Résumé :
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Background. Among the large rearrangements in the DMD gene, the predicted out-of-frame deletions and duplications of the exons 3 to 7 (del/dup3-7) present mainly twoparticular features in the widest DMD gene specific databases (www.dmd.nl: 150 deletions/42 duplications and www.umd.be/DMD: 55 deletions/25 duplications): 1- avariable associated phenotype (DMD, IMD, BMD) with frequent exceptions to the reading frame rules; 2- a comparable proportion of deletions and duplications suggestingthat these rearrangements could occur by recurrent events, i.e. Non-Allelic Homologous Recombination (NAHR). Subjects and methods. Among the families referred formolecular diagnosis in our laboratory we selected 10 samples (8 affected males and 2 female carriers) from 7 non-related pedigrees with previously identified del/dup3-7(n=5/2), respectively related to BMD (for deletions) and DMD phenotype (for duplications). These samples underwent high-resolution custom-designed oligonucleotidecomparative genomic hybridisation array (array-CGH, Roche NimbleGen®) allowing to map the breakpoints and to analyse subsequently the junction fragments byPCR/sequencing. Results. All 7 breakpoint sequences were obtained with no clustering, being distributed over 148kb of the 270kb-intron 2 and 66kb of the 110kb-intron7. The two dup3-7 were confirmed to be in tandem and out-of-frame and the 5 del3-7 were also out-of-frame but identified in BMD patients (exception to the reading-framerule). Analysis of the breakpoints sequences showed a high frequency of repeated elements such as LINEs (L1MA8, L2, L1MA4A, L1P2), LTRs (MER51A, MLT1H,THE1D-int) and SINEs (AluSx, MIR) and microhomology from 2 to 9nt or small insertions up to 11nt. Discussion. Our findings argue for a non-recurrent mechanism ofoccurrence of del/dup3-7 such as Non-Homologous or Microhomology Mediated End-Joining (NHEJ/MMEJ) rather than NAHR. However, particular sequence motivesmight also predispose to the genomic instability of this region. The frequently reported exception to the reading-frame rule for del/dup3-7 is not related to a breakpointposition effect. Other mechanisms, such as re-initiation from optional start sites or particular splicing events may explain the unclear genotype/phenotype correlations forthese particular DMD gene defects.
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