Imagine receiving a genetic diagnosis that doesn't match what you're seeing in the mirror. You're told one thing by your genes, but your body tells a different, more frightening story. This is the reality for a small but significant number of individuals with Duchenne and Becker muscular dystrophy (DMD/BMD), and their stories are rewriting the rules of genetic understanding. Let's delve into a fascinating case study that unravels a complex genetic puzzle, highlighting the critical need for comprehensive diagnostic approaches in these challenging conditions.
Dystrophinopathy, a condition stemming from mutations in the DMD gene located on the X chromosome, presents a spectrum of muscle-wasting disorders. Because it's X-linked recessive, it primarily affects males. This spectrum includes the severe Duchenne muscular dystrophy (DMD), the milder Becker muscular dystrophy (BMD), and even X-linked dilated cardiomyopathy (a heart condition). Think of the DMD gene as the blueprint for dystrophin, a crucial protein that acts like a shock absorber in muscle fibers. When this blueprint is flawed, the muscles gradually weaken and deteriorate.
Mutations within the DMD gene are notoriously diverse. The most frequent types are intragenic deletions (missing pieces) and duplications (extra copies) of gene segments. To complicate matters further, the "reading-frame rule," proposed by Dr. Monaco, attempts to correlate the type of genetic mutation with the severity of the disease. This rule suggests that if a deletion or duplication disrupts the gene's reading frame (like scrambling the letters in a sentence), it usually leads to DMD. Why? Because the resulting dystrophin mRNA is unstable, leading to either no dystrophin production or the creation of a non-functional, truncated protein. Imagine trying to build a house with missing or jumbled instructions – the result would be disastrous. Conversely, if the reading frame is preserved, a partially functional dystrophin protein is produced, typically resulting in the milder BMD. Think of it as using slightly incorrect instructions – the house may be flawed, but still somewhat functional.
However, here's where it gets controversial... the reading-frame rule isn't always accurate. It's a helpful guideline, but not an absolute law. It's generally accepted that there isn't a direct relationship between the size of the deleted or duplicated segment and the severity of the disease. While widely used to differentiate DMD from BMD, exceptions occur in approximately 4% to 9% of patients. Specifically, about 10% of BMD cases and 5% of DMD cases show this genotype-phenotype mismatch, and this is more common near the 5' end of the gene. This means that a person's genetic test might suggest a milder form of the disease, while their symptoms point to a more severe one, or vice versa.
This discordance often stems from the difference between what we predict based on the DNA sequence and what actually happens at the mRNA level. And this is the part most people miss... Standard genetic tests, such as multiplex ligation-dependent probe amplification (MLPA) for large deletions/duplications and short-read sequencing for smaller variants, are great for identifying most pathogenic DMD variants, but an estimated 2-7% of cases remain undiagnosed. More importantly, even when variants are found, these methods often fail to accurately predict their effects on the transcript – the mRNA molecule that serves as the template for protein production. This is particularly true for deep intronic variants (mutations in the non-coding regions within the gene) or complex structural rearrangements that disrupt splicing, the process where the mRNA is correctly assembled. Therefore, a stepwise diagnostic strategy is essential for clarifying complex genotype-phenotype relationships, achieving precise molecular diagnosis, and resolving apparent contradictions between genetic and clinical findings.
Consider this case study: A 7.7-year-old boy presented with severe symptoms and MRI findings strongly suggesting DMD. But initial genetic testing revealed only an in-frame deletion, typically associated with BMD. This presented a significant diagnostic challenge. How could a "milder" genetic mutation cause such a severe disease presentation? To solve this mystery, a comprehensive diagnostic re-evaluation was conducted. This included dystrophin protein analysis, full-length dystrophin mRNA analysis, long-read sequencing of genomic dystrophin, and splicing prediction analysis. The case highlights the difficulties posed by these exceptions and emphasizes the need for a stepwise approach to achieve accurate diagnoses in dystrophinopathy.
