ESPE Abstracts

Introduction: CAH-X syndrome refers to a subset of Congenital adrenal hyperplasia (CAH) patients who display the hypermobility phenotype of Ehlers–Danlos syndrome (hEDS) due to the monoallelic/biallelic presence of a CYP21A2 deletion extending into the TNXB gene (chimeric TNXA/TNXB gene). To date, three different TNXA/TNXB chimeras have been described, CH-1 (presence of a TNXA-derived 120bp deletion in exon 35 of the TNXB gene), CH-2 (presence of p.Cys4058Trp in the TNXB gene) and CH-3 (presence of one or more of the three variants cluster: p.Arg4073His, p.Asp4172Asn and p.Ser4175Asn in the TNXB gene). Herein, we present the molecular investigation of TNXA/TNXB chimeras in subjects carrying deletion of CYP21A2 gene in a period of 7 years (2017-2023).

Patients and Methods: Amongst 805 individuals referred for genetic analysis of the CYP21A2 gene, twenty-two cases (16 CAH patients and 6 CAH heterozygotes) harbored deletion of the CYP21A2 gene in monoallelic or biallelic constellation. These twenty-two cases were further analyzed for the presence of TNXA/TNXB chimeras. The CYP779f/Tena32F PCR primers were employed to amplify a 8.5kb PCR product that was then subjected to nested PCR and bidirectional sequencing. MLPA analysis was also employed to detect the CH-1 chimera (MLPA kit P050-CAH).

Results: Chimeras were identified in 50% (11/22) of cases studied. In CAH patients, chimeras were found in 37.5% of cases particularly: CH-1 chimera in 12.5% (2/16) and CH-2 chimera in 25% (4/16). CH-3 chimera was not identified in CAH patients. In one CAH patient harboring homozygous deletion of the CYP21A2 gene CH-2 chimera was found also in homozygosity. In heterozygotes for CYP21A2 gene deletion chimeras were identified in 88.3% of cases with CH-1 chimera present in 66.7% (4/6) and CH-3 chimera in 16.7% (1/6) of cases. CH-2 chimera was not identified in any heterozygote.

Discussion: TNXA/TNXB chimeras were identified in 50% of tested cases carrying deletion of CYP21A2 gene underlying the necessity for further clinical evaluation of these individuals. The genetic diagnosis of CAH-X is necessary to provide proper genetic counseling and clinical management of CAH patients preventing long-term symptoms of EDS. Furthermore, heterozygotes carrying TNXA/TNXB chimeras should also be monitored to provide the proper genetic counseling and prognosis since they may present with a varied EDS phenotype than monoallelic/biallelic CAH-X patients.