An Alu retrotransposition-mediated deletion of CHD7 in a patient with CHARGE syndrome

Abstract

CHD7 mutations account for about 60-65% among more than 200 CHARGE syndrome cases. When rare whole gene deletion cases associated with chromosomal abnormalities are excluded, all mutations of CHD7 reported to date have been point mutations and small deletions and insertions, rather than exonic deletions. To test whether exonic deletions represent a common pathogenic mechanism, we assessed exon copy number by using a recently developed method, the multiplex PCR/liquid chromatography assay (MP/LC). Multiple exons were amplified using unlabeled primers, then separated by ion-pair reversed-phase high-performance liquid chromatography, and quantitated by fluorescence detection using a post-column intercalation dye under the premise that the relative peak intensities for each target directly reflect exon copy number. By using MP/LC, we identified one CHARGE syndrome patient who had a de novo deletion encompassing exons 8-12 among 13 classic CHARGE patients in whom screening by denaturing high-performance liquid chromatography (DHPLC) failed to identify point mutations and small insertions/deletions in CHD7. This is the first CHARGE patient who was documented to have exonic deletion of CHD7. The deletion closely recapitulated the Alu-mediated inactivation of the human CMP-N-acetylneuraminic acid hydroxylase gene (CMP-Neu5Ac hydroxylase), which is regarded as a novel molecular mechanism in the evolution from non-human primates to humans. As demonstrated in this study, MP/LC is a promising method for characterizing exonic deletions, which are largely left unexamined in most routine mutation analysis.