# Optical genome mapping identifies a balanced inversion disrupting DMD in a patient with Duchenne muscular dystrophy

## TL;DR

Optical genome mapping helped diagnose a rare genetic cause of Duchenne muscular dystrophy that standard tests missed.

## Contribution

Demonstrates the utility of optical genome mapping in identifying a DMD-disrupting inversion undetected by routine methods.

## Key findings

- A paracentric X-chromosomal inversion was identified in a DMD patient with negative routine genetic tests.
- Optical genome mapping pinpointed the inversion's breakpoints and confirmed disruption of the DMD gene.
- Chromosomal inversions are shown to be a cause of DMD in some undiagnosed cases.

## Abstract

Duchenne muscular dystrophy (DMD) is a severe disorder that primarily affects males due to its X-linked recessive inheritance. It is caused by pathogenic variants of the DMD gene, most commonly exonic deletions, duplications, or point mutations. Current routine genetic testing methods, including next-generation sequencing and multiplex ligation-dependent probe amplification, can identify pathogenic DMD variants in over 90% of clinically diagnosed patients. However, in rare cases, a molecular diagnosis cannot be established using routine methods.

We describe a follow-up genetic analysis, based on karyotyping and optical genome mapping (OGM), of a patient with clinically diagnosed DMD who initially had negative results in extensive routine genetic testing. Karyotyping revealed a paracentric X-chromosomal inversion with estimated breakpoints at p22.31 and p21.2. OGM fine-mapped this alteration as inv(X)(p22.2p21.1) and confirmed its pathogenicity by identifying the proximal breakpoint within intron 41 of DMD, thereby disrupting the gene and providing a definitive molecular genetic diagnosis.

Current results further underscore the important role of chromosomal inversions as causal in a subset of DMD patients who remain without a molecular diagnosis after routine testing. It also demonstrates the utility of OGM in providing detailed, gene-level insights into cytogenetic abnormalities observed in the diagnostics of neuromuscular disorders.