Coexistent CYP24A1 and PHEX Gene Mutations With Hypervitaminosis D Plus Hypercalcemia Treated With Fluconazole

Background: CYP24A1 and PHEX gene mutations are rare and can cause hypercalcemia, hypervitaminosis D and elevated FGF23 levels. Fluconazole, an antifungal medication, has shown therapeutic benefit in achieving normocalcemia plus normalisation of vitamin D levels in this case report. Clinical Case: A 42 year old man was referred to the endocrine clinic with a history of severe nephrocalcinosis and recurrent nephrolithiasis requiring surgical intervention and gradual decline in kidney function over 20 years. Biochemical investigations revealed hypercalcaemia with adjusted calcium levels of 2.83 mmol/L (R 2.2–2.6 nmol/L) and suppressed PTH 1.1 pmol/L(R 1.6–6.9 pmol/L). Twenty-four hour urine calcium/creatinine clearance ratio was above 0.0578 mmol/mmol indicating hypercalciuria. Vitamin D metabolites 25 OH Vitamin D was elevated at 201 nmol/L, (R 50–120 nmol/L) along with intermittently elevated 1,25 OH Vitamin D 147 pmol/L(R 55–139 pml/L). 24,25 Vitamin D was low at 2.0 nmol/L producing a 25:24,25 dihydroxyvitamin D ratio of 80 (n<25). This biochemical data was highly suggestive of a loss of function mutation in the CYP24A1 gene that codes for the enzyme 24-hydroxylase, which is responsible for conversion of 1,25 vitamin D to 24,25 vitamin D. A pathogenic variant (heterozygous c.756G>A) was confirmed on genetic testing. Plasma FGF23 (immutopics) was raised (with a peak of 596 RU/mL, n<100 RU/mL) but a full body octreotide scan did not reveal malignancy or other paraneoplastic syndromes such as oncogenic osteomalacia. A pathogenic variant in his PHEX gene (homozygous c.1874A>T) was also identified that has been associated with increased levels of FGF23 plus hypophosphataemia. Fluconazole at 50 mg once daily was initiated. Azoles inhibit cytochrome P450 enzymes and have been used in sarcoidosis to block vitamin D-synthesizing enzymes such as 25-hydroxylases and 1-α-hydroxylase that are P450 dependent. Few cases of CYP24A1 gene defects have been treated with fluconazole, which has a favourable side effect profile and yields good results. Adjusted calcium reduced to 2.62 nmol/L, 25 OH Vitamin D normalised to 111 nmol/L and 24:24,25 dihydroxyvitamin D ratio is now 17. Patient’s liver functions and full blood count has been monitored regularly during the course of treatment and the drug was well tolerated. Conclusion: Genetic causes of hypercalcemia can be left undiagnosed for long periods and there is a lack of proven or definitive therapeutic agents for correction of elevated calcium. Here fluconazole has been shown to reduce the hypercalcaemic burden and effectively lowered the Vitamin D levels in this case of a CYP24A1 mutation. This study augments fluconazole use in these cases but further studies are needed to elucidate the long term safe usage.