Novel CYP27B1 Gene Mutations in Patients with Vitamin D-Dependent Rickets Type 1A

Abstract

The CYP27B1 gene encodes 25-hydroxyvitamin D-1α-hydroxylase. Mutations of this gene cause vitamin D-dependent rickets type 1A (VDDR-IA, OMIM 264700), which is a rare autosomal recessive disorder. To investigate CYP27B1 mutations, we studied 8 patients from 7 unrelated families. All coding exons and intron-exon boundaries of CYP27B1 gene were amplified by PCR from peripheral leukocyte DNA and subsequently sequenced. Homozygous mutations in the CYP27B1 gene were found in all the patients and heterozygous mutations were present in their normal parents. One novel single nucleotide variation (SNV, c.1215 T>C, p.R379R in the last nucleotide of exon 7) and three novel mutations were identified:, a splice donor site mutation (c.1215+2T>A) in intron 7, a 16-bp deletion in exon 6 (c.1022-1037del16), and a 2-bp deletion in exon 5 (c.934_935delAC). Both c.1215 T>C and c.1215+2T>A were present together in homozygous form in two unrelated patients, and caused exon 7 skipping. However, c.1215 T>C alone has no effect on pre-mRNA splicing. The skipping of exon 7 resulted in a shift of downstream reading frame and a premature stop codon 57 amino acids from L380 (p.L380Afs*57). The intra-exon deletions of c.1022-1037del16 and c.934_935delAC also resulted in a frameshift and the creation of premature stop codons at p.T341Rfs*5, and p.T312Rfs*19, respectively, leading to the functional inactivation of the CYP27B1 gene. Clinically, all the patients required continued calcitriol treatment and the clinical presentations were consistent with the complete loss of vitamin D1α-hydroxylase activity. In conclusion, three novel mutations have been identified. All of them caused frameshift and truncated proteins. The silent c.1215 T>C SNV has no effect on pre-mRNA splicing and it is likely a novel SNP. The current study further expands the CYP27B1 mutation spectrum.

A Case of Vitamin-D-Dependent Rickets Type 1A with Normal 1,25-Dihydroxyvitamin D Caused by Two Novel Mutations of the <b><i>CYP27B1</i></b> Gene

“…Most cases stem from the deficient function of the 1α-hydroxylase, also described as vitamin-D-dependent rickets type 1A (VDDR-1A), which is encoded by the CYP27B1 gene at chromosome 12 [2][3][4] . So far, 64 different mutations in CYP27B1 have been reported in more than 100 patients [5,6] . Children with VDDR-1A present in early infancy with muscle weakness, joint deformity, bowed legs, growth failure, seizures or fractures, and metabolic abnormalities of the calcium homeostasis, with the vast majority of the patients having as a hallmark low levels of 1,25-(OH) 2 vitamin D in the presence of normal 25-hydroxyvitamin D (25-OH vitamin D) levels [6] .…”

A Case of Vitamin D-Dependent Rickets Type 1A with a Novel Mutation in the Uzbek Population

“…VDDR-1A is an autosomal recessive disorder due to an inactivating mutation in the CYP27B1 gene on chromosome 12q14 (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21). The CYP27B1 gene encodes 25(OH) D-1-α-hydroxylase which catalyzes the hormonally regulated, rate limiting step in the bioactivation of vitamin D (1,2,3,4).…”

“…The CYP27B1 gene is mapped on chromosome 12q14 (4). In different ethnic groups, several mutations (homozygous or compound heterozygous) have been identified in patients with VDDR-1A (3,5,7,8,9,10,11,g,13,14,15,16,17,18,19,20,21). In some ethnic groups, certain mutations are more frequent (2,8,14,20).…”

Genetic and clinical characteristics of the patients with Vitamin D Dependent Rickets Type 1A

“…The second step occurs mainly in the kidney, where 25-OHD is hydroxylated by the mitochondrial vitamin D 1α-hydroxylase to the biologically active hormone 1, 25-dihydroxyvitamin D (1, 25-OH 2 D), which binds to its nuclear receptor and exerts its biological activities (1,2,3). The biologically active 1, 25-OH 2 D plays a central role in calcium homeostasis and bone metabolism, and also has a significant influence on cell proliferation and u n c o r r e c t e d p r o o f differentiation of a variety of tissues (1,3,4). The renal synthesis of 1, 25-OH 2 D from its precursor 25-OHD is a rate-limiting step and is tightly regulated by serum 1, 25-OH 2 D, parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), calcium and phosphate, with renal 1α-hydroxylase being stimulated by PTH, hypophosphatemia, or hypocalcaemia, and inhibited by FGF23 (4).…”