LAUSR

BACKGROUND AND OBJECTIVES CYP24A1 encodes a 24-hydroxylase involved in vitamin D catabolism, whose loss-of-function results in vitamin D-dependent hypercalcemia. Since the identification of CYP24A1 variants as a cause of idiopathic infantile hypercalcemia (IIH), a large body of literature has emerged indicating heterogeneity in penetrance, symptoms, biochemistry, and treatments. The objectives of the present research work were to investigate the clinical heterogeneity of the disease, the possibility of a relevant phenotype for monoallelic carriers, and to compare the hypocalcemic effect of the available therapies. METHODS Two reviewers searched different databases for studies published between the identification of CYP24A1 variants and December 31st,2020. Eligible studies included clinical trials and reports describing carriers of CYP24A1 variants. RESULTS Fifty eligible studies were identified, accounting for 221 patients. Genetic data were retrieved and allele frequencies calculated. Acute hypercalcemia was the typical presentation during the first year of life (76%, P=0.0005), and nephrocalcinosis was more frequent in infancy (P<0.0001). Pregnancy was associated with symptomatic hypercalcemia in 81.8%, and high rates of obstetric complications. Monoallelic carriers displayed significant rates of nephrolithiasis (19.4%), nephrocalcinosis (4.9%), and symptomatic hypercalcemia (5.6%). The effect size of the most-used medications administered to control hypercalcemia ranged from 18 to 29%. CONCLUSIONS CYP24A1 loss-of-function results in an age-dependent phenotype, which can be exacerbated by triggering factors, such as pregnancy. Although biallelic carriers present more significant clinical and biochemical features, monoallelic carriers have an increased risk of calcium-related conditions. The highly-variable tested therapeutic approaches did not allow to draw conclusions on preferable therapeutic regime.