5α-Reductase 2 deficiency (5αR2D) is an autosomal recessive condition caused by a mutation in SRD5A2, a gene encoding the enzyme 5α-reductase type 2 (5αR2). The condition is rare, affects only genetic males, and has a broad spectrum.
5αR2 is expressed in specific tissues and catalyzes the transformation of testosterone (T) to 5α-dihydrotestosterone (DHT). DHT plays a key role in the process of sexual differentiation in the external genitalia and prostate during development of the male fetus. 5αR2D is a result of impaired 5αR2 activity resulting in decreased DHT levels. This defect results in a spectrum of phenotypes including overt genital ambiguity, hypospadias, and micropenis. Affected males still develop typical masculine features at puberty (deep voice, facial hair, muscle bulk) since most aspects of pubertal virilization are driven by testosterone, not DHT.
Management of this condition in the context of sex assignment is a challenging and controversial area. Diagnostic availability, local laws, and parental anxiety all play roles in treatment decisions.
The investigation of 5αR2D as a disease has played a key role in the biochemical characterization of the SRD5A2 gene, the 5αR2 enzyme, and DHT in male sexual differentiation.
Mutations in the SRD5A2 gene can result in a 46,XY disorder of sex development (46,XY DSD) called 5α-reductase 2 deficiency (5αR2D). The mutations are inherited in an autosomal recessive pattern can be either homozygous or, less frequently, compound heterozygous loss-of-function. Affected males exhibit a broad spectrum of presentation including atypical genitalia (ranging from female-appearing to undervirilized male), hypospadias, and isolated micropenis. The internal reproductive structures (vasa deferentia, seminal vesicles, epididymides and ejaculatory ducts) are normal but testes are usually undescended and prostate hypoplasia is common. Males with the same mutations in SRD5A2 can have different phenotypes suggesting additional factors that are involved in clinical presentation.
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Androgen insensitivity syndrome (AIS) is a difference in sex development involving hormonal resistance due to androgen receptor dysfunction. It affects 1 in 20,000 to 64,000 XY (karyotypically male) births. The condition results in the partial or complete inability of cells to respond to androgens. This unresponsiveness can impair or prevent the development of male genitals, as well as impairing or preventing the development of male secondary sexual characteristics at puberty.
Dihydrotestosterone (DHT, 5α-dihydrotestosterone, 5α-DHT, androstanolone or stanolone) is an endogenous androgen sex steroid and hormone. The enzyme 5α-reductase catalyzes the formation of DHT from testosterone in certain tissues including the prostate gland, seminal vesicles, epididymides, skin, hair follicles, liver, and brain. This enzyme mediates reduction of the C4-5 double bond of testosterone. DHT may also be synthesized from progesterone and 17α-hydroxyprogesterone via the androgen backdoor pathway in the absence of testosterone.
Gynecomastia (also spelled gynaecomastia) is the abnormal non-cancerous enlargement of one or both breasts in males due to the growth of breast tissue as a result of a hormone imbalance between estrogens and androgens. Gynecomastia can cause significant psychological distress or unease. Gynecomastia can be normal in newborn male babies due to exposure to estrogen from the mother, in adolescent boys going through puberty, in older men over age 50, and/or in obese men. Most occurrences of gynecomastia do not require diagnostic tests.
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