Marshall-Smith Syndrome

Mendelian MONDO:0011244 Pathograph 20 Show in embeddings browser Neurodevelopmental Disorders Skeletal Dysplasias

Marshall-Smith syndrome (MSS) is a rare, severe malformation syndrome caused by heterozygous frameshift and splice-site NFIX variants clustered in exons 6-10. Unlike the NFIX loss-of-function/haploinsufficiency variants that cause Malan syndrome (an overgrowth disorder, exon 2), the MSS-associated mutant transcripts escape nonsense-mediated mRNA decay (NMD) and are translated into truncated proteins that retain the N-terminal DNA-binding/dimerization domain but vary in their C-terminus, acting in a dominant-negative manner. The syndrome is characterized by accelerated and dysharmonic osseous maturation, failure to thrive, characteristic craniofacial dysmorphism (high forehead, underdeveloped midface, proptosis, anteverted nares), moderate-to-severe developmental delay with absent or limited speech, kyphoscoliosis, and respiratory compromise from upper airway obstruction that historically caused high early mortality.

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5
Pathophys.
19
Phenotypes
1
Gaps
20
Pathograph
1
Genes
3
Medical Actions
1
Models
6
References
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Discussions and Knowledge Gaps

1
Does the Nfix-null mouse model MSS, given that it reproduces the skeletal and failure-to-thrive phenotype through loss of function rather than the dominant-negative allelic mechanism that causes human MSS?
HUMAN MODEL MISMATCH OPEN mss_mouse_allelic_mechanism_mismatch
Attached to
pathophysiology#Dysregulated Osseous Maturation animal_models#Mouse
The only reported mouse reproduces the MSS skeletal phenotype (spinal deformation, decreased bone mineralization) and failure to thrive, but it is Nfix loss of function. In humans, NFIX loss of function / haploinsufficiency causes Malan syndrome (overgrowth), not MSS; MSS is caused by NMD-escaping dominant-negative alleles. The mouse therefore models the wrong allelic mechanism while reproducing the phenotype, so its findings cannot be treated as validating the dominant-negative mechanism of human MSS.
Show evidence (1 reference)
PMID:20673863 SUPPORT Model Organism
"Nfix-deficient mice are unable to gain weight and die in the first 3 postnatal weeks, while they also present with a spinal deformation and decreased bone mineralization."
Documents the Nfix-null mouse phenotype whose loss-of-function basis mismatches the dominant-negative allelic mechanism of human MSS.
⚙

Pathophysiology

5
NFIX Truncating Variants Escaping Nonsense-Mediated Decay
Marshall-Smith syndrome is caused by heterozygous frameshift and splice-site NFIX variants (and recurrent deletions of exons 6 and 7) scattered through exons 6-10. In contrast to the exon-2 variants that cause Malan syndrome, the MSS-associated mutant mRNAs are not cleared by nonsense-mediated mRNA decay, so they persist and are translated into aberrant, C-terminally altered NFIX protein.
Genetic context NFIX hgnc:7788 HUGO Gene Nomenclature Committee (hgnc) Relation: this genetic context concerns this gene This genetic context concerns NFIX (hgnc:7788). hgnc:7788 is a gene from the HUGO Gene Nomenclature Committee. variant_origin: DE_NOVO zygosity: HETEROZYGOUS functional_impact_category: DOMINANT_NEGATIVE
Frameshift, splice-site, and exon 6-7 deletion variants in exons 6-10 escape NMD and encode truncated proteins acting in a dominant-negative manner; variants arise de novo.
escape from nonsense-mediated mRNA decay of mutant NFIX transcript GO:0000184 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased escape from nonsense-mediated mRNA decay of mutant NFIX transcript, annotated with nuclear-transcribed mRNA catabolic process, nonsense-mediated decay (GO:0000184). GO:0000184 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Dominant-Negative NFIX Transcriptional Dysfunction
Show evidence (3 references)
PMID:20673863 SUPPORT Human Clinical
"Distinct frameshift and splice NFIX mutations that escaped nonsense-mediated mRNA decay (NMD) were identified in nine MSS subjects."
Original identification of NFIX as the MSS gene, showing MSS mutations escape NMD, unlike the NMD-triggering variants of Malan syndrome.
PMID:24924640 SUPPORT In Vitro
"MSS-associated mutant mRNAs are not cleared by nonsense-mediated mRNA decay"
RT-PCR experiments confirm MSS-associated mutant transcripts escape NMD and persist.
PMID:26200704 SUPPORT Human Clinical
"Marshall-Smith mutations are scattered through exons 6-10 of NFIX gene, while most point mutations causing an overgrowth syndrome are clustered in exon 2."
Establishes the exon 6-10 clustering that distinguishes MSS from the exon-2 overgrowth (Malan/Sotos-like) variants.
Dominant-Negative NFIX Transcriptional Dysfunction
The NMD-escaping mutant NFIX proteins retain a preserved DNA-binding and dimerization domain but differ grossly in their C-terminal portion, so they dimerize with and interfere with wild-type NFIX transcription-factor function. NFIX is a nuclear factor I family transcription factor normally expressed prenatally during human brain development and skeletogenesis; its dominant-negative disruption dysregulates the transcriptional programs of those tissues.
regulation of DNA-templated transcription GO:0006355 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal regulation of DNA-templated transcription (GO:0006355). GO:0006355 is a biological process from the Gene Ontology. ⚠ ABNORMAL
DNA-binding transcription factor activity GO:0003700 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves abnormal DNA-binding transcription factor activity (GO:0003700). GO:0003700 is a molecular function from the Gene Ontology. ⚠ ABNORMAL
Downstream
Dysregulated Osseous Maturation
Impaired Neurodevelopment
Craniofacial and Upper Airway Malformation
Aortic Root Dilatation
Precocious Puberty
Show evidence (2 references)
PMID:24924640 SUPPORT Computational
"Predicted MSS-associated mutant NFIX proteins consistently have a preserved DNA binding and dimerization domain, whereas they grossly vary in their C-terminal portion."
Establishes the structural basis for a dominant-negative effect: retained dimerization/DNA-binding but a variable, dysfunctional C-terminus.
PMID:20673863 SUPPORT Human Clinical
"We demonstrate that NFIX is normally expressed prenatally during human brain development and skeletogenesis."
Shows NFIX is normally active in the brain and skeleton, the tissues subsequently disrupted in MSS.
Dysregulated Osseous Maturation
Dominant-negative NFIX dysfunction dysregulates skeletal system development, producing accelerated and dysharmonic (uneven) osseous maturation with advanced bone age, together with osteopenia, kyphoscoliosis, and failure to thrive.
osteoblast CL:0000062 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves osteoblast (CL:0000062). CL:0000062 is a cell type from the Cell Ontology. chondrocyte CL:0000138 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves chondrocyte (CL:0000138). CL:0000138 is a cell type from the Cell Ontology.
skeletal system development GO:0001501 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal skeletal system development (GO:0001501). GO:0001501 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Animal models
Nfix-null mouse Mouse partially recapitulates Nfix-null mice show decreased bone mineralization, a skeletal correlate of the dysregulated osseous maturation (with osteopenia) of MSS.
Downstream
Accelerated Skeletal Maturation
Kyphoscoliosis
Failure to Thrive
Short Stature
Show evidence (1 reference)
PMID:20673863 SUPPORT Human Clinical
"a severe malformation syndrome characterized by failure to thrive, respiratory insufficiency, accelerated osseous maturation, kyphoscoliosis, osteopenia, and unusual facies"
Lists accelerated osseous maturation, kyphoscoliosis, osteopenia and failure to thrive as core skeletal/growth features of MSS.
Impaired Neurodevelopment
NFIX is required for normal human brain development; dominant-negative dysfunction impairs nervous system development, producing moderate-to-severe developmental delay with absent or limited speech and structural brain anomalies such as hypoplasia of the corpus callosum.
neural progenitor cell CL:0011020 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neural progenitor cell (CL:0011020). CL:0011020 is a cell type from the Cell Ontology. neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
nervous system development GO:0007399 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal nervous system development (GO:0007399). GO:0007399 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Intellectual Disability
Absent Speech
Corpus Callosum Hypoplasia
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"Minor abnormalities of brain morphology such as hypoplasia of the corpus callosum are common."
Documents structural brain anomalies (corpus callosum hypoplasia) as a common finding, reflecting impaired neurodevelopment.
Craniofacial and Upper Airway Malformation
MSS produces a characteristic craniofacial gestalt (high forehead, underdeveloped midface, proptosis, anteverted nares, everted lips) and associated upper-airway anomalies. The midface and airway malformation causes upper airway obstruction and respiratory compromise, historically the leading cause of early death. Progressive craniosynostosis with raised intracranial pressure can also occur.
Downstream
Upper Airway Obstruction
Respiratory Insufficiency
Midface Retrusion
Craniosynostosis
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"Facial features are characteristic with high forehead, underdeveloped midface, proptosis, anteverted nares, and everted lips."
Defines the characteristic craniofacial dysmorphism underlying this node.
⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Marshall-Smith Syndrome Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
●

