Congenital Adrenal Hyperplasia Due To 17 Alpha-hydroxylase Deficiency - Pathophysiology

Pathophysiology

This form of CAH results from deficiency of the enzyme 17α-hydroxylase (also called CYP17A1). It accounts for less than 5% of the cases of congenital adrenal hyperplasia and is inherited in an autosomal recessive manner with a reported incidence of about 1 in 1,000,000 births.

The most common abnormal alleles of this condition impair both the 17α-hydroxylase activity and the 17,20-lyase activity of CYP17A1. Like other forms of CAH, 17α-hydroxylase deficiency impairs the efficiency of cortisol synthesis, resulting in high levels of ACTH secretion and hyperplasia of the adrenal glands. Clinical effects of this condition include overproduction of mineralocorticoids and deficiency of prenatal and pubertal sex steroids.

CYP17A1 functions in steroidogenesis, where it converts pregnenolone and progesterone to their 17-hydroxy forms. The enzyme itself is attached to the smooth endoplasmic reticulum of the steroid-producing cells of the adrenal cortex and gonads. CYP17A1 functions as both a 17α-hydroxylase and a 17,20-lyase. The dual activities mediate three key transformations in cortisol and sex steroid synthesis:

• As 17α-hydroxylase it mediates pregnenolone → 17-hydroxypregnenolone
• and progesterone → 17-hydroxyprogesterone.
• As 17,20-lyase it mediates 17-hydroxypregnenolone → DHEA.
• An expected second 17,20-lyase reaction (17-hydroxyprogesterone → androstenedione) is mediated so inefficiently in humans as to be of no known significance.

The hydroxylase reactions are part of the synthetic pathway to cortisol as well as sex steroids, but the lyase reaction is only necessary for sex steroid synthesis. Different alleles of the CYP17A1 gene result in enzyme molecules with a range of impaired or reduced function that produces a range of clinical problems. The OMIM number for diseases arising from mutations of this gene is 202110.

The dual enzyme activities were for many decades assumed to represent two entirely different genes and enzymes. Thus, medical textbooks and nosologies until quite recently described two different diseases: 17α-hydroxylase deficient CAH, and a distinct and even rarer defect of sex steroid synthesis termed 17,20-lyase deficiency (which is not a form of CAH). In the last decade it has become clearer that the two diseases are different forms of defects of the same gene. However, the clinical features of the two types of impairment are distinct enough that they are described separately in the following sections.