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Classifications

Harrison's Part

ICIMD (Inherited Metabolic Disorders)

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Inheritance

1

X-linked recessive inheritance HP:0001419

Only affected males have been reported; heterozygous females have not shown features of SRS to date. Transmission is usually from a carrier mother, but de novo SMS variants are also documented, so an unaffected maternal genotype does not exclude the diagnosis. An apparently de novo variant is not necessarily de novo: low-level maternal somatic/germline mosaicism has been demonstrated in SRS, in one family at ~3% variant allele frequency in maternal blood - a level below the detection limit of Sanger sequencing, which had reported the variant as de novo. This materially raises the recurrence risk above the population baseline quoted for a true de novo event, so deep targeted resequencing of the mother should be considered before counselling a family that recurrence risk is negligible.

X-linked recessive inheritance

Show evidence (7 references)

PMID:23805436 SUPPORT Other

"To date, only affected males have been reported."

GeneReviews states that the disorder manifests only in hemizygous males, consistent with X-linked recessive inheritance.

PMID:34177437 SUPPORT Human Clinical

"This variant in SMS was found to be de novo."

A genetically confirmed case in which the causative hemizygous SMS variant arose de novo, showing that SRS is not invariably maternally transmitted.

PMID:34741636 SUPPORT Human Clinical

"We report a novel case of Snyder-Robinson syndrome, caused by a de novo mutation and first misdiagnosed with osteogenesis imperfecta."

A second independent report of a de novo SMS mutation causing SRS.

+ 4 more references

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Pathophysiology

8

Spermine Synthase Loss of Function

Hemizygous loss-of-function variants in SMS reduce or abolish spermine synthase catalytic activity. Spermine synthase acts only as a homodimer and transfers an aminopropyl group from decarboxylated S-adenosylmethionine to spermidine to make spermine. Most pathogenic variants are hypomorphic missense changes; several act by destabilising the protein or abolishing homodimer formation rather than by disrupting the active site, and a rare complete loss-of-function frameshift allele produces a severe early-lethal phenotype.

SMS hgnc:11123

spermine biosynthetic process GO:0006597 ↓ DECREASED

spermine synthase activity GO:0016768 ↓ DECREASED

cytosol GO:0005829

Downstream

Elevated Spermidine to Spermine Ratio Loss of spermine synthase activity blocks conversion of spermidine to spermine, depleting spermine and letting spermidine accumulate.

Show evidence (4 references)

PMID:14508504 SUPPORT Human Clinical

"We report the first polyamine deficiency syndrome caused by a defect in spermine synthase (SMS)."

Establishes SMS deficiency as the primary molecular lesion of Snyder-Robinson syndrome and as the first recognised human polyamine deficiency.

PMID:41410504 SUPPORT Other

"Snyder-Robinson syndrome results from hemizygous loss-of-function variants in the spermine synthase (SMS) gene, resulting in decreased or complete loss of spermine synthase enzyme activity."

A polyaminopathy review confirms the hemizygous loss-of-function mechanism and the resulting reduction in enzyme activity.

PMID:30544565 SUPPORT Other

"SMS catalyzes the production of spermine (SPM) from its precursor, spermidine (SPD), via the transfer of an aminopropyl group, which is derived from decarboxylated S-adenosylmethionine (dcAdoMet)"

Defines the aminopropyl-transfer reaction that is lost when spermine synthase activity falls.

+ 1 more reference

Elevated Spermidine to Spermine Ratio

Spermine falls and spermidine rises in patient lymphocytes, lymphoblasts and fibroblasts, producing an exaggerated spermidine:spermine ratio that is the biochemical hallmark of SRS and that broadly tracks symptom severity. Ornithine decarboxylase activity and putrescine are secondarily reduced, indicating compensatory down-regulation of upstream polyamine biosynthesis rather than a simple accumulation defect.

intracellular polyamine homeostasis GO:0010509 ↕ DYSREGULATED polyamine metabolic process GO:0006595 ↕ DYSREGULATED

Downstream

Excessive Spermidine Catabolism and Oxidative Stress The enlarged spermidine pool is shunted into the SAT1/polyamine oxidase back-conversion route, generating hydrogen peroxide and reactive aldehydes.

Loss of Polyamine-Dependent Ion Channel Modulation Spermine is the principal endogenous blocker of inward-rectifier potassium channels, so its depletion removes a physiological channel-gating influence in excitable tissue.

Impaired Osteoblastic Bone Matrix Mineralisation Excess spermidine in osteogenic cells suppresses matrix mineralisation in a dose-dependent manner, independent of any resorptive stimulus.

Show evidence (3 references)

PMID:37702369 SUPPORT In Vitro

"Reduced SMS activity causes spermidine accumulation while spermine levels are reduced. The resulting exaggerated spermidine:spermine ratio is a biochemical hallmark of SRS that tends to correlate with symptom severity."

Establishes the elevated spermidine-to-spermine ratio as the defining biochemical abnormality and its correlation with severity.

PMID:14508504 SUPPORT Human Clinical

"The affected males have mild-to-moderate mental retardation (MR), hypotonia, cerebellar circuitry dysfunction, facial asymmetry, thin habitus, osteoporosis, kyphoscoliosis, decreased activity of SMS, correspondingly low levels of intracellular spermine in lymphocytes and fibroblasts, and..."

Documents low intracellular spermine and elevated spermidine/spermine ratios directly in patient lymphocytes and fibroblasts.

PMID:30544565 SUPPORT In Vitro

"In addition to decreased spermine and increased spermidine in SRS cells, ornithine decarboxylase activity and its product putrescine were significantly decreased."

Confirms the polyamine pool shift in patient-derived lymphoblasts and documents the secondary fall in ornithine decarboxylase activity and putrescine.

Excessive Spermidine Catabolism and Oxidative Stress

Because spermidine cannot be converted to spermine it is instead routed through spermidine/spermine N1-acetyltransferase 1 (SAT1) and polyamine oxidase. This back-conversion generates hydrogen peroxide and reactive aldehydes, imposing oxidative stress and depleting acetyl-CoA. In Drosophila SRS models and in patient cells these toxic by-products damage lysosomes and mitochondria; enhancing antioxidant activity genetically or pharmacologically suppresses the oxidative stress.

polyamine catabolic process GO:0006598 ↑ INCREASED response to oxidative stress GO:0006979 ↑ INCREASED

Downstream

Lysosomal and Mitochondrial Dysfunction Catabolism-derived aldehydes and reactive oxygen species compromise autophagy-lysosome flux and mitochondrial function.

Show evidence (2 references)

PMID:35801587 SUPPORT Model Organism

"In the Drosophila SRS model, altered spermidine/spermine balance has been associated with increased generation of ROS and aldehydes, consistent with elevated spermidine catabolism. These toxic byproducts cause mitochondrial and lysosomal dysfunction, which are also observed in cells from SRS patients."

Links the polyamine imbalance to catabolism-derived ROS and aldehydes and to the resulting organelle dysfunction, with confirmation in patient cells.

PMID:29097652 SUPPORT Model Organism

"SMS deficiency leads to excessive spermidine catabolism, which generates toxic metabolites that cause lysosomal defects and oxidative stress."

Establishes excessive spermidine catabolism as the source of the toxic metabolites that produce lysosomal defects and oxidative stress.

Lysosomal and Mitochondrial Dysfunction

Autophagy-lysosome flux and mitochondrial oxidative phosphorylation are compromised in SRS models and patient-derived cells. The G56S SRS mouse shows impaired mitochondrial oxidative phosphorylation in cerebral cortex, fibroblasts and Sms-null hippocampal cells, and high-throughput metabolic profiling of 29 patient lymphoblastoid lines shows broad energy-metabolism rewiring, establishing bioenergetic failure as a cellular hallmark of the disorder.

neuron CL:0000540

autophagy GO:0006914 ↓ DECREASED oxidative phosphorylation GO:0006119 ↓ DECREASED

lysosome GO:0005764 mitochondrion GO:0005739

Downstream

Neuronal and Synaptic Dysfunction Compromised autophagy-lysosome flux and mitochondrial bioenergetics in the nervous system lead to synaptic and retinal degeneration in the fly model.

Show evidence (2 references)

PMID:38463005 SUPPORT Model Organism

"impaired mitochondrial oxidative phosphorylation was evident in G56S cerebral cortex, G56S fibroblasts and Sms-null hippocampal cells"

Mouse-model evidence that SMS loss impairs mitochondrial oxidative phosphorylation in brain and in fibroblasts.

PMID:42250346 SUPPORT In Vitro

"These findings highlight global energy metabolism dysregulation as a hallmark of SRS and support PBA as a promising therapeutic candidate for correcting bioenergetic defects in this disorder."

Patient-derived lymphoblastoid profiling establishes global energy-metabolism dysregulation as a cellular hallmark of SRS.

Loss of Polyamine-Dependent Ion Channel Modulation

Spermine is the dominant endogenous polycation that blocks and gates inward-rectifier potassium channels and modulates glutamate receptors, so spermine depletion removes a physiological control on excitable membranes. This was the mechanism originally proposed to connect the polyamine defect to the cognitive phenotype of SRS, and in the spermine synthase-deficient Gy mouse the loss of the endocochlear potential and of vestibular function is attributed to defective polyamine regulation of Kir channels. Note that the direct human evidence is inferential rather than electrophysiological.

neuron CL:0000540

inward rectifier potassium channel activity GO:0005242 ↕ DYSREGULATED

Downstream

Hearing impairment Loss of polyamine-dependent Kir channel regulation abolishes the endocochlear potential in the spermine synthase-deficient mouse; hearing loss is also reported in severe human SRS.

Show evidence (3 references)

PMID:14508504 SUPPORT Human Clinical

"possibly by spermine's ability to function as an 'intrinsic gateway' molecule for inward rectifier K(+) channels"

The original description proposes this mechanism explicitly as a possibility rather than a demonstrated finding, so the support is recorded as partial.

PMID:29097652 SUPPORT Other

"polyamines were able to block or modulate several distinct types of cation channels, including inward-rectifier K+ channels and glutamate receptors"

A background statement in the paper's discussion summarising prior in vitro observations by others, not data generated in this study; it establishes that polyamines modulate these channels but not that this modulation is disrupted in SRS, so support is partial.

PMID:19001365 SUPPORT Model Organism

"Our results are consistent with a critical role for polyamines in regulation of Kir channels that maintain the endocochlear potential and emphasize the importance of normal spermidine:spermine ratio in the hearing and balance functions of the inner ear."

In the spermine synthase-deficient Gy mouse, loss of the endocochlear potential and of balance is attributed to defective polyamine regulation of Kir channels.

Neuronal and Synaptic Dysfunction

Polyamines are required locally within neurons for chromatin, translational and channel-modulatory functions. Loss of spermine synthase in Drosophila causes survival defects and synaptic and retinal degeneration, and the G56S SRS mouse shows impaired learning, increased anxiety, reduced mobility and reduced total and regional brain volumes. Together these establish a neuronal substrate for the intellectual disability, seizures and gait abnormality of SRS, though the human evidence remains largely clinical rather than histological.

neuron CL:0000540

Downstream

Intellectual disability Neuronal polyamine imbalance underlies the cognitive impairment that defines the syndrome.

Seizure Neuronal dysfunction from polyamine imbalance produces the seizures seen in a substantial proportion of affected males.

Show evidence (2 references)

PMID:38463005 SUPPORT Model Organism

"They showed impaired learning capacity, increased anxiety, reduced mobility and heightened fear responses, accompanied by reduced total and regional brain volumes."

Mouse-model evidence of cognitive and neuroanatomical consequences of spermine synthase loss.

PMID:29097652 SUPPORT Model Organism

"Our data show that dSms is expressed in the nervous system, including cell bodies and synapses, and loss of dSms causes retinal and synaptic degeneration."

Establishes a neuronal and synaptic locus for the consequences of spermine synthase loss.

Impaired Osteoblastic Bone Matrix Mineralisation

The skeletal arm of SRS is a formation defect rather than a resorptive one. Bone histopathology in affected brothers showed profound depletion of both osteoblasts and osteoclasts with absent trabecular meshwork, low bone volume and a thin cortex, and patient-derived bone marrow stromal cells failed to mineralise on osteogenic differentiation. Silencing SMS in human MSCs reproduces the defect in vitro and in vivo, and in the SMS G56S mouse dynamic histomorphometry identifies a reduced bone formation rate as the main cause of the low bone mass. Excess spermidine is itself sufficient to suppress mineralisation, and inhibiting the upstream enzyme ornithine decarboxylase with DFMO rescues it.

osteoblast CL:0000062 mesenchymal stem cell CL:0000134

bone mineralization GO:0030282 ↓ DECREASED ossification GO:0001503 ↓ DECREASED

Downstream

Net Bone Loss and Skeletal Fragility Sustained failure of osteoblastic matrix mineralisation produces low bone volume, a thin cortex and loss of trabecular architecture.

Show evidence (5 references)

PMID:25888122 SUPPORT In Vitro

"Osteogenic differentiation of the SRS-derived hBMSCs identified a severe deficiency of calcium phosphate mineralization."

Patient-derived bone marrow stromal cells fail to mineralise in culture, establishing a cell-autonomous osteogenic defect.

PMID:25888122 SUPPORT Human Clinical

"the low bone density arises from a failure of mineralization"

The authors conclude that the low bone density of SRS is a mineralisation failure rather than a resorption excess.

PMID:39331754 SUPPORT Model Organism

"Cellular and dynamic histomorphometry further identified a reduced bone formation rate as a main cause of the low bone mass phenotype."

Dynamic histomorphometry in the SMS G56S mouse attributes the low bone mass specifically to a reduced bone formation rate.

+ 2 more references

Net Bone Loss and Skeletal Fragility

The cumulative formation deficit produces early-onset osteoporosis with reduced bone mineral density, a thin cortex and loss of trabecular architecture, manifesting during the first decade as fractures in the absence of trauma and as progressive kyphoscoliosis. Because osteoclasts are depleted rather than increased, the low bone mass of SRS is not a resorption-driven osteoporosis; this distinction matters clinically, since SRS is misdiagnosed as osteogenesis imperfecta.

bone remodeling GO:0046849 ↕ DYSREGULATED

Downstream

Osteoporosis Net bone loss presents clinically as early-onset osteoporosis.

Reduced bone mineral density The cumulative formation deficit is measured clinically as reduced bone mineral density on DXA.

Recurrent fractures Skeletal fragility produces fractures in the absence of trauma.

Kyphoscoliosis Loss of vertebral bone mass contributes to the progressive spinal deformity.

Show evidence (2 references)

PMID:23805436 SUPPORT Other

"During the first decade, males with SRS develop osteoporosis, resulting in fractures in the absence of trauma."

GeneReviews documents the clinical endpoint of the skeletal chain, low-trauma fractures from early-onset osteoporosis.

PMID:39331754 SUPPORT Model Organism

"our data identify SMS as an enzyme with physiological relevance for osteoblast activity, thereby demonstrating an important role of polyamine metabolism in the control of bone remodeling."

Places polyamine metabolism, acting through osteoblast activity, in control of bone remodelling.

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Histopathology

1

Depletion of osteoblasts and osteoclasts with absent trabecular meshwork

Iliac crest bone histopathology and morphometry in two affected brothers showed profound depletion of BOTH osteoblasts and osteoclasts, absence of a trabecular meshwork, low bone volume and a thin cortex. The simultaneous loss of the resorptive cell population is the finding that distinguishes SRS osteoporosis from resorption-driven osteoporosis and underpins this entry's decision not to declare conformance to the increased-osteoclastic-resorption node of the osteoporosis module.

Show evidence (1 reference)

PMID:25888122 SUPPORT Human Clinical

"Bone histopathology and morphometry identified a profound depletion of osteoblasts and osteoclasts, absence of a trabecular meshwork, a low bone volume and a thin cortex."

Direct human bone histopathology documenting the cellular and architectural basis of the SRS skeletal phenotype.

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Pathograph

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

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Phenotypes

25

Digestive 1

Exocrine pancreatic insufficiency VERY_RARE HP:0001738

Show evidence (1 reference)

PMID:33186760 SUPPORT Human Clinical

"One of the brothers presented with gastrointestinal symptoms, with jejunal stenosis, enteral feeding intolerance, failure to thrive due to a dysfunctional gastrointestinal system, cholestasis and exocrine pancreatic insufficiency."

Single-patient evidence for exocrine pancreatic insufficiency within the proposed digestive expansion of the SRS phenotype; support is partial.

Ear 1

Hearing impairment HP:0000365

Show evidence (2 references)

PMID:33186760 SUPPORT Human Clinical

"Here we describe two maternal half-brothers who both presented with severe neurodevelopmental delay, seizures, hearing loss, facial dysmorphism, renal and ophthalmologic anomalies, failure to thrive and premature death."

Documents hearing loss in two genetically confirmed brothers with severe SRS.

PMID:19001365 SUPPORT Model Organism

"Male gyro (Gy) mice, which have an X chromosomal deletion inactivating the SpmS and Phex genes, were found to be profoundly hearing impaired."

Model-organism support for hearing impairment as a consequence of spermine synthase deficiency; the deletion also removes Phex, so this is corroborative rather than definitive.

Eye 1

Abnormal retinal pigmentation OCCASIONAL HP:0007703

Show evidence (2 references)

PMID:25888122 SUPPORT Human Clinical

"the propositi manifested retinal pigmentary changes, recurrent episodes of hyper- and hypoglycemia, nephrocalcinosis, renal cysts, and frequent respiratory infections."

Documents retinal pigmentary changes in two affected brothers as an extension of the recognised phenotype. Support is partial because the finding is reported in a single sibship rather than established across the disorder.

PMID:25888122 SUPPORT Human Clinical

"Retinal pigment changes 0/13 + + Hypo-/Hyper-glycemia 1/13 + +"

Consecutive rows of Table 1 ("Comparison of clinical features of all reported SRS patients"). The retinal row reads "0/13 + +" - retinal pigment change had not been reported in any of the 13 previously assessed patients, but the two trailing "+" marks are the table's separate columns for this report's own propositi II-1 and II-3, both of whom are positive. The count over every patient the table tabulates is therefore 2/15 = 13.3%, which falls in the OCCASIONAL band (5-29%), not VERY_RARE.

Genitourinary 1

Renal cyst VERY_RARE HP:0000107

Show evidence (2 references)

PMID:25888122 SUPPORT Human Clinical

"the propositi manifested retinal pigmentary changes, recurrent episodes of hyper- and hypoglycemia, nephrocalcinosis, renal cysts, and frequent respiratory infections."

Documents renal cysts in two affected brothers.

PMID:23805436 SUPPORT Other

"Rare findings may include nonspecific kidney manifestations."

GeneReviews classifies kidney manifestations as rare findings, supporting the very rare frequency band.

Head and Neck 2

Facial asymmetry HP:0000324

Show evidence (2 references)

PMID:14508504 SUPPORT Human Clinical

"The affected males have mild-to-moderate mental retardation (MR), hypotonia, cerebellar circuitry dysfunction, facial asymmetry, thin habitus, osteoporosis, kyphoscoliosis, decreased activity of SMS, correspondingly low levels of intracellular spermine in lymphocytes and fibroblasts, and..."

The original description lists facial asymmetry among the core features.

PMID:30544565 SUPPORT Other

"the imbalance that occurs in SRS results in a combination of clinical manifestations including moderate-to-severe cognitive impairment, osteoporosis, asthenic build, low muscle mass, facial asymmetry, speech abnormalities, and seizures"

Confirms facial asymmetry within the recognised clinical spectrum of SRS.

Cleft palate HP:0000175

Show evidence (1 reference)

PMID:23805436 SUPPORT Other

"standard surgical treatment by craniofacial team for those with cleft palate"

GeneReviews management guidance presupposes cleft palate in a subset of affected individuals; because this is inferred from a management recommendation rather than a stated clinical finding, support is partial.

Immune 1

Recurrent respiratory infections OCCASIONAL HP:0002205

Show evidence (2 references)

PMID:25888122 SUPPORT Human Clinical

"the propositi manifested retinal pigmentary changes, recurrent episodes of hyper- and hypoglycemia, nephrocalcinosis, renal cysts, and frequent respiratory infections."

Documents frequent respiratory infections in two affected brothers. Support is partial because the finding is reported in a single sibship rather than established across the disorder.

PMID:25888122 SUPPORT Human Clinical

"Frequent infections 0/13 + + Retinal pigment changes 0/13 + +"

Consecutive rows of Table 1 ("Comparison of clinical features of all reported SRS patients"). The infections row reads "0/13 + +" - frequent infections had not been reported in any of the 13 previously assessed patients, but the two trailing "+" marks are the table's separate columns for this report's own propositi II-1 and II-3, both of whom are positive. The count over every patient the table tabulates is therefore 2/15 = 13.3%, which falls in the OCCASIONAL band (5-29%), not VERY_RARE.

Musculoskeletal 6

Hypotonia HP:0001252

Show evidence (2 references)

PMID:14508504 SUPPORT Human Clinical

"The affected males have mild-to-moderate mental retardation (MR), hypotonia, cerebellar circuitry dysfunction, facial asymmetry, thin habitus, osteoporosis, kyphoscoliosis, decreased activity of SMS, correspondingly low levels of intracellular spermine in lymphocytes and fibroblasts, and..."

The original clinical description lists hypotonia among the core features of affected males.

PMID:31580924 SUPPORT Human Clinical

"The syndrome is characterized by facial dysmorphism, thin body build, kyphoscoliosis, osteoporosis, hypotonia, developmental delay and associated neurological features (seizures, unsteady gait, abnormal speech)."

Confirms hypotonia as a characteristic feature of the syndrome.

Osteoporosis VERY_FREQUENT HP:0000939

Show evidence (2 references)

PMID:23805436 SUPPORT Other

"During the first decade, males with SRS develop osteoporosis, resulting in fractures in the absence of trauma."

GeneReviews states that males with SRS develop osteoporosis during the first decade, an unqualified statement about the affected population that maps to the very frequent band.

PMID:25888122 SUPPORT Human Clinical

"the low bone density arises from a failure of mineralization"

Characterises the osteoporosis of SRS as a mineralisation failure.

Reduced bone mineral density HP:0004349

Show evidence (2 references)

PMID:23805436 SUPPORT Other

"calcium supplementation has slightly improved bone mineral density in a few individuals"

GeneReviews treats bone mineral density as the measured skeletal parameter in SRS and reports a modest response to calcium supplementation.

PMID:38463005 SUPPORT Model Organism

"The lack of SMS protein in the G56S mice resulted in increased spermidine/spermine ratio, failure to thrive, short stature and reduced bone density."

