Spinal Muscular Atrophy Type 3 Showing a Specific Pattern of Selective Vulnerability on Muscle Ultrasound

The Relevance of Spinal Muscular Atrophy Biomarkers in the Treatment Era

Spinal muscular atrophy (SMA) is a severe neuromuscular disorder that currently has an approved treatment for all forms of the disease. Previously, biomarkers were primarily used for diagnostic purposes, such as detecting the presence of the disease or determining a specific clinical type of SMA. Currently, with the availability of therapy, biomarkers have become more valuable due to their potential for prognostic, predictive, and pharmacodynamic applications. This review describes the most promising physiological, functional, imaging and molecular biomarkers for SMA, derived from different patients' tissues. The review summarizes information about classical biomarkers that are already used in clinical practice as well as fresh findings on promising biomarkers that have been recently disclosed. It highlights the usefulness, limitations, and strengths of each potential biomarker, indicating the purposes for which each is best suited and when combining them may be most beneficial.

Case report: Heterozygous variation in the IGHMBP2 gene leading to spinal muscular atrophy with respiratory distress type 1

A rare autosomal recessive genetic disease is spinal muscular atrophy with respiratory distress type 1 (SMARD 1; OMIM #604320), which is characterized by progressive distal limb muscle weakness, muscular atrophy, and early onset of respiratory failure. Herein, we report the case of a 4-month-old female infant with SMARD type 1 who was admitted to our hospital owing to unexplained distal limb muscle weakness and early respiratory failure. This report summarizes the characteristics of SMARD type 1 caused by heterozygous variation in the immunoglobulin mu DNA binding protein 2 (IGHMBP2) gene by analyzing its clinical manifestations, genetic variation characteristics, and related examinations, aiming to deepen clinicians' understanding of the disease, assisting pediatricians in providing medical information to parents and improving the decision-making process involved in establishing life support.

Quantitative muscle ultrasound in adult spinal muscular atrophy. A pilot study

INTRODUCTION/AIMS

Muscle ultrasound has been investigated in children with spinal muscular atrophy (SMA) and proposed as a potential biomarker of disease severity. We studied the ultrasound properties in adults with SMA to see whether they also have potential as markers of disease severity in older patients.

METHODS

Thickness and quantitative echogenicity of muscle and subcutaneous tissue were compared between eight prospectively recruited adult patients with SMA and eight age, sex and body mass index-matched controls. Measurements were made in the dominant deltoid, biceps, triceps, forearm extensors, first dorsal interosseous, quadriceps, tibialis anterior, and gastrocnemius muscles. The muscle-to-subcutaneous (M:S) thickness and echogenicity ratios were also calculated. A mean value across all muscles as well as the individual values for each muscle were then calculated for each parameter in each subject and compared between the two groups. Significance was set at 0.05 after Bonferroni correction.

RESULTS

In the SMA patients, mean muscle thickness was significantly smaller (1.3 vs. 1.9 cm), muscle echogenicity higher (106 vs. 67 on the grayscale level), and subcutaneous thickness larger (0.9 vs. 0.3 cm) than in controls; M:S echogenicity ratio was significantly increased and M:S thickness ratio reduced in the patients. The most abnormal scores were found in the nonambulatory patients and the least abnormal in the ambulatory patients.

DISCUSSION

Ultrasound can detect and quantify the severity of muscle atrophy and structure in adult SMA, suggesting a potential role as a marker of disease severity, which will require validation by larger studies.

A fast Myosin super enhancer dictates muscle fiber phenotype through competitive interactions with Myosin genes

The contractile properties of adult myofibers are shaped by their Myosin heavy chain isoform content. Here, we identify by snATAC-seq a 42 kb super-enhancer at the locus regrouping the fast Myosin genes. By 4C-seq we show that active fast Myosin promoters interact with this super-enhancer by DNA looping, leading to the activation of a single promoter per nucleus. A rainbow mouse transgenic model of the locus including the super-enhancer recapitulates the endogenous spatio-temporal expression of adult fast Myosin genes. In situ deletion of the super-enhancer by CRISPR/Cas9 editing demonstrates its major role in the control of associated fast Myosin genes, and deletion of two fast Myosin genes at the locus reveals an active competition of the promoters for the shared super-enhancer. Last, by disrupting the organization of fast Myosin, we uncover positional heterogeneity within limb skeletal muscles that may underlie selective muscle susceptibility to damage in certain myopathies.

The contractile properties of adult myofibers are shaped by their Myosin heavy chain isoform content. Here the authors show that a super enhancer controls the spatiotemporal expression of the genes at the fast myosin heavy chain locus by DNA looping and that this expression profile is recapitulated in a rainbow transgenic mouse model of the locus.