Complete Genomic Ablation of Cytochrome P450 Oxidoreductase Unveils Endogenous Steroidogenic Shunting and Variant Instability

Cytochrome P450 oxidoreductase (POR) is the obligate electron donor for microsomal cytochrome P450 enzymes. Recessive mutations in the POR gene cause POR deficiency (PORD), a severe metabolic disorder characterized by skeletal malformations, ambiguous genitalia, and adrenal insufficiency. Because global POR knockout is embryonically lethal in mammalian models, the mechanistic study of PORD has historically been restricted to reconstituted biochemical assays or non-steroidogenic cellular backgrounds. Here, we describe complete biallelic POR knockout in human adrenal-derived NCI-H295R and human embryonic kidney HEK293T cells using CRISPR/Cas9. Transcriptomic and mass spectrometric steroid profiling of the adrenal clones revealed a blockade in canonical steroidogenesis, characterized by upstream accumulation of pregnenolone and progesterone, and severe depletion of downstream glucocorticoids and mineralocorticoids. Strikingly, knockout cells maintained low-level synthesis of dehydroepiandrosterone (DHEA) despite the complete absence of canonical CYP17A1 catalytic support, providing critical in vitro validation for the activation of alternative, CYP17A1-independent bypass steroidogenic pathways. Furthermore, utilizing the HEK293T platform, we evaluated the functional complementation of clinically relevant variants (A287P, R457H, P228L, and delP399_E401). Notably, the highly prevalent P228L variant exhibited selective preservation of CYP17A1 activity while severely impairing CYP19A1 aromatase function. A direct comparison between episomal overexpression and endogenous CRISPR prime editing of P228L highlighted critical differences in enzyme efficiency under native regulatory control. Finally, we establish a link between POR loss and altered intracellular Fe(II) storage, indicating perturbed ferroptotic susceptibility. These engineered human cell models provide a highly tractable platform for interrogating mutant-specific pharmacogenomics and developing targeted interventions.