A single-nucleotide polymorphism in the human beta-defensin ... : AIDS (original) (raw)
In this study we show a significant correlation between a single-nucleotide polymorphism in the 5'-untranslated region of the DEFB1 gene, which probably regulates the gene expression of human beta defensin 1 (hBD-1) and the risk of HIV-1 infection in an Italian paediatric population (97 HIV-1 perinatally infected children), pointing to the importance of innate immunity in HIV-1 infection.
In the past few years researchers have recognized a number of novel proteins of the innate immune system that play important roles in conferring resistance to HIV-1 infection, especially in newborns, in whom the acquired immune response is not fully developed [1].
Defensins are perhaps the most important family of antimicrobial peptides in humans, and are known for their antiviral activities [2]. Furthermore, defensins play an important role in linking innate and adaptive immunity acting as ‘mini-chemokines’ [3]. The β-defensins, encoded by genes localized on 8p22–23, are small cationic amphipathic peptides (30–48 amino acids). DEFB1, which encodes the human beta defensin 1 (hBD-1) gene, is constitutively expressed within the oral cavity, skin, and internal epithelia, and its expression varies between individuals [4].
This study shows a significant correlation between a single-nucleotide polymorphism (SNP) in the 5'- untranslated region of the DEFB1 gene and the risk of HIV-1 infection in newborns in an Italian paediatric population. The study highlights the importance of innate immunity in HIV-1 infection. Ninety-seven HIV-1 perinatally infected children (mean age 6.6 years ± 52.2 months) were enrolled for the study. All of these children were born to HIV-1-positive mothers who had not undergone any antiretroviral therapy during pregnancy, and had not undergone caesarean section to prevent transmission. A total of 120 uninfected children, from the same ethnic background and matched for age, were employed as controls after informed consent. These children were known to be not at risk of infection and were not infected as their mothers were HIV-1 negative and they had never been exposed to the routes of virus transmission.
DNA was extracted by using the GenomicPrep kit (Amersham-Pharmacia, Piscataway, NJ, USA) and polymerase chain reaction amplification performed under standard conditions [5] DNA sequences were obtained using the Big Dye Terminator sequencing kit (Applied Biosystems, Foster City, CA, USA), and allele frequencies were calculated by direct gene counting. The significance of differences in allelic and genotype frequencies was calculated by Fisher's exact test using 2 × 2 contingency tables. Holm's correction for multiple tests was performed and only corrected P values less than 0.05 were considered to be significant. All the statistical analyses were carried out using SPSS software (version 6.1.3; SPSS, Inc., Chicago, IL, USA).
We studied the frequency of three SNP in the 5'-untranslated region of the DEFB1 gene, which is likely to regulate gene expression, in both HIV-1-infected children and healthy controls matched for age. The three SNP examined were: (i) −52 (G/A); (ii) −44 (C/G); and iii) −20 (G/A). Allele and genotype frequencies for the three SNP in the two cohorts studied are shown in Table 1.
Frequencies of DEFB1 polymorphisms in infected children perinatally exposed to HIV-1 and pan-ethnic healthy controls.
Allele frequencies of the −44 C/G SNP was significantly different in HIV-1-positive children compared with healthy controls. Moreover, the distribution of the three genotypes was shown to be significantly different from that of healthy controls (P < 0.001; corrected P value < 0.05). This is mainly caused by the difference in the frequency of (−44)CC homozygous individuals. The odds ratio for the (−44)CC genotype in HIV-1-infected children is 3.6 (95% confidence interval 1.89–6.90). Both genotype and allele frequencies of the −20G/A SNP in HIV-1-positive children were similar to the controls. Allele frequencies of the −52 G/A SNP in HIV-1-positive children were significantly different from healthy controls; however, they did not reach the significance level of 0.05 when Holm's correction is applied (Table 1).
The (−44)C allele, located in a region that is highly conserved in other primate species, has already been shown to correlate with the risk of Candida spp. carriage [5], strongly supporting a role for this polymorphism in the modulation of DEFB1 expression. Dorschner et al. [6] recently demonstrated that murine beta defensin 1 (mBD-1), the murine homologous of hBD-1, is upregulated in embryonic and neonatal skin, and provides protection against infections in the earlier stage of the life. The HIV-1-positive children under study here were infected during delivery by contact with HIV-1-infected maternal blood or mucus in the birth canal. We can hypothesize that hBD-1 is very important in protecting the skin and mucosa of newborns in this stage by interacting with the viral particles or with cells of the immune response. In this study design we cannot exclude the possibility that the −44 C/G polymorphism is associated with HIV infection of the mothers or their ability to transmit the virus to their children. Unfortunately, biological samples from the mothers of HIV-1-infected children were not available to us or information on their viral load at the time of delivery. If women with these polymorphisms are more likely to be infected with HIV than women without the polymorphisms, then children of HIV-infected women may be expected to have a higher frequency of this polymorphism than controls, even if there is not a direct effect on vertical transmission. Another possibility is that the polymorphism may be associated with more rapid disease progression, and thus higher viral loads, in women who are infected. As higher viral loads are associated with an increased rate of vertical transmission, again, we may expect to see this relationship even if there is not a direct influence of the polymorphism itself on vertical transmission. To address these points a study on an adult population and a study on transmitter against non-transmitter mothers is necessary.
A very recent study [7] demonstrated that HIV-1 replication is inhibited in human epithelial cells by β-defensins 2 and 3, that both of these molecules are able to interact with viral particles and downregulate the expression of CXCR4 in host cells. However, the same study did not demonstrate a role for hBD-1 in HIV-1 infection. One hypothesis that could explain this discrepancy would be that hBD-1 acts in synergy with other antimicrobial peptides expressed in the skin and at the mucosal surface (such as hBD-2, 3 and LL-37), or it may be that a new role for this peptide has not yet been discovered. Moreover, as DEFB1 is located close to the DEFB4 and DEFB103 genes (that encode for hBD-2 and 3, respectively), this SNP could be in linkage disequilibrium with another protective allele in these genes or could be associated with the polymorphic number of these genes and then with their expression in the skin [8].
To our knowledge, this is the first study that reports a significant association between an SNP in the DEFB1 gene and HIV-1 infection in children born to seropositive mothers; our results could suggest new insights into therapies based on synthetic antimicrobial peptides, to prevent HIV-1 mother-to-child transmission.
Sponsorship: This work was supported by MIUR COFIN 2002 (Italian Ministry of Research) and RC 51/03 research project (IRCCS Burlo Garofolo) grants. A. Pontillo is recipient of a long-term fellowship from IRCCS Burlo Garofolo.
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