High frequency of mosaic CREBBP deletions in RSTS patients and mapping of somatic and germline breakpoints

Rubinstein-Taybi syndrome (RSTS) is a rare malformation disorder caused by mutations in the CREBBP and EP300 genes accounting for up to 60% and 3% of the tested patients, respectively. About 10% of CREBBP mutations are deletions, often extending to flanking regions, with scattered breakpoints. By FISH and microsatellite analyses as first step of CREBBP mutation screening we identified in 60 Italian RSTS patients, six deletions, three of which present in a mosaic condition, a finding yet unreported. Using BAC clones and small-sized CREBBP- probes we could assess the extent of all deletions. Only two of the twelve breakpoints were found to encompass the same region, confirming the heterogeneity in size and boundaries of CREBBP deletions. However four of our five intragenic breakpoints clustered to the 5’ end of CREBBP, around exon 2, where a peak breakpoints underlying rearrangements in RSTS patients and tumors is apparent too. The search of genomic motifs did not evidence LCRs, as expected by the lack of a recurrent RSTS. By contrast, the percentage of interspersed repetitive elements, mainly Alu and LINEs is in the smaller 5’CREBBP region around exon 2 significantly higher than that in the entire gene or the average in the genome suggesting this feature might be implicated in the vulnerability of the region to breaking. Search of EP300 microdeletions was also afforded by FISH , but no deletion carrier was identified among CREBBP-negative cases, pointing to a minor role of this second gene in the aetiology of RSTS. As to genotype phenotype correlation the clinical presentation was typical in all cases, although more severe in the three patients carrying constitutional deletions, raising.the issue of possible underdiagnosis of a subset of mild RSTS patients. No apparent correlations were observed between increase in the deletion size, from150 Kb to 2.6 Mb, and more severe phenotypic spectrum in the three patients carrying constitutional deletions.