June 1, 2021  |  

HLA variant identification techniques

The Human Leukocyte Antigen (HLA) genes located on chromosome 6 are responsible for regulating immune function via antigen presentation and are one of the determining factors for stem cell and organ transplantation compatibility. Additionally various alleles within this region have been implicated in autoimmune disorders, cancer, vaccine response and both non-infectious and infectious disease risk. The HLA region is highly variable; containing repetitive regions; and co-dominantly expressed genes. This complicates short read mapping and means that assessing the effect of variation within a gene requires full phase information to resolve haplotypes.One solution to the problem of HLA identification is the use of statistical inference to suggest the most likely diploid alleles given the genotypes observed. The assumption of this approach is the availability of an extensive reference panel. Whilst there exists good population genetics data for imputing European populations, there remains a paucity of information about variation in African populations. Filling this gap is one of the aims of the Genome Diversity in Africa Project and as a first step we are performing a pilot study to identify the optimal method for determining HLA type information for large numbers of samples from African populations.To that end we have obtained samples from 125 consented African participants selected from 5 populations across Africa (Morrocan, Ashanti, Igbo, Kalenjin, and Zulu). The methods included in our pilot study are Sanger sequencing (ABI), NGS on HiSeqX Ten platform (Illumina); long-range PCR combined with single molecule real-time (SMRT) sequencing (PacBio); and for a subset of samples library preparation on GemCode Platform (10x Genomics), which delivers valuable long range contextual information, combined with Illumina NGS sequencing.Results from capillary sequencing suggests the presence of a minimum of two novel alleles. Long Range PCR have been performed initially on a subset of samples using both primers sourced from GenDX and designed as described in Shiina et al (2012). Initial results from both primer sets were promising on Promega DNA test samples but only the GenDX primers proved effective on the African samples, producing consistently PCR products of the expected size in the Igbo, Ashanti, Morrocan and Zulu samples. We will present early results from our evaluation of the different sequencing technologies


June 1, 2021  |  

The MHC Diversity in Africa Project (MDAP) pilot – 125 African high resolution HLA types from 5 populations

The major histocompatibility complex (MHC), or human leukocyte antigen (HLA) in humans, is a highly diverse gene family with a key role in immune response to disease; and has been implicated in auto-immune disease, cancer, infectious disease susceptibility, and vaccine response. It has clinical importance in the field of solid organ and bone marrow transplantation, where donors and recipient matching of HLA types is key to transplanted organ outcomes. The Sanger based typing (SBT) methods currently used in clinical practice do not capture the full diversity across this region, and require specific reference sequences to deconvolute ambiguity in HLA types. However, reference databases are based largely on European populations, and the full extent of diversity in Africa remains poorly understood. Here, we present the first systematic characterisation of HLA diversity within Africa in the pilot phase of the MHC Diversity in Africa Project, together with an evaluation of methods to carry out scalable cost-effective, as well as reliable, typing of this region in African populations.To sample a geographically representative panel of African populations we obtained 125 samples, 25 each from the Zulu (South Africa), Igbo (Nigeria), Kalenjin (Kenya), Moroccan and Ashanti (Ghana) groups. For methods validation we included two controls from the International Histocompatibility Working Group (IHWG) collection with known typing information. Sanger typing and Illumina HiSeq X sequencing of these samples indicated potentially novel Class I and Class II alleles; however, we found poor correlation between HiSeq X sequencing and SBT for both classes. Long Range PCR and high resolution PacBio RS-II typing of 4 of these samples identified 7 novel Class II alleles, highlighting the high levels of diversity in these populations, and the need for long read sequencing approaches to characterise this comprehensively. We have now expanded this approach to the entire pilot set of 125 samples. We present these confirmed types and discuss a workflow for scaling this to 5000 individuals across Africa.The large number of new alleles identified in our pilot suggests the high level of African HLA diversity and the utility of high resolution methods. The MDAP project will provide a framework for accurate HLA typing, in addition to providing an invaluable resource for imputation in GWAS, boosting power to identify and resolve HLA disease associations.


June 1, 2021  |  

T-cell receptor profiling using PacBio sequencing of SMARTer libraries

T-cells play a central part in the immune response in humans and related species. T-cell receptors (TCRs), heterodimers located on the T-cell surface, specifically bind foreign antigens displayed on the MHC complex of antigen-presenting cells. The wide spectrum of potential antigens is addressed by the diversity of TCRs created by V(D)J recombination. Profiling this repertoire of TCRs could be useful from, but not limited to, diagnosis, monitoring response to treatments, and examining T-cell development and diversification.


April 21, 2020  |  

Biochemical characterization of a novel cold-adapted agarotetraose-producing a-agarase, AgaWS5, from Catenovulum sediminis WS1-A.

