Background: Mutations in the gene HSD17B3 encoding the 17-beta hydroxysteroid dehydrogenase 3 enzyme cause testosterone insufficiency leading to XY DSD. In this study the clinical characteristics and molecular etiology of 3 new severe XY DSD cases from consanguineous families are elucidated.
Clinical report: Three female patients (2 sisters and a single unrelated female) presented at ages 0.1, 8 and 0.7 years with ambiguous or complete external female genitalia. All 3 had palpated gonads in the inguinal canal, consistent with testes in imaging studies and XY Karyotype. Endocrine workup showed normal cortisol and mineralocorticoid levels with a low testosterone/ androstenedione ratio. Urine metabolom for ketosteroids in the third female ruled out 5 alpha reductase dysfunction, and the mutation in 17HSDB3 gene was found by sequencing the gene.
Molecular Studies: Whole exome sequencing, carried out in both sisters and parents, revealed a homozygous novel mutation in the HSD17B3 gene - c. 673G>A, p. V225M. The same mutation was found by Sanger sequencing in the third unrelated patient. Haplotype analysis of a 4Mb region surrounding the HSD17B3 gene on chromosome 9 using short tandem repeat markers revealed that the mutation resides on the same allele in all 3 patients. . The mutation was found to affect the splicing as Gel electrophoresis of RT-PCR products from one of our patients' testes cDNA revealed the expected 347bp amplicon, when using primers from exons 9 and 11, but also a shorter transcript of only 196bp. Sequencing of the shorter amplicon showed that this transcript did not include exon 10. Using capillary electrophoresis, we demonstrated that the relative proportion of transcripts retaining exon 10 to transcripts skipping exon 10 in the cDNA of healthy testes compared to affected testis was substantially (9.8x) higher. The disruption of the tightly regulated splicing of an exon may result in an imbalance in the relevant protein isoforms and lead to the significant testosterone deficiency.
Conclusion: The founder novel homozygous c. 673G>A, p. V225M mutation in the 17HSDB3 gene causes severe XY-DSD. Other than changing a conserved amino acid residue this mutation alters 17HSDB3 gene transcription by skipping exon 10 and thereby contributing to significantly decreased 17HDSB3 enzymatic activity.
19 - 21 Sep 2019
European Society for Paediatric Endocrinology