Kohlschütter-Tönz Syndrome (KTZS) is an autosomal recessive disorder caused by mutations in the ROGDI gene. This syndrome is characterized by epilepsy, psychomotor regression and amelogenesis imperfecta. In this paper, we report a case of a 13-year-old Malian girl presenting with this rare disease. By genetic analysis, we identified a novel ROGDI homozygous mutation NM_024589.1: c.117+1G>T [Chr16 (GRCh37): g.4852382C>A] which confirmed the diagnosis of Kohlschütter-Tönz syndrome. The mutation abolishes the usual splice donor site of intron 2 which leads to the deletion of exon 2 and in-frame assembly of exon 3. Exon 2 encodes a highly conserved leucine-rich region that is essential for ROGDI protein function. Hence, this deletion may affect the function of the ROGDI protein.

The ryanodine receptor RyR1 is the main sarcoplasmic reticulum Ca2+ channel in skeletal muscle and acts as a connecting link between electrical stimulation and Ca2+-dependent muscle contraction. Abnormal RyR1 activity compromises normal muscle function and results in various human disorders including malignant hyperthermia, central core disease, and centronuclear myopathy. However, RYR1 is one of the largest genes of the human genome and accumulates numerous missense variants of uncertain significance (VUS), precluding an efficient molecular diagnosis for many patients and families. Here we describe a recurrent RYR1 mutation previously classified as VUS, and we provide clinical, histological, and genetic data supporting its pathogenicity. The heterozygous c.12083C>T (p.Ser4028Leu) mutation was found in thirteen patients from nine unrelated congenital myopathy families with consistent clinical presentation, and either segregated with the disease in the dominant families or occurred de novo. The affected individuals essentially manifested neonatal or infancy-onset hypotonia, delayed motor milestones, and a benign disease course differing from classical RYR1-related muscle disorders. Muscle biopsies showed unspecific histological and ultrastructural findings, while RYR1-typical cores and internal nuclei were seen only in single patients. In conclusion, our data evidence the causality of the RYR1 c.12083C>T (p.Ser4028Leu) mutation in the development of an atypical congenital myopathy with gradually improving motor function over the first decades of life, and may direct molecular diagnosis for patients with comparable clinical presentation and unspecific histopathological features on the muscle biopsy.

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