The progression rate of an HIV-1 infection is dependent on both viral  and host genetic factors [2,3], including at least five different co-receptors or primary ligands; CCR5Δ32 , CCR5 59029G/A , CCR2-64I , SDF-1 3′A  and CCR5 P1 . A relationship has also been suggested between reduced risk of heterosexual transmission of HIV-1 and polymorphism in the α(1,2)fucosyltransferase FUT2 gene, regulating the secretor status. A study by Ali et al., showed association between the non-secretor genotype and a reduced risk to be infected with HIV-1, among Senegalese commercial sex workers . Similarly, Blackwell et al. reported an association between non-secretors and resistance to HIV infection . Furthermore, a recent observation have not only illustrated the importance of histo-blood group antigens on host cells, but also that HIV-1 incorporates the ABH antigen and that infection of lymphocytes by HIV-1 appears to activate expression of glycosyltransferases , further supporting a role of fucosyl and glycosyltransferases in the pathogenesis of HIV-1.
The secretor status of an individual relates to the presence of H-antigen, a precursor oligosaccharide molecule of the ABO-antigens that is determined by the Secretor (FUT2) gene, encoding a fucosyltransferase . Secretor-negative individuals are termed non-secretors, and lack H-antigen both on gastro-intestinal epithelium and in body fluids .
Several different polymorphisms are known in the FUT2 gene, some are so-called silent mutations, while others give rise to non-functional enzymes. Homozygous individuals with non-functional enzymes are termed non-secretors (se−/−). Heterozygous individuals on the other hand, carrying one functional allele, have secretion similar to the wild-type and are termed secretors (se+) .
Single nucleotide polymorphisms (SNP) in the FUT2 gene are related to ethnicity , with 20% of the Caucasian population being homozygous non-secretors at position 428 . This mutation causes a switch of amino acid 143 from a tryptophan to a stop codon, which results in a truncated, non-functional protein .
The majority of HIV-infected patients experience an asymptomatic period after seroconversion and most develop AIDS within 10 years of HIV infection without antiretroviral treatment . However some infected individuals remain asymptomatic for a long period and these are called long-term non-progressors (LTNP) . Typically for these patients are more than 7 years of documented HIV infection, a stable, normal CD4+ cell count over time, no symptoms of HIV-induced disease, low levels of HIV virus in the peripheral blood and no history of antiretroviral treatment .
Earlier studies of Swedish homosexual men have shown that the CCR5Δ32 allele has a protective effect against HIV-1 disease progression for several individuals . This protection may be limited to the first years of infection until phenotype switch, from viruses using CCR5 as co-receptor to viruses using CXCR4 . From these studies we also knew that the CCR5 32-base-pair deletion could not explain the slower than expected disease progression among the LTNP individuals examined in this study . Since FUT2 regulates the expression of a carbohydrate on the gastrointestinal epithelium, we decided to investigate the possible association between secretor status and disease progression among LTNP.
All HIV-1 infected study participants were homosexual men from the Venhälsan, Karolinska University Hospital, mean age 40 years, (> 400 × CD4 cells/μl). The definition of the slow progressor group and the rapid progressors were defined as described. Slow progressors (LTNP) were defined as male homosexuals with documented HIV-1 infection over 6 years (median time from HIV-1 diagnosis: 7 years, range, 6–16 years) duration. They were individuals with a positive or stable monthly CD4-cell slope (CD4-cell slope, +2.12 × CD4 cells/μl; range, +0.06–7.86 × CD4 cells/μl) during the study period with no antiviral treatment. Rapid progressors were defined as having reached an AIDS-defining event in less than 5 years (median time from HIV-1 diagnosis, range 2–5 years) or having a monthly CD4 cell slope of ≥ −7 × CD4 cells/μl (mean, + 1SD; range, −7 to −12.9) and having reached a need of anti-retroviral drug treatment in less then 6 years.