The SRS mouse model reproduces the reduced bone density, supporting it as a direct consequence of SMS loss.

Recurrent fractures HP:0002757

Show evidence (2 references)

PMID:23805436 SUPPORT Other

"During the first decade, males with SRS develop osteoporosis, resulting in fractures in the absence of trauma."

GeneReviews documents atraumatic fractures as the clinical consequence of the osteoporosis.

PMID:30237987 SUPPORT Human Clinical

"this syndrome is characterized by an asthenic body habitus, facial dysmorphism, broad-based gait, and osteoporosis with frequent fractures."

Confirms frequent fractures as a characteristic feature.

Kyphoscoliosis HP:0002751

Course: PROGRESSIVE

Show evidence (2 references)

PMID:23805436 SUPPORT Other

"Snyder-Robinson syndrome (SRS) is an X-linked intellectual disability syndrome characterized by facial dysmorphism, asthenic build, progressive kyphoscoliosis, early-onset osteoporosis, and seizures."

GeneReviews lists progressive kyphoscoliosis among the defining features, supporting both the phenotype and the progressive clinical course.

PMID:14508504 SUPPORT Human Clinical

"The affected males have mild-to-moderate mental retardation (MR), hypotonia, cerebellar circuitry dysfunction, facial asymmetry, thin habitus, osteoporosis, kyphoscoliosis, decreased activity of SMS, correspondingly low levels of intracellular spermine in lymphocytes and fibroblasts, and..."

The original description lists kyphoscoliosis among the features of affected males.

Flexion contracture HP:0001371

Show evidence (2 references)

PMID:25888122 SUPPORT Human Clinical

"kyphoscoliosis and flexion contractures of most large and small joints"

Direct clinical documentation of flexion contractures involving most large and small joints in a genetically confirmed affected male.

PMID:23805436 SUPPORT Other

"standard management of kyphoscoliosis and contractures by orthopedics"

GeneReviews management guidance presupposes contractures as an ongoing orthopaedic problem in affected individuals; because this is inferred from a management recommendation rather than a stated clinical finding, support is partial.

Nervous System 4

Intellectual disability OBLIGATE HP:0001249

Show evidence (2 references)

PMID:23805436 SUPPORT Other

"Developmental delay usually presents as failure to meet early developmental milestones and then evolves to mild-to-profound intellectual disability (which appears to remain stable over time) and variable motor disability."

GeneReviews documents the developmental trajectory from early delay to established intellectual disability in affected males. The OBLIGATE band rests on the definitional argument in the second evidence item, not on a quantitative claim here.

PMID:23805436 SUPPORT Other

"Snyder-Robinson syndrome (SRS) is an X-linked intellectual disability syndrome characterized by facial dysmorphism, asthenic build, progressive kyphoscoliosis, early-onset osteoporosis, and seizures."

The disorder is defined as an intellectual disability syndrome, so the phenotype is definitional rather than variable.

Global developmental delay HP:0001263

Show evidence (1 reference)

PMID:31580924 SUPPORT Human Clinical

"The syndrome is characterized by facial dysmorphism, thin body build, kyphoscoliosis, osteoporosis, hypotonia, developmental delay and associated neurological features (seizures, unsteady gait, abnormal speech)."

Lists developmental delay among the characteristic features of the syndrome.

Seizure FREQUENT HP:0001250

Show evidence (3 references)

PMID:25888122 SUPPORT Human Clinical

"Cognitive impairment 15/15 + + Seizures 8/15 + +"

Consecutive rows of Table 1 ("Comparison of clinical features of all reported SRS patients"). The "Reported patients" column records seizures in 8 of the 15 SRS patients reported at that time; 8/15 = 53%, which falls in the FREQUENT band (30-79%). The two trailing "+" marks are the table's separate columns for the report's own propositi II-1 and II-3, so counting them gives 10/17 = 59% and leaves the band unchanged.

PMID:23805436 SUPPORT Other

"Seizure onset varies but typically occurs in early childhood."

GeneReviews documents seizures as a feature of the syndrome with typical early-childhood onset.

PMID:30237987 SUPPORT Human Clinical

"We report here a pediatric autopsy of a 4 year old male with a history of intellectual disability, gait abnormalities, multiple fractures, and seizures previously diagnosed with Snyder-Robinson syndrome"

An autopsy case report documents seizures in a genetically confirmed patient.

Speech abnormality VERY_FREQUENT Abnormal speech pattern HP:0002167

Show evidence (2 references)

PMID:25888122 SUPPORT Human Clinical

"Speech abnormalities 15/15 + ** + ** Diminished body bulk 15/15 + +"

Consecutive rows of Table 1 ("Comparison of clinical features of all reported SRS patients"); the first records speech abnormalities in 15 of 15 reported patients. 15/15 = 100%, which maps by default to OBLIGATE; the band is deliberately set one tier lower at VERY_FREQUENT (80-99%) because a 15-patient literature tabulation is too small a denominator to assert complete penetrance across the disorder, per the "departing from the default mapping" provision of docs/frequency-evidence-guidelines.md.

PMID:25888122 SUPPORT Human Clinical

"The clinical features of SRS include intellectual disability, dysmorphic facies, speech and gait abnormalities, seizures"

Speech abnormality is listed among the defining clinical features of SRS in the same paper's introduction.

Growth 1

Failure to thrive HP:0001508

Show evidence (2 references)

PMID:33186760 SUPPORT Human Clinical

"Here we describe two maternal half-brothers who both presented with severe neurodevelopmental delay, seizures, hearing loss, facial dysmorphism, renal and ophthalmologic anomalies, failure to thrive and premature death."

Documents failure to thrive in two genetically confirmed brothers with severe SRS.

PMID:38463005 SUPPORT Model Organism

"The lack of SMS protein in the G56S mice resulted in increased spermidine/spermine ratio, failure to thrive, short stature and reduced bone density."

The SRS mouse model reproduces failure to thrive.

Other 7

Decreased muscle mass HP:0003199

Show evidence (1 reference)

PMID:23805436 SUPPORT Other

"Asthenic habitus and low muscle mass usually develop during the first year."

GeneReviews documents low muscle mass and dates its onset to the first year of life.

Slender build HP:0001533

Show evidence (2 references)

PMID:23805436 SUPPORT Other

"Asthenic habitus and low muscle mass usually develop during the first year."

GeneReviews documents the asthenic habitus; the HPO term Slender build is the closest available match, so the preferred_term retains the clinical wording.

PMID:30237987 SUPPORT Human Clinical

"this syndrome is characterized by an asthenic body habitus, facial dysmorphism, broad-based gait, and osteoporosis with frequent fractures."

Confirms the asthenic body habitus as a characteristic feature.

Unsteady gait HP:0002317

Show evidence (1 reference)

PMID:31580924 SUPPORT Human Clinical

"The syndrome is characterized by facial dysmorphism, thin body build, kyphoscoliosis, osteoporosis, hypotonia, developmental delay and associated neurological features (seizures, unsteady gait, abnormal speech)."

Lists unsteady gait among the characteristic neurological features.

Broad-based gait HP:0002136

Show evidence (1 reference)

PMID:30237987 SUPPORT Human Clinical

"this syndrome is characterized by an asthenic body habitus, facial dysmorphism, broad-based gait, and osteoporosis with frequent fractures."

Explicitly lists broad-based gait as a characteristic feature of SRS.

Hypernasal speech HP:0001611

Show evidence (1 reference)

PMID:31580924 SUPPORT Human Clinical

"The syndrome is characterized by facial dysmorphism, thin body build, kyphoscoliosis, osteoporosis, hypotonia, developmental delay and associated neurological features (seizures, unsteady gait, abnormal speech)."

The source documents abnormal speech as a characteristic feature but does not specify hypernasality, so support for this specific HPO term is partial.

Nephrocalcinosis VERY_RARE HP:0000121

Show evidence (2 references)

PMID:25888122 SUPPORT Human Clinical

"the propositi manifested retinal pigmentary changes, recurrent episodes of hyper- and hypoglycemia, nephrocalcinosis, renal cysts, and frequent respiratory infections."

Documents nephrocalcinosis in two affected brothers as an extension of the recognised phenotype.

PMID:23805436 SUPPORT Other

"Rare findings may include nonspecific kidney manifestations."

GeneReviews classifies kidney manifestations as rare findings, supporting the very rare frequency band.

Jejunal stenosis VERY_RARE Small intestinal stenosis HP:0012848

Show evidence (1 reference)

PMID:33186760 SUPPORT Human Clinical

"One of the brothers presented with gastrointestinal symptoms, with jejunal stenosis, enteral feeding intolerance, failure to thrive due to a dysfunctional gastrointestinal system, cholestasis and exocrine pancreatic insufficiency."

A single genetically confirmed patient with jejunal stenosis; the authors themselves frame the digestive involvement as a possible phenotype expansion, so support is partial and the frequency band is the lowest available.

🧬

Genetic Associations

1

SMS (CAUSAL)

Gene: SMS hgnc:11123 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (3 references)

PMID:41410504 SUPPORT Other

"Snyder-Robinson syndrome results from hemizygous loss-of-function variants in the spermine synthase (SMS) gene, resulting in decreased or complete loss of spermine synthase enzyme activity."

Establishes the gene-disease relationship and the loss-of-function mechanism.

PMID:31580924 SUPPORT Human Clinical

"Here we describe the first complete LoF variant, Met303Lysfs*, in a male patient with a severe form of Snyder-Robinson syndrome."

Documents the single reported complete loss-of-function allele and its severe phenotype, underpinning the genotype-severity correlation.

PMID:26761001 SUPPORT Computational

"In silico modeling predicted that all studied mutations in this work destabilize SpmSyn and some of them abolish homo-dimer formation."

Supports protein destabilisation and loss of dimerisation as the mechanism for non-active-site missense variants.

💊

Medical Actions

9

Developmental and educational support

Category: Therapeutic Action: developmental and educational support Ontology label: Supportive Care NCIT:C15747

Developmental and educational support is the mainstay of management for the intellectual disability and motor disability of SRS.

Target Phenotypes: Intellectual disability HP:0001249 Global developmental delay HP:0001263

Show evidence (1 reference)

PMID:23805436 SUPPORT Other

"Treatment of manifestations: Developmental and educational support; treatment of seizures per neurologist"

GeneReviews lists developmental and educational support as first-line management of manifestations.

Anticonvulsant therapy

Category: Therapeutic Action: anticonvulsant agent therapy Ontology label: Anticonvulsant Therapy NCIT:C64172

Seizures are treated per neurologist with standard anticonvulsants. GeneReviews advises weighing risk against benefit for medications associated with increased osteoporosis, explicitly including anticonvulsants, because SRS already carries an early-onset osteoporosis burden.

Target Phenotypes: Seizure HP:0001250

Show evidence (2 references)

PMID:23805436 SUPPORT Other

"Treatment of manifestations: Developmental and educational support; treatment of seizures per neurologist"

GeneReviews directs seizure treatment to a neurologist as part of standard management.

PMID:23805436 SUPPORT Other

"Assess the risk vs benefit of medications associated with increased osteoporosis (e.g., anticonvulsants), particularly when alternative treatments are limited."

GeneReviews Agents/Circumstances to Avoid guidance flags the bone-density cost of anticonvulsants in this population.

Calcium supplementation

Category: Therapeutic Action: calcium supplementation Ontology label: Nutritional Support NCIT:C15433

Agent: calcium supplement CHEBI:29108

Calcium supplementation has slightly improved bone mineral density in a few individuals. Because of the risk of ectopic calcification, individuals receiving calcium should be evaluated regularly by an endocrinologist.

Target Phenotypes: Reduced bone mineral density HP:0004349 Osteoporosis HP:0000939

Show evidence (2 references)

PMID:23805436 SUPPORT Other

"calcium supplementation has slightly improved bone mineral density in a few individuals"

GeneReviews reports only a slight benefit in a few individuals, so the support for efficacy is partial.

PMID:23805436 SUPPORT Other

"while receiving calcium supplementation, individuals should be evaluated regularly for ectopic calcification by endocrinologist"

GeneReviews attaches an explicit safety-monitoring requirement to calcium supplementation in SRS.

Osteoporosis pharmacotherapy

Category: Therapeutic Action: bisphosphonate, teriparatide and denosumab sequence Ontology label: Bisphosphonate Therapy NCIT:C198585

Agent: teriparatide NCIT:C61966 denosumab NCIT:C61313

A reported adult was treated with bisphosphonates for a decade until an atypical femoral fracture, then with teriparatide for two years, then denosumab six-monthly, with improvement in bone density and no further fractures. This is single-case evidence and no controlled trial exists in SRS.

Target Phenotypes: Osteoporosis HP:0000939 Recurrent fractures HP:0002757 Reduced bone mineral density HP:0004349

Show evidence (1 reference)

PMID:34741636 SUPPORT Human Clinical

"The patient was treated with bisphosphonates for a decade, until developing an atypical femoral fracture. Teriparatide was then administered for 2 years and then changed to denosumab every 6 months, improving his bone density mass and preventing further fractures."

A single case report describing benefit from an anabolic-then-antiresorptive sequence after bisphosphonate failure; support is partial because this is n-of-1 evidence.

Orthopaedic management of kyphoscoliosis and contractures

Category: Therapeutic Action: orthopaedic management Ontology label: Therapeutic Procedure NCIT:C49236

Standard orthopaedic management of kyphoscoliosis and contractures, with clinical examination for kyphoscoliosis and assessment of mobility at each visit.

Target Phenotypes: Kyphoscoliosis HP:0002751

Show evidence (1 reference)

PMID:23805436 SUPPORT Other

"standard management of kyphoscoliosis and contractures by orthopedics"

GeneReviews specifies standard orthopaedic management for the spinal deformity and contractures.

Skeletal surveillance

Category: Monitoring Action: clinical examination and DXA surveillance Ontology label: Clinical Evaluation NCIT:C124351

Clinical examination and DXA scanning to track progression of osteoporosis, with imaging for fracture when clinically indicated, plus examination for kyphoscoliosis and assessment of mobility at each visit.

Show evidence (1 reference)

PMID:23805436 SUPPORT Other

"clinical examination and DXA scans to evaluate for progression of osteoporosis and investigate for factures if medically indicated"

GeneReviews specifies the skeletal surveillance protocol for SRS.

Renal surveillance

Category: Monitoring Action: nephrological surveillance Ontology label: Clinical Evaluation NCIT:C124351

Monitoring for nephrocalcinosis, renal cysts and kidney function by a nephrologist. GeneReviews notes that creatinine must be interpreted in the context of the low muscle mass of SRS, which otherwise makes creatinine-based estimates of kidney function falsely reassuring.

Show evidence (1 reference)

PMID:23805436 SUPPORT Other

"monitor for nephrocalcinosis and renal cysts and kidney function per nephrologist, considering creatine levels in the context of low muscle mass"

GeneReviews specifies renal surveillance and flags the creatinine confound created by the low muscle mass of the syndrome.

Difluoromethylornithine (DFMO, eflornithine) — investigational

Category: Therapeutic Action: Pharmacotherapy NCIT:C15986

Agent: eflornithine (DFMO) CHEBI:41948

DFMO, an FDA-approved ornithine decarboxylase inhibitor, rebalances the spermidine:spermine ratio in SRS patient cells by reducing spermidine biosynthesis, stimulating residual conversion of spermidine to spermine in hypomorphic cells, and inducing uptake of exogenous spermine. It extends lifespan in the Drosophila SRS model and rescues the mineralisation defect of polyamine-loaded human stromal cells. This is preclinical only; DFMO is profoundly toxic in the spermine synthase-deficient Gy mouse, which suffers catastrophic loss of motor function and death within days, so translation requires caution.

Mechanism Target:

INHIBITS Elevated Spermidine to Spermine Ratio — DFMO inhibits ornithine decarboxylase upstream of the block, lowering spermidine synthesis and so correcting the exaggerated spermidine:spermine ratio rather than replacing the missing enzyme.

Show evidence (1 reference)

PMID:37702369 SUPPORT In Vitro

"Here we report the repurposing of 2-difluoromethylornithine (DFMO), an FDA-approved inhibitor of polyamine biosynthesis, in rebalancing spermidine:spermine ratios in SRS patient cells."

Directly demonstrates that DFMO acts on the elevated spermidine-to-spermine ratio node in patient-derived cells.

Show evidence (3 references)

PMID:37702369 SUPPORT In Vitro

"Here we report the repurposing of 2-difluoromethylornithine (DFMO), an FDA-approved inhibitor of polyamine biosynthesis, in rebalancing spermidine:spermine ratios in SRS patient cells."

Demonstrates correction of the defining biochemical abnormality in patient-derived cells.

PMID:38473716 SUPPORT In Vitro

"inhibition of polyamine synthesis with DFMO rescues most, but not all of these defects"

Rescue of the skeletal cellular phenotype by upstream polyamine-synthesis inhibition is explicitly incomplete in the source, so support is partial.

PMID:19001365 REFUTE Model Organism

"Within 2-3 days of exposure to DFMO in the drinking water, the Gy mice suffered a catastrophic loss of motor function resulting in death within 5 days."

Counterweight evidence, a severe toxicity of DFMO specific to spermine synthase deficiency in vivo, which argues against uncritical translation of the in vitro rescue.

Phenylbutyrate — investigational

Category: Therapeutic Action: Pharmacotherapy NCIT:C15986

Agent: phenylbutyrate (PBA) CHEBI:41500

Phenylbutyrate, an FDA-approved drug, down-regulates the rate-limiting spermidine catabolic enzyme SAT1, reduces production of toxic catabolic metabolites and preserves acetyl-CoA. It restored mitochondrial and autolysosomal function and extended lifespan in the Drosophila SRS model, ameliorated autolysosome dysfunction in SRS patient fibroblasts, and partially restored metabolic flexibility in 29 patient lymphoblastoid lines. Preclinical only.

Mechanism Target:

INHIBITS Excessive Spermidine Catabolism and Oxidative Stress — Phenylbutyrate down-regulates SAT1, the rate-limiting enzyme of the spermidine back-conversion route, throttling the catabolic overflow that generates the toxic metabolites.

Show evidence (1 reference)

PMID:35801587 SUPPORT In Vitro

"We further explored the mechanism of drug action and found that PBA downregulates the first and rate-limiting spermidine catabolic enzyme spermidine/spermine N1-acetyltransferase 1 (SAT1), reduces the production of toxic metabolites, and inhibits the reduction of the substrate acetyl-CoA."

Identifies SAT1 down-regulation as the mechanism by which phenylbutyrate acts on the excessive-spermidine-catabolism node.

Show evidence (3 references)

PMID:35801587 SUPPORT Model Organism

"PBA treatment significantly restored the function of mitochondria and autolysosomes and extended life span in vivo in the Drosophila SRS model."

Demonstrates rescue of the organelle phenotypes and of survival in the fly SRS model.

PMID:35801587 SUPPORT In Vitro

"We further explored the mechanism of drug action and found that PBA downregulates the first and rate-limiting spermidine catabolic enzyme spermidine/spermine N1-acetyltransferase 1 (SAT1), reduces the production of toxic metabolites, and inhibits the reduction of the substrate acetyl-CoA."

Establishes SAT1 down-regulation as the mechanism by which phenylbutyrate addresses the catabolic-overflow node.

PMID:42250346 SUPPORT In Vitro

"Treatment with phenylbutyrate (PBA), previously shown to modulate polyamine catabolism, partially restored metabolic flexibility and normalized several impaired nutrient pathways."

Independent patient-cell evidence that phenylbutyrate partially corrects the bioenergetic phenotype.

🔬

Biochemical Markers

2

Elevated spermidine to spermine ratio (PRESENT)

Show evidence (2 references)

PMID:37702369 SUPPORT In Vitro

"Reduced SMS activity causes spermidine accumulation while spermine levels are reduced. The resulting exaggerated spermidine:spermine ratio is a biochemical hallmark of SRS that tends to correlate with symptom severity."

Establishes the ratio as the biochemical hallmark and its correlation with severity.

PMID:33186760 SUPPORT Human Clinical

"A novel p.(Gly203Asp) variant was found at the hemizygous state in the two boys, and an elevated Spermidine/Spermine ratio confirmed the diagnosis of Snyder-Robinson syndrome."

Demonstrates diagnostic use of the elevated ratio in confirmed patients.

Elevated plasma N8-acetylspermidine (PRESENT)

Show evidence (1 reference)

PMID:26174906 SUPPORT Human Clinical

"Untargeted plasma metabolome analysis revealed significantly elevated levels of N(8)-acetylspermidine, a precursor derivative of spermine biosynthesis, as a potential novel plasma biomarker for SRS."

Reports the elevated plasma analyte and its proposed biomarker role in SRS.

🔬

Diagnosis

1

Molecular genetic testing of SMS

The diagnosis is established in a male proband by identification of a hemizygous loss-of-function SMS pathogenic variant. Biochemical confirmation uses the elevated spermidine:spermine ratio in cultured cells; elevated plasma N(8)-acetylspermidine has been proposed as a less invasive supporting marker.

Show evidence (1 reference)

PMID:23805436 SUPPORT Other

"The diagnosis of SRS is established in a male proband with a hemizygous loss-of-function SMS pathogenic variant identified by molecular genetic testing."

GeneReviews states the diagnostic criterion for SRS.

📈

Progression

3

Infancy

Age: First year of life

Hypotonia, low muscle mass and an asthenic habitus emerge during the first year, with failure to meet early developmental milestones.

Show evidence (1 reference)

PMID:23805436 SUPPORT Other

"Asthenic habitus and low muscle mass usually develop during the first year."

GeneReviews places the onset of the asthenic build and low muscle mass in the first year of life.

Early childhood

Seizure onset typically occurs in early childhood, and developmental delay evolves into intellectual disability that then appears to remain stable.

Show evidence (2 references)

PMID:23805436 SUPPORT Other

"Seizure onset varies but typically occurs in early childhood."

GeneReviews dates typical seizure onset to early childhood.

PMID:23805436 SUPPORT Other

"Developmental delay usually presents as failure to meet early developmental milestones and then evolves to mild-to-profound intellectual disability (which appears to remain stable over time) and variable motor disability."

GeneReviews describes the developmental trajectory from early delay to a stable intellectual disability.

First decade — skeletal phase

Age: First decade

Osteoporosis develops during the first decade and produces fractures in the absence of trauma; kyphoscoliosis is progressive.

Show evidence (1 reference)

PMID:23805436 SUPPORT Other

"During the first decade, males with SRS develop osteoporosis, resulting in fractures in the absence of trauma."

GeneReviews times the emergence of osteoporosis and low-trauma fractures to the first decade.