Although many ß-agarases that hydrolyze the ß-1,4 linkages of agarose have been biochemically characterized, only three a-agarases that hydrolyze the a-1,3 linkages are reported to date. In this study, a new a-agarase, AgaWS5, from Catenovulum sediminis WS1-A, a new agar-degrading marine bacterium, was biochemically characterized. AgaWS5 belongs to the glycoside hydrolase (GH) 96 family. AgaWS5 consists of 1295 amino acids (140 kDa) and has the 65% identity to an a-agarase, AgaA33, obtained from an agar-degrading bacterium Thalassomonas agarivorans JAMB-A33. AgaWS5 showed the maximum activity at a pH and temperature of 8 and 40 °C, respectively. AgaWS5 showed a cold-tolerance, and it retained more than 40% of its maximum enzymatic activity at 10 °C. AgaWS5 is predicted to have several calcium-binding sites. Thus, its activity was slightly enhanced in the presence of Ca2+, and was strongly inhibited by EDTA. The Km and Vmax of AgaWS5 for agarose were 10.6 mg/mL and 714.3 U/mg, respectively. Agarose-liquefication, thin layer chromatography, and mass and NMR spectroscopic analyses demonstrated that AgaWS5 is an endo-type a-agarase that degrades agarose and mainly produces agarotetraose. Thus, in this study, a novel cold-adapted GH96 agarotetraose-producing a-agarase was identified.


April 21, 2020  |  

Transcriptional initiation of a small RNA, not R-loop stability, dictates the frequency of pilin antigenic variation in Neisseria gonorrhoeae.

Neisseria gonorrhoeae, the sole causative agent of gonorrhea, constitutively undergoes diversification of the Type IV pilus. Gene conversion occurs between one of the several donor silent copies located in distinct loci and the recipient pilE gene, encoding the major pilin subunit of the pilus. A guanine quadruplex (G4) DNA structure and a cis-acting sRNA (G4-sRNA) are located upstream of the pilE gene and both are required for pilin antigenic variation (Av). We show that the reduced sRNA transcription lowers pilin Av frequencies. Extended transcriptional elongation is not required for Av, since limiting the transcript to 32 nt allows for normal Av frequencies. Using chromatin immunoprecipitation (ChIP) assays, we show that cellular G4s are less abundant when sRNA transcription is lower. In addition, using ChIP, we demonstrate that the G4-sRNA forms a stable RNA:DNA hybrid (R-loop) with its template strand. However, modulating R-loop levels by controlling RNase HI expression does not alter G4 abundance quantified through ChIP. Since pilin Av frequencies were not altered when modulating R-loop levels by controlling RNase HI expression, we conclude that transcription of the sRNA is necessary, but stable R-loops are not required to promote pilin Av. © 2019 John Wiley & Sons Ltd.


April 21, 2020  |  

Acquired N-Linked Glycosylation Motifs in B-Cell Receptors of Primary Cutaneous B-Cell Lymphoma and the Normal B-Cell Repertoire.

Primary cutaneous follicle center lymphoma (PCFCL) is a rare mature B-cell lymphoma with an unknown etiology. PCFCL resembles follicular lymphoma (FL) by cytomorphologic and microarchitectural criteria. FL B cells are selected for N-linked glycosylation motifs in their B-cell receptors (BCRs) that are acquired during continuous somatic hypermutation. The stimulation of mannosylated BCR by lectins on the tumor microenvironment is therefore a candidate driver in FL pathogenesis. We investigated whether the same mechanism could play a role in PCFCL pathogenesis. Full-length functional variable, diversity, and joining gene sequences of 18 PCFCL and 8 primary cutaneous diffuse large B-cell lymphoma, leg-type were identified by unbiased Anchoring Reverse Transcription of Immunoglobulin Sequences and Amplification by Nested PCR and BCR reconstruction from RNA sequencing data. Low BCR variation demonstrated negligible ongoing somatic hypermutation in PCFCL and primary cutaneous diffuse large B-cell lymphoma, leg-type, and indicated that the PCFCL microarchitecture does not act as a functional germinal center. Similar to FL but in contrast to primary cutaneous diffuse large B-cell lymphoma, leg-type, BCR genes of 15 PCFCLs (83%) had acquired N-linked glycosylation motifs. These motifs were located at the BCR positions converted to N-linked glycosylation motifs in normal B-cell repertoires with low prevalence but mostly at different positions than those found in FL. The cutaneous localization of PCFCL might suggest a role for lectins from commensal skin bacteria in PCFCL lymphomagenesis.Copyright © 2019 The Authors. Published by Elsevier Inc. All rights reserved.


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