Lipoid Congenital Adrenal Hyperplasia - Pathophysiology

Pathophysiology

The deficiency results in impaired synthesis of all three categories of adrenal steroids (cortisol, mineralocorticoids, sex steroids) and high levels of adrenocorticotropic hormone (ACTH). A low level of steroid synthesis proceeds even without efficient transport, but is rarely enough to prevent the consequences of deficiency. While severe loss of steroid production results in manifestation of the disease within a few weeks of birth, milder forms (late onset) can present years after birth. Unlike in models of the disease in mice, patients with lipoid CAH do not always have enlarged adrenals due to lipid accumulation. This may in part be due to hormone replacement used to keep them alive preventing hyperstimulation of the gland by the pituitary.

ACTH stimulates growth of the adrenal cells and increases LDL receptors to amplify transport of cholesterol into the cells of the adrenal cortex which make adrenal steroids, where it accumulates since little can enter the mitochondria for conversion to steroid. Normally, adrenal steroids then signal their presence to the brain to moderate ACTH levels (feedback inhibition). However, in the absence of this, ACTH levels are elevated and cholesterol uptake by the cortical cells continues unabated. The adrenals become markedly enlarged (hyperplastic) by the accumulated lipid. Lipid accumulation is thought to damage the cells further (“second hit hypothesis”).

Because P450scc and StAR are also essential for sex steroid synthesis in the testis and ovary, the production of testosterone by Leydig cells in the testis and androgen (which leads to estrogen production by granulosa cells) and progesterone by ovarian theca cells and luteal cells, respectively, can also be impaired. Similar to the adrenal gland, cholesterol accumulation damages the Leydig cells of the testes. In the ovary, the damage begins after puberty, the time when the ovary starts making steroid with follicle development. The placenta also makes steroid to help maintain pregnancy. However, since StAR is not required for placental steroid production, pregnancy goes to term. When the mutation in P450scc that causes lipoid CAH is either heterozygous or its presence on both alleles does not completely destroy all function, affected babies can survive to birth as well. Also of note, enlargement of the adrenal gland is not always found in the patient, especially in cases where a mutation in the gene for P450scc is the cause.

The pathophysiology of lipoid CAH differs from other forms of CAH in certain aspects. First, the affected gene in most cases is that for a transport protein (StAR) rather than a steroidogenic enzyme. Second, because the defect compromises all steroid synthesis. Thus, there are no problems due to excessive mineralocorticoids or androgens. Third, lipid accumulation damages the testes and ovaries so that even with appropriate adrenal hormone replacement (and in the absence of other intervention), gonadal function and fertility cannot be preserved.