407 |
| Phenotype | Polymicrogyria | HP:0002126 |
| Phenotype | Corpus callosum hypoplasia | HP:0002079 |
| Phenotype | Ventricular septal defect | HP:0001629 |
| Phenotype | Double outlet right ventricle | HP:0001719 |
| Phenotype | Pulmonary atresia | HP:0006530 |
| Phenotype | Short stature | HP:0004322 |
| Phenotype | Seizure | HP:0001250 |
| Phenotype | Pes planus | HP:0001769 |
| Phenotype | Fifth finger clinodactyly | HP:0004209 |
| GO (BP) | Sister chromatid cohesion | GO:0007062 |
| GO (BP) | Heart development | GO:0007507 |
| GO (CC) | Cohesin complex | GO:0008278 |
| CL | Cardiac neural crest cell | CL:0008034 |
| UBERON | Corpus callosum | UBERON:0002336 |
| UBERON | Heart outflow tract | UBERON:0002080 |
| MAXO | Genetic counseling | MAXO:0000079 |
| MAXO | Surgical procedure | MAXO:0000004 |
mechanistic_hypotheses framing rather than an established-fact framing.