DMD and BMD in the same family due to distinct mutations

Identification of Biallelic dystrophin gene variants during maternal carrier testing for Becker muscular dystrophy and review of the DMD exon 49-51 deletion phenotype

BACKGROUND

Dystrophinopathies are X-linked recessive conditions caused by pathogenic variants in the dystrophin (DMD) gene. In a family that included two boys with Becker muscular dystrophy (BMD) due to a DMD deletion of exons 45-47, maternal carrier testing unexpectedly identified biallelic DMD deletions of exons 45-47 and 49-51.

METHODS

The patient's mild phenotype in the setting of biallelic DMD variants prompted further investigation of the exon 49-51 deletion in particular, via literature review and retrospective chart review of patients who have been evaluated in our institution's comprehensive neuromuscular center and/or diagnosed in our clinical genetic testing laboratory.

RESULTS

To our knowledge, this is only the fifth case of confirmed biallelic DMD variants in a female. In males, the DMD exon 49-51 deletion appears to result in a mild BMD phenotype with low or normal creatine kinase levels. This deletion comprised 19% (4/21) of dystrophinopathies diagnosed by chromosomal microarray (CMA) in males during the past ten years in our clinical laboratory. Most individuals identified by chart review were diagnosed through CMA, despite the fact that microarray was genome-wide and not DMD-specific. This case raised important genetic counseling issues.

CONCLUSION

The DMD exon 49-51 deletion appears to cause a variable but generally mild BMD phenotype. Its relatively frequent detection by CMA suggests it may be underdiagnosed.

Intra-individual plasticity of the TAZ gene leading to different heritable mutations in siblings with Barth syndrome

Infantile-onset skeletal myopathy Barth syndrome (OMIM #302060) is caused by mutations in the X-linked TAZ gene and hence usually manifests itself only in hemizygous males. Confirmatory testing is provided by mutational analysis of the TAZ gene and/or by biochemical dosage of the monolysocardiolipin/tetralinoleoyl cardiolipin ratio. Heterozygous females do not usually display a clinical phenotype but may undergo molecular genetic prenatal diagnosis during pregnancy. We characterized two novel and non-identical TAZ gene rearrangements in the offspring of a single female carrier of Barth syndrome. The hg19chrX:g.153634427_153644361delinsKP_123427.1 TAZ gene rearrangement was identified in her affected son, whereas the NM_000116.3(TAZ)c.-72_109+51del TAZ gene deletion was identified in a male foetus during a subsequent pregnancy. The unaffected mother was surprisingly found to harbour both variants in addition to a wild-type TAZ allele. A combination of breakpoint junction sequencing, linkage analysis and assessment of allelic dosage revealed that the two variants had originated independently from an apparently unstable/mutable TAZ maternal allele albeit via different mutational mechanisms. We conclude that molecular prenatal diagnosis in Barth syndrome families with probands carrying TAZ gene rearrangements should include investigation of the entire coding region of the TAZ gene. The identification of the breakpoint junctions of such gross gene rearrangements is important to ensure accurate ascertainment of carriership with a view to providing appropriate genetic counselling.

Becker muscular dystrophy patients with deletions around exon 51; a promising outlook for exon skipping therapy in Duchenne patients

Theoretically, 13% of patients with Duchenne muscular dystrophy may benefit from antisense-mediated skipping of exon 51 to restore the reading frame, which results in the production of a shortened dystrophin protein. We give a detailed description with longitudinal follow up of three patients with Becker muscular dystrophy with in-frame deletions in the DMD gene encompassing exon 51. Their internally deleted, but essentially functional, dystrophins are identical to those that are expected as end products in DMD patients treated with the exon 51 skipping therapy. The mild phenotype encourages further development of exon 51 skipping therapy.

