Title: The Role of SNORD116 in the Neuromolecular Pathogenesis of the Prader-Willi Syndrome
Description: Prader-Willi syndrome (PWS) is caused by a loss of paternally expressed genes in an imprinted region of chromosome 15q. Among the canonical PWS phenotypes are hyperphagic obesity, central hypogonadism, and low growth hormone. Rare microdeletion PWS patients define a 91 kb minimum critical deletion region encompassing three genes, including the non-coding SNORD116. Induced pluripotent stem cells were generated from skin cells of three large deletion (5-6 Mb) PWS patients and one novel microdeletion (118 kb) PWS patient. We found that genes within the PWS region, including SNRPN and NDN, showed persistence of DNA methylation after iPSC reprogramming and differentiation to neurons. Genes within the PWS minimum critical deletion region remain silenced in both PWS large deletion and microdeletion iPSC following reprogramming. We find that NHLH2 and PC1 (protein and transcript) are reduced in PWS patient iPSC-derived neurons. Nhlh2 and Pcsk1 expression are reduced in hypothalami of fasted Snord116p-/m+ mice while hypothalamic AgRP and Npy remain elevated following refeeding in association with relative hyperphagia.

Nhlh2-/- mice have growth deficiencies from 4-7 weeks of age, develop hyperphagic obesity as adults, and are hypogonadal. Nhlh2 promotes expression of the prohormone convertase, Pcsk1 (PC1). PC1 is a neuroendocrine prohormone convertase that catalyzes the processing of hormones to “mature,” active hormones. Humans and mice deficient in PC1 display hyperphagic obesity, hypogonadism, decreased growth hormone, and hypoinsulinemic diabetes due to impaired prohormone processing. Snord116p-/m+ mice display in vivo functional defects in prohormone processing of proinsulin, proGHRH, and proghrelin in association with reductions in islet, hypothalamic, and stomach PC1 content. Our findings suggest that the major neuroendocrine features of PWS are due to PC1 deficiency which results from absence of functional SNORD116. In addition to hyperphagic obesity and endocrinopathies, global developmental delay (delayed motor milestones, delayed language development) is a major characteristic of the Prader-Willi syndrome (PWS).

We identified neuroanatomical defects in iPSC-derived neurons of individuals with PWS and mice deficient for Snord116. iPSC-derived neurons from PWS patients and neurons from Snord116p-/m+ mice, have smaller soma and decreased numbers of neurites. Reduced neuron cell body size is apparent in utero and persists at least until 4 weeks of age in Snord116p-/m+ mice. The reduction in neuronal soma size is associated with smaller neuronal nucleoli. There are also developmental defects in the endocrine pancreas of Snord116p-/m+ animals that persist into adulthood (≥20 weeks). Mice lacking Snord116 have smaller pancreatic islets and within the islet the percentage of δ-cells is increased, while the percentage of α-cells is reduced. In Snord116p-/m+ isolated islets, Sst and Hhex are upregulated while Ins1, Ins2, Pdx1, Nkx6-1, and Pax6 are downregulated. There is a 3-fold increase in the percentage of polyhormonal cells in the neonatal islets of Snord116p-/m+ mice, which was due to an increase in cells co-positive with somatostatin.

Snord116 may play a role in islet cell lineage specification. Overall, this work suggests that the Snord116 gene cluster is important for developmental processes in the brain as well as endocrine pancreas and prohormone processing in multiple tissues. Loss of elements within this cluster could account for the PWS by virtue of effects on the expression of PCSK1.