Pathogenic variants in the SMC1A gene are often dominant-negative and cause an X-linked form of Cornelia de Lange syndrome (CdLS) with growth retardation and typical facial features. However, rare SMC1A variants cause a developmental and epileptic encephalopathy (DEE) with intractable early-onset epilepsy that is absent in CdLS. Here we describe an 11-year-old girl with epilepsy, walking disorder, and neurodevelopmental disorder. A neurophysiological examination of nerve conduction velocity showed a mixed, sensory–motor, chronic 4-limb polyneuropathy. Whole-exome sequencing identified the variant c.3145C > T p.(Arg1049*) in SMC1A (NM_006306.3), which can be classified as pathogenic. To the best of our knowledge, among 79 individuals with SMC1A-related DEE reported in the literature, altered peripheral nerve conduction has never been described. In this article, we propose that severe sensory–motor polyneuropathy could be an expansion of the SMC1A-related phenotype.

The authors declare that this paper complies with internationally accepted standards for research practice and reporting. Written informed consent to participate and written consent to publish were obtained from the patient's parents for the publication of this report.

Conceptualization C.A.C., C.F.; clinical data collection and data curation A.D.L., C.A.C., M.P., G.C., S.G.C., A.L., C.S., D.F., S.R., A.C., C.F.; writing—original draft preparation, A.D.L., C.A.C., M.P., S.G.C.; writing—review and editing, C.A.C., M.P., S.G.C., C.S.; supervision, L.G., C.F. All authors have read and agreed to the published version of the manuscript.

The data used for this paper will be made available by the corresponding author upon reasonable request.

Received: 16 August 2024

Accepted: 21 October 2024

Article published online:
14 November 2024

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