Evaluation of the Value of Long-read Genome Sequencing for the Molecular Diagnosis of Dystonia: a Prospective Multicenter Study

Dystonia is a motor disorder characterized by abnormal movements and/or postures. It is caused by involuntary, intermittent, or sustained muscle contractions. Dystonia can affect all body segments, leading to highly variable and often severe functional impairment with a significant burden on healthcare systems.

The familial nature of some generalized dystonias was recognized as early as the 1940s. With the development and increased availability of high-throughput DNA sequencing technologies since the early 2010s, there has been an exponential rise in the discovery of genes associated with neurological phenotypes in which dystonia is either the main manifestation or part of a broader clinical picture. Dystonia is now known to be associated with all modes of inheritance, including autosomal dominant forms (e.g., TOR1A, KMT2B), autosomal recessive forms often presenting with complex phenotypes, X-linked dominant (e.g., WDR45) and recessive (e.g., TAF1) forms, and even mitochondrial inheritance.

To date, over 100 genes have been linked to dystonic phenotypes, highlighting the significant genetic heterogeneity of this condition. Many genetically determined dystonic syndromes are associated with structural variants (SVs) or copy number variations (CNVs), which are difficult to detect using routine short-read sequencing technologies.

Molecular diagnosis is crucial in the management of dystonia. It provides a definitive and precise diagnosis, bringing an end to the diagnostic odyssey for patients and their families. It also enables accurate genetic counseling, offering information on inheritance risks and the potential for other family members to be affected. In some cases, a molecular diagnosis directly influences therapeutic decisions. For example, pathogenic variants in GNAO1 are associated with early-onset dystonic syndromes that respond well to deep brain stimulation of the globus pallidus. Specific symptomatic treatments have been proposed for certain dystonias, such as levodopa in dopa-responsive dystonia. At the population level, identifying new genes or molecular causes can reveal novel pathogenic mechanisms and facilitate the development of animal or cellular models, paving the way for future gene therapies.

Over the past decade, the growing availability of short-read DNA sequencing technologies has enabled the simultaneous analysis of numerous dystonia-associated genes. In routine diagnostics, short-read sequencing involves fragmenting DNA into small pieces (usually a few hundred base pairs), which are then sequenced from both ends.