Phenotypes

19
Cardiovascular 1
Aortic Root Dilatation VERY_RARE Aortic root aneurysm HP:0002616 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Aortic root dilatation, annotated with Aortic root aneurysm (HP:0002616). HP:0002616 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:28442439 SUPPORT Human Clinical
"Additionally, the patient showed progressive dilatation of the aortic root."
Reports progressive aortic root dilatation in an MSS patient.
Endocrine 1
Precocious Puberty VERY_RARE HP:0000826 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Central precocious puberty, annotated with Precocious puberty (HP:0000826). HP:0000826 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:28442439 SUPPORT Human Clinical
"The patient also presented with precocious puberty diagnosed at five years of age and had an abnormal GnRH stimulation test indicative of central precocious puberty."
Reports central precocious puberty confirmed by GnRH stimulation testing.
Eye 1
Proptosis VERY_FREQUENT HP:0000520 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Proptosis (HP:0000520). HP:0000520 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"Facial features are characteristic with high forehead, underdeveloped midface, proptosis, anteverted nares, and everted lips."
Documents proptosis as a characteristic facial feature.
Head and Neck 5
High Forehead VERY_FREQUENT HP:0000348 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is High forehead (HP:0000348). HP:0000348 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"Facial features are characteristic with high forehead, underdeveloped midface, proptosis, anteverted nares, and everted lips."
Documents high forehead as a characteristic facial feature.
Midface Retrusion VERY_FREQUENT HP:0011800 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Underdeveloped midface, annotated with Midface retrusion (HP:0011800). HP:0011800 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"Facial features are characteristic with high forehead, underdeveloped midface, proptosis, anteverted nares, and everted lips."
Documents underdeveloped midface as a characteristic feature.
Anteverted Nares VERY_FREQUENT HP:0000463 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Anteverted nares (HP:0000463). HP:0000463 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"Facial features are characteristic with high forehead, underdeveloped midface, proptosis, anteverted nares, and everted lips."
Documents anteverted nares as a characteristic facial feature.
Everted Lower Lip FREQUENT Everted lower lip vermilion HP:0000232 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Everted lips, annotated with Everted lower lip vermilion (HP:0000232). HP:0000232 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"Facial features are characteristic with high forehead, underdeveloped midface, proptosis, anteverted nares, and everted lips."
Documents everted lips as a characteristic facial feature.
Craniosynostosis VERY_RARE HP:0001363 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Craniosynostosis (HP:0001363). HP:0001363 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38647663 SUPPORT Human Clinical
"We report a case of a 9-year-old MSS patient with progressive craniosynostosis, elevated intracranial pressure, and catastrophic ocular complications."
Reports progressive craniosynostosis with elevated intracranial pressure in MSS.
Musculoskeletal 3
Accelerated Skeletal Maturation VERY_FREQUENT HP:0005616 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Accelerated skeletal maturation (HP:0005616). HP:0005616 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:28442439 SUPPORT Human Clinical
"characterized by failure to thrive and characteristic dysmorphic features associated with accelerated osseous maturation"
Confirms accelerated osseous maturation as a defining feature of MSS.
Kyphoscoliosis FREQUENT HP:0002751 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Kyphoscoliosis (HP:0002751). HP:0002751 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"respiratory compromise secondary to upper airway obstruction, short stature, and kyphoscoliosis"
Lists kyphoscoliosis among the main clinical features.
Osteopenia FREQUENT HP:0000938 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Osteopenia (HP:0000938). HP:0000938 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20673863 SUPPORT Human Clinical
"a severe malformation syndrome characterized by failure to thrive, respiratory insufficiency, accelerated osseous maturation, kyphoscoliosis, osteopenia, and unusual facies"
Lists osteopenia among the core skeletal features that characterize MSS.
Nervous System 4
Intellectual Disability VERY_FREQUENT HP:0001249 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Intellectual disability (HP:0001249). HP:0001249 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"Main clinical features are moderate to severe developmental delay with absent or limited speech, unusual behavior, dysharmonic bone maturation, respiratory compromise secondary to upper airway obstruction, short stature, and kyphoscoliosis."
Large delineation cohort lists moderate-to-severe developmental delay as a main clinical feature.
Absent Speech VERY_FREQUENT HP:0001344 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Absent or limited speech, annotated with Absent speech (HP:0001344). HP:0001344 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"moderate to severe developmental delay with absent or limited speech"
Documents absent or limited speech as a main feature.
Corpus Callosum Hypoplasia FREQUENT Hypoplasia of the corpus callosum HP:0002079 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypoplasia of the corpus callosum (HP:0002079). HP:0002079 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"Minor abnormalities of brain morphology such as hypoplasia of the corpus callosum are common."
Documents corpus callosum hypoplasia as a common brain-morphology anomaly.
Atypical Behavior FREQUENT HP:0000708 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Unusual behavior, annotated with Atypical behavior (HP:0000708). HP:0000708 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"Main clinical features are moderate to severe developmental delay with absent or limited speech, unusual behavior, dysharmonic bone maturation, respiratory compromise secondary to upper airway obstruction, short stature, and kyphoscoliosis."
Names unusual behavior among the main clinical features of MSS.
Respiratory 2
Upper Airway Obstruction VERY_FREQUENT HP:0002781 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Upper airway obstruction (HP:0002781). HP:0002781 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"respiratory compromise secondary to upper airway obstruction"
Documents upper airway obstruction as the basis of respiratory compromise.
Respiratory Insufficiency VERY_FREQUENT HP:0002093 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Respiratory insufficiency (HP:0002093). HP:0002093 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"Mortality from respiratory complications is high, but airway support increasingly allows survival into adulthood."
Establishes respiratory failure as the principal cause of mortality in MSS.
Growth 2
Failure to Thrive VERY_FREQUENT HP:0001508 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Failure to thrive (HP:0001508). HP:0001508 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:28442439 SUPPORT Human Clinical
"characterized by failure to thrive and characteristic dysmorphic features associated with accelerated osseous maturation"
Names failure to thrive as a characterizing feature of MSS.
Short Stature FREQUENT HP:0004322 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Short stature (HP:0004322). HP:0004322 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"respiratory compromise secondary to upper airway obstruction, short stature, and kyphoscoliosis"
Lists short stature among the main clinical features.
🧬

