NOVEL MUTATIONS IN PATIENTS WITH VITAMIN D-DEPENDENT RICKETS TYPE 2A: A MILD DISEASE PHENOTYPE CAUSED BY A NOVEL CANONICAL SPLICE SITE MUTATION.

Vitamin D-dependent rickets type 2A (VDDR2A) is a rare autosomal recessive disorder and is caused by mutations in the vitamin D receptor gene (), leading to end-organ resistance to 1,25(OH)D. To investigate mutation in 11 patients from 8 Turkish-Arab families. All coding exons and intron-exon boundary of gene were amplified by PCR from peripheral leukocyte DNA and sequenced. The effect of splice site mutation on mRNA splicing was evaluated by a customized mini-gene assay. Homozygous mutations were found in all the patients including four novel mutations: c.473G>T (p.R158L), c.1-4A>G (IVS3-2A>G), c.755+1G>T, and c.352_356delGACAG (p.D118Sfs*7). The c.1-4A>G was located in the canonical splice acceptor site and 4bp from original ATG start codon. The mutation resulted in both complete (60% transcripts) and partial exon 4 skipping (15% transcripts). The latter was due to activation of a cryptic splice acceptor site and did not disrupt the open reading frame. Both c.755+1G>T and c.352_356delGACAG resulted in frameshift and a premature stop codon. Clinically, all the patients required continued treatment except for patient IV-3, who presented with alopecia, hypocalcemia, and increased 1,25(OH)D at 1.5 years of age as a result of c.1-4A>G mutation. He stopped taking medication at 6 years of age and still maintained normal height and biochemical profile. We have identified four novel mutations. Although canonical splice site mutations cause pre-mRNA splicing errors that usually lead to severe disease phenotype, mild disease can also occur due to activation of a cryptic splice site.