Duplication of dystrophin gene and dissimilar clinical phenotype in the same family

Novel Rod Domain Duplication in Dystrophin Resulting in X-Linked Dilated Cardiomyopathy

BACKGROUND

X-linked dilated cardiomyopathy is a rare, cardio-specific form of dystrophinopathy allelic to Duchenne and Becker muscular dystrophy that results in heart failure without skeletal muscle weakness.

PATIENT DESCRIPTION

We describe a previously healthy 16-year-old boy who presented with palpitations progressing to heart failure who was ultimately found to have a novel duplication of exons 13-16 in the dystrophin gene resulting in diagnosis of X-linked dilated cardiomyopathy.

RESULTS

The patient was diagnosed with X-linked dilated cardiomyopathy through clinical diagnosis and genetic testing.

CONCLUSION

X-linked dilated cardiomyopathy shares genotypic overlap with Duchenne and Becker muscular dystrophy, with its distinctive feature being a lack of progressive muscular weakness.

Two novel missense mutations in the myostatin gene identified in Japanese patients with Duchenne muscular dystrophy

Background

Myostatin is a negative regulator of skeletal muscle growth. Truncating mutations in the myostatin gene have been reported to result in gross muscle hypertrophy. Duchenne muscular dystrophy (DMD), the most common lethal muscle wasting disease, is a result of an absence of muscle dystrophin. Although this disorder causes a rather uniform pattern of muscle wasting, afflicted patients display phenotypic variability. We hypothesized that genetic variation in myostatin is a modifier of the DMD phenotype.

Methods

We analyzed 102 Japanese DMD patients for mutations in the myostatin gene.

Results

Two polymorphisms that are commonly observed in Western countries, p.55A>T and p.153K>R, were not observed in these Japanese patients. An uncommon polymorphism of p.164E>K was uncovered in four cases; each patient was found to be heterozygous for this polymorphism, which had the highest frequency of the polymorphism observed in the Japanese patients. Remarkably, two patients were found to be heterozygous for one of two novel missense mutations (p.95D>H and p.156L>I). One DMD patient carrying a novel missense mutation of p.95D>H was not phenotypically different from the non-carriers. The other DMD patient was found to carry both a novel mutation (p.156L>I) and a known polymorphism (p.164E>K) in one allele, although his phenotype was not significantly modified. Any nucleotide change creating a target site for micro RNAs was not disclosed in the 3' untranslated region.

Conclusion

Our results indicate that heterozygous missense mutations including two novel mutations did not produce an apparent increase in muscle strength in Japanese DMD cases, even in a patient carrying two missense mutations.

Heterogous dystrophin mRNA produced by a novel splice acceptor site mutation in intermediate dystrophinopathy

The molecular background of an intermediate type of dystrophinopathy [Duchenne and Becker muscular dystrophy (DMD/BMD)] remains to be clarified, and out-of -frame and in-frame mutations of the dystrophin gene are shown to be causes of DMD and BMD, respectively. In a boy with this disease, dystrophin mRNA extracted from lymphocytes and muscle were analyzed both qualitatively and quantitatively using reverse transcription PCR. Three different dystrophin mRNA were found to be produced via the use of three cryptic splice acceptor sites resulting from a novel point mutation of 2831-2A>G at the conserved splice acceptor site of intron 20. One of three mRNA showed an insertion of six nucleotides of intron 20 between exons 20 and 21 (dys+6) that encoded two novel amino acids in the rod domain of dystrophin. Two other mRNA species showed an insertion of seven nucleotides of intron 20 between exons 20 and 21 (dys+7) or a seven-nucleotide deletion in exon 21 (dys-7). Quantitative analysis of each dystrophin mRNA expressed in the boy's skeletal muscle disclosed that around 95% and 5% of dystrophin mRNAs were dys-7 and dys+6, respectively, whereas these two mRNA were almost equally expressed in lymphocytes. It is suggested that production of a small fraction of in-frame mRNA in muscle explains the molecular background of the intermediate type of dystrophinopathy in the index case. This finding underlines the potential of genetic therapeutic strategies aimed to modify mRNA in DMD to generate a much milder disease.

Muscular dystrophy: centronucleation may reflect a compensatory activation of defective myonuclei

Muscular dystrophy has long been believed to be characterized by degeneration and abortive regeneration of muscle fibers (the muscle degeneration theory), but unfortunately its pathogenesis is still unclear and an effective treatment has yet to be developed. As a challenge to the theory, we have proposed an alternative muscle-defective-growth theory and a further bone muscle growth imbalance hypothesis supposing possible defects in bone-growth-dependent muscle growth based on our findings in hereditary dystrophic dy mice (C57BL/6J dy/dy). This review presents some new insights into the pathogenesis of the disease along with our hypothesis, focusing on the physiological meaning of centronucleation, one of the major pathological changes commonly observed in dystrophic muscles of man and experimental animals.

Alternative dystrophin gene transcripts in golden retriever muscular dystrophy

Golden retriever muscular dystrophy (GRMD), the canine model of Duchenne muscular dystrophy (DMD), is caused by a splice site mutation in the dystrophin gene. This mutation predicts a premature termination codon in exon 8 and a peptide that is 5% the size of normal dystrophin. Western blot analysis of skeletal muscle from GRMD dogs reveals a slightly truncated 390-kD protein that is approximately 91% the size of normal dystrophin. This 390-kD dystrophin suggests that GRMD dogs, like some DMD patients, employ a mechanism to overcome their predicted frameshift. Reverse-transcriptase polymerase chain reaction on GRMD muscle has revealed two in-frame dystrophin transcripts which lack either exons 3-9 or exons 5-12. Both transcripts could be translated into a dystrophin protein of approximately 390 kD. An understanding of how truncated dystrophin is produced in GRMD may allow this mechanism to be manipulated toward a potential therapy for DMD.

Dystrophinopathies

The discovery of the subsarcolemmal muscle fiber protein dystrophin has, to a certain extent, replaced former nosological terms of Duchenne (DMD) and Becker (BMD) muscular dystrophies by the term dystrophinopathies. The immunohistochemical and Western blot analysis of dystrophin has not only enlarged the clinical spectrum of dystrophinopathies, but has also made carrier detection of DMD more reliable, particularly in manifesting carriers without family history. Moreover, prenatal muscle biopsy, under selected circumstances, can show presence or absence of dystrophin, ie, in the latter case an affected male fetus. Molecular genetics have provided a wealth of genetic details in the dystrophinopathies, but therapy has not yet succeeded to a similar extent, on the contrary, myoblast transplantation has not resulted in any clinical improvement.