Do the heterozygous carriers of a CYP24A1 mutation display a different biochemical phenotype than wild types?

Context: Human cytochrome P450 24 subfamily A member 1 (CYP24A1) loss-of-function mutations result in impaired activity of the 24-hydroxylase involved in vitamin D catabolism, thus inducing a vitamin D-dependent hypercalcemia. Homozygotes often present an overt clinical phenotype named Idiopathic Infantile hypercalcemia (IIH), whereas it is debated whether heterozygotes display an abnormal phenotype. Objectives: To compare the clinical and biochemical features of heterozygous carriers of CYP24A1 variant and healthy wild-type controls, sharing the same genetic and environmental exposure. Methods: A large family harboring the nonsense c.667A>T, p.Arg223* pathogenic variant in the CYP24A1 gene was evaluated. All subjects underwent clinical and biochemical evaluation and complete analysis of vitamin D metabolites using mass-spectroscopy including 1,24,25(OH)3D3. Subjects were divided according to their genotype in two groups: heterozygotes and wild-type for the CYP24A1 variant. Results: The proband, a 40-year-old man, homozygous for p.Arg223* pathogenic variant, had a history of mild hypercalcemia with a seasonal trend, recurrent nephrolithiasis, and no episodes of acute hypercalcemia. He showed the highest serum levels of FGF23, the highest 25(OH)D3/24,25(OH)2D3 ratio and undetectable levels of 1,24,25(OH)3D3 which represent indicators of a loss-of function CYP24A1. Compared to the wild-types, heterozygotes had higher serum calcium and 25(OH)D3 concentrations (P=0.017 and P=0.025, respectively), without any difference in the other biochemical parameters and in the rate of nephrolithiasis. Conclusions: Heterozygotes exhibit a biochemical phenotype different from that of wild-type subjects. In clinical practice, these individuals might require surveillance because of the potential risk of developing hypercalcemia and related clinical manifestations if exposed to triggering factors.