📊

Prevalence

1

Worldwide

Cases In Literature Ultra Rare

No population-based prevalence estimate has been published. The literature consists of individual families and small case series, and secondary sources describe the condition only qualitatively as extremely rare. No numeric rate is asserted here because none is documented.

Show evidence (1 reference)

PMID:34741636 SUPPORT Other

"Snyder-Robinson syndrome is an extremely rare genetic disorder, caused by mutations of the spermine synthase gene."

Supports a qualitative ultra-rare occurrence band; the source gives no numeric prevalence, so only the qualitative tier is populated.

🔀

Differential Diagnoses

1

Conditions with similar clinical presentations that must be differentiated from Snyder-Robinson Syndrome:

Osteogenesis imperfecta

Overlapping Features SRS presents with early-onset osteoporosis and recurrent low-trauma fractures and has been misdiagnosed as osteogenesis imperfecta for years before molecular testing established the correct diagnosis.

Show evidence (1 reference)

PMID:34741636 SUPPORT Human Clinical

"We report a novel case of Snyder-Robinson syndrome, caused by a de novo mutation and first misdiagnosed with osteogenesis imperfecta."

Documents a case initially misdiagnosed as osteogenesis imperfecta, establishing the differential.

🐁

Animal Models

3

Sms G56S knock-in Mouse

A knock-in mouse carrying the Sms G56S missense variant lacks SMS protein and shows an increased spermidine/spermine ratio, failure to thrive, short stature, reduced bone density, impaired learning, increased anxiety, reduced mobility and reduced total and regional brain volumes, with impaired mitochondrial oxidative phosphorylation in cortex and fibroblasts. It is the current preclinical model for SRS therapeutic development.

Species

Mouse

Genotype

Sms G56S knock-in

Show evidence (1 reference)

PMID:38463005 SUPPORT Model Organism

"Collectively, our study establishes the suitability of the G56S mice as a preclinical model for SRS and provides a set of molecular and functional outcome measures that can be used to evaluate therapeutic interventions for SRS."

Establishes the G56S mouse as the validated preclinical model for SRS.

Gyro (Gy), X-chromosomal deletion inactivating SpmS and Phex Mouse

The Gy mouse carries an X-chromosomal deletion inactivating both SpmS and the neighbouring Phex gene. It is profoundly hearing impaired with near-complete loss of the endocochlear potential, rescued by a spermine synthase transgene, and is catastrophically sensitive to DFMO. Because the deletion also removes Phex, skeletal findings in this model are confounded by X-linked hypophosphataemia.

Species

Mouse

Genotype

Gyro (Gy), X-chromosomal deletion inactivating SpmS and Phex

Show evidence (1 reference)

PMID:39331754 SUPPORT Model Organism

"Since the impact of a murine SMS deficiency has so far only been analyzed in Gy mice, where a large genomic deletion also includes the neighboring Phex gene, there is only limited knowledge about the potential role of SMS in bone cell regulation."

States the Phex confound that limits interpretation of skeletal phenotypes in the Gy model, motivating the newer G56S mouse.

dSms loss of function Drosophila melanogaster

Loss of dSms in Drosophila recapitulates the polyamine imbalance of SRS and causes survival defects, synaptic and retinal degeneration, lysosomal defects and oxidative stress. It is the model in which both DFMO and phenylbutyrate rescue were first demonstrated.

Species

Drosophila melanogaster

Genotype

dSms loss of function

Show evidence (1 reference)

PMID:29097652 SUPPORT Model Organism

"Here we show that loss of dSms in Drosophila recapitulates the pathological polyamine imbalance of SRS and causes survival defects and synaptic degeneration."

Establishes the Drosophila model and the phenotypes it reproduces.

{ }

Source YAML

click to show

name: Snyder-Robinson Syndrome
category: Mendelian
creation_date: '2026-07-31T00:00:00Z'
synonyms:
- Snyder-Robinson syndrome
- SRS
- MRXSSR
- syndromic X-linked intellectual disability Snyder type
- spermine synthase deficiency
- X-linked intellectual disability Snyder-Robinson type
description: 'Snyder-Robinson syndrome (SRS) is an X-linked recessive syndromic intellectual
  disability caused by hemizygous loss-of-function variants in SMS, the gene encoding
  spermine synthase. It is the only known human disorder of spermine synthase deficiency
  and was the first recognised inborn error of polyamine metabolism. Loss of spermine
  synthase activity depletes spermine and causes spermidine to accumulate, so that
  an exaggerated spermidine:spermine ratio becomes the biochemical hallmark of the
  disease and broadly correlates with clinical severity. Affected males show early
  hypotonia and low muscle mass with an asthenic (thin) habitus, developmental delay
  evolving to mild-to-profound intellectual disability, early-onset osteoporosis with
  low-trauma fractures, progressive kyphoscoliosis, seizures, facial asymmetry and
  other facial dysmorphism, unsteady or broad-based gait, and abnormal (often nasal,
  dysarthric) speech. Nonspecific kidney findings, and in severe alleles hearing loss,
  failure to thrive and premature death, are also reported. Mechanistically the altered
  polyamine pool drives excessive spermidine catabolism with generation of reactive
  oxygen species and toxic aldehydes, lysosomal and mitochondrial dysfunction, loss
  of polyamine-dependent modulation of inward-rectifier potassium channels in excitable
  tissue, and a cell-autonomous failure of osteoblastic bone matrix mineralisation.
  MONDO classifies SRS both as an X-linked syndromic intellectual disability and as
  a disorder of polyamine metabolism.

  '
disease_term:
  preferred_term: Snyder-Robinson syndrome
  term:
    id: MONDO:0010664
    label: syndromic X-linked intellectual disability Snyder type
parents:
- Disorder of Polyamine Metabolism
- X-linked Syndromic Intellectual Disability
- Inborn Error of Metabolism
classifications:
  harrisons_chapter:
  - classification_value: ENDOCRINOLOGY_METABOLISM
    notes: 'SRS is an inborn error of metabolism, specifically the only recognised
      human disorder of spermine synthase deficiency.

      '
    evidence:
    - reference: PMID:14508504
      reference_title: "X-linked spermine synthase gene (SMS) defect: the first polyamine deficiency syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: We report the first polyamine deficiency syndrome caused by a defect in
        spermine synthase (SMS).
      explanation: Identifies SRS as an inherited metabolic deficiency syndrome, placing
        it in the endocrinology and metabolism part.
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
    notes: 'A Mendelian, X-linked recessive single-gene disorder.

      '
    evidence:
    - reference: PMID:41410504
      reference_title: "Genetic and Phenotypic Features of the Five Known Polyaminopathies: A Critical Narrative Review."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: Snyder-Robinson syndrome results from hemizygous loss-of-function variants
        in the spermine synthase (SMS) gene, resulting in decreased or complete loss
        of spermine synthase enzyme activity.
      explanation: Confirms SRS as a single-gene Mendelian disorder, supporting the genetics
        part assignment.
  - classification_value: NEUROLOGIC
    notes: 'Intellectual disability, seizures and gait abnormality dominate the clinical
      presentation.

      '
    evidence:
    - reference: PMID:23805436
      reference_title: "Snyder-Robinson Syndrome."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: Snyder-Robinson syndrome (SRS) is an X-linked intellectual disability syndrome
        characterized by facial dysmorphism, asthenic build, progressive kyphoscoliosis,
        early-onset osteoporosis, and seizures.
      explanation: The defining features include intellectual disability and seizures,
        supporting a neurologic part assignment alongside the metabolic one.
  icimd_category:
  - classification_value: polyamine_metabolism
    notes: 'ICIMD places spermine synthase deficiency among disorders of polyamine
      metabolism, within the peptide and amine metabolism category. This matches the
      MONDO placement of MONDO:0010664 under MONDO:0800159 (disorder of polyamine
      metabolism).

      '
    evidence:
    - reference: PMID:41410504
      reference_title: "Genetic and Phenotypic Features of the Five Known Polyaminopathies: A Critical Narrative Review."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: Snyder-Robinson syndrome results from hemizygous loss-of-function variants
        in the spermine synthase (SMS) gene, resulting in decreased or complete loss
        of spermine synthase enzyme activity.
      explanation: A review of the polyaminopathies places SRS among the inherited disorders
        of polyamine metabolism.
references:
- reference: PMID:23805436
  title: "Snyder-Robinson Syndrome."
  tags:
  - GeneReviews
inheritance:
- name: X-linked recessive inheritance
  inheritance_term:
    preferred_term: X-linked recessive inheritance
    term:
      id: HP:0001419
      label: X-linked recessive inheritance
  description: 'Only affected males have been reported; heterozygous females have not shown
    features of SRS to date. Transmission is usually from a carrier mother, but de novo
    SMS variants are also documented, so an unaffected maternal genotype does not exclude
    the diagnosis. An apparently de novo variant is not necessarily de novo: low-level
    maternal somatic/germline mosaicism has been demonstrated in SRS, in one family at
    ~3% variant allele frequency in maternal blood - a level below the detection limit of
    Sanger sequencing, which had reported the variant as de novo. This materially raises
    the recurrence risk above the population baseline quoted for a true de novo event, so
    deep targeted resequencing of the mother should be considered before counselling a
    family that recurrence risk is negligible.

    '
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: To date, only affected males have been reported.
    explanation: GeneReviews states that the disorder manifests only in hemizygous males,
      consistent with X-linked recessive inheritance.
  - reference: PMID:34177437
    reference_title: "Novel Hemizygous Missense Variant of Spermine Synthase (SMS) Gene Causes Snyder-Robinson Syndrome in a Four-Year-Old Boy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: This variant in SMS was found to be de novo.
    explanation: A genetically confirmed case in which the causative hemizygous SMS variant
      arose de novo, showing that SRS is not invariably maternally transmitted.
  - reference: PMID:34741636
    reference_title: "Snyder-Robinson syndrome: differential diagnosis of osteogenesis imperfecta."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: We report a novel case of Snyder-Robinson syndrome, caused by a de novo mutation
      and first misdiagnosed with osteogenesis imperfecta.
    explanation: A second independent report of a de novo SMS mutation causing SRS.
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: 'If the mother of the proband has an SMS pathogenic variant, the chance of
      transmitting it in each pregnancy is 50%: males who inherit the pathogenic variant
      will be affected; females who inherit the pathogenic variant will be heterozygotes'
    explanation: GeneReviews states the 50% per-pregnancy transmission risk from a carrier
      mother and the sex-specific outcome, the defining recurrence pattern of X-linked
      recessive inheritance.
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Affected males are not known to reproduce.
    explanation: Absence of male reproduction excludes male-to-female obligate-carrier
      transmission, so affected males do not propagate the variant.
  - reference: PMID:34667072
    reference_title: "Maternal mosaicism for a missense variant in the SMS gene that causes Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: 'Although Sanger sequencing confirmed the de novo status in our proband,
      polymerase chain reaction (PCR) and deep targeted resequencing to ∼84,000×-175,000×
      depth revealed that the variant is present in blood from the unaffected mother at
      ∼3% variant allele frequency.'
    explanation: Demonstrates low-level maternal mosaicism for a pathogenic SMS variant that
      standard Sanger sequencing had classified as de novo, establishing that an apparently
      de novo SRS variant can carry a recurrence risk and that deep resequencing of the
      mother may be warranted before counselling.
  - reference: PMID:34667072
    reference_title: "Maternal mosaicism for a missense variant in the SMS gene that causes Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Our findings thus provided a long-sought diagnosis for the family while
      highlighting the role of parental mosaicism in severe genetic disorders.
    explanation: The authors' own framing of the counselling implication of parental
      mosaicism in this disorder.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: 'No population-based prevalence estimate has been published. The literature
    consists of individual families and small case series, and secondary sources describe
    the condition only qualitatively as extremely rare. No numeric rate is asserted
    here because none is documented.

    '
  evidence:
  - reference: PMID:34741636
    reference_title: "Snyder-Robinson syndrome: differential diagnosis of osteogenesis imperfecta."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Snyder-Robinson syndrome is an extremely rare genetic disorder, caused
      by mutations of the spermine synthase gene.
    explanation: Supports a qualitative ultra-rare occurrence band; the source gives no
      numeric prevalence, so only the qualitative tier is populated.
progression:
- phase: Infancy
  age_range: First year of life
  notes: 'Hypotonia, low muscle mass and an asthenic habitus emerge during the first
    year, with failure to meet early developmental milestones.

    '
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Asthenic habitus and low muscle mass usually develop during the first year.
    explanation: GeneReviews places the onset of the asthenic build and low muscle mass
      in the first year of life.
- phase: Early childhood
  notes: 'Seizure onset typically occurs in early childhood, and developmental delay
    evolves into intellectual disability that then appears to remain stable.

    '
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Seizure onset varies but typically occurs in early childhood.
    explanation: GeneReviews dates typical seizure onset to early childhood.
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Developmental delay usually presents as failure to meet early developmental
      milestones and then evolves to mild-to-profound intellectual disability (which
      appears to remain stable over time) and variable motor disability.
    explanation: GeneReviews describes the developmental trajectory from early delay to
      a stable intellectual disability.
- phase: First decade — skeletal phase
  age_range: First decade
  notes: 'Osteoporosis develops during the first decade and produces fractures in the
    absence of trauma; kyphoscoliosis is progressive.

    '
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: During the first decade, males with SRS develop osteoporosis, resulting in
      fractures in the absence of trauma.
    explanation: GeneReviews times the emergence of osteoporosis and low-trauma fractures
      to the first decade.
pathophysiology:
- name: Spermine Synthase Loss of Function
  description: 'Hemizygous loss-of-function variants in SMS reduce or abolish spermine
    synthase catalytic activity. Spermine synthase acts only as a homodimer and transfers
    an aminopropyl group from decarboxylated S-adenosylmethionine to spermidine to
    make spermine. Most pathogenic variants are hypomorphic missense changes; several
    act by destabilising the protein or abolishing homodimer formation rather than
    by disrupting the active site, and a rare complete loss-of-function frameshift
    allele produces a severe early-lethal phenotype.

    '
  biological_scale: MOLECULAR
  role: trigger
  genes:
  - preferred_term: SMS
    term:
      id: hgnc:11123
      label: SMS
  molecular_functions:
  - preferred_term: spermine synthase activity
    term:
      id: GO:0016768
      label: spermine synthase activity
    modifier: DECREASED
  biological_processes:
  - preferred_term: spermine biosynthetic process
    term:
      id: GO:0006597
      label: spermine biosynthetic process
    modifier: DECREASED
  cellular_components:
  - preferred_term: cytosol
    term:
      id: GO:0005829
      label: cytosol
  chemical_entities:
  - preferred_term: decarboxylated S-adenosylmethionine
    term:
      id: CHEBI:57443
      label: S-adenosylmethioninaminium
  evidence:
  - reference: PMID:14508504
    reference_title: "X-linked spermine synthase gene (SMS) defect: the first polyamine deficiency syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: We report the first polyamine deficiency syndrome caused by a defect in
      spermine synthase (SMS).
    explanation: Establishes SMS deficiency as the primary molecular lesion of Snyder-Robinson
      syndrome and as the first recognised human polyamine deficiency.
  - reference: PMID:41410504
    reference_title: "Genetic and Phenotypic Features of the Five Known Polyaminopathies: A Critical Narrative Review."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Snyder-Robinson syndrome results from hemizygous loss-of-function variants
      in the spermine synthase (SMS) gene, resulting in decreased or complete loss of
      spermine synthase enzyme activity.
    explanation: A polyaminopathy review confirms the hemizygous loss-of-function mechanism
      and the resulting reduction in enzyme activity.
  - reference: PMID:30544565
    reference_title: "Polyamine Homeostasis in Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: SMS catalyzes the production of spermine (SPM) from its precursor, spermidine
      (SPD), via the transfer of an aminopropyl group, which is derived from decarboxylated
      S-adenosylmethionine (dcAdoMet)
    explanation: Defines the aminopropyl-transfer reaction that is lost when spermine
      synthase activity falls.
  - reference: PMID:26761001
    reference_title: "Revealing the Effects of Missense Mutations Causing Snyder-Robinson Syndrome on the Stability and Dimerization of Spermine Synthase."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: In silico modeling predicted that all studied mutations in this work destabilize
      SpmSyn and some of them abolish homo-dimer formation.
    explanation: Supports protein destabilisation and loss of obligate homodimer formation
      as a route to loss of enzyme function for SRS missense variants situated away from
      the active site.
  downstream:
  - target: Elevated Spermidine to Spermine Ratio
    description: Loss of spermine synthase activity blocks conversion of spermidine to
      spermine, depleting spermine and letting spermidine accumulate.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:41175261
      reference_title: "Spermine synthase in Snyder-Robinson syndrome and cancer."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: Loss of Sms leads to spermine deficiency, elevated spermidine levels, and
        metabolic imbalances, contributing to SRS pathology.
      explanation: Directly links loss of spermine synthase to spermine depletion with
        spermidine elevation.
- name: Elevated Spermidine to Spermine Ratio
  description: 'Spermine falls and spermidine rises in patient lymphocytes, lymphoblasts
    and fibroblasts, producing an exaggerated spermidine:spermine ratio that is the
    biochemical hallmark of SRS and that broadly tracks symptom severity. Ornithine
    decarboxylase activity and putrescine are secondarily reduced, indicating compensatory
    down-regulation of upstream polyamine biosynthesis rather than a simple accumulation
    defect.

    '
  biological_scale: MOLECULAR
  role: intermediate
  biological_processes:
  - preferred_term: intracellular polyamine homeostasis
    term:
      id: GO:0010509
      label: intracellular polyamine homeostasis
    modifier: DYSREGULATED
  - preferred_term: polyamine metabolic process
    term:
      id: GO:0006595
      label: polyamine metabolic process
    modifier: DYSREGULATED
  chemical_entities:
  - preferred_term: spermine
    term:
      id: CHEBI:15746
      label: spermine
    modifier: DECREASED
  - preferred_term: spermidine
    term:
      id: CHEBI:16610
      label: spermidine
    modifier: INCREASED
  - preferred_term: putrescine
    term:
      id: CHEBI:17148
      label: putrescine
    modifier: DECREASED
  evidence:
  - reference: PMID:37702369
    reference_title: "Difluoromethylornithine rebalances aberrant polyamine ratios in Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: 'Reduced SMS activity causes spermidine accumulation while spermine levels
      are reduced. The resulting exaggerated spermidine:spermine ratio is a biochemical
      hallmark of SRS that tends to correlate with symptom severity.'
    explanation: Establishes the elevated spermidine-to-spermine ratio as the defining
      biochemical abnormality and its correlation with severity.
  - reference: PMID:14508504
    reference_title: "X-linked spermine synthase gene (SMS) defect: the first polyamine deficiency syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The affected males have mild-to-moderate mental retardation (MR), hypotonia,
      cerebellar circuitry dysfunction, facial asymmetry, thin habitus, osteoporosis,
      kyphoscoliosis, decreased activity of SMS, correspondingly low levels of intracellular
      spermine in lymphocytes and fibroblasts, and elevated spermidine/spermine ratios.
    explanation: Documents low intracellular spermine and elevated spermidine/spermine
      ratios directly in patient lymphocytes and fibroblasts.
  - reference: PMID:30544565
    reference_title: "Polyamine Homeostasis in Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: In addition to decreased spermine and increased spermidine in SRS cells,
      ornithine decarboxylase activity and its product putrescine were significantly
      decreased.
    explanation: Confirms the polyamine pool shift in patient-derived lymphoblasts and
      documents the secondary fall in ornithine decarboxylase activity and putrescine.
  downstream:
  - target: Excessive Spermidine Catabolism and Oxidative Stress
    description: The enlarged spermidine pool is shunted into the SAT1/polyamine oxidase
      back-conversion route, generating hydrogen peroxide and reactive aldehydes.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:29097652
      reference_title: "Spermine synthase deficiency causes lysosomal dysfunction and oxidative stress in models of Snyder-Robinson syndrome."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: SMS deficiency leads to excessive spermidine catabolism, which generates
        toxic metabolites that cause lysosomal defects and oxidative stress.
      explanation: Directly links the spermidine excess of SMS deficiency to catabolic
        overflow and toxic metabolite generation.
  - target: Loss of Polyamine-Dependent Ion Channel Modulation
    description: Spermine is the principal endogenous blocker of inward-rectifier potassium
      channels, so its depletion removes a physiological channel-gating influence in
      excitable tissue.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:14508504
      reference_title: "X-linked spermine synthase gene (SMS) defect: the first polyamine deficiency syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: possibly by spermine's ability to function as an 'intrinsic gateway' molecule
        for inward rectifier K(+) channels
      explanation: The original description offers this link as an explicit possibility
        ("possibly"), not a demonstrated finding, so support for the causal edge is partial.
  - target: Impaired Osteoblastic Bone Matrix Mineralisation
    description: Excess spermidine in osteogenic cells suppresses matrix mineralisation
      in a dose-dependent manner, independent of any resorptive stimulus.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:38473716
      reference_title: "Effects of Spermine Synthase Deficiency in Mesenchymal Stromal Cells Are Rescued by Upstream Inhibition of Ornithine Decarboxylase."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: Employing N-cyclohexyl-1,3-propanediamine (CDAP) to chemically inhibit spermine
        synthase (SMS), the enzyme catalyzing conversion of spermidine into spermine, also
        suppresses mineralization.
      explanation: Pharmacological inhibition of spermine synthase itself, which reproduces
        the SRS polyamine shift, suppresses matrix mineralisation in human stromal cells,
        supporting this specific causal edge rather than a generic polyamine-overload effect.
- name: Excessive Spermidine Catabolism and Oxidative Stress
  description: 'Because spermidine cannot be converted to spermine it is instead routed
    through spermidine/spermine N1-acetyltransferase 1 (SAT1) and polyamine oxidase.
    This back-conversion generates hydrogen peroxide and reactive aldehydes, imposing
    oxidative stress and depleting acetyl-CoA. In Drosophila SRS models and in patient
    cells these toxic by-products damage lysosomes and mitochondria; enhancing antioxidant
    activity genetically or pharmacologically suppresses the oxidative stress.

