| CYP7B1 is a microsomal cytochrome P450 monooxygenase that acts as an oxysterol and steroid 7α‑hydroxylase, catalyzing the NADPH‑dependent insertion of oxygen at the 7α (and in some cases 6α) position of diverse cholesterol‑derived substrates and thereby linking oxysterol, neurosteroid, and bile acid metabolism. The enzyme is expressed in liver and in multiple steroidogenic or steroid‑responsive tissues including brain, testis, ovary, prostate, kidney, and intestine, where it hydroxylates oxysterols such as 25‑hydroxycholesterol, 27‑hydroxycholesterol, and cholest-5-ene-3β,25-diol, as well as neurosteroids like dehydroepiandrosterone and pregnenolone, with structural requirements that include a 3β‑hydroxy group, a polar substituent at C17, and absence of additional side‑chain hydroxyls beyond C20–C24. In extrahepatic tissues, CYP7B1 initiates a cholesterol catabolic route by converting oxysterols to 7α‑hydroxy derivatives that can be transported to the liver and further metabolized to bile acids, providing an “alternative” bile acid synthesis pathway complementary to the classic CYP7A1‑dependent route; in the immune system, the CYP7B1 product 7α,25‑dihydroxycholesterol functions as a high‑affinity ligand for the chemotactic receptor GPR183/EBI2 and guides B‑cell positioning within germinal centers, directly coupling CYP7B1 activity to humoral immune architecture. In the brain, CYP7B1 hydroxylates neuroactive steroids and oxysterols, modulating their signaling via nuclear receptors and helping to maintain cholesterol and neurosteroid balance, and Cyp7b1‑null mice display loss of oxysterol 7α‑hydroxylation in liver and brain along with altered levels of multiple oxysterols, indicating that this enzyme is the major 7α‑hydroxylase for these substrates in vivo. Human loss‑of‑function CYP7B1 mutations cause autosomal recessive hereditary spastic paraplegia type 5, where biochemical studies document accumulation of 25‑ and 27‑hydroxycholesterol and related cholestenoic acids, and clinical cohorts show both pure and complicated spastic paraplegia phenotypes with variable white‑matter changes, supporting a pathogenic link between impaired CYP7B1‑mediated detoxification of oxysterols, disturbed bile acid and neurosteroid metabolism, and long‑tract motor neuron degeneration. |
