Stimulation of mTORC1 with L-leucine Rescues Defects Associated with Roberts Syndrome

Abstract

Roberts syndrome (RBS) is a human disease characterized by defects in limb and craniofacial development and growth and mental retardation. RBS is caused by mutations in ESCO2, a gene which encodes an acetyltransferase for the cohesin complex. While the essential role of the cohesin complex in chromosome segregation has been well characterized, it plays additional roles in DNA damage repair, chromosome condensation, and gene expression. The developmental phenotypes of Roberts syndrome and other cohesinopathies suggest that gene expression is impaired during embryogenesis. It was previously reported that ribosomal RNA production and protein translation were impaired in immortalized RBS cells. It was speculated that cohesin binding at the rDNA was important for nucleolar form and function. We have explored the hypothesis that reduced ribosome function contributes to RBS in zebrafish models and human cells. Two key pathways that sense cellular stress are the p53 and mTOR pathways. We report that mTOR signaling is inhibited in human RBS cells based on the reduced phosphorylation of the downstream effectors S6K1, S6 and 4EBP1, and this correlates with p53 activation. Nucleoli, the sites of ribosome production, are highly fragmented in RBS cells. We tested the effect of inhibiting p53 or stimulating mTOR in RBS cells. The rescue provided by mTOR activation was more significant, with activation rescuing both cell division and cell death. To study this cohesinopathy in a whole animal model we used ESCO2-mutant and morphant zebrafish embryos, which have developmental defects mimicking RBS. Consistent with RBS patient cells, the ESCO2 mutant embryos show p53 activation and inhibition of the TOR pathway. Stimulation of the TOR pathway with L-leucine rescued many developmental defects of ESCO2-mutant embryos. Our data support the idea that RBS can be attributed in part to defects in ribosome biogenesis, and stimulation of the TOR pathway has therapeutic potential.

Requirement of Neuronal Ribosome Synthesis for Growth and Maintenance of the Dendritic Tree

“…reported in non-neuronal cell culture models of these conditions (71,72). Therefore, the pathological sequel triggered by neuronal ribosome deficiency may underlie the neurological phenotype not only in Rett syndrome but also in cohesinopathies.…”

“…Reduced brain growth and mental retardation are also found in Roberts and Cornelia DeLange syndromes that are caused by deficits in chromatin structure proteins that regulate sister chromatid cohesion (18). As in the case of MeCP2 deficiency, these mutations are proposed to disrupt transcription, including rRNA production (71,72). Indeed, ribosomal deficits were FIGURE 9.…”

Functionally conserved effects of rapamycin exposure on zebrafish

“…In vivo studies indicate that rapamycin results in developmental delay in different organisms including Drosophila melanogaster, zebrafish and mice (6,13,14). Previous studies using zebrafish have identified significant effects of rapamycin on autophagy (15), prevention of hepatic steatosis (16), heart development (17) and demonstrated the importance of zebrafish as a mitochondrial and ribosomal disease model (18,19). Although the zebrafish is an emerging model in drug-screening and in vivo disease models (20), evolutionarily conserved effects of rapamycin on the zebrafish transcriptome in addition to the dose-dependency in embryonic/larval size and pigmentation have not previously been studied.…”

Nucleolar Enrichment of Brain Proteins with Critical Roles in Human Neurodevelopment

“…At least for one of such factors, the cohesin SMC1A, deficient rRNA transcription has been proposed as a mechanism that could explain cohesonipathy pathology in non-dividing cells including neurons (74,75). Of note, two other cohesinopathy-associated proteins including NIPBL and SMC3 were also enriched in cerebro-cortical nucleoli, albeit less than 1.3-fold (OMIM #122470 and #610759, supplemental Table S2).…”