    '
  biological_scale: CELLULAR
  role: intermediate
  biological_processes:
  - preferred_term: polyamine catabolic process
    term:
      id: GO:0006598
      label: polyamine catabolic process
    modifier: INCREASED
  - preferred_term: response to oxidative stress
    term:
      id: GO:0006979
      label: response to oxidative stress
    modifier: INCREASED
  evidence:
  - reference: PMID:35801587
    reference_title: "Phenylbutyrate modulates polyamine acetylase and ameliorates Snyder-Robinson syndrome in a Drosophila model and patient cells."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: In the Drosophila SRS model, altered spermidine/spermine balance has been
      associated with increased generation of ROS and aldehydes, consistent with elevated
      spermidine catabolism. These toxic byproducts cause mitochondrial and lysosomal
      dysfunction, which are also observed in cells from SRS patients.
    explanation: Links the polyamine imbalance to catabolism-derived ROS and aldehydes
      and to the resulting organelle dysfunction, with confirmation in patient cells.
  - reference: PMID:29097652
    reference_title: "Spermine synthase deficiency causes lysosomal dysfunction and oxidative stress in models of Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: SMS deficiency leads to excessive spermidine catabolism, which generates
      toxic metabolites that cause lysosomal defects and oxidative stress.
    explanation: Establishes excessive spermidine catabolism as the source of the toxic
      metabolites that produce lysosomal defects and oxidative stress.
  downstream:
  - target: Lysosomal and Mitochondrial Dysfunction
    description: Catabolism-derived aldehydes and reactive oxygen species compromise autophagy-lysosome
      flux and mitochondrial function.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:29097652
      reference_title: "Spermine synthase deficiency causes lysosomal dysfunction and oxidative stress in models of Snyder-Robinson syndrome."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: Consequently, autophagy-lysosome flux and mitochondrial function are compromised
        in the Drosophila nervous system and SRS patient cells.
      explanation: Directly supports the causal edge from polyamine catabolic stress to
        combined lysosomal and mitochondrial failure, in both the fly model and patient
        cells.
- name: Lysosomal and Mitochondrial Dysfunction
  description: 'Autophagy-lysosome flux and mitochondrial oxidative phosphorylation
    are compromised in SRS models and patient-derived cells. The G56S SRS mouse shows
    impaired mitochondrial oxidative phosphorylation in cerebral cortex, fibroblasts
    and Sms-null hippocampal cells, and high-throughput metabolic profiling of 29 patient
    lymphoblastoid lines shows broad energy-metabolism rewiring, establishing bioenergetic
    failure as a cellular hallmark of the disorder.

    '
  biological_scale: CELLULAR
  role: intermediate
  biological_processes:
  - preferred_term: autophagy
    term:
      id: GO:0006914
      label: autophagy
    modifier: DECREASED
  - preferred_term: oxidative phosphorylation
    term:
      id: GO:0006119
      label: oxidative phosphorylation
    modifier: DECREASED
  cellular_components:
  - preferred_term: lysosome
    term:
      id: GO:0005764
      label: lysosome
  - preferred_term: mitochondrion
    term:
      id: GO:0005739
      label: mitochondrion
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  evidence:
  - reference: PMID:38463005
    reference_title: "Impaired polyamine metabolism causes behavioral and neuroanatomical defects in a mouse model of Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: impaired mitochondrial oxidative phosphorylation was evident in G56S cerebral
      cortex, G56S fibroblasts and Sms-null hippocampal cells
    explanation: Mouse-model evidence that SMS loss impairs mitochondrial oxidative phosphorylation
      in brain and in fibroblasts.
  - reference: PMID:42250346
    reference_title: "Metabolic alterations in Snyder-Robinson syndrome lymphoblasts are ameliorated by phenylbutyrate treatment."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: These findings highlight global energy metabolism dysregulation as a hallmark
      of SRS and support PBA as a promising therapeutic candidate for correcting bioenergetic
      defects in this disorder.
    explanation: Patient-derived lymphoblastoid profiling establishes global energy-metabolism
      dysregulation as a cellular hallmark of SRS.
  downstream:
  - target: Neuronal and Synaptic Dysfunction
    description: Compromised autophagy-lysosome flux and mitochondrial bioenergetics in
      the nervous system lead to synaptic and retinal degeneration in the fly model.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - impaired autophagy-lysosome flux
    - neuronal energy failure
    evidence:
    - reference: PMID:29097652
      reference_title: "Spermine synthase deficiency causes lysosomal dysfunction and oxidative stress in models of Snyder-Robinson syndrome."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: Here we show that loss of dSms in Drosophila recapitulates the pathological
        polyamine imbalance of SRS and causes survival defects and synaptic degeneration.
      explanation: Model-organism evidence that the SRS polyamine imbalance produces synaptic
        degeneration.
- name: Loss of Polyamine-Dependent Ion Channel Modulation
  description: 'Spermine is the dominant endogenous polycation that blocks and gates
    inward-rectifier potassium channels and modulates glutamate receptors, so spermine
    depletion removes a physiological control on excitable membranes. This was the
    mechanism originally proposed to connect the polyamine defect to the cognitive
    phenotype of SRS, and in the spermine synthase-deficient Gy mouse the loss of the
    endocochlear potential and of vestibular function is attributed to defective polyamine
    regulation of Kir channels. Note that the direct human evidence is inferential
    rather than electrophysiological.

    '
  biological_scale: MOLECULAR
  role: intermediate
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  molecular_functions:
  - preferred_term: inward rectifier potassium channel activity
    term:
      id: GO:0005242
      label: inward rectifier potassium channel activity
    modifier: DYSREGULATED
  chemical_entities:
  - preferred_term: spermine
    term:
      id: CHEBI:15746
      label: spermine
    modifier: DECREASED
  evidence:
  - reference: PMID:14508504
    reference_title: "X-linked spermine synthase gene (SMS) defect: the first polyamine deficiency syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: possibly by spermine's ability to function as an 'intrinsic gateway' molecule
      for inward rectifier K(+) channels
    explanation: The original description proposes this mechanism explicitly as a possibility
      rather than a demonstrated finding, so the support is recorded as partial.
  - reference: PMID:29097652
    reference_title: "Spermine synthase deficiency causes lysosomal dysfunction and oxidative stress in models of Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: polyamines were able to block or modulate several distinct types of cation
      channels, including inward-rectifier K+ channels and glutamate receptors
    explanation: A background statement in the paper's discussion summarising prior in
      vitro observations by others, not data generated in this study; it establishes that
      polyamines modulate these channels but not that this modulation is disrupted in SRS,
      so support is partial.
  - reference: PMID:19001365
    reference_title: "Spermine synthase deficiency leads to deafness and a profound sensitivity to alpha-difluoromethylornithine."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: 'Our results are consistent with a critical role for polyamines in regulation
      of Kir channels that maintain the endocochlear potential and emphasize the importance
      of normal spermidine:spermine ratio in the hearing and balance functions of the
      inner ear.'
    explanation: In the spermine synthase-deficient Gy mouse, loss of the endocochlear
      potential and of balance is attributed to defective polyamine regulation of Kir
      channels.
  downstream:
  - target: Hearing impairment
    description: Loss of polyamine-dependent Kir channel regulation abolishes the endocochlear
      potential in the spermine synthase-deficient mouse; hearing loss is also reported
      in severe human SRS.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - loss of endocochlear potential
    evidence:
    - reference: PMID:19001365
      reference_title: "Spermine synthase deficiency leads to deafness and a profound sensitivity to alpha-difluoromethylornithine."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: There was an almost complete loss of the endocochlear potential in the
        Gy mice, which parallels the hearing deficiency, and this was also reversed by
        the production of spermine from the spermine synthase transgene.
      explanation: Demonstrates that restoring spermine rescues both the endocochlear potential
        and the hearing deficit, establishing the causal chain in the mouse.
- name: Neuronal and Synaptic Dysfunction
  description: 'Polyamines are required locally within neurons for chromatin, translational
    and channel-modulatory functions. Loss of spermine synthase in Drosophila causes
    survival defects and synaptic and retinal degeneration, and the G56S SRS mouse
    shows impaired learning, increased anxiety, reduced mobility and reduced total
    and regional brain volumes. Together these establish a neuronal substrate for the
    intellectual disability, seizures and gait abnormality of SRS, though the human
    evidence remains largely clinical rather than histological.

    '
  biological_scale: CELLULAR
  role: intermediate
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  evidence:
  - reference: PMID:38463005
    reference_title: "Impaired polyamine metabolism causes behavioral and neuroanatomical defects in a mouse model of Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: They showed impaired learning capacity, increased anxiety, reduced mobility
      and heightened fear responses, accompanied by reduced total and regional brain volumes.
    explanation: Mouse-model evidence of cognitive and neuroanatomical consequences of
      spermine synthase loss.
  - reference: PMID:29097652
    reference_title: "Spermine synthase deficiency causes lysosomal dysfunction and oxidative stress in models of Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Our data show that dSms is expressed in the nervous system, including cell
      bodies and synapses, and loss of dSms causes retinal and synaptic degeneration.
    explanation: Establishes a neuronal and synaptic locus for the consequences of spermine
      synthase loss.
  downstream:
  - target: Intellectual disability
    description: Neuronal polyamine imbalance underlies the cognitive impairment that defines
      the syndrome.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:14508504
      reference_title: "X-linked spermine synthase gene (SMS) defect: the first polyamine deficiency syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Additionally, the presence of MR reflects a role for spermine in cognitive
        function
      explanation: Attributes the intellectual disability of SRS to the loss of spermine's
        role in cognitive function; the intermediate steps remain undefined.
  - target: Seizure
    description: Neuronal dysfunction from polyamine imbalance produces the seizures seen
      in a substantial proportion of affected males.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:30544565
      reference_title: "Polyamine Homeostasis in Snyder-Robinson Syndrome."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: the imbalance that occurs in SRS results in a combination of clinical manifestations
        including moderate-to-severe cognitive impairment, osteoporosis, asthenic build,
        low muscle mass, facial asymmetry, speech abnormalities, and seizures
      explanation: A background statement in the paper's introduction attributing the seizure
        phenotype, among others, to the SRS polyamine imbalance.
    - reference: PMID:23805436
      reference_title: "Snyder-Robinson Syndrome."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: Snyder-Robinson syndrome (SRS) is an X-linked intellectual disability syndrome
        characterized by facial dysmorphism, asthenic build, progressive kyphoscoliosis,
        early-onset osteoporosis, and seizures.
      explanation: GeneReviews establishes seizures as a defining clinical feature of the
        syndrome, giving the causal edge a clinical rather than purely review-prose basis.
- name: Impaired Osteoblastic Bone Matrix Mineralisation
  description: 'The skeletal arm of SRS is a formation defect rather than a resorptive
    one. Bone histopathology in affected brothers showed profound depletion of both
    osteoblasts and osteoclasts with absent trabecular meshwork, low bone volume and
    a thin cortex, and patient-derived bone marrow stromal cells failed to mineralise
    on osteogenic differentiation. Silencing SMS in human MSCs reproduces the defect
    in vitro and in vivo, and in the SMS G56S mouse dynamic histomorphometry identifies
    a reduced bone formation rate as the main cause of the low bone mass. Excess spermidine
    is itself sufficient to suppress mineralisation, and inhibiting the upstream enzyme
    ornithine decarboxylase with DFMO rescues it.

    '
  biological_scale: TISSUE
  role: effector
  conforms_to: "osteoporosis_bone_resorption#Impaired Osteoblastic Bone Formation"
  cell_types:
  - preferred_term: osteoblast
    term:
      id: CL:0000062
      label: osteoblast
  - preferred_term: mesenchymal stem cell
    term:
      id: CL:0000134
      label: mesenchymal stem cell
  biological_processes:
  - preferred_term: bone mineralization
    term:
      id: GO:0030282
      label: bone mineralization
    modifier: DECREASED
  - preferred_term: ossification
    term:
      id: GO:0001503
      label: ossification
    modifier: DECREASED
  evidence:
  - reference: PMID:25888122
    reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: Osteogenic differentiation of the SRS-derived hBMSCs identified a severe deficiency
      of calcium phosphate mineralization.
    explanation: Patient-derived bone marrow stromal cells fail to mineralise in culture,
      establishing a cell-autonomous osteogenic defect.
  - reference: PMID:25888122
    reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: the low bone density arises from a failure of mineralization
    explanation: The authors conclude that the low bone density of SRS is a mineralisation
      failure rather than a resorption excess.
  - reference: PMID:39331754
    reference_title: "Inactivation of spermine synthase in mice causes osteopenia due to reduced osteoblast activity."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Cellular and dynamic histomorphometry further identified a reduced bone formation
      rate as a main cause of the low bone mass phenotype.
    explanation: Dynamic histomorphometry in the SMS G56S mouse attributes the low bone
      mass specifically to a reduced bone formation rate.
  - reference: PMID:31659216
    reference_title: "Modeling Snyder-Robinson Syndrome in multipotent stromal cells reveals impaired mitochondrial function as a potential cause for deficient osteogenesis."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: We found that silencing SMS in MSCs led to reduced cell proliferation and
      deficient bone formation in vitro, as evidenced by reduced mineralization and decreased
      bone sialoprotein expression.
    explanation: Knockdown of SMS in healthy human MSCs is sufficient to reproduce the
      mineralisation defect, supporting causality.
  - reference: PMID:38473716
    reference_title: "Effects of Spermine Synthase Deficiency in Mesenchymal Stromal Cells Are Rescued by Upstream Inhibition of Ornithine Decarboxylase."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: this reduced mineralization is rescued with DFMO, an inhibitor of the upstream
      polyamine enzyme ornithine decarboxylase (ODC1)
    explanation: Rescue by upstream polyamine-synthesis inhibition confirms that the mineralisation
      defect is driven by the polyamine imbalance itself.
  downstream:
  - target: Net Bone Loss and Skeletal Fragility
    description: Sustained failure of osteoblastic matrix mineralisation produces low bone
      volume, a thin cortex and loss of trabecular architecture.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:25888122
      reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Bone histopathology and morphometry identified a profound depletion of osteoblasts
        and osteoclasts, absence of a trabecular meshwork, a low bone volume and a thin
        cortex.
      explanation: Directly documents the structural bone deficit that follows the mineralisation
        failure in affected males.
- name: Net Bone Loss and Skeletal Fragility
  description: 'The cumulative formation deficit produces early-onset osteoporosis
    with reduced bone mineral density, a thin cortex and loss of trabecular architecture,
    manifesting during the first decade as fractures in the absence of trauma and as
    progressive kyphoscoliosis. Because osteoclasts are depleted rather than increased,
    the low bone mass of SRS is not a resorption-driven osteoporosis; this distinction
    matters clinically, since SRS is misdiagnosed as osteogenesis imperfecta.

    '
  biological_scale: ORGANISM
  role: consequence
  conforms_to: "osteoporosis_bone_resorption#Net Bone Loss and Skeletal Fragility"
  biological_processes:
  - preferred_term: bone remodeling
    term:
      id: GO:0046849
      label: bone remodeling
    modifier: DYSREGULATED
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: During the first decade, males with SRS develop osteoporosis, resulting in
      fractures in the absence of trauma.
    explanation: GeneReviews documents the clinical endpoint of the skeletal chain, low-trauma
      fractures from early-onset osteoporosis.
  - reference: PMID:39331754
    reference_title: "Inactivation of spermine synthase in mice causes osteopenia due to reduced osteoblast activity."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: our data identify SMS as an enzyme with physiological relevance for osteoblast
      activity, thereby demonstrating an important role of polyamine metabolism in the
      control of bone remodeling.
    explanation: Places polyamine metabolism, acting through osteoblast activity, in control
      of bone remodelling.
  downstream:
  - target: Osteoporosis
    description: Net bone loss presents clinically as early-onset osteoporosis.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:23805436
      reference_title: "Snyder-Robinson Syndrome."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: Snyder-Robinson syndrome (SRS) is an X-linked intellectual disability syndrome
        characterized by facial dysmorphism, asthenic build, progressive kyphoscoliosis,
        early-onset osteoporosis, and seizures.
      explanation: Lists early-onset osteoporosis among the defining features of the syndrome.
  - target: Reduced bone mineral density
    description: The cumulative formation deficit is measured clinically as reduced bone
      mineral density on DXA.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:38463005
      reference_title: "Impaired polyamine metabolism causes behavioral and neuroanatomical defects in a mouse model of Snyder-Robinson syndrome."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: The lack of SMS protein in the G56S mice resulted in increased spermidine/spermine
        ratio, failure to thrive, short stature and reduced bone density.
      explanation: The SRS mouse model links loss of SMS, through the skeletal chain, to
        measurably reduced bone density.
  - target: Recurrent fractures
    description: Skeletal fragility produces fractures in the absence of trauma.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:23805436
      reference_title: "Snyder-Robinson Syndrome."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: During the first decade, males with SRS develop osteoporosis, resulting in
        fractures in the absence of trauma.
      explanation: Directly links the osteoporosis of SRS to atraumatic fractures.
  - target: Kyphoscoliosis
    description: Loss of vertebral bone mass contributes to the progressive spinal deformity.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - vertebral bone loss
    - axial hypotonia
    evidence:
    - reference: PMID:23805436
      reference_title: "Snyder-Robinson Syndrome."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: Snyder-Robinson syndrome (SRS) is an X-linked intellectual disability syndrome
        characterized by facial dysmorphism, asthenic build, progressive kyphoscoliosis,
        early-onset osteoporosis, and seizures.
      explanation: Establishes progressive kyphoscoliosis as a core skeletal feature alongside
        the osteoporosis.
histopathology:
- name: Depletion of osteoblasts and osteoclasts with absent trabecular meshwork
  description: 'Iliac crest bone histopathology and morphometry in two affected brothers
    showed profound depletion of BOTH osteoblasts and osteoclasts, absence of a trabecular
    meshwork, low bone volume and a thin cortex. The simultaneous loss of the resorptive
    cell population is the finding that distinguishes SRS osteoporosis from resorption-driven
    osteoporosis and underpins this entry''s decision not to declare conformance to the
    increased-osteoclastic-resorption node of the osteoporosis module.

    '
  diagnostic: false
  evidence:
  - reference: PMID:25888122
    reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Bone histopathology and morphometry identified a profound depletion of osteoblasts
      and osteoclasts, absence of a trabecular meshwork, a low bone volume and a thin cortex.
    explanation: Direct human bone histopathology documenting the cellular and architectural
      basis of the SRS skeletal phenotype.
phenotypes:
- name: Intellectual disability
  category: Neurologic
  description: 'Developmental delay presenting as failure to meet early milestones
    then evolving to mild-to-profound intellectual disability, which appears to remain
    stable over time.

    '
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  frequency: OBLIGATE
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Developmental delay usually presents as failure to meet early developmental
      milestones and then evolves to mild-to-profound intellectual disability (which
      appears to remain stable over time) and variable motor disability.
    explanation: GeneReviews documents the developmental trajectory from early delay to
      established intellectual disability in affected males. The OBLIGATE band rests on the
      definitional argument in the second evidence item, not on a quantitative claim here.
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Snyder-Robinson syndrome (SRS) is an X-linked intellectual disability syndrome
      characterized by facial dysmorphism, asthenic build, progressive kyphoscoliosis,
      early-onset osteoporosis, and seizures.
    explanation: The disorder is defined as an intellectual disability syndrome, so the
      phenotype is definitional rather than variable.
- name: Global developmental delay
  category: Neurologic
  description: Failure to meet early developmental milestones precedes the established
    intellectual disability.
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: PMID:31580924
    reference_title: "The complete loss of function of the SMS gene results in a severe form of Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The syndrome is characterized by facial dysmorphism, thin body build, kyphoscoliosis,
      osteoporosis, hypotonia, developmental delay and associated neurological features
      (seizures, unsteady gait, abnormal speech).
    explanation: Lists developmental delay among the characteristic features of the syndrome.
- name: Hypotonia
  category: Neurologic
  description: Generalized muscular hypotonia with low muscle mass, apparent from the
    first year of life.
  phenotype_term:
    preferred_term: Hypotonia
    term:
      id: HP:0001252
      label: Hypotonia
  evidence:
  - reference: PMID:14508504
    reference_title: "X-linked spermine synthase gene (SMS) defect: the first polyamine deficiency syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The affected males have mild-to-moderate mental retardation (MR), hypotonia,
      cerebellar circuitry dysfunction, facial asymmetry, thin habitus, osteoporosis,
      kyphoscoliosis, decreased activity of SMS, correspondingly low levels of intracellular
      spermine in lymphocytes and fibroblasts, and elevated spermidine/spermine ratios.
    explanation: The original clinical description lists hypotonia among the core features
      of affected males.
  - reference: PMID:31580924
    reference_title: "The complete loss of function of the SMS gene results in a severe form of Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The syndrome is characterized by facial dysmorphism, thin body build, kyphoscoliosis,
      osteoporosis, hypotonia, developmental delay and associated neurological features
      (seizures, unsteady gait, abnormal speech).
    explanation: Confirms hypotonia as a characteristic feature of the syndrome.
- name: Decreased muscle mass
  category: Musculoskeletal
  description: Low muscle mass develops during the first year and contributes to the
    asthenic habitus; it also confounds creatinine-based assessment of kidney function.
  phenotype_term:
    preferred_term: Decreased muscle mass
    term:
      id: HP:0003199
      label: Decreased muscle mass
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Asthenic habitus and low muscle mass usually develop during the first year.
    explanation: GeneReviews documents low muscle mass and dates its onset to the first
      year of life.
- name: Slender build
  category: Musculoskeletal
  description: Asthenic, thin body habitus, one of the recognisable features of the
    syndrome.
  phenotype_term:
    preferred_term: Asthenic habitus
    term:
      id: HP:0001533
      label: Slender build
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Asthenic habitus and low muscle mass usually develop during the first year.
    explanation: GeneReviews documents the asthenic habitus; the HPO term Slender build
      is the closest available match, so the preferred_term retains the clinical wording.
  - reference: PMID:30237987
    reference_title: "Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: this syndrome is characterized by an asthenic body habitus, facial dysmorphism,
      broad-based gait, and osteoporosis with frequent fractures.
    explanation: Confirms the asthenic body habitus as a characteristic feature.
- name: Osteoporosis
  category: Musculoskeletal
  description: 'Early-onset osteoporosis developing during the first decade, arising
    from failure of osteoblastic mineralisation rather than excess resorption. SRS
    is a differential diagnosis of osteogenesis imperfecta.

