Exon 10 skipping caused by intron 10 splice donor site mutation in cholesteryl ester transfer protein gene results in abnormal downstream splice site selection

Abstract

Cholesteryl ester transfer protein (CETP) deficiency is the most common cause of hyperalphalipoproteinemia in Japan. However, the genetic basis of this disorder has not been fully characterized. We have studied a 49-year-old Japanese male presenting with total cholesterol, HDL-cholesterol, and apolipoprotein A-I levels of 300, 236, and 233 mg/dl, respectively, and total absence of CETP activity and mass in plasma. Sequence analysis of the patient's CETP gene revealed that the splice donor consensus GT was substituted by GG in intron 10 (intron 10 splice defect) and by AT in intron 14 (intron 14 splice defect). Restriction digestion of PCR-amplified DNA using NdeI and MaeIII established that the patient was a compound heterozygote for both gene defects. Sequencing of cDNA amplified by RT-PCR from the patient's monocyte-derived macrophage RNA demonstrated abnormal splicing with deletion of exon 10 as well as alternative splicing at a native AG site located 31 nucleotides 5' of the normal splice acceptor in intron 13. Thus, the intron 10 splice defect results in exon 10 skipping and the insertion of a 31 bp fragment between exon 13 and exon 14, which contains an in frame stop codon. The presence of abnormally spliced mRNA was further confirmed by amplification of patient cDNA using CETP specific primers. Abnormal splicing of exon 14 as a result of the intron 14 splice defect was not detected, indicating potential unstable CETP mRNA derived from that mutation. These findings demonstrate that a novel splice site mutation in intron 10 of the CETP gene results in the skipping of exon 10, as well as disruption of downstream splicing at intron 13 identifying a novel mechanism leading to CETP deficiency.