Identification of the Molecular and Genetic Basis of PX2, a Glycosphingolipid Blood Group Antigen Lacking on Globoside-deficient Erythrocytes

ABO blood group antigens and differential glycan expression: Perspective on the evolution of common human enzyme deficiencies

Enzymes catalyze biochemical reactions and play critical roles in human health and disease. Enzyme variants and deficiencies can lead to variable expression of glycans, which can affect physiology, influence predilection for disease, and/or directly contribute to disease pathogenesis. Although certain well-characterized enzyme deficiencies result in overt disease, some of the most common enzyme deficiencies in humans form the basis of blood groups. These carbohydrate blood groups impact fundamental areas of clinical medicine, including the risk of infection and severity of infectious disease, bleeding risk, transfusion medicine, and tissue/organ transplantation. In this review, we examine the enzymes responsible for carbohydrate-based blood group antigen biosynthesis and their expression within the human population. We also consider the evolutionary selective pressures, e.g. malaria, that may account for the variation in carbohydrate structures and the implications of this biology for human disease.

Comprehensive characterization of complex glycosphingolipids in human pancreatic cancer tissues

Pancreatic ductal adenocarcinoma (PDAC) is one of the most common causes of cancer-related deaths worldwide, accounting for 90% of primary pancreatic tumors with an average 5-year survival rate of less than 10%. PDAC exhibits aggressive biology, which, together with late detection, results in most PDAC patients presenting with unresectable, locally advanced, or metastatic disease. In-depth lipid profiling and screening of potential biomarkers currently appear to be a promising approach for early detection of PDAC or other cancers. Here, we isolated and characterized complex glycosphingolipids (GSL) from normal and tumor pancreatic tissues of patients with PDAC using a combination of TLC, chemical staining, carbohydrate-recognized ligand-binding assay, and LC/ESI-MS². The major neutral GSL identified were GSL with the terminal blood groups A, B, H, Le^a, Le^b, Le^x, Le^y, P1, and PX2 determinants together with globo- (Gb₃ and Gb₄) and neolacto-series GSL (nLc₄ and nLc₆). We also revealed that the neutral GSL profiles and their relative amounts differ between normal and tumor tissues. Additionally, the normal and tumor pancreatic tissues differ in type 1/2 core chains. Sulfatides and GM₃ gangliosides were the predominant acidic GSL along with the minor sialyl-nLc₄/nLc₆ and sialyl-Le^a/Le^x. The comprehensive analysis of GSL in human PDAC tissues extends the GSL coverage and provides an important platform for further studies of GSL alterations; therefore, it could contribute to the development of new biomarkers and therapeutic approaches.

Characterization of Human Medullary Thyroid Carcinoma Glycosphingolipids Identifies Potential Cancer Markers

Medullary thyroid carcinoma (MTC) accounts for only 1–2% of thyroid cancers; however, metastatic MTC is a mortal disease with no cure. In this study, glycosphingolipids were isolated from human MTCs and characterized by mass spectrometry and binding of carbohydrate recognizing ligands. The tissue distribution of selected compounds was investigated by immunohistochemistry. The amount of acid glycosphingolipids in the MTCs was higher than in the normal thyroid glands. The major acid glycosphingolipid was the GD3 ganglioside. Sulfatide and the gangliosides GM3 and GD1a were also present. The majority of the complex non-acid glycosphingolipids had type 2 (Galβ4GlcNAc) core chains, i.e., the neolactotetraosylceramide, the Le^x, H type 2 and x₂ pentaosylceramides, the Le^y and A type 2 hexaosylceramides, and the A type 2 heptaosylceramide. There were also compounds with globo (GalαGalβ4Glc) core, i.e., globotriaosylceramide, globotetraosylceramide, the Forssman pentaosylceramide, and the Globo H hexaosylceramide. Immunohistochemistry demonstrated an extensive expression av Le^y in the MTC cells and also a variable intensity and prevalence of Globo H and Le^x. One individual with multiple endocrine neoplasia type 2B expressed the Forssman determinant, which is rarely found in humans. This study of human MTC glycosphingolipids identifies glycans that could serve as potential tumor-specific markers.

