In this study, we took a candidate gene approach to evaluate whether a human endogenous retroviral K (HERV-K) dUTPase sequence in the MHC region is associated with psoriasis. Sequencing the dUTPase in 708 cases and 349 controls, we found that HERV-K dUTPase variants at PSORS1 are strongly associated with psoriasis but that this association was not as strong as the association of psoriasis with HLA-C*06:02. To determine whether dUTPase variants were associated with psoriasis independent of psoriasis-associated HLA alleles, we determined the LD between the HERV variants and these HLA alleles and identified HLA-C*06:02 and HLA-B*57:01 as the most likely potential confounders. We then performed association testing of HERV variants conditional on HLA-C*06:02 and HLA-B*57:01 and found that two variants within the dUTPase, rs9264082 and rs3134774, remained independently associated. The minor allele A of the missense SNP rs114780460 was also protective against psoriasis (OR=0.78), but its effect was not quite independent (p=0.051).
rs9264082 encodes the synonymous change L161L, whereas rs114780460 and rs3134774 encode missense SNPs corresponding to G155R and K158R substitutions, respectively. Our haplotype analysis shows that the haplotype 155G/158R, previously termed “high-risk” (Foerster et al., 2005
), is the most common haplotype in psoriasis patients (). Foerster et al.’s “low-risk” haplotype 155R/158K corresponds to protective haplotype H3 in our analysis; however, the haplotype 155G/158K which was not reported by Foerster et al. also corresponds to protective haplotypes H2 and H4 (). Ariza and Williams studied whether the 155G/158R substitutions and the 155R/158K substitutions (the latter associated with psoriasis protection) influenced the ability of recombinant HERV-K dUTPase protein to stimulate cytokine responses when added to human primary cells (Ariza and Williams, 2011
). In agreement with the genetic prediction from our data, recombinant protein containing 155G/158R showed a significantly increased ability to stimulate Th1 and Th17 cytokines compared to protection-associated 155R/158K recombinant protein in human primary dendritic cells. The authors also tested the effects of the single substitutions 158R and 158K which did not show the expected trend; however, in these two cases 158R and 158K were paired with 155K, a situation which is never observed at the PSORS1 HERV-K dUTPase in which amino acid position 155 can only harbor a G (Glycine) or R (Arginine). Indeed, it is important to note that the recombinant protein experiments conducted by Ariza and Williams employed as their baseline protein the wild-type HERV-K dUTPase sequence, which is present in many copies throughout the human genome but which differs in sequence slightly from the PSORS1 HERV-K dUTPase at the N-terminus and C-terminus. Therefore, interpretation of the functional effects of the G155R and K158R polymorphisms require additional studies which examine these changes within the context of the PSORS1 dUTPase specifically.
We tested whether psoriasis patients mounted humoral responses against the HERV-K dUTPase protein. Using an ELISA assay, we found higher antibody responses against wild-type HERV-K dUTPase in psoriasis patients compared to healthy controls. The response was specific for HERV-K dUTPase, as human dUTPase did not elicit reactivity. Future work is needed to define the relevant reactive epitopes and to determine whether there is a differential antibody response to dUTPase protein containing lysine at position 158, corresponding to the protective genetic variant identified here. We also tested whether HERV-K dUTPase peptides could elicit immunologic responses from PBMCs of psoriasis patients and healthy controls. We found T cell responsivity to several of the peptides, with peptide P04 being recognized in 5/13 psoriasis patients, including 2 at moderate strength. This 13 amino acid sequence has several potential HLA anchor motifs, including that of HLA-C*06:02, amongst others. That our screen did not detect a differential response against the psoriasis-associated genetic variants does not rule out a potential functional role for these variants. For efficiency, we tested 13-mer peptides staggered by 5 amino acids, but this does not cover all possible peptide sequences. We also did not examine the possibility that skin residents immune cells might react to these peptides. In addition, it is possible that these genetic variants might be important for the secondary or tertiary structure of the dUTPase protein which could affect interaction with innate pattern receptors such as TLR-2, a mechanism which is not addressed with peptide experiments.
Apart from modulating immune response, endogenous retroviral dUTPase may harbor alternative psoriasis-relevant properties. Related dUTPases have been found to interact with peroxisome proliferator-activated receptor (PPAR) isoforms (Chu et al., 1996
) and human dUTPase specifically with PPAR delta (Rual et al., 2005
), previously implicated in psoriasis (Romanowska et al., 2010
; Westergaard et al., 2001
). In addition, human dUTPase was recently shown to act as an unconventional high-affinity DNA binding protein that interacts with specific target sequences independently of enzymatic function (Hu et al., 2009
). It is quite possible that such functions are retained in truncated homologous HERV-K dUTPase fragments.
In conclusion, using a large case-control cohort, we have identified variants within the HERV-K dUTPase gene at PSORS1 which have an effect on psoriasis susceptibility independent of HLA loci known to be associated with psoriasis. Our functional studies preliminarily suggest that certain HERV-K dUTPase epitopes and peptides may elicit immunologic responses more frequently in psoriasis patients than in healthy controls. Given the extensive linkage disequilibrium within the MHC region, replication of our genetic findings in additional cohorts is warranted and further mapping studies are needed to evaluate the possibility that the independent HERV variants identified here are markers of other causal variants in the region. Nevertheless, this study advances the evidence that the HERV-K dUTPase at PSORS1 may be involved in the pathogenesis of psoriasis and suggests the importance of additional genetic and immunologic work to clarify the role of this dUTPase in psoriasis.