2.1 17 Alpha-Hydroxylase/17,20-Lyase Deficiency

Dozens of mutations in the CYP17A1 gene have been found to cause 17α-hydroxylase/17,20-lyase deficiency. This condition affects the function of certain hormone-producing glands, leading to high blood pressure (hypertension) and abnormal sexual development. Mutations associated with this condition reduce or eliminate both 17α-hydroxylase and 17,20-lyase activity. Reduction of these activities leads to partial 17α-hydroxylase/17,20-lyase deficiency, while total loss of these activities leads to the more severe form of the disorder known as complete 17α-hydroxylase/17,20-lyase deficiency.

Without 17α-hydroxylase activity, pregnenolone and progesterone are not converted to 17-hydroxypregnenolone or 17-hydroxyprogesterone, impairing production of glucocorticoids. Instead pregnenolone and progesterone are processed to form mineralocorticoids. An excess of these salt-regulating hormones leads to hypertension and low levels of potassium in the blood (hypokalemia).

A loss of 17,20-lyase activity impairs sex hormone production. In females, a lack of female sex hormones disrupts development of internal reproductive organs (the ovaries and uterus) and secondary sex characteristics, such as breasts and menstrual periods. In chromosomal males (individuals with an X and a Y chromosome), a lack of male sex hormones leads to abnormal development of external genitalia. Depending on the severity of the condition, these affected individuals can have abnormal male genitalia, genitalia that do not look clearly male or clearly female (ambiguous genitalia), or characteristically female genitalia.

2.2 Other Disorders

A small number of CYP17A1 gene mutations have been found to cause isolated 17,20-lyase deficiency, which is characterized by abnormal sexual development without hypertension or hypokalemia. These mutations alter a region of the CYP17A1 protein that plays a role in the enzyme's 17,20-lyase function but not its 17α-hydroxylase function. As a result, 17,20-lyase activity is severely reduced but 17α-hydroxylase activity is normal. As in 17α-hydroxylase/17,20-lyase deficiency (described above), impairment of 17,20-lyase activity disrupts sex hormone production, leading to abnormal development of internal or external reproductive organs and delayed or absent puberty in affected individuals.