Vitamin D-dependent rickets type II: extreme end organ resistance to 1,25-dihydroxy vitamin D3 in a patient without alopecia

Abstract

Vitamin D-dependent rickets type II (VDDR II) is a rare syndrome resulting in severe rickets and is resistant to treatment with vitamin D and its derivatives. Patient with this disease, who are frequently the children of consanguinous marriages, present with elevated circulating concentrations of 1,25-dihydroxy vitamin D, the active metabolite of vitamin D, and in vitro studies have indicated a failure of intracellular binding of the hormone. Alopecia has been noted in many of these patients and it has been suggested that this feature may indicate a more marked resistance to treatment. However we describe a 3-year-old boy with this disease who, although having normal hair growth, displayed extreme resistance to treatment with active vitamin D metabolites. In vitro studies of skin fibroblasts disclosed not only an absence of hormone binding or 1,25(OH)2D3-induced 24-hydroxylase activity but reduced metabolism of 1,25(OH)2D3 itself. In this child, treatment with exogenous 1,25-dihydroxy vitamin D3 at doses of up to 24 micrograms/day, which increased the circulating concentration of the metabolite to greater than 100 times the normal adult mean, failed to alleviate his condition and he died at the age of 39 months. This would therefore suggest that absence of alopecia, in this condition, cannot be regarded as a constant predictive sign of a lesser resistance and of responsiveness to Vitamin D treatment.