Genetic Associations

1
NFIX (Pathogenic Variants)
Gene: NFIX hgnc:7788 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is NFIX (hgnc:7788). hgnc:7788 is a gene from the HUGO Gene Nomenclature Committee.
Autosomal dominant
Show evidence (3 references)
PMID:24924640 SUPPORT Human Clinical
"we identified a recurrent deletion of NFIX exon 6 and 7 in five individuals"
Documents the recurrent Alu-mediated exon 6-7 deletion as a cause of MSS.
PMID:24924640 SUPPORT Human Clinical
"These findings show that MSS is a genetically homogeneous Mendelian disorder."
Establishes NFIX as the single causal gene for a genetically homogeneous MSS.
PMID:20673863 SUPPORT Human Clinical
"Distinct frameshift and splice NFIX mutations that escaped nonsense-mediated mRNA decay (NMD) were identified in nine MSS subjects."
Original NFIX identification in MSS, showing the NMD-escaping variant class.
💊

Medical Actions

3
Airway and Respiratory Support
Action: supportive careNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is supportive care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. Ontology label: Supportive Care NCIT:C15747
Airway support (including tracheostomy where needed) manages the upper airway obstruction and respiratory compromise that are the leading causes of mortality; such support increasingly allows survival into adulthood.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"Mortality from respiratory complications is high, but airway support increasingly allows survival into adulthood."
Supports airway/respiratory support as the intervention that reduces respiratory mortality and prolongs survival.
Craniosynostosis Surgery
Action: cranial surgical procedureNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is cranial surgical procedure, annotated with Surgical Procedure (NCIT:C15329). NCIT:C15329 is a clinical intervention from the NCI Thesaurus. Ontology label: Surgical Procedure NCIT:C15329
Platform: Surgery
Surgical management of progressive craniosynostosis and raised intracranial pressure, with associated ophthalmologic care.
Show evidence (1 reference)
PMID:38647663 SUPPORT Human Clinical
"We report a case of a 9-year-old MSS patient with progressive craniosynostosis, elevated intracranial pressure, and catastrophic ocular complications."
Documents the progressive craniosynostosis and raised intracranial pressure that require surgical management.
Developmental and Rehabilitative Support
Action: rehabilitationNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is rehabilitation (NCIT:C15315). NCIT:C15315 is a clinical intervention from the NCI Thesaurus. Ontology label: Rehabilitation NCIT:C15315
Platform: Behavioral / lifestyle
Multidisciplinary developmental support and rehabilitation for the moderate-to-severe developmental delay, absent/limited speech, and skeletal complications.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"Main clinical features are moderate to severe developmental delay with absent or limited speech, unusual behavior, dysharmonic bone maturation, respiratory compromise secondary to upper airway obstruction, short stature, and kyphoscoliosis."
Documents the developmental delay and speech impairment that developmental and rehabilitative support addresses.
🔬

Diagnosis

2
Molecular genetic testing of NFIX
MSS is confirmed by molecular analysis of NFIX. Conventional sequencing detects the frameshift and splice-site variants; multiplex ligation-dependent probe amplification (MLPA) is additionally required to detect the recurrent Alu-mediated deletion of exons 6 and 7 and other copy-number changes that sequencing misses.
molecular genetic testing NCIT:C15709 NCI Thesaurus (NCIT)
Results: A heterozygous NFIX frameshift or splice-site variant, or a recurrent exon 6-7 deletion, in exons 6-10.
Show evidence (2 references)
PMID:24924640 SUPPORT Human Clinical
"In our study cohort of 17 patients with a clinical diagnosis of MSS, conventional sequencing of NFIX revealed frameshift and splice-site mutations in 10 individuals."
Establishes conventional NFIX sequencing as the first-line molecular diagnostic test in clinically diagnosed MSS.
PMID:24924640 SUPPORT Human Clinical
"Using multiplex ligation-dependent probe amplification analysis, we identified a recurrent deletion of NFIX exon 6 and 7 in five individuals."
Establishes that MLPA is needed to detect the recurrent exon 6-7 deletion that sequencing does not capture.
Cardiovascular surveillance by echocardiography
Echocardiographic surveillance is warranted given reports of progressive aortic root dilatation.
echocardiography NCIT:C16525 NCI Thesaurus (NCIT)
Results: Progressive aortic root dilatation on serial echocardiography.
Show evidence (1 reference)
PMID:28442439 SUPPORT Human Clinical
"Additionally, the patient showed progressive dilatation of the aortic root."
Progressive aortic root dilatation supports cardiovascular (echocardiographic) surveillance.
📊

Prevalence

1
Worldwide
Cases In Literature Ultra Rare
Fewer than 50 patients described in the medical literature as of the 2010 international delineation study; MSS is a very rare malformation syndrome.
Show evidence (1 reference)
PMID:20949508 SUPPORT Human Clinical
"Marshall-Smith syndrome (MSS) is a distinctive entity of unknown etiology with fewer than 50 patients described in the medical literature to date."
Establishes the ultra-rare status with fewer than 50 reported patients.
🐁