    '
  phenotype_term:
    preferred_term: Osteoporosis
    term:
      id: HP:0000939
      label: Osteoporosis
  frequency: VERY_FREQUENT
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: During the first decade, males with SRS develop osteoporosis, resulting in
      fractures in the absence of trauma.
    explanation: GeneReviews states that males with SRS develop osteoporosis during the
      first decade, an unqualified statement about the affected population that maps to
      the very frequent band.
  - reference: PMID:25888122
    reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: the low bone density arises from a failure of mineralization
    explanation: Characterises the osteoporosis of SRS as a mineralisation failure.
- name: Reduced bone mineral density
  category: Musculoskeletal
  description: Reduced bone mineral density on DXA, monitored as the quantitative index
    of the skeletal phenotype and the target of calcium and antiresorptive/anabolic
    therapy.
  phenotype_term:
    preferred_term: Reduced bone mineral density
    term:
      id: HP:0004349
      label: Reduced bone mineral density
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: calcium supplementation has slightly improved bone mineral density in a few
      individuals
    explanation: GeneReviews treats bone mineral density as the measured skeletal parameter
      in SRS and reports a modest response to calcium supplementation.
  - reference: PMID:38463005
    reference_title: "Impaired polyamine metabolism causes behavioral and neuroanatomical defects in a mouse model of Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: The lack of SMS protein in the G56S mice resulted in increased spermidine/spermine
      ratio, failure to thrive, short stature and reduced bone density.
    explanation: The SRS mouse model reproduces the reduced bone density, supporting it
      as a direct consequence of SMS loss.
- name: Recurrent fractures
  category: Musculoskeletal
  description: Fractures occurring in the absence of trauma, a consequence of the early-onset
    osteoporosis.
  phenotype_term:
    preferred_term: Recurrent fractures
    term:
      id: HP:0002757
      label: Recurrent fractures
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: During the first decade, males with SRS develop osteoporosis, resulting in
      fractures in the absence of trauma.
    explanation: GeneReviews documents atraumatic fractures as the clinical consequence
      of the osteoporosis.
  - reference: PMID:30237987
    reference_title: "Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: this syndrome is characterized by an asthenic body habitus, facial dysmorphism,
      broad-based gait, and osteoporosis with frequent fractures.
    explanation: Confirms frequent fractures as a characteristic feature.
- name: Kyphoscoliosis
  category: Musculoskeletal
  description: Progressive kyphoscoliosis, one of the defining skeletal features, requiring
    standard orthopaedic management.
  phenotype_term:
    preferred_term: Kyphoscoliosis
    term:
      id: HP:0002751
      label: Kyphoscoliosis
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Snyder-Robinson syndrome (SRS) is an X-linked intellectual disability syndrome
      characterized by facial dysmorphism, asthenic build, progressive kyphoscoliosis,
      early-onset osteoporosis, and seizures.
    explanation: GeneReviews lists progressive kyphoscoliosis among the defining features,
      supporting both the phenotype and the progressive clinical course.
  - reference: PMID:14508504
    reference_title: "X-linked spermine synthase gene (SMS) defect: the first polyamine deficiency syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The affected males have mild-to-moderate mental retardation (MR), hypotonia,
      cerebellar circuitry dysfunction, facial asymmetry, thin habitus, osteoporosis,
      kyphoscoliosis, decreased activity of SMS, correspondingly low levels of intracellular
      spermine in lymphocytes and fibroblasts, and elevated spermidine/spermine ratios.
    explanation: The original description lists kyphoscoliosis among the features of affected
      males.
- name: Seizure
  category: Neurologic
  description: Seizures with onset typically in early childhood; status epilepticus
    has been a cause of death in a reported case.
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  frequency: FREQUENT
  evidence:
  - reference: PMID:25888122
    reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Cognitive impairment 15/15 + + Seizures 8/15 + +
    explanation: Consecutive rows of Table 1 ("Comparison of clinical features of all reported
      SRS patients"). The "Reported patients" column records seizures in 8 of the 15 SRS
      patients reported at that time; 8/15 = 53%, which falls in the FREQUENT band (30-79%).
      The two trailing "+" marks are the table's separate columns for the report's own
      propositi II-1 and II-3, so counting them gives 10/17 = 59% and leaves the band unchanged.
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Seizure onset varies but typically occurs in early childhood.
    explanation: GeneReviews documents seizures as a feature of the syndrome with typical
      early-childhood onset.
  - reference: PMID:30237987
    reference_title: "Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: We report here a pediatric autopsy of a 4 year old male with a history of
      intellectual disability, gait abnormalities, multiple fractures, and seizures previously
      diagnosed with Snyder-Robinson syndrome
    explanation: An autopsy case report documents seizures in a genetically confirmed patient.
- name: Facial asymmetry
  category: Craniofacial
  description: Asymmetric facies, part of the facial dysmorphism recognised in the
    syndrome.
  phenotype_term:
    preferred_term: Facial asymmetry
    term:
      id: HP:0000324
      label: Facial asymmetry
  evidence:
  - reference: PMID:14508504
    reference_title: "X-linked spermine synthase gene (SMS) defect: the first polyamine deficiency syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The affected males have mild-to-moderate mental retardation (MR), hypotonia,
      cerebellar circuitry dysfunction, facial asymmetry, thin habitus, osteoporosis,
      kyphoscoliosis, decreased activity of SMS, correspondingly low levels of intracellular
      spermine in lymphocytes and fibroblasts, and elevated spermidine/spermine ratios.
    explanation: The original description lists facial asymmetry among the core features.
  - reference: PMID:30544565
    reference_title: "Polyamine Homeostasis in Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: the imbalance that occurs in SRS results in a combination of clinical manifestations
      including moderate-to-severe cognitive impairment, osteoporosis, asthenic build,
      low muscle mass, facial asymmetry, speech abnormalities, and seizures
    explanation: Confirms facial asymmetry within the recognised clinical spectrum of SRS.
- name: Unsteady gait
  category: Neurologic
  description: Unsteady, often broad-based gait, historically attributed to cerebellar
    circuitry dysfunction.
  phenotype_term:
    preferred_term: Unsteady gait
    term:
      id: HP:0002317
      label: Unsteady gait
  evidence:
  - reference: PMID:31580924
    reference_title: "The complete loss of function of the SMS gene results in a severe form of Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The syndrome is characterized by facial dysmorphism, thin body build, kyphoscoliosis,
      osteoporosis, hypotonia, developmental delay and associated neurological features
      (seizures, unsteady gait, abnormal speech).
    explanation: Lists unsteady gait among the characteristic neurological features.
- name: Broad-based gait
  category: Neurologic
  description: Broad-based gait, described as a characteristic feature of the syndrome.
  phenotype_term:
    preferred_term: Broad-based gait
    term:
      id: HP:0002136
      label: Broad-based gait
  evidence:
  - reference: PMID:30237987
    reference_title: "Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: this syndrome is characterized by an asthenic body habitus, facial dysmorphism,
      broad-based gait, and osteoporosis with frequent fractures.
    explanation: Explicitly lists broad-based gait as a characteristic feature of SRS.
- name: Speech abnormality
  category: Neurologic
  description: 'Speech is abnormal in essentially every reported affected male, ranging
    from nasal and dysarthric speech at the milder end to absent vocalisation in the most
    severely affected. This is the primary, best-quantified speech claim; the hypernasal
    quality below is the specific sub-phenotype.

    '
  phenotype_term:
    preferred_term: Speech abnormality
    term:
      id: HP:0002167
      label: Abnormal speech pattern
  frequency: VERY_FREQUENT
  evidence:
  - reference: PMID:25888122
    reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Speech abnormalities 15/15 + ** + ** Diminished body bulk 15/15 + +
    explanation: Consecutive rows of Table 1 ("Comparison of clinical features of all reported
      SRS patients"); the first records speech abnormalities in 15 of 15 reported patients. 15/15 = 100%, which
      maps by default to OBLIGATE; the band is deliberately set one tier lower at
      VERY_FREQUENT (80-99%) because a 15-patient literature tabulation is too small a
      denominator to assert complete penetrance across the disorder, per the "departing
      from the default mapping" provision of docs/frequency-evidence-guidelines.md.
  - reference: PMID:25888122
    reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The clinical features of SRS include intellectual disability, dysmorphic facies,
      speech and gait abnormalities, seizures
    explanation: Speech abnormality is listed among the defining clinical features of SRS
      in the same paper's introduction.
- name: Hypernasal speech
  category: Neurologic
  description: Abnormal, characteristically nasal and dysarthric speech; the specific
    speech quality within the broader speech abnormality above.
  phenotype_term:
    preferred_term: Nasal, dysarthric speech
    term:
      id: HP:0001611
      label: Hypernasal speech
  evidence:
  - reference: PMID:31580924
    reference_title: "The complete loss of function of the SMS gene results in a severe form of Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The syndrome is characterized by facial dysmorphism, thin body build, kyphoscoliosis,
      osteoporosis, hypotonia, developmental delay and associated neurological features
      (seizures, unsteady gait, abnormal speech).
    explanation: The source documents abnormal speech as a characteristic feature but does
      not specify hypernasality, so support for this specific HPO term is partial.
- name: Cleft palate
  category: Craniofacial
  description: Cleft palate occurs in a subset of affected males and requires craniofacial
    surgical management.
  phenotype_term:
    preferred_term: Cleft palate
    term:
      id: HP:0000175
      label: Cleft palate
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: standard surgical treatment by craniofacial team for those with cleft palate
    explanation: GeneReviews management guidance presupposes cleft palate in a subset of
      affected individuals; because this is inferred from a management recommendation rather
      than a stated clinical finding, support is partial.
- name: Nephrocalcinosis
  category: Renal
  description: Nephrocalcinosis is among the nonspecific kidney manifestations reported
    in SRS and is a surveillance target, particularly during calcium supplementation.
  phenotype_term:
    preferred_term: Nephrocalcinosis
    term:
      id: HP:0000121
      label: Nephrocalcinosis
  frequency: VERY_RARE
  evidence:
  - reference: PMID:25888122
    reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: the propositi manifested retinal pigmentary changes, recurrent episodes of
      hyper- and hypoglycemia, nephrocalcinosis, renal cysts, and frequent respiratory
      infections.
    explanation: Documents nephrocalcinosis in two affected brothers as an extension of
      the recognised phenotype.
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Rare findings may include nonspecific kidney manifestations.
    explanation: GeneReviews classifies kidney manifestations as rare findings, supporting
      the very rare frequency band.
- name: Renal cyst
  category: Renal
  description: Renal cysts are reported among the nonspecific kidney manifestations
    and are a nephrological surveillance target.
  phenotype_term:
    preferred_term: Renal cyst
    term:
      id: HP:0000107
      label: Renal cyst
  frequency: VERY_RARE
  evidence:
  - reference: PMID:25888122
    reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: the propositi manifested retinal pigmentary changes, recurrent episodes of
      hyper- and hypoglycemia, nephrocalcinosis, renal cysts, and frequent respiratory
      infections.
    explanation: Documents renal cysts in two affected brothers.
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Rare findings may include nonspecific kidney manifestations.
    explanation: GeneReviews classifies kidney manifestations as rare findings, supporting
      the very rare frequency band.
- name: Abnormal retinal pigmentation
  category: Ophthalmologic
  description: 'Retinal pigmentary change was documented in both propositi of the NIH
    series, one of whom had frank retinitis pigmentosa on ophthalmologic examination. It
    had not been recorded in any previously reported SRS patient, so it sits at the
    uncommon end of the phenotype. This is also the human counterpart of the retinal degeneration
    seen in the Drosophila dSms model, which the pathograph otherwise leaves without a
    human endpoint.

    '
  phenotype_term:
    preferred_term: Retinal pigmentary changes
    term:
      id: HP:0007703
      label: Abnormal retinal pigmentation
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:25888122
    reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: the propositi manifested retinal pigmentary changes, recurrent episodes of
      hyper- and hypoglycemia, nephrocalcinosis, renal cysts, and frequent respiratory
      infections.
    explanation: Documents retinal pigmentary changes in two affected brothers as an
      extension of the recognised phenotype. Support is partial because the finding is
      reported in a single sibship rather than established across the disorder.
  - reference: PMID:25888122
    reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Retinal pigment changes 0/13 + + Hypo-/Hyper-glycemia 1/13 + +
    explanation: Consecutive rows of Table 1 ("Comparison of clinical features of all reported
      SRS patients"). The retinal row reads "0/13 + +" - retinal pigment change had not been
      reported in any of the 13 previously assessed patients, but the two trailing "+" marks
      are the table's separate columns for this report's own propositi II-1 and II-3, both of
      whom are positive. The count over every patient the table tabulates is therefore
      2/15 = 13.3%, which falls in the OCCASIONAL band (5-29%), not VERY_RARE.
- name: Recurrent respiratory infections
  category: Immunologic
  description: 'Frequent respiratory infections were documented in both propositi of the
    NIH series. Like the retinal and renal findings from the same report, they had not
    been recorded in previously reported patients.

    '
  phenotype_term:
    preferred_term: Frequent respiratory infections
    term:
      id: HP:0002205
      label: Recurrent respiratory infections
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:25888122
    reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: the propositi manifested retinal pigmentary changes, recurrent episodes of
      hyper- and hypoglycemia, nephrocalcinosis, renal cysts, and frequent respiratory
      infections.
    explanation: Documents frequent respiratory infections in two affected brothers.
      Support is partial because the finding is reported in a single sibship rather than
      established across the disorder.
  - reference: PMID:25888122
    reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Frequent infections 0/13 + + Retinal pigment changes 0/13 + +
    explanation: Consecutive rows of Table 1 ("Comparison of clinical features of all reported
      SRS patients"). The infections row reads "0/13 + +" - frequent infections had not been
      reported in any of the 13 previously assessed patients, but the two trailing "+" marks
      are the table's separate columns for this report's own propositi II-1 and II-3, both of
      whom are positive. The count over every patient the table tabulates is therefore
      2/15 = 13.3%, which falls in the OCCASIONAL band (5-29%), not VERY_RARE.
- name: Flexion contracture
  category: Musculoskeletal
  description: 'Joint contractures are part of the musculoskeletal burden of SRS and are
    named by GeneReviews as requiring ongoing orthopaedic management alongside
    kyphoscoliosis. In the severe end of the spectrum they involve most large and small
    joints.

    '
  phenotype_term:
    preferred_term: Joint contractures
    term:
      id: HP:0001371
      label: Flexion contracture
  evidence:
  - reference: PMID:25888122
    reference_title: "Impaired osteoblast and osteoclast function characterize the osteoporosis of Snyder - Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: kyphoscoliosis and flexion contractures of most large and small joints
    explanation: Direct clinical documentation of flexion contractures involving most large
      and small joints in a genetically confirmed affected male.
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: standard management of kyphoscoliosis and contractures by orthopedics
    explanation: GeneReviews management guidance presupposes contractures as an ongoing
      orthopaedic problem in affected individuals; because this is inferred from a
      management recommendation rather than a stated clinical finding, support is partial.
- name: Hearing impairment
  category: Audiologic
  description: 'Hearing loss is reported in severe SRS. The spermine synthase-deficient
    Gy mouse is profoundly hearing impaired through loss of the endocochlear potential,
    providing a mechanistic counterpart.

    '
  phenotype_term:
    preferred_term: Hearing impairment
    term:
      id: HP:0000365
      label: Hearing impairment
  evidence:
  - reference: PMID:33186760
    reference_title: "Digestive involvement in a severe form of Snyder-Robinson syndrome: Possible expansion of the phenotype."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Here we describe two maternal half-brothers who both presented with severe
      neurodevelopmental delay, seizures, hearing loss, facial dysmorphism, renal and
      ophthalmologic anomalies, failure to thrive and premature death.
    explanation: Documents hearing loss in two genetically confirmed brothers with severe
      SRS.
  - reference: PMID:19001365
    reference_title: "Spermine synthase deficiency leads to deafness and a profound sensitivity to alpha-difluoromethylornithine."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Male gyro (Gy) mice, which have an X chromosomal deletion inactivating the
      SpmS and Phex genes, were found to be profoundly hearing impaired.
    explanation: Model-organism support for hearing impairment as a consequence of spermine
      synthase deficiency; the deletion also removes Phex, so this is corroborative rather
      than definitive.
- name: Jejunal stenosis
  category: Gastrointestinal
  description: 'Jejunal stenosis with enteral feeding intolerance was reported in one of
    two brothers with a severe SRS allele, proposed by the authors as a possible expansion
    of the phenotype. Reported in a single individual, so this is a candidate rather than
    an established feature.

    '
  phenotype_term:
    preferred_term: Jejunal stenosis
    term:
      id: HP:0012848
      label: Small intestinal stenosis
  frequency: VERY_RARE
  evidence:
  - reference: PMID:33186760
    reference_title: "Digestive involvement in a severe form of Snyder-Robinson syndrome: Possible expansion of the phenotype."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: One of the brothers presented with gastrointestinal symptoms, with jejunal
      stenosis, enteral feeding intolerance, failure to thrive due to a dysfunctional gastrointestinal
      system, cholestasis and exocrine pancreatic insufficiency.
    explanation: A single genetically confirmed patient with jejunal stenosis; the authors
      themselves frame the digestive involvement as a possible phenotype expansion, so support
      is partial and the frequency band is the lowest available.
- name: Exocrine pancreatic insufficiency
  category: Gastrointestinal
  description: 'Exocrine pancreatic insufficiency and cholestasis accompanied the digestive
    involvement described in a severe SRS case, again in a single individual.

    '
  phenotype_term:
    preferred_term: Exocrine pancreatic insufficiency
    term:
      id: HP:0001738
      label: Exocrine pancreatic insufficiency
  frequency: VERY_RARE
  evidence:
  - reference: PMID:33186760
    reference_title: "Digestive involvement in a severe form of Snyder-Robinson syndrome: Possible expansion of the phenotype."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: One of the brothers presented with gastrointestinal symptoms, with jejunal
      stenosis, enteral feeding intolerance, failure to thrive due to a dysfunctional gastrointestinal
      system, cholestasis and exocrine pancreatic insufficiency.
    explanation: Single-patient evidence for exocrine pancreatic insufficiency within the
      proposed digestive expansion of the SRS phenotype; support is partial.
- name: Failure to thrive
  category: Growth
  description: Failure to thrive is described in severe SRS and is reproduced in the
    G56S mouse model.
  phenotype_term:
    preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  evidence:
  - reference: PMID:33186760
    reference_title: "Digestive involvement in a severe form of Snyder-Robinson syndrome: Possible expansion of the phenotype."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Here we describe two maternal half-brothers who both presented with severe
      neurodevelopmental delay, seizures, hearing loss, facial dysmorphism, renal and
      ophthalmologic anomalies, failure to thrive and premature death.
    explanation: Documents failure to thrive in two genetically confirmed brothers with
      severe SRS.
  - reference: PMID:38463005
    reference_title: "Impaired polyamine metabolism causes behavioral and neuroanatomical defects in a mouse model of Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: The lack of SMS protein in the G56S mice resulted in increased spermidine/spermine
      ratio, failure to thrive, short stature and reduced bone density.
    explanation: The SRS mouse model reproduces failure to thrive.
biochemical:
- name: Elevated spermidine to spermine ratio
  notes: 'The spermidine:spermine ratio measured in cultured fibroblasts, lymphocytes
    or lymphoblasts is the biochemical hallmark of SRS and is used to confirm the diagnosis
    alongside molecular testing. The magnitude of the elevation broadly correlates
    with symptom severity.

    '
  biomarker_term:
    preferred_term: spermidine
    term:
      id: CHEBI:16610
      label: spermidine
    modifier: INCREASED
  presence: PRESENT
  specificity: Highly specific for spermine synthase deficiency; no other human disorder
    is known to produce this pattern.
  evidence:
  - reference: PMID:37702369
    reference_title: "Difluoromethylornithine rebalances aberrant polyamine ratios in Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: 'Reduced SMS activity causes spermidine accumulation while spermine levels
      are reduced. The resulting exaggerated spermidine:spermine ratio is a biochemical
      hallmark of SRS that tends to correlate with symptom severity.'
    explanation: Establishes the ratio as the biochemical hallmark and its correlation
      with severity.
  - reference: PMID:33186760
    reference_title: "Digestive involvement in a severe form of Snyder-Robinson syndrome: Possible expansion of the phenotype."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: A novel p.(Gly203Asp) variant was found at the hemizygous state in the two
      boys, and an elevated Spermidine/Spermine ratio confirmed the diagnosis of Snyder-Robinson
      syndrome.
    explanation: Demonstrates diagnostic use of the elevated ratio in confirmed patients.
- name: Elevated plasma N8-acetylspermidine
  biomarker_term:
    preferred_term: N(8)-acetylspermidine
    term:
      id: CHEBI:27911
      label: N(8)-acetylspermidine
    modifier: INCREASED
  presence: PRESENT
  notes: 'Untargeted plasma metabolomics in twin boys with epileptic encephalopathy
    identified markedly elevated N(8)-acetylspermidine, verified in a third genetically
    confirmed patient, as a candidate plasma biomarker for SRS accessible without a
    cell culture assay.

    '
  evidence:
  - reference: PMID:26174906
    reference_title: "N(8)-acetylspermidine as a potential plasma biomarker for Snyder-Robinson syndrome identified by clinical metabolomics."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Untargeted plasma metabolome analysis revealed significantly elevated levels
      of N(8)-acetylspermidine, a precursor derivative of spermine biosynthesis, as a
      potential novel plasma biomarker for SRS.
    explanation: Reports the elevated plasma analyte and its proposed biomarker role in
      SRS.
genetic:
- name: SMS
  gene_term:
    preferred_term: SMS
    term:
      id: hgnc:11123
      label: SMS
  association: CAUSAL
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  notes: 'SMS at Xp22.11 encodes spermine synthase. Hemizygous loss-of-function
    variants cause Snyder-Robinson syndrome. Nearly all reported pathogenic variants
    are hypomorphic missense changes retaining partial activity; a single complete
    loss-of-function frameshift allele (Met303Lysfs*) produced a severe phenotype with
    multiple malformations and death at four months, supporting a genotype-severity
    correlation. Several missense variants outside the active site act by destabilising
    the protein or abolishing obligate homodimer formation.

    '
  evidence:
  - reference: PMID:41410504
    reference_title: "Genetic and Phenotypic Features of the Five Known Polyaminopathies: A Critical Narrative Review."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Snyder-Robinson syndrome results from hemizygous loss-of-function variants
      in the spermine synthase (SMS) gene, resulting in decreased or complete loss of
      spermine synthase enzyme activity.
    explanation: Establishes the gene-disease relationship and the loss-of-function mechanism.
  - reference: PMID:31580924
    reference_title: "The complete loss of function of the SMS gene results in a severe form of Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Here we describe the first complete LoF variant, Met303Lysfs*, in a male patient
      with a severe form of Snyder-Robinson syndrome.
    explanation: Documents the single reported complete loss-of-function allele and its
      severe phenotype, underpinning the genotype-severity correlation.
  - reference: PMID:26761001
    reference_title: "Revealing the Effects of Missense Mutations Causing Snyder-Robinson Syndrome on the Stability and Dimerization of Spermine Synthase."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: In silico modeling predicted that all studied mutations in this work destabilize
      SpmSyn and some of them abolish homo-dimer formation.
    explanation: Supports protein destabilisation and loss of dimerisation as the mechanism
      for non-active-site missense variants.
diagnosis:
- name: Molecular genetic testing of SMS
  description: 'The diagnosis is established in a male proband by identification of
    a hemizygous loss-of-function SMS pathogenic variant. Biochemical confirmation
    uses the elevated spermidine:spermine ratio in cultured cells; elevated plasma
    N(8)-acetylspermidine has been proposed as a less invasive supporting marker.

