Novel CUL3 Variant Causing Familial Hyperkalemic Hypertension Impairs Regulation and Function of Ubiquitin Ligase Activity.

Familial hyperkalemic hypertension is caused by pathogenic variants in genes of the CUL3 (cullin-3)-KLHL3 (kelch-like-family-member-3)-WNK (with no-lysine [K] kinase) pathway, manifesting clinically as hyperkalemia, metabolic acidosis, and high systolic blood pressure. The ubiquitin E3 ligase CUL3-KLHL3 targets WNK kinases for degradation to limit activation of the thiazide-sensitive NCC (Na-Cl cotransporter). All known variants in CUL3 lead to exon 9 skipping (CUL3Δ9) and typically result in severe familial hyperkalemic hypertension and growth disturbances in patients. Whether other variants in CUL3 cause familial hyperkalemic hypertension is unknown. Here, we identify a novel de novo heterozygous CUL3 variant (CUL3Δ474-477) in a pediatric familial hyperkalemic hypertension patient with multiple congenital anomalies and reveal molecular mechanisms by which CUL3Δ474-477 leads to dysregulation of the CUL3-KLHL3-WNK signaling axis. Using patient-derived urinary extracellular vesicles and dermal fibroblasts, in vitro assays, and cultured kidney cells, we demonstrate that CUL3Δ474-477 causes reduced total CUL3 levels due to increased autoubiquitination. The CUL3Δ474-477 that escapes autodegradation shows enhanced modification with NEDD8 (neural precursor cell expressed developmentally down-regulated protein 8) and increased formation of CUL3-KLHL3 complexes that are impaired in ubiquitinating WNK4. Proteomic analysis of CUL3 complexes revealed that, in addition to increased KLHL3 binding, the CUL3Δ474-477 variant also exhibits increased interactions with other BTB (Bric-a-brac, Tramtrack, and Broad complex) substrate adaptors, providing a rationale for the patient's diverse phenotypes. We conclude that the pathophysiological effects of CUL3Δ474-477 are caused by reduced CUL3 levels and formation of catalytically impaired CUL3 ligase complexes.