Disorders of vitamin D metabolism include vitamin D deficiency, resistance, and biosynthesis, which include mutations in CYP27B1 and CYP2R1. These disorders result in the clinical and biochemical manifestations of rickets. There are two forms of vitamin D dependent rickets, type 1 (VDDR-I) and 2 (VDDR-II). VDDR-IA is common in French-Canadians from the Charlevoix-Saguenay-Lac Saint Jean area of Quebec with a carrier frequency of 1 in 26. It is characterized clinically by hypotonia, delayed motor development, seizures; due to hypocalcemia in early infancy, growth retardation, rickets and osteomalacia. The age of onset of clinical symptoms is 2 years of age.
Laboratory findings include hypocalcemia, elevated serum PTH, markedly decreased or absent serum 1,25-dihydroxyvitamin D3 and normal serum 25-hydroxyvitamin D3. VDDR-I can be treated with physiologic levels of 1,25-dihydroxyvitamin D3 or 1-alpha-hydroxyvitamin D3. Some studies had reported relatively strong genotype-phenotype correlation. In addition, the presence of a residual enzyme activity may contribute to the mild phenotype in some affected individuals.