June 1, 2021  |  

Unique haplotype structure determination in human genome using Single Molecule, Real-Time (SMRT) Sequencing of targeted full-length fosmids.

Determination of unique individual haplotypes is an essential first step toward understanding how identical genotypes having different phases lead to different biological interpretations of function, phenotype, and disease. Genome-wide methods for identifying individual genetic variation have been limited in their ability to acquire phased, extended, and complete genomic sequences that are long enough to assemble haplotypes with high confidence. We explore a recombineering approach for isolation and sequencing of a tiling of targeted fosmids to capture interesting regions from human genome. Each individual fosmid contains large genomic fragments (~35?kb) that are sequenced with long-read SMRT technology to generate contiguous long reads. These long reads can be easily de novo assembled for targeted haplotype resolution within an individual’s genomes. The P5-C3 chemistry for SMRT Sequencing generated contiguous, full-length fosmid sequences of 30 to 40 kb in a single read, allowing assembly of resolved haplotypes with minimal data processing. The phase preserved in fosmid clones spanned at least two heterozygous variant loci, providing the essential detail of precise haplotype structures. We show complete assembly of haplotypes for various targeted loci, including the complex haplotypes of the KIR locus (~150 to 200 kb) and conserved extended haplotypes (CEHs) of the MHC region. This method is easily applicable to other regions of the human genome, as well as other genomes.

June 1, 2021  |  

Immune regions are no longer incomprehensible with SMRT Sequencing

The complex immune regions of the genome, including MHC and KIR, contain large copy number variants (CNVs), a high density of genes, hyper-polymorphic gene alleles, and conserved extended haplotypes (CEH) with enormous linkage disequilibrium (LDs). This level of complexity and inherent biases of short-read sequencing make it challenging for extracting immune region haplotype information from reference-reliant, shotgun sequencing and GWAS methods. As NGS based genome and exome sequencing and SNP arrays have become a routine for population studies, numerous efforts are being made for developing software to extract and or impute the immune gene information from these datasets. Despite these efforts, the fine mapping of causal variants of immune genes for their well-documented association with cancer, drug-induced hypersensitivity and immune-related diseases, has been slower than expected. This has in many ways limited our understanding of the mechanisms leading to immune disease. In the present work, we demonstrate the advantages of long reads delivered by SMRT Sequencing for assembling complete haplotypes of MHC and KIR gene clusters, as well as calling correct genotypes of genes comprised within them. All the genotype information is detected at allele- level with full phasing information across SNP-poor regions. Genotypes were called correctly from targeted gene amplicons, haplotypes, as well as from a completely assembled 5 Mb contig of the MHC region from a de novo assembly of whole genome shotgun data. De novo analysis pipeline used in all these approaches allowed for reference-free analysis without imputation, a key for interrogation without prior knowledge about ethnic backgrounds. These methods are thus easily adoptable for previously uncharacterized human or non-human species.

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