Diagnostic analysis of the Rubinstein-Taybi syndrome: five cosmids should be used for microdeletion detection and low number of protein truncating mutations

Rubinstein-Taybi syndrome (RTS) is a malformation syndrome characterised by facial abnormalities, broad thumbs, broad big toes, and mental retardation . In a subset of RTS patients, microdeletions, translocations, and inversio ns involving chromosome band 16p13.3 can be detected. We have previously sh own that disruption of the human CREB binding protein (CREBBP or CBP) gene, either by these gross chromosomal rearrangements or by point mutations, le ads to RTS. CBP is a large nuclear protein involved in transcription regula tion, chromatin remodelling, and the integration of several different signa l transduction pathways. Here we report diagnostic analysis of CBP in 194 R TS patients, divided into several subsets. In one case the mother is also s uspect of having RTS. Analyses of the entire CBP gene by the protein trunca tion test showed 4/37 truncating mutations. Two point mutations, one 11 bp deletion, and one mutation affecting the splicing of the second exon were d etected by subsequent sequencing. Screening the CBP gene for larger deletio ns, by using different cosmid probes in FISH, showed 14/171 microdeletions. Using five cosmid probes that contain the entire gene, we found 8/89 micro deletions of which 4/8 were 5' or interstitial. This last subset of microde letions would not have been detected using the commonly used 3' probe RT1, showing the necessity of using all five probes.