Familial 46,XY disorder of sex development due to a novel CYB5A variant: a rare cause of isolated 17,20-lyase deficiency.

Primary amenorrhea may be the first clinical presentation of an underlying 46,XY disorder of sex development (DSD). We report 2 siblings assigned female at birth; the index case presented with primary amenorrhea and the younger sibling was identified through family screening. Both had female external genitalia with minimal virilization, absent Mullerian structures, and testes located in the inguinal/labioscrotal region on imaging. Hormonal evaluation showed impaired androgen synthesis with very low testosterone and markedly elevated 17-hydroxyprogesterone:androstenedione ratios, suggestive of isolated 17,20-lyase deficiency. Whole-exome sequencing identified a novel homozygous missense variant in exon 2 of the CYB5A gene (c.140G>T; p.Gly47Val) in both siblings, and segregation analysis confirmed the same variant in both parents as heterozygous carriers. Subsequently, methemoglobin levels were found to be above the normal reference range in both siblings despite the absence of clinical symptoms. Multidisciplinary evaluation confirmed female gender identity in both siblings without any gender incongruence and advised continuation of the assigned female gender role. Gonadectomy and vaginoplasty were planned after 18 years of age. These cases highlight cytochrome b5 deficiency due to a novel variant as a rare cause of 46,XY DSD and the importance of recognizing methemoglobinemia in a DSD.