DNA was obtained from 15 LTNP, 19 HIV-1 infected individuals with rapid or expected disease progression rate and 276 Swedish blood donors. DNA was purified from the first two groups using a QIAamp DNA Blood Minikit (Qiagen, Hilden, Germany), and DNA from blood donors by the DTAB/CTAB (dodecyltrimethylammoniumbromide/cetyltrimethylammoniumbromide) method . Purified DNA from blood donors was stored frozen in micro-titre plates (10 ng/μl in TE buffer) until analysed.
PCR and sequencing primers
Primers (Invitrogen AB, Lidingö, Sweden) for the initial PCR amplification and the pyrosequencing reaction were designed using conventional standard criteria. Primer specificity was checked with published nucleotide sequences using BLAST (http://www.ncbi.nlm.nih.gov/BLAST/) and analysed for the risk of primer–dimer formation and secondary structures using Pyrosequencing Assay Design Software version 1.0. The following pair of primers was used for the FUT2 PCR: 5′—BIOTIN–GATGGAGGAGGAATACCGCCAC—3′ (F) and 5′—TGGGCCTCCTCCCGCACGT—3′ (R). For sequencing the 5′—GGTGGTGGTAGAAGGTC—3′ primer was used. For CCR5 genotyping the following pair of amplification primers were used: 5′—CACCTGCAGCTCTCATTTTCC—3′ (F) and 5′—BIOTIN–GTTTTTAGGATTCCCGAGTAGCA—3′ (R). The sequencing primer was 5′—CAGCTCTCATTTTCCAT—3′.
PCR (FUT2 and CCR5)
To 1 μl DNA (≥ 10 μg), 17.3 μl water, 0.5 μl 20 μM forward primer, 0.5 μl 20 μM reverse primer, 2.5 μl PCR buffer, 1 μl dNTPs, 2 μl MgCl2 and 0.2 μl TaqGold (Applied Biosystems, Branchburg, New Jersey, USA) was added. Amplification conditions for the FUT2 and CCR5 sequences were one cycle of 300 s at 95°C followed by 50 cycles of 15 s at 95°C, 30 s at 65 °C and 30 s at 72 °C and finally one cycle of 300 s at 72°C, followed by 4°C. The PCR was performed by Model 9600 from Applied Biosystems.
Separation by electrophoresis
Amplified products were separated by electrophoresis (Electrophoresis Power Supply, EPS 301, Amersham Biosciences, Buckinghamshire, UK) on a 1.5% agarose gel and visualized by ethidium bromide staining after exposure to ultraviolet light. The sizes of the final fragments were 131 bp (FUT2) and 132 bp (CCR5).
Pyrosequencing was performed essentially as described by Ronaghi et al. . Briefly amplified DNA from the PCR reactions was mixed with a pyrosequencing mastermix containing 3 μl sepharose–streptavidin conjugate, 37 μl binding buffer (10 mM Tris–HCl, 2 M NaCl, 1 mM EDTA, 0.1% Tween 20 pH 7.6) and 15 μl water for each 25 μl PCR product. The mixture was then mix-incubated for 20 min, 22°C at 1300 rpm (Thermomixer Comfort, Eppendorf, Hamburg, Germany). Samples were set to a mix of 43.5 μl annealing buffer (Biotage AB, Uppsala, Sweden) containing 20 mM Tris–acetate and 2 mM MgAc2 and 1.5 μl 10 μM primer per sample. Annealing was performed for 2 min at 80°C, followed by addition of substrates, enzymes (50 μl + 5 μl per number of samples) and dNTPs (50 μl + 1 μl per number of samples of each dNTP) from the SNP Reagent Kit (code number 40–0023, Biotage AB). The pyrosequencing reaction was carried out with the dispension order GCTCAGAGC (FUT2) and GACAGTC (CCR5), in a PSQ 96 MA Instrument (Biotage AB).
Conformity with Hardy–Weinberg equilibrium  of genotypic frequencies was tested using allelic and genotypic proportions. The significance of phenotypic differences between populations was calculated by Fisher exact test with 95% confidence interval.