    '
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: The diagnosis of SRS is established in a male proband with a hemizygous loss-of-function
      SMS pathogenic variant identified by molecular genetic testing.
    explanation: GeneReviews states the diagnostic criterion for SRS.
differential_diagnoses:
- name: Osteogenesis imperfecta
  description: 'SRS presents with early-onset osteoporosis and recurrent low-trauma
    fractures and has been misdiagnosed as osteogenesis imperfecta for years before
    molecular testing established the correct diagnosis.

    '
  evidence:
  - reference: PMID:34741636
    reference_title: "Snyder-Robinson syndrome: differential diagnosis of osteogenesis imperfecta."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: We report a novel case of Snyder-Robinson syndrome, caused by a de novo mutation
      and first misdiagnosed with osteogenesis imperfecta.
    explanation: Documents a case initially misdiagnosed as osteogenesis imperfecta, establishing
      the differential.
treatments:
- name: Developmental and educational support
  description: Developmental and educational support is the mainstay of management
    for the intellectual disability and motor disability of SRS.
  action_category: THERAPEUTIC
  treatment_term:
    preferred_term: developmental and educational support
    term:
      id: NCIT:C15747
      label: Supportive Care
  target_phenotypes:
  - preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  - preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: 'Treatment of manifestations: Developmental and educational support; treatment
      of seizures per neurologist'
    explanation: GeneReviews lists developmental and educational support as first-line
      management of manifestations.
- name: Anticonvulsant therapy
  description: 'Seizures are treated per neurologist with standard anticonvulsants.
    GeneReviews advises weighing risk against benefit for medications associated with
    increased osteoporosis, explicitly including anticonvulsants, because SRS already
    carries an early-onset osteoporosis burden.

    '
  action_category: THERAPEUTIC
  treatment_term:
    preferred_term: anticonvulsant agent therapy
    term:
      id: NCIT:C64172
      label: Anticonvulsant Therapy
  target_phenotypes:
  - preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: 'Treatment of manifestations: Developmental and educational support; treatment
      of seizures per neurologist'
    explanation: GeneReviews directs seizure treatment to a neurologist as part of standard
      management.
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Assess the risk vs benefit of medications associated with increased osteoporosis
      (e.g., anticonvulsants), particularly when alternative treatments are limited.
    explanation: GeneReviews Agents/Circumstances to Avoid guidance flags the bone-density
      cost of anticonvulsants in this population.
- name: Calcium supplementation
  description: 'Calcium supplementation has slightly improved bone mineral density
    in a few individuals. Because of the risk of ectopic calcification, individuals
    receiving calcium should be evaluated regularly by an endocrinologist.

    '
  action_category: THERAPEUTIC
  treatment_term:
    preferred_term: calcium supplementation
    term:
      id: NCIT:C15433
      label: Nutritional Support
    therapeutic_agent:
    - preferred_term: calcium supplement
      term:
        id: CHEBI:29108
        label: calcium(2+)
  target_phenotypes:
  - preferred_term: Reduced bone mineral density
    term:
      id: HP:0004349
      label: Reduced bone mineral density
  - preferred_term: Osteoporosis
    term:
      id: HP:0000939
      label: Osteoporosis
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: calcium supplementation has slightly improved bone mineral density in a few
      individuals
    explanation: GeneReviews reports only a slight benefit in a few individuals, so the
      support for efficacy is partial.
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: while receiving calcium supplementation, individuals should be evaluated regularly
      for ectopic calcification by endocrinologist
    explanation: GeneReviews attaches an explicit safety-monitoring requirement to calcium
      supplementation in SRS.
- name: Osteoporosis pharmacotherapy
  description: 'A reported adult was treated with bisphosphonates for a decade until
    an atypical femoral fracture, then with teriparatide for two years, then denosumab
    six-monthly, with improvement in bone density and no further fractures. This is
    single-case evidence and no controlled trial exists in SRS.

    '
  action_category: THERAPEUTIC
  treatment_term:
    preferred_term: bisphosphonate, teriparatide and denosumab sequence
    term:
      id: NCIT:C198585
      label: Bisphosphonate Therapy
    therapeutic_agent:
    - preferred_term: teriparatide
      term:
        id: NCIT:C61966
        label: Teriparatide
    - preferred_term: denosumab
      term:
        id: NCIT:C61313
        label: Denosumab
  target_phenotypes:
  - preferred_term: Osteoporosis
    term:
      id: HP:0000939
      label: Osteoporosis
  - preferred_term: Recurrent fractures
    term:
      id: HP:0002757
      label: Recurrent fractures
  - preferred_term: Reduced bone mineral density
    term:
      id: HP:0004349
      label: Reduced bone mineral density
  evidence:
  - reference: PMID:34741636
    reference_title: "Snyder-Robinson syndrome: differential diagnosis of osteogenesis imperfecta."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The patient was treated with bisphosphonates for a decade, until developing
      an atypical femoral fracture. Teriparatide was then administered for 2 years and
      then changed to denosumab every 6 months, improving his bone density mass and preventing
      further fractures.
    explanation: A single case report describing benefit from an anabolic-then-antiresorptive
      sequence after bisphosphonate failure; support is partial because this is n-of-1
      evidence.
- name: Orthopaedic management of kyphoscoliosis and contractures
  description: Standard orthopaedic management of kyphoscoliosis and contractures,
    with clinical examination for kyphoscoliosis and assessment of mobility at each
    visit.
  action_category: THERAPEUTIC
  treatment_term:
    preferred_term: orthopaedic management
    term:
      id: NCIT:C49236
      label: Therapeutic Procedure
  target_phenotypes:
  - preferred_term: Kyphoscoliosis
    term:
      id: HP:0002751
      label: Kyphoscoliosis
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: standard management of kyphoscoliosis and contractures by orthopedics
    explanation: GeneReviews specifies standard orthopaedic management for the spinal deformity
      and contractures.
- name: Skeletal surveillance
  description: 'Clinical examination and DXA scanning to track progression of osteoporosis,
    with imaging for fracture when clinically indicated, plus examination for kyphoscoliosis
    and assessment of mobility at each visit.

    '
  action_category: MONITORING
  treatment_term:
    preferred_term: clinical examination and DXA surveillance
    term:
      id: NCIT:C124351
      label: Clinical Evaluation
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: clinical examination and DXA scans to evaluate for progression of osteoporosis
      and investigate for factures if medically indicated
    explanation: GeneReviews specifies the skeletal surveillance protocol for SRS.
- name: Renal surveillance
  description: 'Monitoring for nephrocalcinosis, renal cysts and kidney function by a
    nephrologist. GeneReviews notes that creatinine must be interpreted in the context
    of the low muscle mass of SRS, which otherwise makes creatinine-based estimates of
    kidney function falsely reassuring.

    '
  action_category: MONITORING
  treatment_term:
    preferred_term: nephrological surveillance
    term:
      id: NCIT:C124351
      label: Clinical Evaluation
  evidence:
  - reference: PMID:23805436
    reference_title: "Snyder-Robinson Syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: monitor for nephrocalcinosis and renal cysts and kidney function per nephrologist,
      considering creatine levels in the context of low muscle mass
    explanation: GeneReviews specifies renal surveillance and flags the creatinine confound
      created by the low muscle mass of the syndrome.
- name: Difluoromethylornithine (DFMO, eflornithine) — investigational
  description: 'DFMO, an FDA-approved ornithine decarboxylase inhibitor, rebalances
    the spermidine:spermine ratio in SRS patient cells by reducing spermidine biosynthesis,
    stimulating residual conversion of spermidine to spermine in hypomorphic cells,
    and inducing uptake of exogenous spermine. It extends lifespan in the Drosophila
    SRS model and rescues the mineralisation defect of polyamine-loaded human stromal
    cells. This is preclinical only; DFMO is profoundly toxic in the spermine synthase-deficient
    Gy mouse, which suffers catastrophic loss of motor function and death within days,
    so translation requires caution.

    '
  action_category: THERAPEUTIC
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: eflornithine (DFMO)
      term:
        id: CHEBI:41948
        label: eflornithine
  target_mechanisms:
  - target: Elevated Spermidine to Spermine Ratio
    treatment_effect: INHIBITS
    description: DFMO inhibits ornithine decarboxylase upstream of the block, lowering
      spermidine synthesis and so correcting the exaggerated spermidine:spermine ratio
      rather than replacing the missing enzyme.
    evidence:
    - reference: PMID:37702369
      reference_title: "Difluoromethylornithine rebalances aberrant polyamine ratios in Snyder-Robinson syndrome."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: 'Here we report the repurposing of 2-difluoromethylornithine (DFMO), an
        FDA-approved inhibitor of polyamine biosynthesis, in rebalancing spermidine:spermine
        ratios in SRS patient cells.'
      explanation: Directly demonstrates that DFMO acts on the elevated spermidine-to-spermine
        ratio node in patient-derived cells.
  evidence:
  - reference: PMID:37702369
    reference_title: "Difluoromethylornithine rebalances aberrant polyamine ratios in Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: 'Here we report the repurposing of 2-difluoromethylornithine (DFMO), an FDA-approved
      inhibitor of polyamine biosynthesis, in rebalancing spermidine:spermine ratios in
      SRS patient cells.'
    explanation: Demonstrates correction of the defining biochemical abnormality in patient-derived
      cells.
  - reference: PMID:38473716
    reference_title: "Effects of Spermine Synthase Deficiency in Mesenchymal Stromal Cells Are Rescued by Upstream Inhibition of Ornithine Decarboxylase."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: 'inhibition of polyamine synthesis with DFMO rescues most, but not all of these
      defects'
    explanation: Rescue of the skeletal cellular phenotype by upstream polyamine-synthesis
      inhibition is explicitly incomplete in the source, so support is partial.
  - reference: PMID:19001365
    reference_title: "Spermine synthase deficiency leads to deafness and a profound sensitivity to alpha-difluoromethylornithine."
    supports: REFUTE
    evidence_source: MODEL_ORGANISM
    snippet: Within 2-3 days of exposure to DFMO in the drinking water, the Gy mice suffered
      a catastrophic loss of motor function resulting in death within 5 days.
    explanation: Counterweight evidence, a severe toxicity of DFMO specific to spermine
      synthase deficiency in vivo, which argues against uncritical translation of the in
      vitro rescue.
- name: Phenylbutyrate — investigational
  description: 'Phenylbutyrate, an FDA-approved drug, down-regulates the rate-limiting
    spermidine catabolic enzyme SAT1, reduces production of toxic catabolic metabolites
    and preserves acetyl-CoA. It restored mitochondrial and autolysosomal function
    and extended lifespan in the Drosophila SRS model, ameliorated autolysosome dysfunction
    in SRS patient fibroblasts, and partially restored metabolic flexibility in 29
    patient lymphoblastoid lines. Preclinical only.

    '
  action_category: THERAPEUTIC
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: phenylbutyrate (PBA)
      term:
        id: CHEBI:41500
        label: 4-phenylbutyric acid
  target_mechanisms:
  - target: Excessive Spermidine Catabolism and Oxidative Stress
    treatment_effect: INHIBITS
    description: Phenylbutyrate down-regulates SAT1, the rate-limiting enzyme of the
      spermidine back-conversion route, throttling the catabolic overflow that generates
      the toxic metabolites.
    evidence:
    - reference: PMID:35801587
      reference_title: "Phenylbutyrate modulates polyamine acetylase and ameliorates Snyder-Robinson syndrome in a Drosophila model and patient cells."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: We further explored the mechanism of drug action and found that PBA downregulates
        the first and rate-limiting spermidine catabolic enzyme spermidine/spermine N1-acetyltransferase
        1 (SAT1), reduces the production of toxic metabolites, and inhibits the reduction
        of the substrate acetyl-CoA.
      explanation: Identifies SAT1 down-regulation as the mechanism by which phenylbutyrate
        acts on the excessive-spermidine-catabolism node.
  evidence:
  - reference: PMID:35801587
    reference_title: "Phenylbutyrate modulates polyamine acetylase and ameliorates Snyder-Robinson syndrome in a Drosophila model and patient cells."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: PBA treatment significantly restored the function of mitochondria and autolysosomes
      and extended life span in vivo in the Drosophila SRS model.
    explanation: Demonstrates rescue of the organelle phenotypes and of survival in the
      fly SRS model.
  - reference: PMID:35801587
    reference_title: "Phenylbutyrate modulates polyamine acetylase and ameliorates Snyder-Robinson syndrome in a Drosophila model and patient cells."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: We further explored the mechanism of drug action and found that PBA downregulates
      the first and rate-limiting spermidine catabolic enzyme spermidine/spermine N1-acetyltransferase
      1 (SAT1), reduces the production of toxic metabolites, and inhibits the reduction
      of the substrate acetyl-CoA.
    explanation: Establishes SAT1 down-regulation as the mechanism by which phenylbutyrate
      addresses the catabolic-overflow node.
  - reference: PMID:42250346
    reference_title: "Metabolic alterations in Snyder-Robinson syndrome lymphoblasts are ameliorated by phenylbutyrate treatment."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: Treatment with phenylbutyrate (PBA), previously shown to modulate polyamine
      catabolism, partially restored metabolic flexibility and normalized several impaired
      nutrient pathways.
    explanation: Independent patient-cell evidence that phenylbutyrate partially corrects
      the bioenergetic phenotype.
animal_models:
- species: Mouse
  genotype: Sms G56S knock-in
  description: 'A knock-in mouse carrying the Sms G56S missense variant lacks SMS protein
    and shows an increased spermidine/spermine ratio, failure to thrive, short stature,
    reduced bone density, impaired learning, increased anxiety, reduced mobility and
    reduced total and regional brain volumes, with impaired mitochondrial oxidative
    phosphorylation in cortex and fibroblasts. It is the current preclinical model
    for SRS therapeutic development.

    '
  evidence:
  - reference: PMID:38463005
    reference_title: "Impaired polyamine metabolism causes behavioral and neuroanatomical defects in a mouse model of Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Collectively, our study establishes the suitability of the G56S mice as a
      preclinical model for SRS and provides a set of molecular and functional outcome
      measures that can be used to evaluate therapeutic interventions for SRS.
    explanation: Establishes the G56S mouse as the validated preclinical model for SRS.
- species: Mouse
  genotype: Gyro (Gy), X-chromosomal deletion inactivating SpmS and Phex
  description: 'The Gy mouse carries an X-chromosomal deletion inactivating both SpmS
    and the neighbouring Phex gene. It is profoundly hearing impaired with near-complete
    loss of the endocochlear potential, rescued by a spermine synthase transgene, and
    is catastrophically sensitive to DFMO. Because the deletion also removes Phex,
    skeletal findings in this model are confounded by X-linked hypophosphataemia.

    '
  evidence:
  - reference: PMID:39331754
    reference_title: "Inactivation of spermine synthase in mice causes osteopenia due to reduced osteoblast activity."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Since the impact of a murine SMS deficiency has so far only been analyzed
      in Gy mice, where a large genomic deletion also includes the neighboring Phex gene,
      there is only limited knowledge about the potential role of SMS in bone cell regulation.
    explanation: States the Phex confound that limits interpretation of skeletal phenotypes
      in the Gy model, motivating the newer G56S mouse.
- species: Drosophila melanogaster
  genotype: dSms loss of function
  description: 'Loss of dSms in Drosophila recapitulates the polyamine imbalance of
    SRS and causes survival defects, synaptic and retinal degeneration, lysosomal defects
    and oxidative stress. It is the model in which both DFMO and phenylbutyrate rescue
    were first demonstrated.

    '
  evidence:
  - reference: PMID:29097652
    reference_title: "Spermine synthase deficiency causes lysosomal dysfunction and oxidative stress in models of Snyder-Robinson syndrome."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Here we show that loss of dSms in Drosophila recapitulates the pathological
      polyamine imbalance of SRS and causes survival defects and synaptic degeneration.
    explanation: Establishes the Drosophila model and the phenotypes it reproduces.
notes: 'Two mechanistic threads are deliberately recorded with restraint. First, the
  ion-channel arm rests on an explicitly hypothetical proposal in the original human
  description plus mouse cochlear physiology; no human electrophysiological study
  of Kir channel function in SRS exists. Second, the skeletal arm declares conformance
  to the osteoporosis module only at the impaired-formation and net-bone-loss nodes.
  It deliberately does NOT conform to `osteoporosis_bone_resorption#Increased Osteoclastic
  Bone Resorption`, because bone histopathology in SRS shows osteoclasts to be depleted
  rather than increased and dynamic histomorphometry in the SMS mouse attributes the
  low bone mass to a reduced bone formation rate; asserting the resorptive node would
  contradict the primary evidence.

  '

📚

References & Deep Research

References

1

PMID:23805436

Snyder-Robinson Syndrome.

No top-level findings curated for this source.

Deep Research

1

Falcon ▸

Snyder–Robinson Syndrome: Disease Characteristics Research Report

Edison Scientific Literature 23 citations 2026-07-31T01:34:46.299211

Snyder–Robinson Syndrome: Disease Characteristics Research Report

Executive summary and evidence scope

Snyder–Robinson syndrome (SRS) is an ultra-rare, X-linked recessive, multisystem neurodevelopmental and skeletal disorder caused by loss-of-function variants in SMS, which encodes spermine synthase. The defining biochemical lesion is reduced conversion of spermidine to spermine, producing low spermine, excess spermidine, and an elevated spermidine:spermine ratio. The strongest human evidence concerns developmental disability, hypotonia, seizures, thin habitus, osteoporosis, fractures, and kyphoscoliosis. Mechanistic evidence additionally implicates excessive spermidine catabolism, reactive oxygen species and aldehydes, lysosomal/autophagic failure, mitochondrial dysfunction, and defective bone mineralization. Most evidence comes from fewer than 30 historically reported individuals, patient-derived cells, and Drosophila or mouse models; therefore frequencies, natural history, prognosis, and treatment effects remain imprecise. No disease-modifying therapy has established human efficacy (dontaine2021digestiveinvolvementin pages 1-7, albert2015impairedosteoblastand pages 1-2, li2017sperminesynthasedeficiency pages 1-2, akinyele2024impairedpolyaminemetabolism pages 1-3).

The following compact curation table summarizes the principal evidence and proposed ontology mappings. Ontology terms labeled “suggested” should be checked against the current ontology release before database ingestion.

