Genotype-phenotype correlation in Becker muscular dystrophy in Chinese patients

Investigating genotype-phenotype correlation of limb-girdle muscular dystrophy R8: association of clinical severity, protein biological function and protein oligomerization

Limb-girdle muscular dystrophy R8 (LGMD R8) is a hereditary muscle disease caused by biallelic defects in E3 ubiquitinated ligase gene (TRIM32). LGMD R8 is featured by high genetic heterogeneity and phenotypic diversity, most pathogenic variants are missense variants located in the NHL domain, but the genotype-phenotype correlation remains unclear. We hypothesized that various missense variants in NHL domain might have different degrees of impact on the structure and function of the protein, thus resulting in disease variability. Firstly, by analyzing present patients' clinical data, we screen out 4 variants: R394H, D487N, V591M and P619S. Patients homozygous for the aforementioned variants exhibited significant phenotypic variability, including variations in age of onset and age of any walking aid (AWA). Then, bioinformatics analysis, cellular functional experiment and biophysical assay were used to measure the effect of above variants in TRIM32 protein oligomerization and ubiquitination to target substrates. And they revealed distinct differences in the intrinsic E3 ligase activity among various mutant TRIM32 proteins, which corresponded to differences in their oligomerization status. In conclusion, our results showed a correlation between clinical severity, protein function and oligomerization state in patients homozygous for missense variants in NHL domain. It is the first time to reveal a connection between TRIM32 variant with LGMD R8 phenotype and this finding provided valuable reference in predicting disease severity and more precise guidance to affected family on genetic counseling.

An Updated Analysis of Exon-Skipping Applicability for Duchenne Muscular Dystrophy Using the UMD-DMD Database

BACKGROUND/OBJECTIVES

Antisense oligonucleotide (ASO)-mediated exon-skipping is an effective approach to restore the disrupted reading frame of the dystrophin gene for the treatment of Duchenne muscular dystrophy (DMD). Currently, four FDA-approved ASOs can target three different exons, but these therapies are mutation-specific and only benefit a subset of patients. Understanding the broad applicability of exon-skipping approaches is essential for prioritizing the development of additional therapies with the greatest potential impact on the DMD population. This review offers an updated analysis of all theoretical exon-skipping strategies and their applicability across the patient population, with a specific focus on DMD-associated mutations documented in the UMD-DMD database. Unlike previous studies, this approach leverages the inclusion of phenotypic data for each mutation, providing a more comprehensive and clinically relevant perspective.

METHODS

The theoretical applicability of all single and double exon-skipping strategies, along with multi exon-skipping strategies targeting exons 3-9 and 45-55, was evaluated for all DMD mutations reported in the UMD-DMD database.

RESULTS

Single and double exon-skipping approaches were applicable for 92.8% of large deletions, 93.7% of small lesions, 72.4% of duplications, and 90.3% of all mutations analyzed. Exon 51 was the most relevant target and was applicable for 10.6% of all mutations and 17.2% of large deletions. Additionally, two multi-exon-skipping approaches, targeting exons 45-55 and 3-9, were relevant for 70.6% of large deletions and 19.2% of small lesions.

CONCLUSIONS

Current FDA-approved ASOs were applicable to 27% of the UMD-DMD population analyzed, leaving a significant portion of patients without access to exon-skipping therapies. The clinical translation of alternative approaches is critical to expanding the accessibility of these therapies for the DMD population.

Findings from the Longitudinal CINRG Becker Natural History Study

BACKGROUND

Becker muscular dystrophy is an X-linked, genetic disorder causing progressive degeneration of skeletal and cardiac muscle, with a widely variable phenotype.

OBJECTIVE

A 3-year, longitudinal, prospective dataset contributed by patients with confirmed Becker muscular dystrophy was analyzed to characterize the natural history of this disorder. A better understanding of the natural history is crucial to rigorous therapeutic trials.