To investigate the possible association between the nonsense mutation (428 G→A) in the FUT2 gene and HIV disease progression, 276 Swedish blood donors were first genotyped by pyrosequencing to establish the allele frequency in the Swedish population. The genotyping revealed that 82 (29.7%) of the 276 blood donors were homozygous wild types (se+/+), 136 (49.3%) were heterozygous (se+/−) and 58 (21.0%) were found to be non-secretors (se−/−) (Table 1). Of the 15 LTNP examined, two (13.3%) were carriers of two wild-type alleles, three (20.0%) were heterozygous and 10 (66.7%) were non-secretors (Table 1). This was in strong contrast to the FUT2 genotype profile of HIV infected individuals with expected or rapid disease progression rate which was similar to the profile of the blood donors; five individuals (26%) were homozygous wild-types, 10 (53%) were heterozygous and four (21%) were homozygous (Table 1). Furthermore, the allele frequency 428A was significantly higher in LTNP, than in progressors and blood donors (Table 1).
While the nonsense 428 G→A mutation was significantly over represented among LTNP, the Δ32 CCR5 allele frequency was similar in all three groups investigated (Table 1). Of 265 blood donors, 213 (80.3%) were homozygous wild-types, 51 (19.2%) were heterozygous and one (0.4%) was homozygous for the deletion. The allele frequency of the Δ32 CCR5 in the blood donor material was 0.1 and agreed with the expected frequencies of Hardy–Weinberg equilibrium.
The frequency of the Δ32 CCR5 allele, known to affect the disease progression rate of an HIV-1 infection , was the same (∼10%) in all three groups studied (Table 1) and agrees with earlier studies [19,20]. This suggests that CCR5 could not explain the difference in disease progression rate between the two groups of HIV-1 infected individuals examined in this study. Instead we found that secretor status was highly associated with slow but not rapid HIV-1 progression, with LTNP patients being significantly more often non-secretors than secretors. Of 276 Swedish blood donors, 21.0% were non-secretors compared with 66.7% among LTNP.
In contrast to LTNP, the FUT2 genotype profile of the HIV infected individuals with expected or rapid disease progression were not different from the blood donors (Table 1). Our findings can be compared with those of Blackwell et al. , and those of Ali et al. , showing that non-secretors run a lower risk of being infected by HIV-1. A more recent study by Puissant et al. observed however that the Lewisb antigen, present in secretor positive individuals, had a slight protective effect against HIV infection, at least among blood group A individuals . In support of those observations showing that non-secretors are at a lower risk of being infected, we report for the first time that slow rate of disease progression is strongly associated with the nonsense mutation 428G→A in the FUT2 gene.
As many pathogens including HIV use carbohydrate-binding proteins as primary or secondary receptor, we hypothesize that a glycolipid receptor used by HIV is regulated by the fucosyltransferase FUT2. It cannot be ruled out that polymorphism in glycosyl and fucosyl transferases influence the outcome of disease progression by interference both on host cell and viral glycoprotein synthesis. Indeed glycosylation, which is encoded by the host cell is important for proper maturation of gp120 . One possibility is that the lack of fucosylation, normally catalysed by the FUT2 enzyme, prevents the formation of a glycolipid receptor for HIV-1 on the epithelial surface. When this receptor is not properly folded the increase of viral load may not occur as expected. The fact that FUT2 polymorphisms and secretor status have a ethnical and geographical correlation  with 20% of Europeans being non-secretors and only a few cases in Asia, raises interesting questions about HIV and host cell adaptation. An example of pathogen–host cell co-evolution is Helicobacter pylori and the Lewisb antigen, with the Lewisb antigen being dependent on the secretor H-antigen. It has been shown that blood group O-dependent H. pylori dominate in populations where O is the dominating human blood group, while strains binding A, B and O generally have a broader distribution .
This study was supported by the Swedish Research Council (grant 8266), the Swedish Medical Society, and by the Health research Council in the South-East of Sweden.
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