Duchenne muscular dystrophy caused by a frame-shift mutation in the acceptor splice site of intron 26

Mechanisms of Action of the US Food and Drug Administration-Approved Antisense Oligonucleotide Drugs

Antisense oligonucleotides (ASOs) are single stranded nucleic acids that target RNA. The US Food and Drug Administration has approved ASOs for several diseases. ASOs utilize three principal modes of action (MOA). The first MOA is initiated by base-pairing between the ASO and its target mRNA, followed by RNase H-dependent mRNA degradation. The second MOA is triggered by ASOs that occlude splice acceptor sites in pre-mRNAs leading to skipping of a mutation-bearing exon. The third MOA involves ASOs that sterically hinder mRNA function, often inhibiting translation. ASOs contain a variety of modifications to the sugar-phosphate backbone and bases that stabilize the ASO or render them resistant to RNase activity. RNase H-dependent ASOs include inotersen and eplontersen (for hereditary transthyretin amyloidosis), fomiversen (for opportunistic cytomegalovirus infection), mipomersen (for familial hypercholesterolemia), and tofersen [for amyotrophic lateral sclerosis (ALS)]. Splice modulating ASOs include nursinersen (for spinal muscular atrophy) and eteplirsen, golodirsen, viltolarsen, and casimersen (all for the treatment of Duchenne muscular dystrophy). In addition, a designer ASO, milasen, was used to treat a single individual afflicted with Batten disease. Since ASO design relies principally upon knowledge of mRNA sequence, the bench to bedside pipeline for ASOs is expedient compared with protein-directed drugs. [Graphical abstract available.].

Identification of two novel insertion abnormal transcripts in two Chinese families affected with Dystrophinopathy

Background

Duchenne muscular dystrophy (DMD) is an X‐linked recessive inheritance muscle dystrophy disease, associated with pathogenic variants in the DMD gene. MLPA, DHPLC and DMD sequence studies fail to found the causative alteration in two cases. This study intends to evaluate the disease‐causing mutations and explains the correlation genotype‐phenotype.

Methods

The mRNA analysis and Long‐range PCR with sequencing were used for molecular diagnosis.

Results

In case one, an insertion of 78 nucleotides between exons 40 and 41 (r.5739_5740insMN602429:r415_492) was identified in case one. The insertion sequences were highly homologous to the intron 40 (NG_012232.1:g.1001760_g.1001837). Long‐range PCR with sequencing analysis showed that a novel deep intronic DMD mutation (NG_012232.1:g.1001838A>G) was identified, generating a premature stop codon and terminating protein translation. The likely pathogenic mutation was detected in fetal sample. In case two, an insertion of 74 nucleotides which located inside the consensus sequence AG/GT was detected between exons 2 and 3 (r.93_94insMN584887:r61_134), which resulted in a premature stop codon. The insertion sequences were traceable in the intron 2 of DMD gene (NG_012232.1:g.415926_g.415999). We did not perform prenatal DMD gene diagnosis for case two due to lack of sufficient genetic information.

Conclusion

These findings clarify importance of proceeding to the mRNA analysis when no causative mutations were found neither by MLPA/DHPLC nor gene sequencing so as to reach the molecular confirmation of DMD and carry out an accurate genetic assessment/ carrier status testing.

Peptide-conjugate antisense based splice-correction for Duchenne muscular dystrophy and other neuromuscular diseases

Duchenne muscular dystrophy (DMD) is an X-linked disorder characterized by progressive muscle degeneration, caused by the absence of dystrophin. Exon skipping by antisense oligonucleotides (ASOs) has recently gained recognition as therapeutic approach in DMD. Conjugation of a peptide to the phosphorodiamidate morpholino backbone (PMO) of ASOs generated the peptide-conjugated PMOs (PPMOs) that exhibit a dramatically improved pharmacokinetic profile. When tested in animal models, PPMOs demonstrate effective exon skipping in target muscles and prolonged duration of dystrophin restoration after a treatment regime. Herein we summarize the main pathophysiological features of DMD and the emergence of PPMOs as promising exon skipping agents aiming to rescue defective gene expression in DMD and other neuromuscular diseases. The listed PPMO laboratory findings correspond to latest trends in the field and highlight the obstacles that must be overcome prior to translating the animal-based research into clinical trials tailored to the needs of patients suffering from neuromuscular diseases.

Application whole exome sequencing for the clinical molecular diagnosis of patients with Duchenne muscular dystrophy; identification of four novel nonsense mutations in four unrelated Chinese DMD patients

Background

Duchenne muscular dystrophy (DMD) is the most common form of inherited muscular dystrophy. Germline mutations in dystrophin (DMD) gene cause DMD, with a X‐linked recessive mode of inheritance. Patients with DMD are usually characterized by weakness of muscle, usually started since childhood and gradually the patient will unable to stand and walk.

Methods

In our present study, we identified four unrelated Chinese patients with DMD from four Chinese families. Whole exome sequencing was performed for genetic molecular analysis for these probands.

Results

Whole exome sequencing and confirmatory Sanger sequencing identified four novel nonsense mutations in these four unrelated Chinese patients, respectively. All these four mutations lead to the formation of a truncated DMD protein by formation of a premature stop codon. According to the variant interpretation guidelines of American College of Medical Genetics and Genomics (ACMG), these four novel nonsense mutations are categorized as “likely pathogenic” variants.

Conclusion

Our present finding not only identified four novel loss‐of‐function mutations in dystrophin (DMD) gene but also strongly emphasized the significance of whole exome sequencing as the most efficient way of identifying the candidate genes and mutations which enables us for easy and rapid clinical diagnosis, follow‐up, and management of DMD patients.