Materials and Methods
The study involved a 7.7-year-old boy displaying clinical features consistent with DMD. A detailed neurological examination was performed to assess the pattern of muscle weakness. Serum creatine kinase (CK) levels, an indicator of muscle damage, were measured. Thigh muscle MRI was performed using a standardized protocol.
Routine genetic testing began with MLPA analysis of the DMD gene due to the patient’s DMD phenotype. To rule out small pathogenic variants (single-nucleotide variants and small insertions/deletions) in DMD and other genes, whole exome sequencing was conducted at a mean depth of >100x. Copy number variant (CNV) analysis was also performed using CNVkit.
Muscle biopsy samples were collected from the biceps brachii of the patient and an age-matched healthy control, and subjected to histological and molecular analysis. The control muscle sample was obtained from an anonymized individual with no documented neuromuscular pathology who had provided written informed consent for the use of residual tissue in future research. The use of this sample was approved by the ethics committee. Standard procedures were followed for histological and immunohistochemical staining of muscle sections. Dystrophin expression was assessed by immunohistochemical staining, using a panel of monoclonal antibodies against different domains of the dystrophin protein: dystrophin-N (amino-terminal), dystrophin-C (carboxyl-terminal), and dystrophin-R (rod-domain).
From the remaining biopsy tissue, total muscle RNA was extracted and reverse-transcribed into cDNA by reverse transcription polymerase chain reaction (RT-PCR). Full-length dystrophin cDNA was amplified in 22 overlapping fragments and analyzed by agarose gel electrophoresis as described previously. Sequences of aberrant transcripts were determined by Sanger sequencing.
Long-read sequencing of genomic DMD gene was performed to identify large-scale or structural variants, followed by validation through Sanger sequencing. In silico splicing prediction was carried out using Human Splicing Finder and Maximum Entropy Scan algorithms to evaluate the effect of detected variants on splicing.
Results
The patient presented with progressive lower limb weakness at 7.7 years of age. There was no reported consanguinity or family history of neuromuscular disorders. Neurological examination confirmed proximal muscle weakness, a positive Gowers’ sign (using arms to stand up), and bilateral tendon contractures. Serum creatine kinase (CK) levels were significantly elevated, fluctuating between 8820 and 14132 IU/L (normal range: 25–195 IU/L), indicating substantial muscle damage. Muscle MRI revealed severe fatty infiltration of the pelvis and thigh muscles. A specific fatty infiltration pattern known as the "trefoil with single fruit sign" was observed on his thigh muscle MRI, which is highly specific for dystrophinopathies.
Whole exome sequencing revealed no small pathogenic variants in the DMD gene or in other genes known to cause neuromuscular disorders. MLPA analysis detected an in-frame deletion of DMD exons 50–51, which was confirmed by CNV analysis.
Given the phenotype-genotype discordance, a stepwise molecular analysis strategy was implemented. Histopathological examination of the patient’s biopsy revealed a dystrophic pattern characterized by increased variation in myofiber diameter, connective tissue deposition, and signs of muscle degeneration and regeneration. Dystrophin protein analysis by immunohistochemical staining showed negative dystrophin-N and dystrophin-C expression, along with severe reduction of dystrophin-R. No revertant fibers were observed, confirming the molecular diagnosis of DMD at the protein level.
Agarose gel electrophoresis analysis of the amplified 22 overlapping dystrophin cDNA fragments showed two aberrant splicing transcripts that were shorter than the normal band. Sanger sequencing of the aberrant splicing transcripts revealed two truncated DMD transcripts. One truncated transcript was the out-of-frame skipping of DMD exons 50–52, while the other was the skipping of DMD exons 50–51 and a part of DMD exon 52, along with a partial intron inclusion of DMD intron 49. Both truncated transcripts were out-of-frame and harbored premature termination codons, which would be targeted by nonsense-mediated mRNA decay, resulting in the absence or severe reduction of dystrophin-N, -C, and -R observed in the patient, demonstrating his genetic diagnosis of DMD at the mRNA level.
