# SNUPN‐Related Muscular Dystrophy: Novel Phenotypic, Pathological and Functional Protein Insights

## TL;DR

This paper describes a new type of muscular dystrophy linked to SNUPN gene mutations, with adult-onset symptoms and unique muscle and cellular changes.

## Contribution

The study reports the first adult-onset case of SNUPN-related muscular dystrophy and novel ultrastructural and functional protein findings.

## Key findings

- Two siblings with SNUPN mutations showed adult-onset muscle weakness and respiratory issues.
- Muscle MRI and histopathology revealed dystrophic changes and abnormal protein patterns.
- SNUPN variants impair snurportin-1 function, affecting nuclear import and splicing.

## Abstract

SNUPN‐related muscular dystrophy or LGMDR29 is a new entity that covers from a congenital or childhood onset pure muscular dystrophy to more complex phenotypes combining neurodevelopmental features, cataracts, or spinocerebellar ataxia. So far, 12 different variants have been described. Here we report the first family with SNUPN‐related muscular dystrophy presenting an adult‐onset myopathy as well as novel ultrastructural findings.

Clinical evaluation, muscle and brain magnetic resonance imaging (MRI), and muscle histopathological and electron microscopy analysis were conducted. Functional studies including protein modelling and interaction, immunofluorescence and splicing analysis were also performed.

Two siblings carrying two novel deleterious variants in the SNUPN gene (p.Arg27Cys and p.Cys174Tyr) showed adult‐onset proximo‐distal and axial muscle weakness with early respiratory involvement. One patient presented with asymptomatic cerebellar atrophy. Muscle MRI identified involvement in the paravertebral, triceps brachii, sartorius and gracilis muscles. The histopathology revealed dystrophic changes and an abnormal pattern of cytoskeletal and myofibrillar proteins, while electron microscopy disclosed the proliferation of granules and vesicles associated with features of nuclear envelope and sarcolemma remodelling. Functional studies showed that SNUPN variants impair snurportin‐1 function through reduced binding affinity to importin‐β and impaired folding, leading to disturbed nuclear import of small nuclear ribonucleoproteins and downstream splicing.

Our work expands the phenotype of SNUPN‐related muscular dystrophy and provides more insights into their pathological profile. We advise SNUPN testing in patients with late‐onset proximo‐distal and axial weakness with early respiratory impairment and features reminding inclusion body myositis (IBM). Granular deposits suggestive of biomolecular condensates perturbed cell organelle traffic and membrane homeostasis, opening new avenues to understand the pathomechanisms involved in this novel disease.