Background: The CYP27B1 gene encodes 25-hydroxyvitamin D-1α-hydroxylase. Mutations of this gene cause a rare autosomal recessive disorder, vitamin D-dependent rickets type 1A.
Objective and hypotheses: To investigate CYP27B1 mutations in children when rickets was associated with normal or high vitamin D levels and low or inappropriately normal calcitriol levels.
Method: All coding exons and intron-exon boundaries of CYP27B1 gene from eight patients and their parents from seven unrelated families were sequenced. RNA extraction and mini-gene analysis were performed.
Results: Clinically, all the patients (M/F:4/4, age range 12 months-11 years) required continued calcitriol treatment and the clinical presentations were consistent with the complete loss of vitamin D1α-hydroxylase activity. Biallelic mutations in the CYP27B1 gene were found in all the patients and monoallelic mutations were present in their asymptomatic parents. Four novel mutations were identified: c.1215 T>C (p.R379R) in the last nucleotide of exon 7, a splice donor site mutation (c.1215+2T>A) in intron 7, a 16-bp deletion in exon 6 (c.1022-1037del16, p.T341Rfs*346), and a 2-bp deletion in exon 5 (c.934_935delAC, p.T312Rfs*331). Both c.1215 T>C and c.1215+2T>A were present together in two unrelated patients, and caused exon 7 skipping. However, c.1215 T>C alone has no effect on RNA splicing. The skipping of exon 7 resulted in a shift of the downstream reading frame and a premature stop codon 57 amino acids from L380 (p.L380Afs*437). The intra-exon deletions of c.1022-1037del16 and c.934_935delAC also resulted in frameshift and the creation of premature stop codons at p.T341Rfs*346, and p.T312Rfs*331, respectively, leading to the functional inactivation of the CYP27B1 gene.
Conclusion: Four novel mutations have been identified. Three of them caused frameshift and truncated proteins. The silent c.1215 T>C has no effect on pre-mRNA splicing and may be considered as a novel SNP.
01 - 03 Oct 2015
European Society for Paediatric Endocrinology