To further characterize the aberrant DMD splicing-causing variant(s) at the genomic DNA level, long-read sequencing of genomic dystrophin was performed. This identified a novel large-scale deletion variant (~97kb) in the DMD gene, which was validated by Sanger sequencing. This deletion variant has not been reported in the Database of Genomic Variants or published literature. Apart from this deletion, no other pathogenic structural variants were detected in the DMD gene.
Splicing prediction analysis revealed that the novel deletion variant created a new acceptor splice site within DMD intron 49, causing the partial intron inclusion of DMD intron 49 (46-bp sequence). The novel deletion variant was classified as pathogenic, as it complies with the PVS1, PM2, and PP4 criteria based on the ACMG guidelines, thereby establishing the genetic diagnosis of DMD at the DNA level for the proband.
Discussion
This integrated analysis provides a definitive molecular explanation for a case of dystrophinopathy with genotype-phenotype discordance. It demonstrates that a seemingly in-frame deletion of DMD exons 50–51, initially suggestive of BMD, can result in DMD because the underlying genomic deletion induces out-of-frame splicing. This case highlights a key diagnostic principle: the reading-frame rule must be applied cautiously when inferred solely from genomic DNA, as clinical severity is mainly dictated by transcriptional and translational outcomes.
These findings expand on previously reported exceptions to the reading-frame rule. Earlier studies have attributed discordant phenotypes to disruption of critical functional domains or to compensatory splicing events that restore the reading frame. By contrast, this case is distinguished by the mechanism through which a large genomic deletion produces multiple aberrant, out-of-frame transcripts. Unlike cases where large in-frame deletions disrupt utrophin localization, this patient’s phenotype is directly explained by the induction of premature termination codons via aberrant splicing—a mechanism more consistent with classic DMD pathophysiology. This contrasts with both domain disruption and frame-preserving mechanisms, adding a novel example to the spectrum of variants that produce exceptional phenotypes through predominant out-of-frame splicing.
This case delineates a clear diagnostic pathway for resolving genotype-phenotype discordances in dystrophinopathy. The process begins when routine genetic testing identifies a variant inconsistent with the clinical presentation. Muscle biopsy with dystrophin immunohistochemistry then provides crucial protein-level confirmation of pathogenicity. Subsequent muscle-derived mRNA analysis is essential for detecting the aberrant splicing events underlying the discrepancy. Finally, long-read DMD sequencing enables definitive characterization of the complex genomic variant driving the aberrant splicing. This stepwise strategy emphasizes that, despite advances in genetic technologies, muscle biopsy remains indispensable for guiding RNA-based molecular analyses.
This study has limitations. As a single-case report, the generalizability of the specific molecular mechanism described here may be limited. Moreover, the proposed diagnostic framework requires access to specialized methodologies such as muscle biopsy and long-read sequencing, which may not be available in all clinical settings. In addition, the study proceeded directly to long-read sequencing without prior short-read analysis of the DMD gene. While long-read technology offers superior resolution of complex structural variants, bypassing short-read sequencing may pose cost-effectiveness limitations in clinical practice.
In conclusion, by employing the stepwise strategy, this study resolved a diagnostically challenging case and identified a novel variant underlying phenotypic exception to the reading-frame rule in dystrophinopathy. The strategy applied—combining histopathological evaluation, mRNA-level analysis, and advanced genomic technologies including long-read sequencing—provides an effective approach for achieving precise molecular diagnoses.
What do you think about the continued reliance on muscle biopsies in the age of advanced genetic sequencing? Is it a necessary step, or could future technologies eventually replace it? And how can we improve access to advanced diagnostic tools like long-read sequencing for all patients with suspected dystrophinopathy? Share your thoughts and experiences in the comments below!