Two sisters with Rett syndrome and non-identical paternally-derived microdeletions in the MECP2 gene

The unique case of two sisters with symptoms of RTT and two quite distinct, novel, and apparently de novo microdeletions of the MECP2 gene is described. One sister possessed an 18 base-pair (bp) deletion (c.1155_1172del18) within the deletion hotspot region of exon 4, whereas the other sister exhibited a 43 bp deletion at a different location in the same exon (c.1448_1461del14+29). Although these lesions occurred on the same paternally-derived X chromosome, this is probably due to chance co-occurrence owing to the relatively high mutation rate of the MECP2 gene rather than to a constitutional mutator phenotype.

Renal dysgenesis and KAL1 gene defects in patients with sporadic Kallmann syndrome

OBJECTIVE

To correlate the presence of renal agenesis/dysgenesis to the prevalence of KAL1 gene defects in patients with sporadic Kallmann syndrome (KS).

DESIGN

Prospective assessment of renal structure and DNA sequence analysis of the KAL1 gene.

SETTING

Outpatient clinics of the divisions of endocrinology of university hospitals.

PATIENT(S)

Sixteen male patients with sporadic KS.

INTERVENTION(S)

Assessment of renal structure by abdominal ultrasounds scans and DNA extraction, polymerase chain reaction amplification, and DNA sequence analysis of all 14 exons of the KAL1 gene.

MAIN OUTCOME MEASURE(S)

KAL 1 gene structure and presence of renal dysgenesis.

RESULT(S)

Renal dysgenesis was identified in only two of 16 KS patients. Genetic defects were found in only two patients with KS, that is, in those with the identified renal dysgenesis. The first gene defect was identified in a patient with associated right renal agenesis who had two point mutations in the KAL1 gene: the first was a G to A transition in exon 11, turning codon 514 encoding glutamic acid into lysine; and the second was a G to A transition in exon 13, turning codon 660 encoding alanine into threonine. The second gene defect was identified in a patient with ichthyosis, right renal agenesis, and mirror movements of the upper limbs (synkinesia) and comprised a deletion of exons 5-10 of the KAL1 gene and a complete deletion of the steroid sulphatase gene.

CONCLUSION(S)

The phenotype of renal agenesis/dysgenesis strongly indicates the existence of KAL1 gene defects in the genotype of patients with sporadic KS, providing evidence for the X-linked mode of inheritance and offering the opportunity for genetic counseling.

Grandpa and I have dystrophinopathy?: approach to asymptomatic hyperCKemia

This report describes three males from a single kinship, ages 7, 8, and 67 years with clinically asymptomatic dystrophinopathy. The index case was an 8-year-old male evaluated for asymptomatic but persistently elevated serum creatine kinase levels. Muscle biopsy demonstrated a mild myopathy, without necrotic fibers. Immunostaining for dystrophin revealed a slight reduction in sarcolemmal reactivity for the amino terminus of dystrophin. Dystrophin gene analysis revealed a deletion of exon 45 to exon 51. Genetic analysis identified two other affected males (age 7 years and 67 years), as well as four female carriers in the same family. The 7-year-old male had mildly increased creatine kinase levels with normal muscle strength. The 67-year-old grandfather had normal neuromuscular examination and serum creatine kinase levels. Asymptomatic dystrophinopathy in late adulthood is exceptionally rare, and highlights the importance of consideration of dystrophin mutation analysis in patients with hyperCKemia, even in the absence of muscle weakness.

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.

Molecular basis of genetic heterogeneity: role of the clinical neurologist

Advances in molecular genetics have disclosed many different explanations for allelic heterogeneity, how different clinical syndromes arise from mutations in the same gene. The converse, how similar clinical syndromes arise from mutations of different genes on different chromosomes is called locus heterogeneity. Both, however, give rise to some disease-defining mutations, as in childhood spinal muscular atrophy or Duchenne muscular dystrophy. Nevertheless, new problems have been created, including what might be called "diagnosis by the number," diverse syndromes from mutations in the same gene without current explanation, or siblings with different clinical syndromes. These discoveries have transformed the clinical neurology of heritable diseases. They also provide clinicians with new responsibilities and opportunities in defining clinical syndromes and influencing the evolution of our clinical language.