Characterization of complex structural variation in the CYP2D6-CYP2D7-CYP2D8 gene loci using single-molecule long-read sequencing.

Complex regions in the human genome such as repeat motifs, pseudogenes and structural (SVs) and copy number variations (CNVs) present ongoing challenges to accurate genetic analysis, particularly for short-read Next-Generation-Sequencing (NGS) technologies. One such region is the highly polymorphic CYP2D loci, containing CYP2D6, a clinically relevant pharmacogene contributing to the metabolism of >20% of common drugs, and two highly similar pseudogenes, CYP2D7 and CYP2D8. Multiple complex SVs, including CYP2D6/CYP2D7-derived hybrid genes are known to occur in different configurations and frequencies across populations and are difficult to detect and characterize accurately. This can lead to incorrect enzyme activity assignment and impact drug dosing recommendations, often disproportionally affecting underrepresented populations. To improve CYP2D6 genotyping accuracy, we developed a PCR-free CRISPR-Cas9 based enrichment method for targeted long-read sequencing that fully characterizes the entire CYP2D6-CYP2D7-CYP2D8 loci. Clinically relevant sample types, including blood, saliva, and liver tissue were sequenced, generating high coverage sets of continuous single molecule reads spanning the entire targeted region of up to 52 kb, regardless of SV present (n = 9). This allowed for fully phased dissection of the entire loci structure, including breakpoints, to accurately resolve complex CYP2D6 diplotypes with a single assay. Additionally, we identified three novel CYP2D6 suballeles, and fully characterized 17 CYP2D7 and 18 CYP2D8 unique haplotypes. This method for CYP2D6 genotyping has the potential to significantly improve accurate clinical phenotyping to inform drug therapy and can be adapted to overcome testing limitations of other clinically challenging genomic regions.

Characterization of Novel CYP2D6 Alleles across Sub-Saharan African Populations.

The CYP2D6 gene has been widely studied to characterize variants and/or star alleles, which account for a significant portion of variability in drug responses observed within and between populations. However, African populations remain under-represented in these studies. The increasing availability of high coverage genomes from African populations has provided the opportunity to fill this knowledge gap. In this study, we characterized computationally predicted novel CYP2D6 star alleles in 30 African subjects for whom DNA samples were available from the Coriell Institute. CYP2D6 genotyping and resequencing was performed using a variety of commercially available and laboratory-developed tests in a collaborative effort involving three laboratories. Fourteen novel CYP2D6 alleles and multiple novel suballeles were identified. This work adds to the growing catalogue of validated African ancestry CYP2D6 allelic variation in pharmacogenomic databases, thus laying the foundation for future functional studies and improving the accuracy of CYP2D6 genotyping, phenotype prediction, and the refinement of clinical pharmacogenomic implementation guidelines in African and global settings.

Identification of CYP2D6 Haplotypes that Interfere with Commonly Used Assays for Copy Number Variation Characterization.

Cytochrome P450 2D6 (CYP2D6) copy number (CN) variation affects the metabolism of numerous prescribed drugs. Sequence variation within primer or probe target regions of hydrolysis probe CN assays can generate false-positive calls for CN loss. Furthermore, CYP2D6-CYP2D7 hybrids and gene conversions can cause difficult to interpret discordant CN calls. The identification of haplotypes with CN variations and structural arrangements is important to predict phenotype accurately. During clinical testing with hydrolysis probe assays targeting three CYP2D6 regions (intron 2, intron 6, and exon 9), samples with haplotypes causing inconsistent CN calls were identified. To resolve these cases, next-generation sequencing and allele-specific Sanger sequencing was performed. Sequence analysis of 16 samples, all but one from subjects of African descent, identified six novel suballeles containing single-nucleotide polymorphisms, which cause false-positive calls for CN loss in introns 2 and 6. Five samples with an exon 9 CN loss contained CYP2D6/CYP2D7 hybrids (∗13 or ∗36) and one sample was found to have a novel haplotype, CYP2D6∗141. Interestingly, CYP2D6∗141 contains a CYP2D7-derived exon 9 conversion and core single-nucleotide polymorphisms that are otherwise found in CYP2D6∗17 and ∗27. Although these variants are rare, they can cause inconsistent CN calls that typically are reported as no calls or indeterminant, and thus may deprive patients, particularly those of African descent, from taking full benefit of pharmacogenetic testing.