Globoside Is Dispensable for Parvovirus B19 Entry but Essential at a Postentry Step for Productive Infection

Globoside (Gb4) is considered the primary receptor of parvovirus B19 (B19V); however, its expression does not correlate well with the attachment and restricted tropism of the virus. The N terminus of VP1 (VP1u) of B19V interacts with an as-yet-unknown receptor required for virus internalization. In contrast to Gb4, the VP1u cognate receptor is expressed exclusively in cells that B19V can internalize. With the aim of clarifying the role of Gb4 as a B19V receptor, we knocked out the gene B3GalNT1 coding for the enzyme globoside synthase in UT7/Epo cells. Consequently, B3GalNT1 transcripts and Gb4 became undetectable in the knockout (KO) cells without affecting cell viability and proliferation. Unexpectedly, virus attachment, internalization, and nuclear targeting were not disturbed in the KO cells. However, NS1 transcription failed, and consequently, genome replication and capsid protein expression were abrogated. The block could be circumvented by transfection with a B19V infectious clone, indicating that Gb4 is not required after the generation of viral double-stranded DNA with resolved inverted terminal repeats. While in wild-type (WT) cells, occupation of the VP1u cognate receptor with recombinant VP1u disturbed virus binding and blocked the infection, antibodies against Gb4 had no significant effect. In a mixed population of WT and KO cells, B19V selectively infected WT cells. This study demonstrates that Gb4 does not have the expected receptor function, as it is dispensable for virus entry; however, it is essential for productive infection, explaining the resistance of the rare individuals lacking Gb4 to B19V infection.IMPORTANCE Globoside has long been considered the primary receptor of B19V. However, its expression does not correlate well with B19V binding and uptake and cannot explain the pathogenesis or the remarkable narrow tissue tropism of the virus. By using a knockout cell line, we demonstrate that globoside does not have the expected function as a cell surface receptor required for B19V entry, but it has an essential role at a postentry step for productive infection. This finding explains the natural resistance to infection associated with individuals lacking globoside, contributes to a better understanding of B19V restricted tropism, and offers novel strategies for the development of antiviral therapies.

Separation of glycosphingolipids with titanium dioxide

We introduce the principle of a new technique to isolate glycosphingolipids (GSLs) from phospholipids. Neutral and acidic GSLs in organic solvent bind to titanium dioxide under neutral pH and can be eluted with 5 mg/ml of 2,5-dihydroxybenzoic acid in methanol. This special property is applicable for eliminating phospholipids, including sphingomyelin, which cannot be eliminated by a typical mild alkaline treatment. By using this technique, we demonstrated the rapid separation of minor components of GSLs, namely sulfatide and gangliosides from rabbit serum and liver, respectively. The minor GSL components were effectively purified despite both sources containing tremendous amount of phospholipids and simple lipids such as cholesterol, cholesteryl esters and triglycerides.

Multiple miscarriages in two sisters of Thai origin with the rare Pk phenotype caused by a novel nonsense mutation at the B3GALNT1 locus

OBJECTIVES

To determine the genetic background underlying the P^k phenotype in two Thai sisters suffering from multiple spontaneous abortions.

BACKGROUND

The P antigen is carried by globoside, an abundant glycosphingolipid in the red blood cell (RBC) membrane. Inactivating mutations in the 3-β-N-acetylgalactosaminyltransferase gene (B3GALNT1) give rise to the rare P^k phenotype, which lack the P and PX2 antigens. Consequently, naturally occurring anti-P may cause recurrent miscarriages following the cytotoxic attack of the globoside-rich fetal portion of the placenta.

METHODS/MATERIALS

P/P1/PX2/P^k antigens on RBCs and their corresponding antibodies were detected by haemagglutination and flow cytometry. The B3GALNT1 coding region was sequenced, and an allele-specific polymerase chain reaction (PCR) was developed.

RESULTS

The two sisters had suffered 8 and 11 miscarriages, most of which occurred in the first trimester. Anti-P and anti-PX2 were identified in their plasmas, and the RBCs typed as P-PX2-P^k +, i.e. had the P^k phenotype. Sequencing revealed homozygosity for a nonsense mutation, c.420T>G, in B3GALNT1. This substitution introduces a premature stop codon, p.Tyr140Ter, which is predicted to abolish enzymatic activity. Screening of 384 Thai donors indicated an allele frequency of 0·13%.

CONCLUSION

We describe a novel nonsense mutation (c.420T>G) in B3GALNT1 (GLOB*01N·13), which was added to the 12 alleles already known in the GLOB system.

