Introduction
HIV infection causes neurological and psychiatric complications manifesting in HIV-associated neurocognitive disorders (HAND) despite antiretroviral therapy (Mothobi and Brew
2012). They occur in various forms and affect medication adherence and quality of life. Research focuses on host factors that may be implicated in HAND pathogenesis, the etiology of HAND; however, remains so far unclear.
Disorganization of dopaminergic circuits in HIV infection has been implicated in the neuropathophysiology of HIV infection, and has been described extensively in the reviews of (Koutsilieri et al.
2002; Purohit et al.
2011). HIV as well as the simian immunodeficiency virus (SIV) causes apparent reduction in postmortem brain dopamine (DA) levels (Kumar et al.
2009; Sardar et al.
1996; Scheller et al.
2005), suggesting that intracellular DA levels are depleted. An explanation for this may be the dysfunction of the DA transporter (DAT) known to regulate DA availability through DA uptake and release. Indeed, decreased DAT has been found in brains of HIV-associated dementia patients with positron emission tomography (PET) (Wang et al.
2004), and HIV proteins have been shown to impair DAT in animals (Maragos et al.
2002). In contrast, increased DAT protein has been detected in striatal specimens of subjects with HIV encephalitis (Gelman et al.
2006) and no alterations of DAT were found with single-photon emission computed tomography (SPECT) in brains of HIV/treatment-naive patients (Scheller et al.
2010). However, DA levels were increased in CSF of HIV/treatment-naive patients compared with uninfected subjects (Scheller et al.
2010), suggesting an altered regulation of extracellular concentrations of DA.
In the current study, we thought that the increase of CSF DA may have been due to the genetic constitution of the subjects and may not have been a direct causal effect of the virus. We specifically hypothesized that humans carrying a specific allele of the DAT functional genetic polymorphism associated with higher availability of DA would (a) have higher levels of CSF DA (b) be present more frequently within HIV individuals and (c) show adverse development of HIV infection compared to those carrying alleles associated with reduced DA availability. To assess our hypothesis we analyzed CSF DA, DAT mRNA expression, determined DAT genetic polymorphisms in uninfected and HIV-infected subjects and investigated whether different ethnicities would influence our assumption. Progression of disease in the infected groups was based on routine virological and immunological measures as well as scores in the HIV dementia scale (HDS), international HIV dementia scale (IHDS) and global deficit score (GDS). Detailed neuropsychological testing has been assessed but will be reported elsewhere.
Discussion
In this study, we demonstrate for the first time that people infected with HIV carry significantly more often the DAT 10/10-repeat allele compared to uninfected subjects, indicating that this genotype confers a significant risk factor for HIV infection. The reason for studying both German and SA groups was that we initially identified the higher frequency of the DAT 10/10-repeat allele in our German cohort and we wanted to exclude that this could be a feature based on homosexuality as the German participants are men having sex with men. This is why we chose to investigate further this phenomenon in a completely different setting, with heterosexuals with different ethnicity, gender and even virus subtype. Indeed our finding on the frequency of DAT 10/10-repeat allele in HIV-infected subjects is independent on ethnicity, gender, route of sexual transmission and viral subtype (HIV infections in SA are predominantly attributed to subtype C (Jacobs et al.
2009) and in Germany to subtype B (Paraskevis et al.
2009). The independent detection in these different study populations emphasizes the importance of our findings, even though the number of screened patients is comparably low.
One possible explanation for the higher frequency of the 10/10-repeat allele in HIV patients could be that people carrying this allele have a different behavior including a risky, impulsive way of life. In this line, the DAT 10/10-repeat allele has been associated with the attention deficit hyperactivity disorder (ADHD) (Hawi et al.
2005), a disease with enhanced impulsivity (Cornish et al.
2005). Individuals with the highest hyperactivity and impulsivity scores were found to be homozygous for the DAT 10-repeat allele along with one 7-repeat variant for DRD4 (Roman et al.
2001). The same genetic variants were associated with reduced inhibition control (Congdon et al.
2008). Interestingly, people with the 10/10-repeat allele showed more risk taking in a risk-taking task (Mata et al.
2012). In studies with humans, the DAT 10/10-repeat allele has been associated with changes in DAT availability, however, without consistent results. Individuals with DAT 10-repeat allele have lower (van de Giessen et al.
2009) or higher DAT availability (Heinz et al.
