Screening for Congenital Myasthenic Syndromes in Adults With Seronegative Myasthenia Gravis Using Next-Generation...

Myasthenia gravis (MG) is a disorder of the neuromuscular junction, typically associated with autoantibodies (Abs) that impair neuromuscular transmission. However, approximately 10% of cases are seronegative. Emerging evidence suggests that seronegative MG (SNMG) may be mimicked by hereditary conditions, particularly congenital myasthenic syndromes (CMSs), which require different treatments. In this study, we aimed to determine the proportion of CMS among patients diagnosed with SNMG.

We used whole-exome sequencing (WES) in adult patients (aged ≥18 years) diagnosed with SNMG who were enrolled at 3 Austrian tertiary neuromuscular centers between August 2022 and January 2024. Genetic testing was conducted in individuals who remained seronegative after comprehensive serologic testing to exclude Abs against (clustered) acetylcholine receptors, muscle-specific kinase, lipoprotein receptor-related protein 4, and voltage-gated calcium channels. Moreover, we aimed to analyze clinical and demographic factors associated with the likelihood of receiving a molecular diagnosis.

A total of 50 patients with SNMG (35 [70%] female) were referred for exome-based genetic screening. The median age at disease onset was 35 years (interquartile range 24.0-46.0 years). Seven patients (14%) were genetically diagnosed with CMS through WES (4 with CHRNE and 3 with RAPSN variants). In addition, findings of uncertain clinical significance were reported in 4 cases, implicating CACNA1S, DOK7, DPAGT1, and RAPSN. Although patients with CMS tended to have a younger age at disease onset in univariate analysis (p = 0.04, r = 0.29), no clinical or demographic factors remained significantly associated with a molecular diagnosis after correction for multiple testing. Only 1 patient with a confirmed CMS diagnosis reported a positive family history. Six individuals with CMS (86%) had either received immunomodulatory treatments (n = 4) or undergone thymectomy (n = 4). Of the 4 patients with CMS receiving immunotherapies, 3 were reported to have experienced at least a partial response.

Our findings provide evidence that a considerable proportion of patients diagnosed with SNMG have an underlying hereditary etiology. Notably, a (subjective) response to immunotherapies does not exclude a molecular CMS diagnosis. In conclusion, offering genetic testing to seronegative patients with myasthenic syndromes may have profound therapeutic implications.