Identification of human glycosyltransferase genes expressed in erythroid cells predicts potential carbohydrate blood group loci

Glycans are biologically important structures synthesised by glycosyltransferase (GT) enzymes. Disruptive genetic null variants in GT genes can lead to serious illness but benign phenotypes are also seen, including antigenic differences on the red blood cell (RBC) surface, giving rise to blood groups. To characterise known and potential carbohydrate blood group antigens without a known underlying gene, we searched public databases for human GT loci and investigated their variation in the 1000 Genomes Project (1000 G). We found 244 GT genes, distributed over 44 families. All but four GT genes had missense variants or other variants predicted to alter the amino acid sequence, and 149 GT genes (61%) had variants expected to cause null alleles, often associated with antigen-negative blood group phenotypes. In RNA-Seq data generated from erythroid cells, 155 GT genes were expressed at a transcript level comparable to, or higher than, known carbohydrate blood group loci. Filtering for GT genes predicted to cause a benign phenotype, a set of 30 genes remained, 16 of which had variants in 1000 G expected to result in null alleles. Our results identify potential blood group loci and could serve as a basis for characterisation of the genetic background underlying carbohydrate RBC antigens.

Comparison of the glycosphingolipids of human-induced pluripotent stem cells and human embryonic stem cells

High expectations are held for human-induced pluripotent stem cells (hiPSC) since they are established from autologous tissues thus overcoming the risk of allogeneic immune rejection when used in regenerative medicine. However, little is known regarding the cell-surface carbohydrate antigen profile of hiPSC compared with human embryonic stem cells (hESC). Here, glycosphingolipids were isolated from an adipocyte-derived hiPSC line, and hiPSC and hESC glycosphingolipids were compared by concurrent characterization by binding assays with carbohydrate-recognizing ligands and mass spectrometry. A high similarity between the nonacid glycosphingolipids of hiPSC and hESC was found. The nonacid glycosphingolipids P1 pentaosylceramide, x2 pentaosylceramide and H type 1 heptaosylceramide, not previously described in human pluripotent stem cells (hPSC), were characterized in both hiPSC and hESC. The composition of acid glycosphingolipids differed, with increased levels of GM3 ganglioside, and reduced levels of GD1a/GD1b in hiPSC when compared with hESC. In addition, the hESC glycosphingolipids sulf-globopentaosylceramide and sialyl-globotetraosylceramide were lacking in hiPSC. Neural stem cells differentiating from hiPSC had a reduced expression of sialyl-lactotetra, whereas expression of the GD1a ganglioside was significantly increased. Thus, while sialyl-lactotetra is a marker of undifferentiated hPSC, GD1a is a novel marker of neural differentiation.

Human Genetic Determinants of Viral Diseases

Much progress has been made in the identification of specific human gene variants that contribute to enhanced susceptibility or resistance to viral diseases. Herein we review multiple discoveries made with genome-wide or candidate gene approaches that have revealed significant insights into virus-host interactions. Genetic factors that have been identified include genes encoding virus receptors, receptor-modifying enzymes, and a wide variety of innate and adaptive immunity-related proteins. We discuss a range of pathogenic viruses, including influenza virus, respiratory syncytial virus, human immunodeficiency virus, human T cell leukemia virus, human papilloma virus, hepatitis B and C viruses, herpes simplex virus, norovirus, rotavirus, parvovirus, and Epstein-Barr virus. Understanding the genetic underpinnings that affect infectious disease outcomes should allow tailored treatment and prevention approaches in the future.

CD1: A Singed Cat of the Three Antigen Presentation Systems

Contrary to general view that the MHC Class I and II are the kapellmeisters of recognition and response to antigens, there is another big player in that part of immunity, represented by CD1 glycoproteins. In contrast to MHC Class I or II, which present peptides, CD1 molecules present lipids. Humans express five CD1 proteins (CD1a-e), four of which (CD1a-d) are trafficked to the cell surface, where they may display lipid antigens to T-cell receptors. This interaction may lead to both non-cognate and cognate T cell help to B cells, the latter eliciting anti-lipid antibody response. All CD1 proteins can bind a broad range of structurally different exogenous and endogenous lipids, but each shows a preference to one or more lipid classes. This unorthodox binding behavior is the result of elaborate architectures of CD1 binding clefts and distinct intracellular trafficking routes. Together, these features make CD1 system a versatile player in immune response, sitting at the crossroads of innate and adaptive immunity. While CD1 system may be involved in numerous infectious, inflammatory, and autoimmune diseases, its involvement may lead to opposite outcomes depending on different pathologies. Despite these ambiguities and complexity, CD1 system draws growing attention and continues to show glimmers of therapeutic potential. In this review, we summarize the current knowledge about CD1 proteins, their structures, lipid-binding profiles, and roles in immunity, and evaluate the role of CD1 proteins in eliciting humoral immune response.