2000) than those with other DAT genotypes. The main role of DAT is to regulate DA availability (Giros and Caron
1993) by forward and reverse transport mechanisms that result in changes of synaptic DA in an opposite mode (Uhl
2003). To check whether concentrations of DA in HIV-infected subjects were attributed to the DAT genotype, we measured concentrations of DA in CSF which is a useful index of central DA activity in humans, particularly for the striatum (Wester et al.
1990). We found that participants with the 10/10-repeat allele had higher levels of CSF DA. This is in accordance with the only study in the literature to our knowledge, which reported on DAT genotypes and CSF DA (Wagner et al.
2007). Those authors, like us here, reported that individuals with the DAT 10/10-repeat allele have higher CSF DA levels compared to DAT 9/10 and DAT 9/9 genotypes in traumatic brain injury patients (Wagner et al.
2007).
We also found that 10/10-repeat allele carriers had a significantly lower PBMC DAT expression compared to all other genotypes. This allows us to assume indirectly on the alterations in DA availability in the SA participants because the peripheral PBMC DAT correlates significantly with the central striatal DAT expression in people with dopaminergic medication (Pontieri and Colosimo
2010). In accordance different genotypes of the DAT1 3′ VNTR influence mRNA expression of the DAT both in brain tissue and lymphocytes, although the expression of DAT in the periphery is approximately 1,000-fold lower than in the brain (Mill et al.
2002).
If the DAT 10/10-repeat allele leads to an elevated DA availability, as we show here, it would be possible that it causes an exacerbation of CNS HIV infection, as it was demonstrated previously that SIV neuropathology was accelerated due to dopaminergic drugs (Czub et al.
2001). This issue could not be approached in living people but neurocognitive scores did not seem to alter in DAT 10/10 carriers with HIV infection. In another study, it was shown that the dopaminergic drug selegiline, known to increase DAT expression (Lamensdorf et al.
1999), was well tolerated by HIV-infected patients with cognitive impairment based on the neuropsychology scores (Evans et al.
2007; Schifitto et al.
2007). Another discussion point is that an increase of DA in the periphery by the DAT 10/10-repeat genotype may result in an increased infection risk due to the effects of DA on the immune system per se. It is known that DA may regulate the initiation and development of immune responses (Buttarelli et al.
2011). It has been reported that stimulation of DRD1 receptors, of which the most prominent agonist is DA itself, inhibits the cytotoxic function of CD8+ T cells (Saha et al.
2001), which play a crucial role in the immune response towards infections. Individuals with higher DA levels may, therefore, be more prone towards sexually-transmitted diseases such as herpes genitalis or syphilis, which correlate with a 2–5-fold higher infection risk towards HIV. Besides this indirect effect of DA, DA may also directly enhance HIV replication. We have shown previously that DA activates HIV replication in latently infected T cells in vitro (Scheller et al.
2000), so that individuals with higher DA levels might have an elevated infection risk towards HIV due to a faster initial virus replication. As we do not have information on the virological and immunological status of the HIV patients in the beginning of their infection, we cannot know whether the participants with 10/10-repeat allele had a different initial disease progression. In addition, as our study was not powered for disease severity, we can only assume from our results that later on the DAT 10/10-repeat allele may have no effect on CD4+ T cells, viral load and neurocognitive impairment. This is in agreement with a previous study which found no effect of DAT genotype on disease severity and brain function in HIV-infected patients (Levine et al.
2012) while another group identified that the DRD3 genetic polymorphism relates with cognitive impairment in methamphetamine/HIV-infected people (Gupta et al.
2011).
Taken together, this is the first study to show that increase in CSF DA levels in HIV-infected subjects is attributed to the DAT genotype and it is not necessarily a secondary effect of the virus. In addition, HIV-infected individuals carry more frequently a genetic variant in a functional element of the dopaminergic synapse such as DAT. This could be due to a behavioral trait with a severe consequence but we cannot rule out the possibility that the population with HIV has another significant risk factor that has enriched the frequency of this repeat in these patients. The strengths of our study are the high odds ratios in both German and SA groups, as well as the fact that the results were similar in different ethnicities, were independent on gender, routes of sexual transmission and viral subtype. The weakness of the study is that the study sample with 167 participants was modest. However, the sample size provided enough power to detect statistical significance between frequency of HIV infection and the DAT variant. New studies have to be planned with a large sample number to confirm our data and experimental studies have to be designed to further elucidate the mechanistic background of these data.