Résumé :
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Communication n° 161 Most of dystrophin gene mutations that cause Duchenne Muscular Dystrophy (DMD) lead to premature stop codons. Pathogenic missense mutations appear to be extremely rare events. We report the identification of two distinct missense mutations (p.Phe3332Cys and p.Cys3340Arg) in the dystrophin gene of a 10-year old DMD patient with mental retardation. Immunohistoanalysis of the muscle biopsy revealed a positive staining of dystrophin in 80% of the fibers with dys-2. Multiplex PCR was negative and transcripts analysis by RT-PCR and use of the Protein Truncation Test did not reveal any chain-terminating mutation or the presence of unusual splicing pattern. Sequence analysis of the whole 11 kb-coding sequence only identified three previously described polymorphisms in addition to the two newly reported missense mutations in exon 69. The Phe3332 and Cys3340 amino acid residues are located in the ZZ domain of the cystein-rich domain, which is a functionally important domain of dystrophin for protein-protein interactions. Both residues are conserved in all known dystrophin, utrophin and DRP2 sequences. Three nearby pathogenic missense mutations have already been described in the ZZ domain (p.Cys3313Phe, p.Cys3340Tyr, p.Asp3335His), one affecting the same aminoacid position as the p.Cys3340Arg reported here. Functional studies have demonstrated that the Cystein3340 was essential for binding of dystrophin to B -dystroglycan. In silico prediction argue for a possibly to probably damaging effect on the protein structure for the p.Phe3332Cys and p.Cys3340Arg changes, respectively (http://tux.embl-heidelberg.de/ramensky/polyphen.cgi ; http://blocks.fhcrc.org/sift/SIFT.htm). None of the two mutations seem to be inherited (result confirmed from a new blood sample of the mother), suggesting the occurrence of a double de novo mutational event. The mechanism which has led to the change of two aminoacids distant from only 8 aminoacids residues is unknown. Moreover, the respective contribution of each change in the phenotype should be further established by functional studies.
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