3-M syndrome is characterised by post-natal growth restriction. We have identified causative mutations in three genes CUL7, OBSL1 and CCDC8. CUL7, a component of an E3 ubiquitin ligase, has a binding domain for p53 and its reduction or absence has a major impact on growth and cell division. OBSL1 is postulated to have a role as a cytoskeletal adaptor, and was not recognised previously to be a growth regulator. The domain structure of CCDC8 predicts a possible role in the regulation of gene expression and it interacts with protein phosphatase 1. We have shown that OBSL1 interacts with both CUL7 and CCDC8 suggesting a shared pathway related to ubiquitination is responsible for the pathogenesis of 3-M. We report a cohort of patients (N=13) with 3-M, 11 CUL7 mutations were identified in 9 patients, 3 OBSL1 mutations in 3 patients and a CCDC8 mutation in one patient. Of the 11 CUL7 mutations 9 were novel and 2 OBSL1 mutations were novel. To isolate the 3-M pathway interactome, V5-OBSL1 or Myc-CUL7 were exogenously expressed in HEK293s in which we identified co-immunoprecipitated proteins by mass spectrometry. Bioinformatic analysis of the 3-M pathway interaction network revealed splicing machinery, ribosome and cell cycle pathways are particularly enriched. We discovered OBSL1 is part of the CUL7-p53 complex and associates with the mini-chromosome maintenance complex, a regulator of DNA replication. Including this study 3-M has been genetically confirmed in 106 families, 67% (n=71) have CUL7 mutations, 27% (n=29) OBSL1 mutations and 6% (n=6) CCDC8 mutations. The discovery of novel CUL7 and OBSL1 mutations in a cohort of phenotypically identical patients enhances the notion that 3-M syndrome is a disorder of a single pathway. We have also identified novel partners associated with this pathway; the impact of these key cell cycle proteins on human growth now requires investigation.
Declaration of interest: There is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.
Funding: No specific grant from any funding agency in the public, commercial or not-for-profit sector.