Clinical and genetic analysis of patients with vitamin D-dependent rickets type 1A.

Context: Vitamin D-dependent rickets type 1A (VDDR-IA, OMIM 264700) is a rare autosomal recessive disorder and is caused by mutations in the CYP27B1 gene.

Objectives: We aim to investigate CYP27B1 mutation in seven patients from four separate families and characterize the genotype-phenotype correlation.

Methods: The entire coding region of the CYP27B1 gene was sequenced, and genotype-phenotype correlation among patients was assessed.

Results: Sequencing analysis identified biallelic CYP27B1 mutations in all patients and monoallelic mutations in their parents. One patient from the first family was compound heterozygous for c.1166G>A (p.Arg389His) and a novel nonsense mutation c.1079 C>A (p.Ser360*). Two patients from the second family were homozygous for a novel splice donor site mutation in intron 1 (c.195 + 2 T>G), causing partial retention of the intron and a shift in the reading frame. Both novel mutations lead to the complete loss of vitamin D1α-hydroxylase activity. Four patients from families 3 and 4 were homozygous for a previously reported duplication mutation in exon 8 (1319-1325dupCCCACCC, Phe443Profs*24). Interestingly, one patient who was presented with severe hypocalcaemia and seizures at 4 months of age as a result of Phe443Profs*24 has improved spontaneously since 11 years of age and does not need regular treatment. Her laboratory tests showed normal serum calcium and 1,25(OH)(2) D after refusing to take medication for 12 months.

Conclusions: There is a good genotype-phenotype correlation in VDDR-IA. However, some patients may recover from the loss of CYP27B1 function, probably due to 1α-hydroxylase activity exerted by a non-CYP27B1 enzyme.