Animal Models

1
Nfix-null mouse
Constitutive Nfix-null mice fail to gain weight, die within the first three postnatal weeks, and develop a spinal deformation with decreased bone mineralization. The mouse reproduces the skeletal and growth phenotype of MSS but through Nfix loss of function rather than the dominant-negative allelic mechanism operating in human MSS (the mechanism that instead underlies Malan syndrome).
Species
Mouse
Genotype
Nfix-deficient (Nfix knockout)
Publication
PMID:20673863
Pathograph links
Dysregulated Osseous Maturation partially recapitulates fidelity: moderate Nfix-null mice show decreased bone mineralization, a skeletal correlate of the dysregulated osseous maturation (with osteopenia) of MSS.
Limitations: Decreased bone mineralization in the mouse reflects Nfix loss of function, not the dominant-negative allelic mechanism of MSS; the mouse also lacks the accelerated, dysharmonic bone maturation that defines the human phenotype.
Show evidence (1 reference)
PMID:20673863 SUPPORT Model Organism
"Nfix-deficient mice are unable to gain weight and die in the first 3 postnatal weeks, while they also present with a spinal deformation and decreased bone mineralization."
Reports decreased bone mineralization in Nfix-null mice as a partial correlate of the MSS skeletal phenotype.
{ }

Source YAML

click to show 
name: Marshall-Smith Syndrome
creation_date: "2026-09-02T00:00:00Z"
description: >
  Marshall-Smith syndrome (MSS) is a rare, severe malformation syndrome caused by
  heterozygous frameshift and splice-site NFIX variants clustered in exons 6-10.
  Unlike the NFIX loss-of-function/haploinsufficiency variants that cause Malan
  syndrome (an overgrowth disorder, exon 2), the MSS-associated mutant transcripts
  escape nonsense-mediated mRNA decay (NMD) and are translated into truncated
  proteins that retain the N-terminal DNA-binding/dimerization domain but vary in
  their C-terminus, acting in a dominant-negative manner. The syndrome is
  characterized by accelerated and dysharmonic osseous maturation, failure to
  thrive, characteristic craniofacial dysmorphism (high forehead, underdeveloped
  midface, proptosis, anteverted nares), moderate-to-severe developmental delay
  with absent or limited speech, kyphoscoliosis, and respiratory compromise from
  upper airway obstruction that historically caused high early mortality.
category: Mendelian
disease_term:
  preferred_term: Marshall-Smith syndrome
  term:
    id: MONDO:0011244
    label: Marshall-Smith syndrome
parents:
- Neurodevelopmental Disorders
- Skeletal Dysplasias
pathophysiology:
- name: NFIX Truncating Variants Escaping Nonsense-Mediated Decay
  description: >
    Marshall-Smith syndrome is caused by heterozygous frameshift and splice-site
    NFIX variants (and recurrent deletions of exons 6 and 7) scattered through
    exons 6-10. In contrast to the exon-2 variants that cause Malan syndrome, the
    MSS-associated mutant mRNAs are not cleared by nonsense-mediated mRNA decay, so
    they persist and are translated into aberrant, C-terminally altered NFIX
    protein.
  biological_scale: MOLECULAR
  genetic_context:
    gene:
      preferred_term: NFIX
      term:
        id: hgnc:7788
        label: NFIX
    variant_origin: DE_NOVO
    zygosity: HETEROZYGOUS
    functional_impact_category: DOMINANT_NEGATIVE
    description: >
      Frameshift, splice-site, and exon 6-7 deletion variants in exons 6-10 escape
      NMD and encode truncated proteins acting in a dominant-negative manner;
      variants arise de novo.
  biological_processes:
  - preferred_term: escape from nonsense-mediated mRNA decay of mutant NFIX transcript
    term:
      id: GO:0000184
      label: nuclear-transcribed mRNA catabolic process, nonsense-mediated decay
    modifier: DECREASED
  downstream:
  - target: Dominant-Negative NFIX Transcriptional Dysfunction
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:24924640
      reference_title: "Deletions in the 3' part of the NFIX gene including a recurrent Alu-mediated deletion of exon 6 and 7 account for previously unexplained cases of Marshall-Smith syndrome."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: COMPUTATIONAL
      snippet: "This is in line with the hypothesis that MSS-associated mutations encode dysfunctional proteins that act in a dominant negative manner."
      explanation: >-
        The dominant-negative mechanism is an in-silico inference from the predicted
        domain structure of the NMD-escaping mutant proteins, supporting the causal
        link through an inference step rather than a direct functional assay.
  evidence:
  - reference: PMID:20673863
    reference_title: "Distinct effects of allelic NFIX mutations on nonsense-mediated mRNA decay engender either a Sotos-like or a Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Distinct frameshift and splice NFIX mutations that escaped nonsense-mediated mRNA decay (NMD) were identified in nine MSS subjects."
    explanation: >-
      Original identification of NFIX as the MSS gene, showing MSS mutations escape
      NMD, unlike the NMD-triggering variants of Malan syndrome.
  - reference: PMID:24924640
    reference_title: "Deletions in the 3' part of the NFIX gene including a recurrent Alu-mediated deletion of exon 6 and 7 account for previously unexplained cases of Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "MSS-associated mutant mRNAs are not cleared by nonsense-mediated mRNA decay"
    explanation: >-
      RT-PCR experiments confirm MSS-associated mutant transcripts escape NMD and persist.
  - reference: PMID:26200704
    reference_title: "Novel mutations of NFIX gene causing Marshall-Smith syndrome or Sotos-like syndrome: one gene, two phenotypes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Marshall-Smith mutations are scattered through exons 6-10 of NFIX gene, while most point mutations causing an overgrowth syndrome are clustered in exon 2."
    explanation: >-
      Establishes the exon 6-10 clustering that distinguishes MSS from the exon-2
      overgrowth (Malan/Sotos-like) variants.
- name: Dominant-Negative NFIX Transcriptional Dysfunction
  description: >
    The NMD-escaping mutant NFIX proteins retain a preserved DNA-binding and
    dimerization domain but differ grossly in their C-terminal portion, so they
    dimerize with and interfere with wild-type NFIX transcription-factor function.
    NFIX is a nuclear factor I family transcription factor normally expressed
    prenatally during human brain development and skeletogenesis; its dominant-negative
    disruption dysregulates the transcriptional programs of those tissues.
  biological_scale: MOLECULAR
  biological_processes:
  - preferred_term: regulation of DNA-templated transcription
    term:
      id: GO:0006355
      label: regulation of DNA-templated transcription
    modifier: ABNORMAL
  molecular_functions:
  - preferred_term: DNA-binding transcription factor activity
    term:
      id: GO:0003700
      label: DNA-binding transcription factor activity
    modifier: ABNORMAL
  evidence:
  - reference: PMID:24924640
    reference_title: "Deletions in the 3' part of the NFIX gene including a recurrent Alu-mediated deletion of exon 6 and 7 account for previously unexplained cases of Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: "Predicted MSS-associated mutant NFIX proteins consistently have a preserved DNA binding and dimerization domain, whereas they grossly vary in their C-terminal portion."
    explanation: >-
      Establishes the structural basis for a dominant-negative effect: retained
      dimerization/DNA-binding but a variable, dysfunctional C-terminus.
  - reference: PMID:20673863
    reference_title: "Distinct effects of allelic NFIX mutations on nonsense-mediated mRNA decay engender either a Sotos-like or a Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We demonstrate that NFIX is normally expressed prenatally during human brain development and skeletogenesis."
    explanation: >-
      Shows NFIX is normally active in the brain and skeleton, the tissues
      subsequently disrupted in MSS.
  downstream:
  - target: Dysregulated Osseous Maturation
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Impaired Neurodevelopment