Domain	Key evidence	Suggested ontology terms/IDs	Evidence type / maturity
Disease identifiers	Snyder-Robinson syndrome is an ultra-rare X-linked syndromic intellectual disability caused by SMS deficiency; disease mappings in retrieved sources include MONDO:0010664, OMIM:309583, Orphanet:3063. Open Targets links MONDO_0010664 / Orphanet_3063 to SMS (dontaine2021digestiveinvolvementin pages 1-7, mouskou2021novelhemizygousmissense pages 1-2, OpenTargets Search: Snyder-Robinson syndrome-SMS)	Exact IDs: MONDO:0010664; OMIM:309583; Orphanet:3063. Term suggestion requiring validation: “Syndromic X-linked intellectual disability, Snyder type”	Aggregated disease resource + human clinical reports; high confidence for identifiers
Causal gene / inheritance	Causal gene is SMS (spermine synthase), X-linked recessive; reported disease-causing variants include missense and nonsense changes, with inherited, de novo, and reported maternal mosaic transmission in the broader literature; examples in retrieved evidence include p.Gln148Arg, p.Gly203Asp, p.Pro112Ala, p.Ser302Leu (dontaine2021digestiveinvolvementin pages 11-16, albert2015impairedosteoblastand pages 1-2, mouskou2021novelhemizygousmissense pages 1-2, qazi2020wholeexomesequencingidentifies pages 1-3, li2017sperminesynthasedeficiency pages 1-2)	Exact gene symbol: SMS. Term suggestions requiring validation: HP:0001417 X-linked inheritance; SO terms such as missense_variant, nonsense_variant, splice_region_variant	Human genetic evidence; high confidence for SMS causality, moderate for complete variant spectrum
Core biochemical mechanism	SMS catalyzes spermidine → spermine; loss of function lowers spermine and raises spermidine, producing an increased spermidine/spermine ratio, the core biochemical hallmark of SRS (dontaine2021digestiveinvolvementin pages 1-7, qazi2020wholeexomesequencingidentifies pages 1-3, li2017sperminesynthasedeficiency pages 1-2, akinyele2024impairedpolyaminemetabolism pages 1-3)	Exact/near-exact suggestions: GO:0006598 polyamine biosynthetic process; CHEBI:15746 spermidine; CHEBI:15729 spermine. Term suggestion requiring validation: “increased spermidine to spermine ratio”	Human cells + animal models; high confidence
Downstream pathophysiology	Evidence supports a causal chain from polyamine imbalance to excessive spermidine catabolism, toxic aldehydes/ROS, lysosomal dysfunction, impaired autophagy-lysosome flux, mitochondrial dysfunction, acetyl-CoA depletion, altered protein acetylation, and tissue injury affecting brain and bone (li2017sperminesynthasedeficiency pages 1-2, tao2022phenylbutyratemodulatespolyamine pages 1-2, tao2022phenylbutyratemodulatespolyamine pages 2-4)	Exact/near-exact GO suggestions: GO:0006979 response to oxidative stress; GO:0000422 autophagy of mitochondrion/mitophagy (validate fit); GO:0005773 vacuole/lysosomal compartment term suggestion; GO:0005739 mitochondrion; GO:0007015 actin filament organization term suggestion if needed. All mechanistic ontology mappings should be validated	Primary mechanistic evidence from Drosophila, patient fibroblasts, and supportive review; moderate-high confidence
Neurologic phenotype	Common neurologic features include developmental delay/intellectual disability, hypotonia, seizures/epilepsy, speech impairment, gait abnormalities, learning impairment, anxiety-like behavior and reduced brain volumes in mouse models (dontaine2021digestiveinvolvementin pages 1-7, mouskou2021novelhemizygousmissense pages 1-2, qazi2020wholeexomesequencingidentifies pages 1-3, akinyele2024impairedpolyaminemetabolism pages 1-3)	Exact HPO suggestions: HP:0001249 Intellectual disability; HP:0001252 Hypotonia; HP:0001250 Seizure; HP:0001263 Global developmental delay; HP:0000750 Delayed speech and language development; HP:0001288 Gait disturbance. Model-only anatomy suggestions requiring validation: reduced total brain volume	Human clinical + mouse model; high confidence for core human neurologic features
Skeletal phenotype	Characteristic skeletal disease includes low bone density/osteoporosis, atraumatic or low-energy fractures, kyphosis/kyphoscoliosis, thin cortex, low bone volume, absent trabecular meshwork, severe mineralization defect, and reduced osteoblast/osteoclast function (mouskou2021novelhemizygousmissense pages 4-5, albert2015impairedosteoblastand pages 1-2)	Exact/near-exact HPO suggestions: HP:0000939 Osteoporosis; HP:0002757 Pathological fracture; HP:0002650 Scoliosis; HP:0002808 Kyphosis. Process/cell suggestions requiring validation: osteoblast differentiation defect, osteoclast defect	Human clinical + bone histopathology; high confidence
Additional/expanded phenotype	Reported additional manifestations include asthenic/thin habitus, facial dysmorphism, long fingers/toes, genital/renal anomalies, respiratory infections, retinal changes, and possible digestive involvement such as jejunal stenosis, feeding intolerance, cholestasis, pancreatic exocrine insufficiency, and failure to thrive in severe cases (dontaine2021digestiveinvolvementin pages 1-7, dontaine2021digestiveinvolvementin pages 11-16, albert2015impairedosteoblastand pages 1-2, mouskou2021novelhemizygousmissense pages 1-2, dontaine2021digestiveinvolvementin pages 16-21)	Exact/near-exact HPO suggestions: HP:0001508 Failure to thrive; HP:0001511 Intrauterine growth restriction/poor growth term suggestions; HP:0001166 Arachnodactyly term suggestion; HP:0002242 Feeding difficulties; HP:0001394 Cholestasis. All require phenotype-level validation	Human case reports/series; moderate confidence for expanded GI phenotype
Diagnostic biomarkers / tests	Diagnosis can be made by identifying a pathogenic SMS variant and/or showing decreased or absent spermine synthase activity with elevated spermidine/spermine ratio. One severe report provided markedly abnormal erythrocyte polyamines: spermidine >50 nmoles/8×10^9 erythrocytes (norm 5–11) and spermine 2.58 (norm 3.5–8.5). WES/Sanger are established; DXA/radiography are used for bone disease (dontaine2021digestiveinvolvementin pages 1-7, dontaine2021digestiveinvolvementin pages 11-16, mouskou2021novelhemizygousmissense pages 4-5, mouskou2021novelhemizygousmissense pages 1-2, qazi2020wholeexomesequencingidentifies pages 1-3)	Exact/near-exact suggestions: biomarker = elevated spermidine:spermine ratio; assay = SMS enzymatic activity test; genetic test = WES / Sanger confirmation. LOINC/MAXO/other codes require validation	Human diagnostic evidence; high confidence
Supportive care	Current management is mainly supportive: antiepileptic drugs for seizures, calcium/vitamin D with caution because of ectopic calcification concerns, orthopedic surveillance, nutritional/feeding support, and multidisciplinary genetic care. No disease-modifying standard therapy is established (mouskou2021novelhemizygousmissense pages 4-5, dontaine2021digestiveinvolvementin pages 16-21, tao2022phenylbutyratemodulatespolyamine pages 1-2)	MAXO term suggestions requiring validation: antiseizure medication therapy; calcium supplementation; vitamin D supplementation; orthopedic monitoring; enteral feeding support; genetic counseling	Human clinical practice from case literature; moderate confidence
Experimental therapy: phenylbutyrate (PBA)	PBA improved SRS-related phenotypes in Drosophila and patient fibroblasts by downregulating SAT1, reducing toxic catabolites, restoring acetyl-CoA/protein acetylation, improving mitochondrial and autolysosomal function, and extending fly lifespan. In flies, 2 mM showed benefit, whereas 10 mM was toxic; glycerol-PBA also showed benefit at 0.6 mM (tao2022phenylbutyratemodulatespolyamine pages 1-2, tao2022phenylbutyratemodulatespolyamine pages 2-4)	CHEBI / drug suggestion requiring validation: phenylbutyrate. MAXO suggestions requiring validation: experimental small-molecule therapy; metabolite-modulating therapy	Preclinical only (patient cells + fly); moderate confidence, not yet human efficacy
Experimental therapy: DFMO	A 2023 EMBO Molecular Medicine study is cited in later reviews as showing DFMO can rebalance aberrant polyamine ratios in SRS, but primary quantitative details were not retrievable in the current tool outputs; therefore translational promise is noted without overclaiming clinical efficacy (wu2024structuralinsightsinto pages 11-12)	Drug suggestion requiring validation: difluoromethylornithine / eflornithine. MAXO suggestion requiring validation: polyamine-pathway inhibition therapy	Secondary/review-level evidence in current retrieval; low-moderate confidence until primary paper details are confirmed
Other experimental approaches	Additional exploratory strategies cited in reviews include direct spermine supplementation, polyamine analogs such as (R,R)-1,12-dimethylspermine, antioxidants/ROS scavengers, and redox-sensitive spermine prodrugs; benefits are partial or preclinical only (tao2022phenylbutyratemodulatespolyamine pages 1-2, wu2024structuralinsightsinto pages 11-12, akinyele2024impairedpolyaminemetabolism pages 21-24)	CHEBI/MAXO suggestions requiring validation: spermine supplementation; antioxidant therapy; polyamine analog therapy; prodrug therapy	Preclinical / review-supported; low-moderate confidence
Model organisms	Drosophila dSms loss recapitulates polyamine imbalance, shortened lifespan, locomotor defects, retinal/synaptic degeneration, oxidative stress, lysosomal and mitochondrial dysfunction. G56S mouse shows failure to thrive, short stature, reduced bone density, impaired learning, anxiety-like behavior, reduced mobility, heightened fear responses, reduced brain volumes, and impaired mitochondrial oxidative phosphorylation (li2017sperminesynthasedeficiency pages 1-2, akinyele2024impairedpolyaminemetabolism pages 1-3)	Exact/near-exact suggestions: Drosophila melanogaster model; Mus musculus G56S Sms model. Ontology suggestions requiring validation: model recapitulates HP:0001249, HP:0001252, HP:0000939; CL terms for neurons, osteoblasts, osteoclasts, fibroblasts	Strong preclinical evidence; high value for mechanism and therapeutic testing
Evidence gaps / curation notes	Prevalence, penetrance, founder effects, standardized diagnostic criteria, and long-term prognosis remain poorly quantified because very few families have been reported. Several ontology mappings above are term suggestions and should be validated against HPO/GO/CL/MAXO/LOINC before database ingestion (dontaine2021digestiveinvolvementin pages 1-7, qazi2020wholeexomesequencingidentifies pages 1-3, wu2024structuralinsightsinto pages 11-12)	Curation note: retain exact IDs only for MONDO:0010664, OMIM:309583, Orphanet:3063, SMS, and high-confidence HPO terms; validate all others	Evidence-synthesis note; high confidence for gap statement

Table: This table summarizes high-yield evidence and ontology mappings for Snyder-Robinson syndrome across identifiers, mechanism, phenotype, diagnostics, treatment, and models. It is designed as a compact curation aid and flags which ontology terms are exact versus suggestions needing validation.

1. Disease information

Definition and classification

SRS is a Mendelian polyaminopathy and syndromic X-linked intellectual-developmental disorder. It is sometimes described as the first recognized inherited disorder of the polyamine pathway or “spermine synthase deficiency syndrome.” Its phenotype combines neurodevelopmental impairment with skeletal fragility, hypotonia, asthenic habitus, dysmorphism, speech and gait abnormalities, and variably epilepsy and visceral involvement (mouskou2021novelhemizygousmissense pages 1-2, dontaine2021digestiveinvolvementin pages 16-21, li2017sperminesynthasedeficiency pages 1-2).

Identifiers and synonyms

MONDO: MONDO:0010664, syndromic X-linked intellectual disability, Snyder type.
OMIM phenotype: 309583, generally indexed as Snyder–Robinson syndrome/X-linked syndromic intellectual disability, Snyder type.
Orphanet: ORPHA:3063, X-linked intellectual disability, Snyder type.
Causal gene: SMS, Ensembl ENSG00000102172; the retrieved literature gives SMS gene records as MIM 300105 in one source and 300015 in another, so the current OMIM gene record should be checked directly before ingestion.
Synonyms: Snyder–Robinson syndrome; SRS; spermine synthase deficiency syndrome; X-linked intellectual disability, Snyder type; syndromic X-linked intellectual disability, Snyder type; MRXSSR (dontaine2021digestiveinvolvementin pages 1-7, mouskou2021novelhemizygousmissense pages 1-2, qazi2020wholeexomesequencingidentifies pages 1-3, OpenTargets Search: Snyder-Robinson syndrome-SMS).

No dedicated ICD-10-CM code, ICD-11 entity, or MeSH descriptor was established in the retrieved evidence. Operational coding will generally require a broader intellectual-disability, congenital-malformation, epilepsy, or osteoporosis code, supplemented by the molecular diagnosis.

Evidence provenance

The knowledge summarized here is primarily aggregated disease-level evidence derived from published pedigrees, individual case reports, small case series, cell studies, and model organisms—not population-scale EHR data. Examples include three affected members of one Pakistani family, two severely affected maternal half-brothers, two brothers studied through detailed bone histology, and isolated de novo cases (dontaine2021digestiveinvolvementin pages 11-16, albert2015impairedosteoblastand pages 1-2, qazi2020wholeexomesequencingidentifies pages 1-3).

2. Etiology, risk, protection, and gene–environment interaction

Causal factor

The necessary initiating lesion is a germline hemizygous pathogenic variant in SMS in an affected male. SMS catalyzes the terminal polyamine-biosynthetic reaction, spermidine to spermine. Reduced or absent activity produces a high spermidine:spermine ratio and initiates downstream cellular toxicity (li2017sperminesynthasedeficiency pages 1-2, akinyele2024impairedpolyaminemetabolism pages 1-3).

Genetic risk factors

Documented variant classes include missense, nonsense, splice-altering, and other loss-of-function alleles. Retrieved examples are c.334C>G (p.Pro112Ala), de novo; c.443A>G (p.Gln148Arg), affecting the 5′-methylthioadenosine-binding region; c.608G>A (p.Gly203Asp); and c.905C>T (p.Ser302Leu) in three related Pakistani males (dontaine2021digestiveinvolvementin pages 11-16, mouskou2021novelhemizygousmissense pages 1-2, qazi2020wholeexomesequencingidentifies pages 1-3, li2017sperminesynthasedeficiency pages 1-2).

Female heterozygosity confers reproductive risk; expression in females may depend on X-inactivation, but penetrance and frequency of symptomatic carriers are insufficiently defined. Most familial cases involve carrier mothers, while de novo cases occur—two of 20 published cases in one 2021 review. Maternal mosaic transmission has also been reported in the literature, making parental testing and consideration of mosaicism important even when a variant appears de novo (mouskou2021novelhemizygousmissense pages 4-5).

No validated modifier gene, susceptibility locus, founder allele, protective allele, or robust genotype–phenotype relationship has been established. Marked intrafamilial variability argues that residual enzyme activity, X-inactivation in females, background genetic variation, and non-genetic factors may modify expression, but these remain hypotheses (dontaine2021digestiveinvolvementin pages 11-16).

Environmental and protective factors

No toxin, infection, diet, occupation, smoking, alcohol, or lifestyle exposure is known to cause SRS. Likewise, no environmental factor has been shown to prevent occurrence in a genetically affected male. Avoidance of falls and bone trauma, adequate—but carefully monitored—calcium/vitamin D, good nutrition, and infection prevention may reduce complications rather than disease penetrance (mouskou2021novelhemizygousmissense pages 4-5).

A formal gene–environment interaction has not been demonstrated. Oxidative load, nutritional status, immobility, antiseizure medications, and mechanical loading could plausibly alter skeletal or neurologic severity, but this has not been quantified in SRS cohorts.

3. Phenotypes

Core neurologic and developmental phenotype

Global developmental delay/intellectual disability—usually evident in infancy or childhood; moderate-to-severe impairment is typical, with major effects on communication, education, independence, and activities of daily living. Suggested HPO: HP:0001263, HP:0001249.
Hypotonia/low muscle mass—early and common, contributing to delayed motor milestones, poor mobility, falls, and asthenic habitus. HPO: HP:0001252, with muscular hypoplasia as an additional term.
Speech impairment—dysarthric, markedly delayed, or absent speech. Suggested HPO: HP:0000750 and dysarthria.
Gait disturbance/motor impairment—unsteady gait and difficulty ambulating may evolve through childhood. HPO: HP:0001288.
Epilepsy/seizures—often begins in early childhood; one synthesis reported seizures in up to 67% of cases, with variable type, frequency, and severity. HPO: HP:0001250. Epileptic encephalopathy has occurred in severe cases (dontaine2021digestiveinvolvementin pages 11-16, mouskou2021novelhemizygousmissense pages 4-5, mouskou2021novelhemizygousmissense pages 1-2).

Skeletal, growth, and connective-tissue manifestations

Low bone density/osteoporosis commonly develops during the first decade and particularly affects long bones. HPO: HP:0000939.
Pathologic or low-energy fractures substantially impair mobility and quality of life. Suggested HPO: HP:0002757.
Kyphosis, scoliosis, or kyphoscoliosis, joint contractures, long fingers/toes, and thin habitus are characteristic. Suggested HPO: HP:0002808, HP:0002650, arachnodactyly.
Histomorphometry in two brothers showed profound osteoblast and osteoclast depletion, absent trabecular meshwork, thin cortex, and low bone volume; patient-derived marrow stromal cells had severely deficient calcium-phosphate mineralization (albert2015impairedosteoblastand pages 1-2).

A 2021 review suggested osteoporosis and fracture burden may remain approximately stable after reaching diagnostic severity rather than invariably worsening, but this conclusion rests on very small numbers and should not be encoded as a universal course (mouskou2021novelhemizygousmissense pages 4-5).

Dysmorphic and multisystem findings

Reported craniofacial features include a long, thin, sometimes asymmetric face, prominent lower lip, high or abnormal palate, and other nonspecific dysmorphism. Additional variably reported findings include cryptorchidism or other genital anomalies, renal cysts/nephrocalcinosis, recurrent respiratory infections, tracheomalacia, hearing loss, retinal pigmentary changes, corpus-callosum abnormalities, cerebral atrophy, and fluctuating hyper-/hypoglycemia (dontaine2021digestiveinvolvementin pages 11-16, albert2015impairedosteoblastand pages 1-2, mouskou2021novelhemizygousmissense pages 1-2).

Severe gastrointestinal/hepatopancreatic disease—jejunal or jejunoduodenal stenosis, feeding intolerance, failure to thrive, cholestasis, hepatic fibrosis, and exocrine pancreatic insufficiency—was reported in two maternal half-brothers. These should presently be curated as possible expanded/rare phenotypes, not obligatory features. Suggested HPO includes feeding difficulty, HP:0001508 failure to thrive, cholestasis, intestinal stenosis, and exocrine pancreatic insufficiency (dontaine2021digestiveinvolvementin pages 1-7, dontaine2021digestiveinvolvementin pages 11-16, dontaine2021digestiveinvolvementin pages 16-21).

Frequency and quality-of-life evidence

Reliable percentages are unavailable for most features because the literature historically comprised approximately 20 individuals in about 10 families and fewer than 30 total reported patients by 2021. No SRS-specific EQ-5D, SF-36, PROMIS, or validated caregiver-burden dataset was retrieved. Nevertheless, recurrent fractures, severe developmental disability, epilepsy, feeding dependence, and impaired ambulation clearly impose substantial functional and caregiver burden (dontaine2021digestiveinvolvementin pages 1-7, albert2015impairedosteoblastand pages 1-2, qazi2020wholeexomesequencingidentifies pages 1-3).

4. Genetic and molecular information

SMS is located at Xp22.11 and encodes spermine synthase. Pathogenic disease alleles are germline; SRS is not a somatic neoplasm. Functional consequence is loss or marked reduction of enzyme activity rather than gain of function or dominant negativity (mouskou2021novelhemizygousmissense pages 1-2, li2017sperminesynthasedeficiency pages 1-2).

Variant interpretation should follow ACMG/AMP criteria using segregation, de novo status, population frequency, computational/structural evidence, enzyme activity, and the polyamine ratio. The p.Pro112Ala and p.Ser302Leu reports illustrate WES discovery followed by Sanger confirmation and in-silico structural analysis. Current ClinVar classifications and gnomAD allele counts must be queried variant-by-variant; exact population frequencies were not available in the retrieved full texts and should not be inferred as zero (mouskou2021novelhemizygousmissense pages 1-2, qazi2020wholeexomesequencingidentifies pages 1-3).

No recurrent chromosomal rearrangement, aneuploidy, repeat expansion, mitochondrial-DNA lesion, validated epigenetic episignature, or established modifier gene is known. Copy-number variants disrupting SMS are biologically plausible and should be detectable by genome sequencing or copy-number analysis, but they were not a major documented class in the retrieved cohort.

5. Environmental information

SRS is not caused by pollution, radiation, occupational exposure, lifestyle behavior, or an infectious agent. There is no zoonotic or transmissible component. Environmental management is relevant only to complications: fall prevention, weight-bearing activity within orthopedic safety limits, adequate nutrition, aspiration/infection precautions, and avoidance of excessive supplementation or medications that worsen bone health.

6. Mechanism and pathophysiology

Upstream biochemical defect

The causal chain begins with SMS loss of function → reduced spermine synthesis → spermine deficiency plus spermidine accumulation → elevated spermidine:spermine ratio. In patient lymphoblasts, secondary changes include reduced ornithine decarboxylase activity and putrescine, showing that the lesion perturbs broader polyamine homeostasis rather than a single metabolite (li2017sperminesynthasedeficiency pages 1-2).

A representative diagnostic measurement in severe disease found erythrocyte spermidine >50 nmol/8×10⁹ erythrocytes (reference 5–11) and spermine 2.58 (reference 3.5–8.5), demonstrating the magnitude of imbalance in at least one individual (dontaine2021digestiveinvolvementin pages 11-16).

Downstream organelle and metabolic injury

Human cells and Drosophila support the following sequence:

Excess spermidine enters catabolism.
Catabolism generates reactive oxygen species and toxic aldehyde metabolites.
Oxidative injury disrupts lysosomes and autophagy–lysosome flux.
Mitochondrial function and oxidative phosphorylation decline.
Cellular survival, synaptic maintenance, and tissue homeostasis fail (li2017sperminesynthasedeficiency pages 1-2).

The 2022 phenylbutyrate study added SAT1 activation, acetyl-CoA depletion, and altered global protein acetylation to this model. Phenylbutyrate or its phenylacetyl-CoA metabolite downregulated SAT1 and restored aspects of acetyl-CoA/protein acetylation, linking polyamine catabolism to central carbon metabolism (tao2022phenylbutyratemodulatespolyamine pages 1-2, tao2022phenylbutyratemodulatespolyamine pages 2-4).

Suggested GO annotations include polyamine biosynthetic process; spermidine metabolic process; spermine biosynthetic process; response to oxidative stress (GO:0006979); autophagy; lysosomal organization/function; mitochondrial respiratory chain/oxidative phosphorylation; and bone mineralization. Suggested cellular components are cytosol, lysosome, autolysosome, mitochondrion (GO:0005739), and synapse. Suggested cell types include neuron, retinal neuron/photoreceptor, osteoblast, osteoclast, bone-marrow stromal cell, fibroblast, and lymphoblast; exact CL identifiers should be validated.

Neural and skeletal tissue specificity

Drosophila loss of dSms causes synaptic and retinal degeneration, locomotor deficits, and reduced survival. The 2024 G56S mouse exhibits reduced whole and regional brain volumes, impaired learning, anxiety-like behavior, reduced mobility, heightened fear responses, and defective cortical mitochondrial oxidative phosphorylation. These model findings plausibly connect cellular bioenergetic injury to human intellectual disability and movement impairment, but mouse behavioral phenotypes are not direct human symptoms (li2017sperminesynthasedeficiency pages 1-2, akinyele2024impairedpolyaminemetabolism pages 1-3).

Bone appears unusually sensitive to polyamine imbalance. Patient osteoblast-lineage cells showed much larger spermine/spermidine disturbances than fibroblasts, deficient mineral deposition, and reduced osteoblast and osteoclast populations. This supports cell-type-specific metabolic vulnerability rather than a collagen-structural defect (albert2015impairedosteoblastand pages 1-2).

Molecular profiling and advanced technologies

Established profiling includes targeted polyamine metabolomics in erythrocytes and cultured cells, metabolic phenotyping, mitochondrial assays, protein-acetylation analysis, and anatomical MRI/volumetry in mice. N8-acetylspermidine has been proposed as a plasma biomarker, but clinical sensitivity and specificity are unvalidated (dontaine2021digestiveinvolvementin pages 16-21).

No replicated SRS-specific single-cell atlas, spatial transcriptomic dataset, human tissue-wide transcriptome/proteome, lipidomic signature, CRISPR therapeutic screen, or integrated multi-omics classifier was identified. These are evidence gaps, not negative biological findings.

7. Anatomical structures affected

Primary systems are:

Central nervous system: cerebral cortex, hippocampal systems, synapses, and possibly cerebellum, corpus callosum, and retina. Suggested UBERON terms: brain, cerebral cortex, hippocampus, cerebellum, corpus callosum, retina.
Skeleton: long bones, vertebral column, cortical and trabecular bone; osteoblast and osteoclast compartments.
Skeletal muscle/connective tissue: generalized low muscle mass and hypotonia.
Secondary/variable: kidney, genital tract, respiratory tract, intestine, liver, and pancreas (dontaine2021digestiveinvolvementin pages 11-16, albert2015impairedosteoblastand pages 1-2, li2017sperminesynthasedeficiency pages 1-2, akinyele2024impairedpolyaminemetabolism pages 1-3).

At the subcellular level, lysosomes/autolysosomes and mitochondria are the best-supported affected compartments. No consistent lateralization is established; disease is systemic and generally bilateral.

8. Temporal development

Onset is congenital or early pediatric and usually insidious rather than acute. Hypotonia and developmental delay appear first; epilepsy may emerge in early childhood, while osteoporosis, fractures, spinal curvature, facial gestalt, and thin habitus become more conspicuous with age. The phenotype therefore evolves from childhood into adulthood (mouskou2021novelhemizygousmissense pages 4-5, qazi2020wholeexomesequencingidentifies pages 1-3).

