AbstractBackgroundCongenital adrenal hyperplasia (CAH) due to 21‐hydroxylase deficiency is an autosomal recessive disease of steroidogenesis that affects 1 in 15,000. Approximately, 10% of the CAH population also suffer from CAH‐X, a connective tissue dysplasia consistent with hypermobility type Ehlers–Danlos syndrome (EDS). Most patients with CAH‐X carry a contiguous gene deletion involving CYP21A2 encoding 21‐hydroxylase and TNXB encoding tenascin‐X (TNX), but some are of unknown etiology.MethodsWe conducted clinical evaluation and medical history review of EDS‐related manifestations in subjects from two unrelated CAH families who carry a heterozygous TNXB c.12463+2T>C variant that alters the splice donor site of intron 42. A next generation sequencing (NGS) based EDS panel composed of 45 genes was performed for index patients from each family. TNX expression in patient skin biopsy tissues and dermal fibroblasts was assessed by qRT‐PCR and Sanger sequencing.ResultsAll three evaluated CAH patients carrying the TNXB splice site variant had moderate EDS manifestations. An NGS panel excluded involvement of other known EDS‐related variants. RNA assay on skin biopsies and dermal fibroblasts did not detect splicing errors in TNX mRNA; however, the removal of intron 42 was less efficient in the allele harboring the splice site variant as evidenced by the existence of a premature TNX RNA form, leading to an allele specific decrease in TNX mRNA.ConclusionsCarrying a TNXB c.12463+2T>C variant at the intron 42 splice donor site causes an allele specific decrease in TNX expression, which can be associated with moderate EDS in CAH patients.