Evaluation of an amino acid residue critical for the specificity and activity of human Gb3/CD77 synthase

Human Gb3/CD77 synthase (α1,4-galactosyltransferase) is the only known glycosyltransferase that changes acceptor specificity because of a point mutation. The enzyme, encoded by A4GALT locus, is responsible for biosynthesis of Gal(α1-4)Gal moiety in Gb3 (CD77, P^k antigen) and P1 glycosphingolipids. We showed before that a single nucleotide substitution c.631C > G in the open reading frame of A4GALT, resulting in replacement of glutamine with glutamic acid at position 211 (substitution p. Q211E), broadens the enzyme acceptor specificity, so it can not only attach galactose to another galactose but also to N-acetylgalactosamine. The latter reaction leads to synthesis of NOR antigens, which are glycosphingolipids with terminal Gal(α1-4)GalNAc sequence, never before described in mammals. Because of the apparent importance of position 211 for enzyme activity, we stably transfected the 2102Ep cells with vectors encoding Gb3/CD77 synthase with glutamine substituted by aspartic acid or asparagine, and evaluated the cells by quantitative flow cytometry, high-performance thin-layer chromatography and real-time PCR. We found that cells transfected with vectors encoding Gb3/CD77 synthase with substitutions p. Q211D or p. Q211N did not express P^k, P1 and NOR antigens, suggesting complete loss of enzymatic activity. Thus, amino acid residue at position 211 of Gb3/CD77 synthase is critical for specificity and activity of the enzyme involved in formation of P^k, P1 and NOR antigens. Altogether, this approach affords a new insight into the mechanism of action of the human Gb3/CD77 synthase.

International society of blood transfusion working party on red cell immunogenetics and terminology: report of the Seoul and London meetings

The Working Party has met twice since the last report: in Seoul, South Korea 2014, and in London, UK 2015, both in association with the International Society of Blood Transfusion (ISBT) Congress. As in previous meetings, matters pertaining to blood group antigen nomenclature were discussed. Eleven new blood group antigens were added to seven blood group systems. This brings the current total of blood group antigens recognized by the ISBT to 346, of which 308 are clustered within 36 blood groups systems. The remaining 38 antigens are currently unassigned to a known blood group system.

Human Gb3/CD77 synthase reveals specificity toward two or four different acceptors depending on amino acid at position 211, creating P(k), P1 and NOR blood group antigens

Human Gb3/CD77 synthase (α1,4-galactosyltransferase, P(k) synthase), encoded by A4GALT gene, is known for synthesis of Gal(α1-4)Gal moiety in globotriaosylceramide (Gb3Cer, CD77, P(k) blood group antigen), a glycosphingolipid of the globo series. Recently, it was shown that c.631C > G mutation in A4GALT, which causes p.Q211E substitution in the open reading frame of the enzyme, broadens the enzyme specificity, making it able also to synthesize Gal(α1-4)GalNAc moiety, which constitutes the defining terminal disaccharide of the NOR antigen (carried by two glycosphingolipids: NOR1 and NOR2). Terminal Gal(α1-4)Gal disaccharide is also present in another glycosphingolipid blood group antigen, called P1, which together with P(k) and NOR comprises the P1PK blood group system. Despite several attempts, it was never clearly shown that P1 antigen is synthesized by Gb3/CD77 synthase, leaving open an alternative hypothesis that there are two homologous α1,4-galactosyltransferases in humans. In this study, using recombinant Gb3/CD77 synthase produced in insect cells, we show that the consensus enzyme synthesizes both the P(k) and P1 antigens, while its p.Q211E variant additionally synthesizes the NOR antigen. This is the first direct biochemical evidence that Gb3/CD77 synthase is able to synthesize two different glycosphingolipid antigens: P(k) and P1, and when p.Q211E substitution is present, the NOR antigen is also synthesized.