Disruption of an exon splicing enhancer in exon 3 of MLH1 is the cause of HNPCC in a Quebec family
- 1Department of Human Genetics, McGill University, Montreal, Quebec, Canada
- 2Programme in Cancer Genetics, Department of Oncology and Human Genetics, McGill University
- 3Department of Surgery, Division of Colorectal Surgery, Sir Mortimer B Davis Jewish General Hospital, McGill University
- 4Department of Diagnostic Medicine, SMBD Jewish General Hospital, McGill University
- 5Cancer Prevention Centre, SMBD-Jewish General Hospital, McGill University
- Correspondence to: Dr William Foulkes Department of Medical Genetics, Room C-107.1, Sir M B Davis-Jewish General Hospital, 3755 Côte Ste Catherine Rd, Montreal, Quebec, Canada H3T 1E2;
- Received 13 February 2005
- Accepted 20 May 2005
- Revised 17 May 2005
- Published Online First 27 May 2005
Background: A 3 bp deletion located at the 5′ end of exon 3 of MLH1, resulting in deletion of exon 3 from RNA, was recently identified.
Hypothesis: That this mutation disrupts an exon splicing enhancer (ESE) because it occurs in a purine-rich sequence previously identified as an ESE in other genes, and ESEs are often found in exons with splice signals that deviate from the consensus signals, as does the 3′ splice signal in exon 3 of MLH1.
Design: The 3 bp deletion and several other mutations were created by polymerase chain reaction mutagenesis and tested using an in vitro splicing assay. Both mutant and wild type exon 3 sequences were cloned into an exon trapping vector and transiently expressed in Cos-1 cells.
Results: Analysis of the RNA indicates that the 3 bp deletion c.213_215delAGA (gi:28559089, NM_000249.2), a silent mutation c.216T→C, a missense mutation c.214G→C, and a nonsense mutation c.214G→T all cause varying degrees of exon skipping, suggesting the presence of an ESE at the 5′ end of exon 3. These mutations are situated in a GAAGAT sequence 3 bp downstream from the start of exon 3.
Conclusions: The results of the splicing assay suggest that inclusion of exon 3 in the mRNA is ESE dependent. The exon 3 ESE is not recognised by all available motif scoring matrices, highlighting the importance of RNA analysis in the detection of ESE disrupting mutations.
- CRC, colorectal cancer
- ESE, exon splicing enhancer
- HNPCC, hereditary non-polyposis colorectal cancer
- NAS, nonsense associated altered splicing
- PTT, protein truncation test
- exon splicing enhancer
- hereditary non-polyposis colorectal neoplasms
- mismatch repair
- molecular diagnostic techniques
Conflicts of interest: none declared.