Clinical features
Snyder Robinson syndrome is a disorder characterized by global developmental delay/intellectual disability of variable degree, neonatal hypotonia, seizures, asthenic body built with diminished muscle mass, distinctive facial characteristics, nasal speech, osteoporosis with recurrent fractures, and progressive kyphoscoliosis. Low levels of spermine and low spermine/spermidine ratios have been identified in all patients tested.
Prevalence
Snyder-Robinson syndrome is a very rare condition. The prevalence is not known. Nearly 60 individuals with a point mutation in the SMS gene have been identified thus far. One male with a frame-shift/truncating mutation has been identified.
Inheritance
Snyder-Robinson syndrome is caused by pathogenic variants in the SMS gene, encoding for spermine synthase. It is inherited in an X-linked recessive fashion. To date only affected males have been reported in the literature. Affected individuals result from a de novo mutation or, more commonly, have inherited the mutation from their mother, who is and healthy carrier. The risk for future pregnancies depends on the mother’s status. Prenatal testing is theoretically feasible once the pathogenic variant is known.