Human Mendelian diseases related to abnormalities of the RNA exosome or its cofactors

The RNA exosome has a key role in RNA decays and RNA quality control. In 2012, two human Mendelian diseases: syndromic diarrhea/tricho-hepato-enteric syndrome (SD/THE) and Ponto-cerebellar hypoplasia type 1(PCH1) were linked to the RNA exosome or its cofactor's defect. SD/THE's main features are an intractable diarrhea of infancy associated with hair abnormalities, facial dysmorphism, intra uterine growth restriction and immune deficiency. SD/THE is caused by a defect of the SKI complex (TTC37 and SKIV2L), the cytoplasmic co-factor of the RNA exosome for mRNA degradation. PCH1's main features are atrophy of the pons and of the cerebellum, a progressive microcephaly with developmental delay and muscle atrophy secondary to spinal anterior horn cell loss. In 30-40% of patients, PCH1 is caused by a defect in EXOSC3 which encodes RRP40, a protein of the cap of the RNA exosome. Thanks to knowledge about other forms of PCH it could be assumed that the altered substrates are probably transfer RNA However, as there exists no patient with two null mutations, residual RNA exosome functionality is probably required to preserve viability. Thus, to date two very different human Mendelian diseases have been related to the dysfunctioning of the RNA exosome. It illustrates the versatility of the RNA exosome function and substrate.