The disorder is chronic and lifelong. There is no accepted staging system or spontaneous remission pattern. Neurologic and skeletal progression is variable; severe visceral disease can produce early mortality, whereas other affected individuals survive into adulthood. Developmental periods of synaptogenesis and bone accrual are plausible therapeutic windows, but no clinical study has defined an optimal intervention age (dontaine2021digestiveinvolvementin pages 11-16, dontaine2021digestiveinvolvementin pages 16-21).

9. Inheritance and population

Inheritance is X-linked recessive: hemizygous males are predominantly affected, carrier females transmit the allele to 50% of sons and 50% of daughters at each pregnancy, subject to standard Mendelian probability. De novo variants and maternal mosaicism mean apparently negative family history does not eliminate recurrence risk (mouskou2021novelhemizygousmissense pages 4-5, mouskou2021novelhemizygousmissense pages 1-2).

Penetrance in hemizygous males appears high, but exact penetrance and age dependence are not quantified. Expressivity is variable, including within families; anticipation is not expected because the disease is not a repeat-expansion disorder. Consanguinity is not mechanistically required for an X-linked condition. No validated founder effect, population enrichment, carrier frequency, or ethnic predisposition is known.

Prevalence and incidence per 100,000 cannot be reliably estimated. Approximately 10 families/20 affected individuals and 11 mutations had been described by 2020, and fewer than 30 cases were cited in 2021. These are literature counts, not epidemiologic prevalence estimates. The observed male predominance follows X-linked inheritance rather than demonstrated sex-specific environmental risk (dontaine2021digestiveinvolvementin pages 1-7, qazi2020wholeexomesequencingidentifies pages 1-3).

10. Diagnostics

Recommended approach

Clinical suspicion: male with developmental delay/intellectual disability, hypotonia, thin habitus, speech/gait impairment, osteoporosis or low-energy fractures, kyphoscoliosis, and possibly epilepsy.
Molecular confirmation: sequence SMS through a neurodevelopmental/epilepsy/bone-fragility panel, WES, WGS, or single-gene testing; confirm candidate variants and segregation by Sanger sequencing.
Functional/biochemical confirmation: measure spermine synthase activity where available and quantify spermine, spermidine, and their ratio in erythrocytes or validated cells.
Phenotypic assessment: DXA, skeletal radiographs, fracture history, spine/orthopedic examination, developmental testing, neurologic examination, EEG when seizures are suspected, nutritional assessment, and organ-directed evaluation (dontaine2021digestiveinvolvementin pages 1-7, mouskou2021novelhemizygousmissense pages 4-5, mouskou2021novelhemizygousmissense pages 1-2, qazi2020wholeexomesequencingidentifies pages 1-3).

WGS is useful when coding sequencing is negative because it can detect noncoding, structural, and copy-number lesions. WES has repeatedly diagnosed SRS but may miss deep intronic and some structural variants. CMA can identify larger Xp22.11 deletions but is not sufficient for most single-nucleotide variants. Karyotyping, FISH, mitochondrial sequencing, and repeat-expansion testing are not routine unless another differential diagnosis is suspected.

RNA sequencing may clarify suspected splice variants. Targeted metabolomics provides functional support. No clinically validated SRS epigenomic, proteomic, liquid-biopsy, or newborn-screening test exists.

Differential diagnosis

Important alternatives include osteogenesis imperfecta and other monogenic bone-fragility disorders; other X-linked intellectual-developmental syndromes; creatine deficiency syndromes; Menkes disease; congenital muscular disorders; mitochondrial disease; and other polyaminopathies. SRS is distinguished by the combination of SMS variation and the characteristic high spermidine:spermine ratio. A collagen defect is not the primary skeletal mechanism (albert2015impairedosteoblastand pages 1-2, li2017sperminesynthasedeficiency pages 1-2).

No formal consensus clinical criteria were identified; molecular confirmation is therefore central. Cascade testing should be offered to at-risk maternal relatives.

11. Outcome and prognosis

No 5-year or 10-year survival rate, median life expectancy, mortality rate, or validated prognostic model exists. Premature death has occurred in severe cases, including the two brothers with major digestive, respiratory, and hepatopancreatic involvement, but this cannot be generalized to all SRS (dontaine2021digestiveinvolvementin pages 11-16, dontaine2021digestiveinvolvementin pages 16-21).

Major long-term morbidity includes intellectual disability, limited communication, impaired mobility, epilepsy, fractures, spinal deformity, feeding difficulty, and dependence in daily living. Recovery of the underlying disorder is not expected with current supportive management. Potential prognostic factors—residual SMS activity, magnitude of the polyamine ratio, seizure burden, nutritional status, bone density, and visceral involvement—are biologically plausible but not prospectively validated.

12. Treatment and current applications

Current clinical management

There is no approved SRS-specific disease-modifying therapy. Care is individualized and multidisciplinary:

Antiseizure medication; reported agents include carbamazepine, phenobarbital, clobazam, levetiracetam, and valproate.
Calcium and vitamin D when indicated, monitored carefully because ectopic calcification/nephrocalcinosis has been reported.
DXA and orthopedic surveillance; fracture prevention and management; scoliosis monitoring.
Physical, occupational, and speech therapy; mobility aids and fall prevention.
Nutrition, swallow/feeding support, and enteral feeding where required.
Monitoring guided by symptoms for renal, respiratory, ophthalmologic, hearing, gastrointestinal, hepatic, and pancreatic complications.
Clinical genetics and reproductive counseling (mouskou2021novelhemizygousmissense pages 4-5, dontaine2021digestiveinvolvementin pages 16-21, tao2022phenylbutyratemodulatespolyamine pages 1-2).

Suggested MAXO annotations include genetic counseling, molecular genetic testing, biochemical assay, EEG, DXA, orthopedic surveillance, antiseizure pharmacotherapy, calcium/vitamin-D supplementation, physical therapy, occupational therapy, speech therapy, enteral feeding, and fracture management; exact MAXO identifiers require validation.

Experimental disease-directed strategies

Phenylbutyrate (PBA): In patient fibroblasts and Drosophila—not patients—PBA reduced SAT1, toxic polyamine catabolism, and acetyl-CoA depletion; improved mitochondrial/autolysosomal function; and prolonged fly lifespan. In flies, 2 mM was beneficial, lower concentrations had milder effects, and 10 mM was toxic; glycerol-PBA prolonged lifespan at 0.6 mM. These concentrations cannot be directly translated into a human dose (tao2022phenylbutyratemodulatespolyamine pages 1-2, tao2022phenylbutyratemodulatespolyamine pages 2-4).

A useful abstract-level quotation is: “PBA treatment significantly restored the function of mitochondria and autolysosomes and extended life span in vivo in the Drosophila SRS model.” The same abstract reports that treatment of patient fibroblasts ameliorated autolysosomal dysfunction. Publication: July 2022; DOI URL: https://doi.org/10.1172/jci.insight.158457 (tao2022phenylbutyratemodulatespolyamine pages 1-2).

Difluoromethylornithine/eflornithine (DFMO): A 2023 EMBO Molecular Medicine study reported rebalancing of abnormal polyamine ratios by inhibiting upstream ornithine decarboxylase. The retrieved current review confirms the study and DOI 10.15252/emmm.202317833, but primary quantitative details were not available in the tool evidence. DFMO should therefore be classified as preclinical/experimental for SRS, not as demonstrated human therapy (wu2024structuralinsightsinto pages 11-12).

Other approaches: Antioxidants partially rescued mitochondrial but not lysosomal defects; direct spermine supplementation was ineffective in the cited cell work; polyamine analogues such as (R,R)-1,12-dimethylspermine and redox-sensitive spermine prodrugs have preclinical rationale. These approaches lack established human efficacy (tao2022phenylbutyratemodulatespolyamine pages 1-2, wu2024structuralinsightsinto pages 11-12, akinyele2024impairedpolyaminemetabolism pages 21-24).

The ClinicalTrials.gov search retrieved no relevant SRS-specific interventional study. Trials of PBA or DFMO in cancer and other diseases do not constitute evidence of efficacy in SRS.

Expert interpretation

The most credible therapeutic principle is pathway rebalancing, not simply replacing spermine. Upstream substrate reduction with DFMO, limiting SAT1-mediated toxic catabolism with PBA, reducing oxidative injury, or restoring organelle function may need to be combined. However, CNS exposure, developmental timing, long-term effects on an essential polyamine pathway, skeletal endpoints, and pediatric safety require dedicated preclinical and clinical evaluation (li2017sperminesynthasedeficiency pages 1-2, tao2022phenylbutyratemodulatespolyamine pages 1-2, wu2024structuralinsightsinto pages 11-12).

13. Prevention

Primary prevention by lifestyle or vaccination is not applicable. Reproductive prevention options include carrier testing, cascade testing, genetic counseling, preimplantation genetic testing, chorionic-villus or amniotic-fluid prenatal diagnosis when the familial variant is known, and discussion of residual recurrence risk from parental mosaicism (mouskou2021novelhemizygousmissense pages 4-5, mouskou2021novelhemizygousmissense pages 1-2).

There is no population newborn screen. Secondary prevention consists of early molecular diagnosis and surveillance for seizures, low bone density, scoliosis, feeding difficulty, and organ complications. Tertiary prevention includes fracture/fall prevention, nutrition and rehabilitation, seizure control, orthopedic care, aspiration precautions, and prompt treatment of respiratory infections. No vaccine or chemoprophylaxis specifically prevents SRS.

14. Other species and natural disease

SMS and polyamine metabolism are deeply conserved across eukaryotes. Orthologues include Sms in Mus musculus (NCBI Taxonomy 10090) and dSms in Drosophila melanogaster (Taxonomy 7227). No well-established naturally occurring companion-animal, livestock, or wildlife disease equivalent was identified, and no breed association or VBO term is currently justified.

SRS is noninfectious and has no zoonotic potential or cross-species transmission. Comparative relevance lies in conserved biochemical susceptibility rather than transmissibility.

15. Model organisms

Drosophila

Loss of dSms recreates the high spermidine/low spermine imbalance and causes reduced viability/lifespan, locomotor impairment, synaptic and retinal degeneration, oxidative stress, lysosomal/autophagic dysfunction, and mitochondrial defects. Genetic or pharmacologic antioxidant enhancement suppresses oxidative stress, and PBA prolongs lifespan, making this a rapid in-vivo platform for mechanism and drug screening (li2017sperminesynthasedeficiency pages 1-2, tao2022phenylbutyratemodulatespolyamine pages 1-2, tao2022phenylbutyratemodulatespolyamine pages 2-4).

Limitations: fly neuroanatomy, bone biology, pharmacokinetics, and dosing differ substantially from humans; rescue of lifespan or climbing is not equivalent to clinical benefit.

Mouse

The G56S Sms mouse carries a missense allele and lacks detectable SMS protein. It shows elevated spermidine:spermine ratio, failure to thrive, short stature, reduced bone density, learning impairment, anxiety-like behavior, reduced mobility, heightened fear responses, reduced total/regional brain volumes, and impaired mitochondrial oxidative phosphorylation in cortex, fibroblasts, and Sms-null hippocampal cells (akinyele2024impairedpolyaminemetabolism pages 1-3).

A useful exact abstract quotation is: “Collectively, our study establishes the suitability of the G56S mice as a preclinical model for SRS and provides a set of molecular and functional outcome measures that can be used to evaluate therapeutic interventions for SRS.” Publication: May 2024; DOI URL: https://doi.org/10.1242/dmm.050639 (akinyele2024impairedpolyaminemetabolism pages 1-3).

Applications: pharmacokinetics, chronic safety, behavioral endpoints, MRI brain volume, bone density, polyamine ratios, and mitochondrial respiration. Limitations: one allele cannot represent the full human variant spectrum; anxiety/fear assays are indirect; lifespan and severe visceral phenotypes require further characterization.

Human cellular models

Patient lymphoblasts, fibroblasts, osteoblast-lineage marrow stromal cells, and engineered hippocampal cells are useful for enzyme activity, polyamine metabolomics, mineralization, lysosomal/autophagic flux, oxidative stress, acetyl-CoA/protein acetylation, and drug-response assays. Their main limitation is the inability to reproduce whole-organism development, blood–brain-barrier pharmacology, biomechanics, and systemic toxicity (albert2015impairedosteoblastand pages 1-2, li2017sperminesynthasedeficiency pages 1-2, tao2022phenylbutyratemodulatespolyamine pages 1-2).

Key evidence gaps and curation cautions

Incidence, prevalence, penetrance, life expectancy, and feature frequencies remain unknown.
The apparent lack of a clear genotype–phenotype relationship is based on very small cohorts.
Gastrointestinal and hepatopancreatic manifestations remain candidate phenotype expansions.
N8-acetylspermidine and the spermidine:spermine ratio require assay standardization and prospective diagnostic validation.
PBA, DFMO, antioxidants, analogues, and prodrugs remain experimental; no human SRS response rate or adverse-event profile exists.
No established SRS single-cell, spatial, epigenomic, or comprehensive multi-omic reference dataset was identified.
Exact PMIDs were not present in the retrieved source metadata; DOI URLs and publication dates are therefore supplied rather than inventing PMID assignments.

Overall, the 2023–2024 literature materially advances SRS from a descriptive syndrome toward a testable metabolic-organelle disease model. The 2024 G56S mouse supplies translational outcome measures, while pathway-rebalancing studies identify SAT1, ornithine decarboxylase, acetyl-CoA homeostasis, oxidative stress, lysosomes, and mitochondrial respiration as candidate intervention points. The central translational challenge is to normalize polyamine flux without disrupting the essential developmental functions of spermine and spermidine.

References

(dontaine2021digestiveinvolvementin pages 1-7): Pauline Dontaine, Elisa Kottos, Martine Dassonville, Ovidiu Balasel, Véronique Catros, Julie Soblet, Pascale Perlot, and Catheline Vilain. Digestive involvement in a severe form of snyder-robinson syndrome: possible expansion of the phenotype. Jan 2021. URL: https://doi.org/10.1016/j.ejmg.2020.104097, doi:10.1016/j.ejmg.2020.104097. This article has 10 citations and is from a peer-reviewed journal.
(albert2015impairedosteoblastand pages 1-2): Jessica S Albert, Nisan Bhattacharyya, Lynne A Wolfe, William P Bone, Valerie Maduro, John Accardi, David R Adams, Charles E Schwartz, Joy Norris, Tim Wood, Rachel I Gafni, Michael T Collins, Laura L Tosi, Thomas C Markello, William A Gahl, and Cornelius F Boerkoel. Impaired osteoblast and osteoclast function characterize the osteoporosis of snyder - robinson syndrome. Orphanet Journal of Rare Diseases, Mar 2015. URL: https://doi.org/10.1186/s13023-015-0235-8, doi:10.1186/s13023-015-0235-8. This article has 64 citations and is from a peer-reviewed journal.
(li2017sperminesynthasedeficiency pages 1-2): Chong Li, Jennifer M. Brazill, Sha Liu, Christofer Bello, Yi Zhu, Marie Morimoto, Lauren Cascio, Rini Pauly, Zoraida Diaz-Perez, May Christine V. Malicdan, Hongbo Wang, Luigi Boccuto, Charles E. Schwartz, William A. Gahl, Cornelius F. Boerkoel, and R. Grace Zhai. Spermine synthase deficiency causes lysosomal dysfunction and oxidative stress in models of snyder-robinson syndrome. Nature Communications, Nov 2017. URL: https://doi.org/10.1038/s41467-017-01289-7, doi:10.1038/s41467-017-01289-7. This article has 107 citations and is from a highest quality peer-reviewed journal.
(akinyele2024impairedpolyaminemetabolism pages 1-3): Oluwaseun Akinyele, Anushe Munir, Marie A. Johnson, Megan S. Perez, Yuan Gao, Jackson R. Foley, Ashley Nwafor, Yijen Wu, Tracy Murray-Stewart, Robert A. Casero, Hülya Bayir, and Dwi U. Kemaladewi. Impaired polyamine metabolism causes behavioral and neuroanatomical defects in a mouse model of snyder–robinson syndrome. Disease Models & Mechanisms, May 2024. URL: https://doi.org/10.1242/dmm.050639, doi:10.1242/dmm.050639. This article has 19 citations and is from a domain leading peer-reviewed journal.
(mouskou2021novelhemizygousmissense pages 1-2): Stella Mouskou, Adamantios Katerelos, Artemis Doulgeraki, Sofia Leka-Emiri, Emmanouil Manolakos, Ioannis Papoulidis, Athina Ververi, Georgios Vartzelis, Anastasia Korona, Sotiria Mastroyanni, and Konstantinos Voudris. Novel hemizygous missense variant of spermine synthase (sms) gene causes snyder-robinson syndrome in a four-year-old boy. Molecular Syndromology, 12:194-200, Apr 2021. URL: https://doi.org/10.1159/000514122, doi:10.1159/000514122. This article has 9 citations and is from a peer-reviewed journal.
(OpenTargets Search: Snyder-Robinson syndrome-SMS): Open Targets Query (Snyder-Robinson syndrome-SMS, 4 results). Buniello, A. et al. (2025). Open Targets Platform: facilitating therapeutic hypotheses building in drug discovery. Nucleic Acids Research.
(dontaine2021digestiveinvolvementin pages 11-16): Pauline Dontaine, Elisa Kottos, Martine Dassonville, Ovidiu Balasel, Véronique Catros, Julie Soblet, Pascale Perlot, and Catheline Vilain. Digestive involvement in a severe form of snyder-robinson syndrome: possible expansion of the phenotype. Jan 2021. URL: https://doi.org/10.1016/j.ejmg.2020.104097, doi:10.1016/j.ejmg.2020.104097. This article has 10 citations and is from a peer-reviewed journal.
(qazi2020wholeexomesequencingidentifies pages 1-3): Talal J. Qazi, Qiao Wu, Ailikemu Aierken, Daru Lu, Ihtisham Bukhari, Hafiz M. J. Hussain, Jingmin Yang, Asif Mir, and Hong Qing. Whole-exome sequencing identifies a novel mutation in spermine synthase gene (sms) associated with snyder-robinson syndrome. BMC Medical Genetics, Aug 2020. URL: https://doi.org/10.1186/s12881-020-01095-x, doi:10.1186/s12881-020-01095-x. This article has 13 citations and is from a peer-reviewed journal.
(tao2022phenylbutyratemodulatespolyamine pages 1-2): Xianzun Tao, Yi Zhu, Zoraida Diaz-Perez, Seok-Ho Yu, Jackson R. Foley, Tracy Murray Stewart, Robert A. Casero, Richard Steet, and R. Grace Zhai. Phenylbutyrate modulates polyamine acetylase and ameliorates snyder-robinson syndrome in a drosophila model and patient cells. JCI Insight, Jul 2022. URL: https://doi.org/10.1172/jci.insight.158457, doi:10.1172/jci.insight.158457. This article has 21 citations and is from a domain leading peer-reviewed journal.
(tao2022phenylbutyratemodulatespolyamine pages 2-4): Xianzun Tao, Yi Zhu, Zoraida Diaz-Perez, Seok-Ho Yu, Jackson R. Foley, Tracy Murray Stewart, Robert A. Casero, Richard Steet, and R. Grace Zhai. Phenylbutyrate modulates polyamine acetylase and ameliorates snyder-robinson syndrome in a drosophila model and patient cells. JCI Insight, Jul 2022. URL: https://doi.org/10.1172/jci.insight.158457, doi:10.1172/jci.insight.158457. This article has 21 citations and is from a domain leading peer-reviewed journal.
(mouskou2021novelhemizygousmissense pages 4-5): Stella Mouskou, Adamantios Katerelos, Artemis Doulgeraki, Sofia Leka-Emiri, Emmanouil Manolakos, Ioannis Papoulidis, Athina Ververi, Georgios Vartzelis, Anastasia Korona, Sotiria Mastroyanni, and Konstantinos Voudris. Novel hemizygous missense variant of spermine synthase (sms) gene causes snyder-robinson syndrome in a four-year-old boy. Molecular Syndromology, 12:194-200, Apr 2021. URL: https://doi.org/10.1159/000514122, doi:10.1159/000514122. This article has 9 citations and is from a peer-reviewed journal.
(dontaine2021digestiveinvolvementin pages 16-21): Pauline Dontaine, Elisa Kottos, Martine Dassonville, Ovidiu Balasel, Véronique Catros, Julie Soblet, Pascale Perlot, and Catheline Vilain. Digestive involvement in a severe form of snyder-robinson syndrome: possible expansion of the phenotype. Jan 2021. URL: https://doi.org/10.1016/j.ejmg.2020.104097, doi:10.1016/j.ejmg.2020.104097. This article has 10 citations and is from a peer-reviewed journal.
(wu2024structuralinsightsinto pages 11-12): Bing Wu and Sen Liu. Structural insights into the mechanisms underlying polyaminopathies. International Journal of Molecular Sciences, 25:6340, Jun 2024. URL: https://doi.org/10.3390/ijms25126340, doi:10.3390/ijms25126340. This article has 8 citations.
(akinyele2024impairedpolyaminemetabolism pages 21-24): Oluwaseun Akinyele, Anushe Munir, Marie A. Johnson, Megan S. Perez, Yuan Gao, Jackson R. Foley, Ashley Nwafor, Yijen Wu, Tracy Murray-Stewart, Robert A. Casero, Hülya Bayir, and Dwi U. Kemaladewi. Impaired polyamine metabolism causes behavioral and neuroanatomical defects in a mouse model of snyder–robinson syndrome. Disease Models & Mechanisms, May 2024. URL: https://doi.org/10.1242/dmm.050639, doi:10.1242/dmm.050639. This article has 19 citations and is from a domain leading peer-reviewed journal.

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Diagnosis

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Snyder–Robinson Syndrome: Disease Characteristics Research Report

Executive summary and evidence scope

1. Disease information

Definition and classification

Identifiers and synonyms

Evidence provenance

2. Etiology, risk, protection, and gene–environment interaction

Causal factor

Genetic risk factors

Environmental and protective factors

3. Phenotypes

Core neurologic and developmental phenotype

Skeletal, growth, and connective-tissue manifestations

Dysmorphic and multisystem findings

Frequency and quality-of-life evidence

4. Genetic and molecular information

5. Environmental information

6. Mechanism and pathophysiology

Upstream biochemical defect

Downstream organelle and metabolic injury

Neural and skeletal tissue specificity

Molecular profiling and advanced technologies

7. Anatomical structures affected

8. Temporal development

9. Inheritance and population

10. Diagnostics

Recommended approach

Differential diagnosis

11. Outcome and prognosis

12. Treatment and current applications

Current clinical management

Experimental disease-directed strategies

Expert interpretation

13. Prevention

14. Other species and natural disease

15. Model organisms

Drosophila

Mouse

Human cellular models

Key evidence gaps and curation cautions

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