Currently available techniques used to recognize point mutations in genetic disease are time consuming and are capable of screening only small pieces of DNA. Moreover, they detect all sequence differences including phenotypically silent changes. Consequently, they are not convenient to analyse mutations in large, multi-exonic genes, where a large fraction of pathological point mutations arises from early termination, as is the case for the one third non-deletion/duplication cases of Duchenne Muscular Dystrophy. We have developed a rapid and sensitive method, the Protein Truncation Test (PTT). PTT is based on a combination of RT-PCR, transcription and translation and selectively detects translation-terminating mutations. We demonstrate its effectiveness to detect point mutations in DMD-patients and carrier females. PTT should be widely applicable diagnostically in any disease where early terminations contribute substantially to the disease cause.
This study consisted of 1) molecular deletion analyses in patients with Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) using the entire cDNA for the DMD gene as hybridization probes, 2) RFLP analyses in a large number of Japanese normal women using 11 DMD-linked cloned DNAs as probes, and 3) segregation analyses with these RFLP data in 17 DMD families in which prenatal or carrier diagnosis was required. The deletion study showed that 18 (43%) of 42 male DMD patients had a deletion within the DMD gene, while no detectable deletion was found in 3 BMD patients. These deletions were preferentially observed at the 5' end of the DMD gene, while no deletion was found in the 3' portion of the gene. Of a total of 15 RFLPs detected with the 11 probes, one was a new RFLP (probe/enzyme: P20/MspI). In 6 RFLPs, the allele frequencies in the Japanese were statistically different from those in the Caucasian. Based on the RFLP data combined with the result of the deletion study, an estimated diagnostic rate for prenatal diagnosis and/or carrier detection in the Japanese DMD families was 63%. The real diagnostic rate obtained from the prenatal and carrier diagnoses, which were practically performed in 17 families, corresponded to the estimation. A protocol useful for the diagnosis in Japanese DMD families is presented.
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