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Craniofacial and Upper Airway Malformation
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Aortic Root Dilatation
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:28442439
      reference_title: "Marshall-Smith syndrome: Novel pathogenic variant and previously unreported associations with precocious puberty and aortic root dilatation."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Additionally, the patient showed progressive dilatation of the aortic root."
      explanation: >-
        Reports aortic root dilatation as a newly recognized association of MSS,
        the intermediate steps between NFIX dysfunction and the aortic phenotype
        being unknown.
  - target: Precocious Puberty
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:28442439
      reference_title: "Marshall-Smith syndrome: Novel pathogenic variant and previously unreported associations with precocious puberty and aortic root dilatation."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The patient also presented with precocious puberty diagnosed at five years of age and had an abnormal GnRH stimulation test indicative of central precocious puberty."
      explanation: >-
        Reports central precocious puberty as a newly recognized association of MSS,
        with the intermediate mechanism linking NFIX dysfunction to the endocrine
        phenotype unknown.
- name: Dysregulated Osseous Maturation
  description: >
    Dominant-negative NFIX dysfunction dysregulates skeletal system development,
    producing accelerated and dysharmonic (uneven) osseous maturation with advanced
    bone age, together with osteopenia, kyphoscoliosis, and failure to thrive.
  biological_scale: TISSUE
  cell_types:
  - preferred_term: osteoblast
    term:
      id: CL:0000062
      label: osteoblast
  - preferred_term: chondrocyte
    term:
      id: CL:0000138
      label: chondrocyte
  biological_processes:
  - preferred_term: skeletal system development
    term:
      id: GO:0001501
      label: skeletal system development
    modifier: ABNORMAL
  evidence:
  - reference: PMID:20673863
    reference_title: "Distinct effects of allelic NFIX mutations on nonsense-mediated mRNA decay engender either a Sotos-like or a Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a severe malformation syndrome characterized by failure to thrive, respiratory insufficiency, accelerated osseous maturation, kyphoscoliosis, osteopenia, and unusual facies"
    explanation: >-
      Lists accelerated osseous maturation, kyphoscoliosis, osteopenia and failure
      to thrive as core skeletal/growth features of MSS.
  downstream:
  - target: Accelerated Skeletal Maturation
    causal_link_type: DIRECT
  - target: Kyphoscoliosis
    causal_link_type: DIRECT
  - target: Failure to Thrive
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Short Stature
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- name: Impaired Neurodevelopment
  description: >
    NFIX is required for normal human brain development; dominant-negative
    dysfunction impairs nervous system development, producing moderate-to-severe
    developmental delay with absent or limited speech and structural brain anomalies
    such as hypoplasia of the corpus callosum.
  biological_scale: TISSUE
  cell_types:
  - preferred_term: neural progenitor cell
    term:
      id: CL:0011020
      label: neural progenitor cell
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: nervous system development
    term:
      id: GO:0007399
      label: nervous system development
    modifier: ABNORMAL
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Minor abnormalities of brain morphology such as hypoplasia of the corpus callosum are common."
    explanation: >-
      Documents structural brain anomalies (corpus callosum hypoplasia) as a common
      finding, reflecting impaired neurodevelopment.
  downstream:
  - target: Intellectual Disability
    causal_link_type: DIRECT
  - target: Absent Speech
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Corpus Callosum Hypoplasia
    causal_link_type: DIRECT
- name: Craniofacial and Upper Airway Malformation
  description: >
    MSS produces a characteristic craniofacial gestalt (high forehead, underdeveloped
    midface, proptosis, anteverted nares, everted lips) and associated upper-airway
    anomalies. The midface and airway malformation causes upper airway obstruction
    and respiratory compromise, historically the leading cause of early death.
    Progressive craniosynostosis with raised intracranial pressure can also occur.
  biological_scale: TISSUE
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Facial features are characteristic with high forehead, underdeveloped midface, proptosis, anteverted nares, and everted lips."
    explanation: >-
      Defines the characteristic craniofacial dysmorphism underlying this node.
  downstream:
  - target: Upper Airway Obstruction
    causal_link_type: DIRECT
  - target: Respiratory Insufficiency
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:20949508
      reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "respiratory compromise secondary to upper airway obstruction"
      explanation: >-
        States that respiratory compromise in MSS is secondary to upper airway
        obstruction, supporting this causal edge.
  - target: Midface Retrusion
    causal_link_type: DIRECT
  - target: Craniosynostosis
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
phenotypes:
- category: Clinical
  name: Accelerated Skeletal Maturation
  description: >
    Accelerated and dysharmonic osseous maturation with advanced bone age is a
    cardinal and near-diagnostic feature of MSS.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Accelerated skeletal maturation
    term:
      id: HP:0005616
      label: Accelerated skeletal maturation
  evidence:
  - reference: PMID:28442439
    reference_title: "Marshall-Smith syndrome: Novel pathogenic variant and previously unreported associations with precocious puberty and aortic root dilatation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "characterized by failure to thrive and characteristic dysmorphic features associated with accelerated osseous maturation"
    explanation: >-
      Confirms accelerated osseous maturation as a defining feature of MSS.
- category: Clinical
  name: Failure to Thrive
  description: >
    Failure to thrive with poor weight gain is a consistent early feature.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  evidence:
  - reference: PMID:28442439
    reference_title: "Marshall-Smith syndrome: Novel pathogenic variant and previously unreported associations with precocious puberty and aortic root dilatation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "characterized by failure to thrive and characteristic dysmorphic features associated with accelerated osseous maturation"
    explanation: >-
      Names failure to thrive as a characterizing feature of MSS.
- category: Clinical
  name: Intellectual Disability
  description: >
    Moderate-to-severe developmental delay and intellectual disability are present
    in affected individuals.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Main clinical features are moderate to severe developmental delay with absent or limited speech, unusual behavior, dysharmonic bone maturation, respiratory compromise secondary to upper airway obstruction, short stature, and kyphoscoliosis."
    explanation: >-
      Large delineation cohort lists moderate-to-severe developmental delay as a
      main clinical feature.
- category: Clinical
  name: Absent Speech
  description: >
    Speech is absent or severely limited in most patients.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Absent or limited speech
    term:
      id: HP:0001344
      label: Absent speech
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "moderate to severe developmental delay with absent or limited speech"
    explanation: >-
      Documents absent or limited speech as a main feature.
- category: Clinical
  name: Kyphoscoliosis
  description: >
    Kyphoscoliosis is a recurrent axial skeletal deformity in MSS.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Kyphoscoliosis
    term:
      id: HP:0002751
      label: Kyphoscoliosis
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "respiratory compromise secondary to upper airway obstruction, short stature, and kyphoscoliosis"
    explanation: >-
      Lists kyphoscoliosis among the main clinical features.
- category: Clinical
  name: Short Stature
  description: >
    Short stature is a recurrent growth feature.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Short stature
    term:
      id: HP:0004322
      label: Short stature
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "respiratory compromise secondary to upper airway obstruction, short stature, and kyphoscoliosis"
    explanation: >-
      Lists short stature among the main clinical features.
- category: Clinical
  name: High Forehead
  description: >
    A high, prominent forehead is part of the characteristic MSS facial gestalt.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: High forehead
    term:
      id: HP:0000348
      label: High forehead
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Facial features are characteristic with high forehead, underdeveloped midface, proptosis, anteverted nares, and everted lips."
    explanation: >-
      Documents high forehead as a characteristic facial feature.
- category: Clinical
  name: Midface Retrusion
  description: >
    Underdeveloped (retruded) midface contributes to both the facial gestalt and
    upper airway compromise.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Underdeveloped midface
    term:
      id: HP:0011800
      label: Midface retrusion
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Facial features are characteristic with high forehead, underdeveloped midface, proptosis, anteverted nares, and everted lips."
    explanation: >-
      Documents underdeveloped midface as a characteristic feature.
- category: Clinical
  name: Proptosis
  description: >
    Prominent, protruding eyes (proptosis) are characteristic.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Proptosis
    term:
      id: HP:0000520
      label: Proptosis
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Facial features are characteristic with high forehead, underdeveloped midface, proptosis, anteverted nares, and everted lips."
    explanation: >-
      Documents proptosis as a characteristic facial feature.
- category: Clinical
  name: Anteverted Nares
  description: >
    Anteverted nares are part of the characteristic facial appearance.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Anteverted nares
    term:
      id: HP:0000463
      label: Anteverted nares
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Facial features are characteristic with high forehead, underdeveloped midface, proptosis, anteverted nares, and everted lips."
    explanation: >-
      Documents anteverted nares as a characteristic facial feature.
- category: Clinical
  name: Everted Lower Lip
  description: >
    Everted lips are part of the facial gestalt. The source describes "everted
    lips" generically; the bound HP term HP:0000232 is the more precise "everted
    lower lip vermilion", so the binding is slightly more specific than the cited
    wording.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Everted lips
    term:
      id: HP:0000232
      label: Everted lower lip vermilion
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Facial features are characteristic with high forehead, underdeveloped midface, proptosis, anteverted nares, and everted lips."
    explanation: >-
      Documents everted lips as a characteristic facial feature.
- category: Clinical
  name: Upper Airway Obstruction
  description: >
    Upper airway obstruction, related to midfacial and airway anomalies, is a
    central clinical problem driving respiratory compromise.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Upper airway obstruction
    term:
      id: HP:0002781
      label: Upper airway obstruction
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "respiratory compromise secondary to upper airway obstruction"
    explanation: >-
      Documents upper airway obstruction as the basis of respiratory compromise.
- category: Clinical
  name: Respiratory Insufficiency
  description: >
    Respiratory insufficiency from upper airway obstruction is the major
    life-threatening complication; mortality from respiratory complications is high,
    though airway support increasingly allows survival into adulthood.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Respiratory insufficiency
    term:
      id: HP:0002093
      label: Respiratory insufficiency
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mortality from respiratory complications is high, but airway support increasingly allows survival into adulthood."
    explanation: >-
      Establishes respiratory failure as the principal cause of mortality in MSS.
- category: Clinical
  name: Corpus Callosum Hypoplasia
  description: >
    Hypoplasia of the corpus callosum is a common structural brain anomaly in MSS.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Hypoplasia of the corpus callosum
    term:
      id: HP:0002079
      label: Hypoplasia of the corpus callosum
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Minor abnormalities of brain morphology such as hypoplasia of the corpus callosum are common."
    explanation: >-
      Documents corpus callosum hypoplasia as a common brain-morphology anomaly.
- category: Clinical
  name: Osteopenia
  description: >
    Osteopenia (reduced bone mineral density) is a recognized skeletal feature of
    MSS alongside the accelerated, dysharmonic osseous maturation.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Osteopenia
    term:
      id: HP:0000938
      label: Osteopenia
  evidence:
  - reference: PMID:20673863
    reference_title: "Distinct effects of allelic NFIX mutations on nonsense-mediated mRNA decay engender either a Sotos-like or a Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a severe malformation syndrome characterized by failure to thrive, respiratory insufficiency, accelerated osseous maturation, kyphoscoliosis, osteopenia, and unusual facies"
    explanation: >-
      Lists osteopenia among the core skeletal features that characterize MSS.
- category: Clinical
  name: Atypical Behavior
  description: >
    Unusual behavior is reported among the main clinical features of MSS in the
    large international delineation cohort.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Unusual behavior
    term:
      id: HP:0000708
      label: Atypical behavior
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Main clinical features are moderate to severe developmental delay with absent or limited speech, unusual behavior, dysharmonic bone maturation, respiratory compromise secondary to upper airway obstruction, short stature, and kyphoscoliosis."
    explanation: >-
      Names unusual behavior among the main clinical features of MSS.
- category: Clinical
  name: Aortic Root Dilatation
  description: >
    Progressive aortic root dilatation has been reported in MSS, consistent with the
    connective-tissue abnormalities of the syndrome.
  frequency: VERY_RARE
  phenotype_term:
    preferred_term: Aortic root dilatation
    term:
      id: HP:0002616
      label: Aortic root aneurysm
  evidence:
  - reference: PMID:28442439
    reference_title: "Marshall-Smith syndrome: Novel pathogenic variant and previously unreported associations with precocious puberty and aortic root dilatation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Additionally, the patient showed progressive dilatation of the aortic root."
    explanation: >-
      Reports progressive aortic root dilatation in an MSS patient.
- category: Clinical
  name: Precocious Puberty
  description: >
    Central precocious puberty has been reported as part of the broadening natural
    history of MSS.
  frequency: VERY_RARE
  phenotype_term:
    preferred_term: Central precocious puberty
    term:
      id: HP:0000826
      label: Precocious puberty
  evidence:
  - reference: PMID:28442439
