A NEW DE NOVO MUTATION IN A NON-HOT SPOT REGION AT THE DMD GENE IN A MEXICAN FAMILY

Summary: A new de novo mutation in a non-hot spot region at the DMD gene in a Mexican family: In this report we present the analysis of a sporadic case of DMD and his family. In the present case, a deletion of exons 18-47 is presented which predicts abolition of the reading frame and is located between the well-known deletion hot spots of the DMD gene. This mutation was not previously reported in the Leiden database (LOVD), Both MLPA and segregation analysis with short tandem repeat markers elucidated the status of the mother, sister and the younger brother of the proband, who were not carriers of the mutation. This case provides a description of a new pathogenic variant presented as de novo mutation in a DMD patient. Haplotype analysis and complete gene screening may improve genetic counseling in cases of germline mosaicism and de novo mutations.Key-words: Deletions DMD gene - Genetic counseling - Duchenne - Hot spot.INTRODUCTIONWhen mutated, the DMD gene causes, in most cases, Duchenne Muscular Dystrophy (DMD) (OMIM 310200). About 60% mutations in DMD are deletions harbored in two hotspots in the proximal and distal regions of the gene (exons 2-20 and 40-79) (3). Multiple-ligation dependent probe amplification (MLPA) allows complete gene analysis. Therefore, information regarding mutations in non-deletion hotspots is now possible. Mutation discovery and how it segregates in families allows the prediction of recurrence risks (3), which are challenging in cases of apparently de novo mutations. On this regard, the theory of the balance between selection of X-linked recessive mutations and the rate of new mutations establishes that for the incidence of a disease to be maintained, new mutations should replace those that were not transmitted by the individuals who carry the lethal X-linked mutation; this holds true for DMD but not necessarily BMD patients (5). Therefore at least for DMD patients since 1/3 of mutations disappear in each generation, 1/3 of mutations must occur as de novo and 2/3 as familial. Nevertheless, this approximation does not take into account the type and location of the mutation which may modify recurrence risk in germline mosaicism cases (3). Herein we report the mutational analysis of the proband with clinical suspicion of DMD and we also performed segregation analysis with informative markers within the gene.I CLINICAL DESCRIPTIONThe index case is a male aged 16 years old; was bom as the 2nd pregnancy from non-consanguineous and apparently healthy parents with no family history of neuromuscular disorders. Motor developmental delay was noticed because of retarded start to walk at 18 months and he was able to climb stairs at 24 months. At four years old abnormal gait and speech were present and it was difficult to climb stairs. He received speech therapy with a favorable progression, however, DMD clinical suspicion was established at 8 years old, physical examination showed independent walking equine support, dorsal scoliosis, lumbar lordosis, generalized hypotrophy, normal muscle tone; positive Gower's sign. Laboratory studies showed 7,606 CPK and CPK-MB 341.4 UI/L1. Respiratory and cardiac evaluations were normal. Stanford test showed intelligence below average. He began Deflazacort treatment (18mg /day) with calcium supplement. At 10 years old, he lost ambulation, but continued Deflazacort treatment; currently the patient presents sleep apnea and uses non-invasive ventilatory support in the night.MATERIALS AND METHODSBlood samples were obtained from the propositus, his mother, sister and a young male brother; genomic DNA was isolated by the CTAB-DTAB method (2). Samples were analyzed by MLPA as described previously (6). An analysis of ten short tandem repeat markers (STR's) in the family was performed using 5' fluorescent labeled primers and endpoint PCR. PCR products were visualized by capillary electrophoresis using an ABI310 sequence detection system. …