METHODS

A cohort of 83 patients with Becker muscular dystrophy (5-75 years at baseline) were followed for up to 3 years with annual assessments. Muscle and pulmonary function outcomes were analyzed herein. Age-stratified statistical analysis and modeling were conducted to analyze cross-sectional data, time-to-event data, and longitudinal data to characterize these clinical outcomes.

RESULTS

Deletion mutations of dystrophin exons 45-47 or 45-48 were most common. Subgroup analysis showed greater pairwise association between motor outcomes at baseline than association between these outcomes and age. Stronger correlations between outcomes for adults than for those under 18 years were also observed. Using cross-sectional binning analysis, a ceiling effect was seen for North Star Ambulatory Assessment but not for other functional outcomes. Longitudinal analysis showed a decline in percentage predicted forced vital capacity over the life span. There was relative stability or improved median function for motor functional outcomes through childhood and adolescence and decreasing function with age thereafter.

CONCLUSIONS

There is variable progression of outcomes resulting in significant heterogeneity of the clinical phenotype of Becker muscular dystrophy. Disease progression is largely manifest in adulthood. There are implications for clinical trial design revealed by this longitudinal analysis of a Becker natural history dataset.

DMD Gene and Dystrophinopathy Phenotypes Associated With Mutations: A Systematic Review for Clinicians

ABSTRACT

The diagnosis of Duchenne and Becker muscular dystrophy (DBMD) is made by genetic testing in approximately 95% of cases. Although specific mutations can be associated with skeletal muscle phenotype, pulmonary and cardiac comorbidities (leading causes of death in Duchenne) have not been associated with Duchenne muscular dystrophy mutation type or location and vary within families. Therefore, identifying predictors for phenotype severity beyond frameshift prediction is important clinically. We performed a systematic review assessing research related to genotype-phenotype correlations in DBMD. While there are severity differences across the spectrum and within mild and severe forms of DBMD, few protective or exacerbating mutations within the dystrophin gene were reported. Except for intellectual disability, clinical test results reporting genotypic information are insufficient for clinical prediction of severity and comorbidities and the predictive validity is too low to be useful when advising families. Including expanded information coupled with proposed severity predictions in clinical genetic reports for DBMD is critical for improving anticipatory guidance.

The CINRG Becker Natural History Study: Baseline characteristics

We performed an observational, natural history study of males with in-frame dystrophin gene deletions causing Becker muscular dystrophy (BMD). A prospective natural history study collected longitudinal medical, strength, and timed function assessments. Eighty-three participants with genetically confirmed BMD were enrolled (age range 5.6-75.4 years). Lower extremity function and the percentage of participants who retained ambulation declined across the age span. The largest single group of participants had in-frame deletions that corresponded to an out-of-frame deletion treated with an exon 45 skip to restore the reading frame. This group of 54 participants showed similarities in baseline motor functional assessments when compared to the group of all others in the study. A prospective natural history cohort with in-frame dystrophin gene deletions offers the potential to contribute to clinical trial readiness for BMD and to analyze therapeutic benefit of exon skipping for Duchenne muscular dystrophy.

Duchenne muscular dystrophy animal models for high-throughput drug discovery and precision medicine

Introduction: Duchenne muscular dystrophy (DMD) is an X-linked handicapping disease due to the loss of an essential muscle protein dystrophin. Dystrophin-null animals have been extensively used to study disease mechanisms and to develop experimental therapeutics. Despite decades of research, however, treatment options for DMD remain very limited.Areas covered: High-throughput high-content screening and precision medicine offer exciting new opportunities. Here, the authors review animal models that are suitable for these studies.Expert opinion: Nonmammalian models (worm, fruit fly, and zebrafish) are particularly attractive for cost-effective large-scale drug screening. Several promising lead compounds have been discovered using these models. Precision medicine for DMD aims at developing mutation-specific therapies such as exon-skipping and genome editing. To meet these needs, models with patient-like mutations have been established in different species. Models that harbor hotspot mutations are very attractive because the drugs developed in these models can bring mutation-specific therapies to a large population of patients. Humanized hDMD mice carry the entire human dystrophin gene in the mouse genome. Reagents developed in the hDMD mouse-based models are directly translatable to human patients.