Uncovering the true features of dystrophin gene rearrangement and improving the molecular diagnosis of Duchenne and Becker muscular dystrophies

Unveiling non-coding DMD variants: synergising RNA sequencing and DNA sequencing for enhanced molecular diagnosis

BACKGROUND

Pathogenic variants in the DMD gene are associated with dystrophinopathy including Duchenne and Becker muscular dystrophy (DMD/BMD). Targeted DMD gene, gene panels, exomes and genome sequencing have advanced genetic diagnostics, yet some cases remain elusive.

METHODS

We performed total RNA sequencing (RNAseq) on muscle biopsy from 13 male patients with a clinical diagnosis of DMD/BMD. Splice aberration events are evaluated using the Integrative Genomics Viewers. Targeted DNA sequencing result was used/re-analysed to confirm complex rearrangement events identified.

RESULTS

RNAseq identified aberration splicing or expression events in the DMD gene of 12 cases. Splice-altering intronic single nucleotide variant events including c.7309+5G>T, c.7309+5G>A, c.3276+1G>A and c.3603+820G>T were identified in four cases. Splice-altering events were also detected in one case with small indel c.94-38_94del and two cases with intragenic deletions (exons 51-52 and 45-47 deletions). Furthermore, complex DNA rearrangements inducing aberration splicing/expression events were identified in five cases involving exons 44-45, 55-56, 2-79, 45-79 or 68-79, which were concordant with their DNA sequencing reanalysis results. Some cases with exon deletions have presented non-canonical transcripts expression. The RNAseq result showing aberrant splicing/expression in different exon regions in most of these cases corresponded with their immunohistochemical dystrophin staining results and/or clinical symptoms.

CONCLUSION

Our data demonstrated RNAseq is a powerful tool to provide functional data for DMD splice aberration events, especially when integrating with immunohistochemical testing and DNA sequencing, for elucidating the pathogenicity of DMD variants and achieving a precise genetic diagnosis in patients with DMD/BMD clinical presentation but without definitive diagnoses after routine genetic testing.

Optical genome mapping: Unraveling complex variations and enabling precise diagnosis in dystrophinopathy

OBJECTIVE

Approximately 7% of individuals with dystrophinopathy remain undiagnosed at the genetic level using conventional genetic tests like multiplex ligation-dependent probe amplification (MLPA) and next-generation sequencing (NGS). We used the optical genome mapping (OGM) technology to detect and analyze uncommon mutations or structural variations (SVs) within the DMD gene, thus contributing to more precise clinical diagnoses.

METHODS

We herein included eight patients with dystrophinopathy (six males and two females) in whom pathogenic variants of the DMD gene could not be accurately identified using MLPA and NGS. Clinical data were collected for all patients and genetic testing was performed using OGM.

RESULTS

Conventional methods (MLPA and NGS) failed to detect pathogenic mutations in six out of eight individuals (four males and two females). OGM testing uncovered rare mutations in the DMD gene in four patients, including a pericentric inversion in chromosome X (one male), a complex rearrangement (one male), and two X-autosome translocations (two females). No mutations were detected in the remaining two male patients. OGM also accurately mapped balanced X-autosome translocations in female patients, defining chromosomal breakpoints. In the other two male patients in whom MLPA suggested non-contiguous exon duplications or deletions in the DMD gene, OGM characterized one case as a complex rearrangement and the other as a deletion within the DMD gene.

INTERPRETATION

OGM is a valuable diagnostic tool for dystrophinopathy patients with negative results from conventional genetic tests. It can effectively elucidate complex SVs and pinpoint breakpoints in X-autosomal translocations in female patients, facilitating prompt and appropriate interventions.

Two novel deep intronic variants cause Duchenne muscular dystrophy by splice-altering mechanism

Duchenne muscular dystrophy (DMD) is a genetic disorder characterized by progressive muscle degeneration and weakness, due to mutations in the DMD gene, which encodes the dystrophin protein. While mutations within the coding regions of DMD have been extensively studied, recent focus has shifted to deep intronic variants for their potential impact on disease severity. Here, we characterize two deep intronic variants, c.8669-19_8669-24del and c.6439-1016_6439-3376del, in unrelated DMD patients. These variants were identified using targeted long-read sequencing on patients' DNA. RNA sequencing/reverse transcription polymerase chain reaction on RNA extracted from muscle biopsies revealed the presence of a pseudoexon or retention of part of the intron in the transcript, resulting in the introduction of premature termination codons. This study enhances our understanding of pseudoexon activation mechanisms in DMD and underscores the diverse genetic abnormalities contributing to the disease's complexity.

Identifying inversions with breakpoints in the Dystrophin gene through long-read sequencing: report of two cases

BACKGROUND

Duchenne Muscular Dystrophy (DMD) is an X-linked disorder caused by mutations in the DMD gene, with large deletions being the most common type of mutation. Inversions involving the DMD gene are a less frequent cause of the disorder, largely because they often evade detection by standard diagnostic methods such as multiplex ligation probe amplification (MLPA) and whole exome sequencing (WES).

CASE PRESENTATION

Our research identified two intrachromosomal inversions involving the dystrophin gene in two unrelated families through Long-read sequencing (LRS). These variants were subsequently confirmed via Sanger sequencing. The first case involved a pericentric inversion extending from DMD intron 47 to Xq27.3. The second case featured a paracentric inversion between DMD intron 42 and Xp21.1, inherited from the mother. In both cases, simple repeat sequences (SRS) were present at the breakpoints of these inversions.

CONCLUSIONS

Our findings demonstrate that LRS is an effective tool for detecting atypical mutations. The identification of SRS at the breakpoints in DMD patients enhances our understanding of the mechanisms underlying structural variations, thereby facilitating the exploration of potential treatments.