    reference_title: "Marshall-Smith syndrome: Novel pathogenic variant and previously unreported associations with precocious puberty and aortic root dilatation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The patient also presented with precocious puberty diagnosed at five years of age and had an abnormal GnRH stimulation test indicative of central precocious puberty."
    explanation: >-
      Reports central precocious puberty confirmed by GnRH stimulation testing.
- category: Clinical
  name: Craniosynostosis
  description: >
    Progressive craniosynostosis with raised intracranial pressure can occur and
    poses management challenges.
  frequency: VERY_RARE
  phenotype_term:
    preferred_term: Craniosynostosis
    term:
      id: HP:0001363
      label: Craniosynostosis
  evidence:
  - reference: PMID:38647663
    reference_title: "Management of an older Marshall-Smith syndrome patient: a review of literature of MSS and craniosynostosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report a case of a 9-year-old MSS patient with progressive craniosynostosis, elevated intracranial pressure, and catastrophic ocular complications."
    explanation: >-
      Reports progressive craniosynostosis with elevated intracranial pressure in MSS.
genetic:
- name: NFIX
  gene_term:
    preferred_term: NFIX
    term:
      id: hgnc:7788
      label: NFIX
  association: Pathogenic Variants
  notes: >
    Marshall-Smith syndrome is caused by heterozygous NFIX frameshift and
    splice-site variants, and recurrent Alu-mediated deletions of exons 6 and 7,
    scattered through exons 6-10. These variants escape nonsense-mediated mRNA
    decay and encode truncated proteins with a preserved DNA-binding/dimerization
    domain that act in a dominant-negative manner. This mechanism is distinct from
    the exon-2 loss-of-function/haploinsufficiency variants that cause Malan
    syndrome. Variants arise de novo; MSS is a genetically homogeneous Mendelian
    disorder.
  inheritance:
  - name: Autosomal dominant
    inheritance_term:
      preferred_term: Autosomal dominant inheritance
      term:
        id: HP:0000006
        label: Autosomal dominant inheritance
  evidence:
  - reference: PMID:24924640
    reference_title: "Deletions in the 3' part of the NFIX gene including a recurrent Alu-mediated deletion of exon 6 and 7 account for previously unexplained cases of Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we identified a recurrent deletion of NFIX exon 6 and 7 in five individuals"
    explanation: >-
      Documents the recurrent Alu-mediated exon 6-7 deletion as a cause of MSS.
  - reference: PMID:24924640
    reference_title: "Deletions in the 3' part of the NFIX gene including a recurrent Alu-mediated deletion of exon 6 and 7 account for previously unexplained cases of Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These findings show that MSS is a genetically homogeneous Mendelian disorder."
    explanation: >-
      Establishes NFIX as the single causal gene for a genetically homogeneous MSS.
  - reference: PMID:20673863
    reference_title: "Distinct effects of allelic NFIX mutations on nonsense-mediated mRNA decay engender either a Sotos-like or a Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Distinct frameshift and splice NFIX mutations that escaped nonsense-mediated mRNA decay (NMD) were identified in nine MSS subjects."
    explanation: >-
      Original NFIX identification in MSS, showing the NMD-escaping variant class.
treatments:
- name: Airway and Respiratory Support
  description: >
    Airway support (including tracheostomy where needed) manages the upper airway
    obstruction and respiratory compromise that are the leading causes of mortality;
    such support increasingly allows survival into adulthood.
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mortality from respiratory complications is high, but airway support increasingly allows survival into adulthood."
    explanation: >-
      Supports airway/respiratory support as the intervention that reduces
      respiratory mortality and prolongs survival.
- name: Craniosynostosis Surgery
  description: >
    Surgical management of progressive craniosynostosis and raised intracranial
    pressure, with associated ophthalmologic care.
  treatment_term:
    preferred_term: cranial surgical procedure
    term:
      id: NCIT:C15329
      label: Surgical Procedure
  therapeutic_modality: SURGERY
  evidence:
  - reference: PMID:38647663
    reference_title: "Management of an older Marshall-Smith syndrome patient: a review of literature of MSS and craniosynostosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report a case of a 9-year-old MSS patient with progressive craniosynostosis, elevated intracranial pressure, and catastrophic ocular complications."
    explanation: >-
      Documents the progressive craniosynostosis and raised intracranial pressure
      that require surgical management.
- name: Developmental and Rehabilitative Support
  description: >
    Multidisciplinary developmental support and rehabilitation for the
    moderate-to-severe developmental delay, absent/limited speech, and skeletal
    complications.
  treatment_term:
    preferred_term: rehabilitation
    term:
      id: NCIT:C15315
      label: Rehabilitation
  therapeutic_modality: BEHAVIORAL
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Main clinical features are moderate to severe developmental delay with absent or limited speech, unusual behavior, dysharmonic bone maturation, respiratory compromise secondary to upper airway obstruction, short stature, and kyphoscoliosis."
    explanation: >-
      Documents the developmental delay and speech impairment that developmental and
      rehabilitative support addresses.
diagnosis:
- name: Molecular genetic testing of NFIX
  description: >
    MSS is confirmed by molecular analysis of NFIX. Conventional sequencing detects
    the frameshift and splice-site variants; multiplex ligation-dependent probe
    amplification (MLPA) is additionally required to detect the recurrent
    Alu-mediated deletion of exons 6 and 7 and other copy-number changes that
    sequencing misses.
  diagnosis_term:
    preferred_term: molecular genetic testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  results: >-
    A heterozygous NFIX frameshift or splice-site variant, or a recurrent exon 6-7
    deletion, in exons 6-10.
  evidence:
  - reference: PMID:24924640
    reference_title: "Deletions in the 3' part of the NFIX gene including a recurrent Alu-mediated deletion of exon 6 and 7 account for previously unexplained cases of Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In our study cohort of 17 patients with a clinical diagnosis of MSS, conventional sequencing of NFIX revealed frameshift and splice-site mutations in 10 individuals."
    explanation: >-
      Establishes conventional NFIX sequencing as the first-line molecular diagnostic
      test in clinically diagnosed MSS.
  - reference: PMID:24924640
    reference_title: "Deletions in the 3' part of the NFIX gene including a recurrent Alu-mediated deletion of exon 6 and 7 account for previously unexplained cases of Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Using multiplex ligation-dependent probe amplification analysis, we identified a recurrent deletion of NFIX exon 6 and 7 in five individuals."
    explanation: >-
      Establishes that MLPA is needed to detect the recurrent exon 6-7 deletion that
      sequencing does not capture.
- name: Cardiovascular surveillance by echocardiography
  description: >
    Echocardiographic surveillance is warranted given reports of progressive aortic
    root dilatation.
  diagnosis_term:
    preferred_term: echocardiography
    term:
      id: NCIT:C16525
      label: Echocardiography Test
  results: >-
    Progressive aortic root dilatation on serial echocardiography.
  evidence:
  - reference: PMID:28442439
    reference_title: "Marshall-Smith syndrome: Novel pathogenic variant and previously unreported associations with precocious puberty and aortic root dilatation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Additionally, the patient showed progressive dilatation of the aortic root."
    explanation: >-
      Progressive aortic root dilatation supports cardiovascular (echocardiographic)
      surveillance.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >
