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Congenital adrenal hyperplasia due to 17a-hydroxylase deficiency is an uncommon form of congenital adrenal hyperplasia resulting from a defect in the gene for the enzyme CYP17A1. It produces decreased synthesis of both cortisol and sex steroids, with resulting increase in mineralocorticoid production. Thus, common symptoms include mild hypocortisolism, ambiguous genitalia in genetic males or failure of the ovaries to function at puberty in genetic females, and hypertension (respectively).
This form of CAH results from deficiency of the enzyme 17a-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 50,000 births.
The most common abnormal alleles of this condition impair both the 17a-hydroxylase activity and the 17,20-lyase activity of CYP17A1. Like other forms of CAH, 17a-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 17a-hydroxylase and a 17,20-lyase. The dual activities mediate three key transformations in cortisol and sex steroid synthesis:
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: 17a-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.
The adrenal cortex is hyperplastic and overstimulated, with no impairment of the mineralocorticoid pathway. Consequently, levels of DOC, corticosterone, and 18-deoxycorticosterone are elevated. Although these precursors of aldosterone are weaker mineralocorticoids, the extreme elevations usually provide enough volume expansion, blood pressure elevation, and potassium depletion to suppress renin and aldosterone production. Some persons with 17a-hydroxylase deficiency develop hypertension in infancy, and nearly 90% do so by late childhood. The low-renin hypertension is often accompanied by hypokalemia due to urinary potassium wasting and metabolic alkalosis. These features of mineralocorticoid excess are the major clinical clue distinguishig the more complete 17a-hydroxylase deficiency from the 17,20-lyase deficiency, which only affects the sex steroids. Treatment with glucocorticoid suppresses ACTH, returns mineralocorticoid production toward normal, and lowers blood pressure.
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