Genetic characterization in symptomatic female DMD carriers: lack of relationship between X-inactivation, transcriptional DMD allele balancing and phenotype

Abstract

BackgroundAlthough Duchenne and Becker muscular dystrophies, X-linked recessive myopathies, predominantly affect males, a clinically significant proportion of females manifesting symptoms have also been reported. They represent an heterogeneous group characterized by variable degrees of muscle weakness and/or cardiac involvement. Though preferential inactivation of the normal X chromosome has long been considered the principal mechanism behind disease manifestation in these females, supporting evidence is controversial.MethodsEighteen females showing a mosaic pattern of dystrophin expression on muscle biopsy were recruited and classified as symptomatic (7) or asymptomatic (11), based on the presence or absence of muscle weakness. The causative DMD gene mutations were identified in all cases, and the X-inactivation pattern was assessed in muscle DNA. Transcriptional analysis in muscles was performed in all females, and relative quantification of wild-type and mutated transcripts was also performed in 9 carriers. Dystrophin protein was quantified by immunoblotting in 2 females.ResultsThe study highlighted a lack of relationship between dystrophic phenotype and X-inactivation pattern in females; skewed X-inactivation was found in 2 out of 6 symptomatic carriers and in 5 out of 11 asymptomatic carriers. All females were characterized by biallelic transcription, but no association was found between X-inactivation pattern and allele transcriptional balancing. Either a prevalence of wild-type transcript or equal proportions of wild-type and mutated RNAs was observed in both symptomatic and asymptomatic females. Moreover, very similar levels of total and wild-type transcripts were identified in the two groups of carriers.ConclusionsThis is the first study deeply exploring the DMD transcriptional behaviour in a cohort of female carriers. Notably, no relationship between X-inactivation pattern and transcriptional behaviour of DMD gene was observed, suggesting that the two mechanisms are regulated independently. Moreover, neither the total DMD transcript level, nor the relative proportion of the wild-type transcript do correlate with the symptomatic phenotype.

Symptomatic female carriers of Duchenne muscular dystrophy (DMD): Genetic and clinical characterization

“…These data suggest a different reason for a skewed X-inactivation than the presence of an autosomic fragment joined to X chromosome with a mutated dystrophin gene. The lack of translocation in the only patient with a severe form of dystrophinopathy (#514) correlates with the partially skewed pattern of X-inactivation, unlike to that observed in patients with an X-autosome translocation [9].…”

“…Segregation of STRs and MLPA analyses allowed for the detection of deletions in three females and a duplication in one of the patients. Two of these mutations were clustered in the central hot spot region and two other mutations, one deletion and one duplication were located at the 5´ region of the gene, which is a common site for duplications [9]. All the mutations were out of frame.…”

“…Such females should have an active X chromosome with the mutation and an inactive normal X chromosome. This hypothesis has been proved in females with an X-autosome translocation, in whom the X-autosome can not be inactivated and thus, the normal X-chromosome became inactivated in all the muscle cells, leading to severe DMD-like symptoms [9]. However, in cases with a skewed X-inactivation in the majority of the fibers (for example 80%) some muscle cells should have dystrophin-positive fibers.…”

“…DMD and BMD usually affect males, with the majority of females being asymptomatic carriers. However, some of these females can reveal symptoms that vary from mild muscle weakness to a more severe clinical course and are classified as manifesting or symptomatic carriers [6,7,8,9,10]. Studies on muscle biopsy suggest that female dystrophinopathy patients show a skewed X inactivation, where the X chromosome that carries the normal dystrophin gene is preferentially inactivated [11,12].…”

Splicing mutation of a gene within the Duchenne muscular dystrophy family

“…The incidence of DMD ranged from 10.71 to 27.78 per 100,000 males (Mah et al, 2014). Although theoretically DMD does not occur in women, both male and female DMD patients have been reported in clinical practice; this might be caused by a secondary mutation or failure in epigenetic regulation in female carriers (Yoon et al, 2011;Brioschi et al, 2012). Therefore, this occurrence in women highlights the importance of conducting genetic testing for differential diagnoses between autosomal recessive limb-girdle muscular dystrophy and DMD.…”