    Fewer than 50 patients described in the medical literature as of the 2010
    international delineation study; MSS is a very rare malformation syndrome.
  evidence:
  - reference: PMID:20949508
    reference_title: "Phenotype and natural history in Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Marshall-Smith syndrome (MSS) is a distinctive entity of unknown etiology with fewer than 50 patients described in the medical literature to date."
    explanation: >-
      Establishes the ultra-rare status with fewer than 50 reported patients.
animal_models:
- name: Nfix-null mouse
  species: Mouse
  genotype: Nfix-deficient (Nfix knockout)
  publication: PMID:20673863
  description: >-
    Constitutive Nfix-null mice fail to gain weight, die within the first three
    postnatal weeks, and develop a spinal deformation with decreased bone
    mineralization. The mouse reproduces the skeletal and growth phenotype of MSS
    but through Nfix loss of function rather than the dominant-negative allelic
    mechanism operating in human MSS (the mechanism that instead underlies Malan
    syndrome).
  modeled_mechanisms:
  - target: Failure to Thrive
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Nfix-null mice fail to gain weight and die in early postnatal life,
      paralleling the failure to thrive of MSS.
    limitations: >-
      The mouse is Nfix loss of function, whereas human MSS is caused by
      dominant-negative NMD-escaping alleles; Nfix haploinsufficiency/loss in
      humans causes Malan syndrome, an overgrowth disorder, so the allelic
      mechanism does not match.
    evidence:
    - reference: PMID:20673863
      reference_title: "Distinct effects of allelic NFIX mutations on nonsense-mediated mRNA decay engender either a Sotos-like or a Marshall-Smith syndrome."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Nfix-deficient mice are unable to gain weight and die in the first 3 postnatal weeks, while they also present with a spinal deformation and decreased bone mineralization."
      explanation: >-
        Reports the failure-to-gain-weight phenotype of Nfix-null mice as a partial
        correlate of MSS failure to thrive.
  - target: Kyphoscoliosis
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Nfix-null mice present with a spinal deformation, paralleling the
      kyphoscoliosis of MSS.
    limitations: >-
      The spinal deformation arises from Nfix loss of function, not the
      dominant-negative mechanism of human MSS; loss of NFIX in humans causes
      Malan syndrome rather than MSS.
    evidence:
    - reference: PMID:20673863
      reference_title: "Distinct effects of allelic NFIX mutations on nonsense-mediated mRNA decay engender either a Sotos-like or a Marshall-Smith syndrome."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Nfix-deficient mice are unable to gain weight and die in the first 3 postnatal weeks, while they also present with a spinal deformation and decreased bone mineralization."
      explanation: >-
        Reports the spinal deformation of Nfix-null mice as a partial correlate of
        MSS kyphoscoliosis.
  - target: Dysregulated Osseous Maturation
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Nfix-null mice show decreased bone mineralization, a skeletal correlate of
      the dysregulated osseous maturation (with osteopenia) of MSS.
    limitations: >-
      Decreased bone mineralization in the mouse reflects Nfix loss of function,
      not the dominant-negative allelic mechanism of MSS; the mouse also lacks the
      accelerated, dysharmonic bone maturation that defines the human phenotype.
    evidence:
    - reference: PMID:20673863
      reference_title: "Distinct effects of allelic NFIX mutations on nonsense-mediated mRNA decay engender either a Sotos-like or a Marshall-Smith syndrome."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Nfix-deficient mice are unable to gain weight and die in the first 3 postnatal weeks, while they also present with a spinal deformation and decreased bone mineralization."
      explanation: >-
        Reports decreased bone mineralization in Nfix-null mice as a partial
        correlate of the MSS skeletal phenotype.
discussions:
- discussion_id: mss_mouse_allelic_mechanism_mismatch
  prompt: >-
    Does the Nfix-null mouse model MSS, given that it reproduces the skeletal and
    failure-to-thrive phenotype through loss of function rather than the
    dominant-negative allelic mechanism that causes human MSS?
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Dysregulated Osseous Maturation
  - animal_models#Mouse
  rationale: >-
    The only reported mouse reproduces the MSS skeletal phenotype (spinal
    deformation, decreased bone mineralization) and failure to thrive, but it is
    Nfix loss of function. In humans, NFIX loss of function / haploinsufficiency
    causes Malan syndrome (overgrowth), not MSS; MSS is caused by NMD-escaping
    dominant-negative alleles. The mouse therefore models the wrong allelic
    mechanism while reproducing the phenotype, so its findings cannot be treated as
    validating the dominant-negative mechanism of human MSS.
  evidence:
  - reference: PMID:20673863
    reference_title: "Distinct effects of allelic NFIX mutations on nonsense-mediated mRNA decay engender either a Sotos-like or a Marshall-Smith syndrome."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Nfix-deficient mice are unable to gain weight and die in the first 3 postnatal weeks, while they also present with a spinal deformation and decreased bone mineralization."
    explanation: >-
      Documents the Nfix-null mouse phenotype whose loss-of-function basis mismatches
      the dominant-negative allelic mechanism of human MSS.
datasets:
notes: >
  Curated from primary literature. Deep-research providers were unavailable in the
  environment used to address the PR review (no API keys), so no research/ artifact
  was generated; the entry was extended by mining the six cached primary references
  (Malan 2010, Shaw 2010, Schanze 2014, Martinez 2015, Aggarwal 2017, Khurana 2024),
  which surfaced the osteopenia and unusual-behavior phenotypes and the Nfix-null
  mouse model that earlier drafts had dropped.
references:
- reference: PMID:20949508
  title: "Phenotype and natural history in Marshall-Smith syndrome."
- reference: PMID:20673863
  title: "Distinct effects of allelic NFIX mutations on nonsense-mediated mRNA decay engender either a Sotos-like or a Marshall-Smith syndrome."
- reference: PMID:24924640
  title: "Deletions in the 3' part of the NFIX gene including a recurrent Alu-mediated deletion of exon 6 and 7 account for previously unexplained cases of Marshall-Smith syndrome."
- reference: PMID:26200704
  title: "Novel mutations of NFIX gene causing Marshall-Smith syndrome or Sotos-like syndrome: one gene, two phenotypes."
- reference: PMID:28442439
  title: "Marshall-Smith syndrome: Novel pathogenic variant and previously unreported associations with precocious puberty and aortic root dilatation."
- reference: PMID:38647663
  title: "Management of an older Marshall-Smith syndrome patient: a review of literature of MSS and craniosynostosis."
📚

References & Deep Research

References

6
PMID:20949508
Phenotype and natural history in Marshall-Smith syndrome.
No top-level findings curated for this source.
PMID:20673863
Distinct effects of allelic NFIX mutations on nonsense-mediated mRNA decay engender either a Sotos-like or a Marshall-Smith syndrome.
No top-level findings curated for this source.
PMID:24924640
Deletions in the 3' part of the NFIX gene including a recurrent Alu-mediated deletion of exon 6 and 7 account for previously unexplained cases of Marshall-Smith syndrome.
No top-level findings curated for this source.
PMID:26200704
Novel mutations of NFIX gene causing Marshall-Smith syndrome or Sotos-like syndrome: one gene, two phenotypes.
No top-level findings curated for this source.
PMID:28442439
Marshall-Smith syndrome: Novel pathogenic variant and previously unreported associations with precocious puberty and aortic root dilatation.
No top-level findings curated for this source.
PMID:38647663
Management of an older Marshall-Smith syndrome patient: a review of literature of MSS and craniosynostosis.
No top-level findings curated for this source.