Sphingomyelinases generate ceramide from sphingomyelin as a second messenger in intracellular signaling pathways involved in cell proliferation, differentiation, or apoptosis. Children from 12 unrelated families presented with microcephaly, simplified gyral pattern of the cortex, hypomyelination, cerebellar hypoplasia, congenital arthrogryposis, and early fetal/postnatal demise. Genomic analysis revealed bi-allelic loss-of-function variants in SMPD4, coding for the neutral sphingomyelinase-3 (nSMase-3/SMPD4). Overexpression of human Myc-tagged SMPD4 showed localization both to the outer nuclear envelope and the ER and additionally revealed interactions with several nuclear pore complex proteins by proteomics analysis. Fibroblasts from affected individuals showed ER cisternae abnormalities, suspected for increased autophagy, and were more susceptible to apoptosis under stress conditions, while treatment with siSMPD4 caused delayed cell cycle progression. Our data show that SMPD4 links homeostasis of membrane sphingolipids to cell fate by regulating the cross-talk between the ER and the outer nuclear envelope, while its loss reveals a pathogenic mechanism in microcephaly.
Sotos syndrome is an overgrowth syndrome characterized by distinctive facial features and intellectual disability caused by haploinsufficiency of the NSD1 gene. Genotype-phenotype correlations have been observed, with major anomalies seen more frequently in patients with 5q35 deletions than those with point mutations in NSD1. Though endocrine features have rarely been described, transient hyperinsulinemic hypoglycemia (HI) of the neonatal period has been reported as an uncommon presentation of Sotos syndrome. Eight cases of 5q35 deletions and one patient with an intragenic NSD1 mutation with transient HI have been reported. Here, we describe seven individuals with HI caused by NSD1 gene mutations with three having persistent hyperinsulinemic hypoglycemia. These patients with persistent HI and Sotos syndrome caused by NSD1 mutations, further dispel the hypothesis that HI is due to the deletion of other genes in the deleted 5q35 region. These patients emphasize that NSD1 haploinsufficiency is sufficient to cause HI, and suggest that Sotos syndrome should be considered in patients presenting with neonatal HI. Lastly, these patients help extend the phenotypic spectrum of Sotos syndrome to include HI as a significant feature. K E Y W O R D S hyperinsulinism, hypoglycemia, NSD1, overgrowth syndrome, sacrococcygeal teratoma, Sotos syndrome
RASopathies are a group of developmental disorders caused by pathogenic variants in the RAS-MAPK pathway. Cardiomyopathy is a major feature of this group of disorders, specifically hypertrophic cardiomyopathy (HCM). HCM can be the first presenting feature in individuals with RASopathies. We conducted a retrospective study of all individuals who have had a cardiomyopathy gene panel ordered through our institution to determine the prevalence of pathogenic or likely pathogenic variants in RAS pathway genes in individuals with cardiomyopathy.We evaluated variants in the following genes: BRAF, CBL, HRAS, KRAS, MAP2K1, MAP2K2, NF1, NRAS, PTPN11, RAF1, SHOC2, and SOS1. We reviewed 74 cases with cardiomyopathy, including 32 with HCM, 24 with dilated cardiomyopathy (DCM), nine with both left ventricular noncompaction (LVNC) and DCM, four with LVNC only, two with arrhythmogenic right ventricular cardiomyopathy (ARVC) and three with unspecified cardiomyopathy. We identified four patients (5.41%) with pathogenic or likely pathogenic variants in HRAS, PTPN11 and RAF1 (two individuals). Indication for testing for all four individuals was HCM. The prevalence of pathogenic or likely pathogenic variants in RASopathy genes in our HCM patient cohort is 12.5% (4/32). We conclude that the RASopathy genes should be included on multi-gene panels for cardiomyopathy to increase diagnostic yield for individuals with HCM.
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