Clinical Differences between Men and Women with Psoriatic Arthritis: Relevance of the Analysis of Genes and Polymorphisms in the Major Histocompatibility Complex Region and of the Age at Onset of Psoriasis

It has been shown that males with spondyloarthritis tend to suffer from more severe spinal disease while females are more likely to have peripheral joint involvement. Nevertheless, gender-related differences have not been thoroughly explored in psoriatic arthritis (PsA). In PsA, males accumulate more peripheral and axial joint damage compared to women. However, it is not clear whether these findings are secondary to differences in occupational physical activity, hormonal changes, or other factors. The present study analyzed the differences in clinical expression of PsA between men and women. We have also evaluated the possible existence of gender-linked differences in the distribution of genes and polymorphisms within the major histocompatibility complex and whether patients' age at the onset of psoriasis established any differences in these aspects. Women suffered more polyarthritis, greater functional impairment, and a larger number of swollen joints during followup. We appreciated a differential expression of certain MHC genes according to gender and age at onset of psoriasis. Our results point to the need to include patient's age at the onset of psoriasis and gender as key stratification elements in future studies of genetic associations in PsA.


Introduction
Psoriasis is a relatively common T-cell mediated cutaneous disease in which both environmental and genetic factors contribute to its pathogenesis [1]. Despite multiple susceptibility loci for psoriasis that have been identified, the PSORS1 locus located in the MHC class I region on chromosome 6p21.3 confers the most risk for psoriasis [1]. However, the extensive ranges of polymorphism and the preservation of HLA haplotypes in most populations have made it difficult to characterize the exact gene responsible for psoriasis susceptibility located in PSORS1. Nevertheless, HLA-C * 06 is the strongest association found in psoriasis and is the only genetic variant repeatedly observed to be associated with psoriasis in most of the populations (1). Additionally, other MHC genes have also been associated with psoriasis independently of HLA-C * 06 [2,3].
It has been shown that males with spondyloarthritis tend to suffer from more severe spinal disease while females are more likely to have peripheral joint involvement [9]. Nevertheless, gender-related differences have not been thoroughly explored in PsA. In a recent investigation among patients with PsA, men tended to accumulate more peripheral and axial joint damage compared to women. However, it was unclear whether these findings were secondary to differences in occupational physical activity, hormonal changes, or other factors [10]. It is also plausible that some of these genderobserved differences in PsA could be attributed to genetic factors. Indeed, some studies have shown a close correlation between male sex, HLA-B27 positivity, and risk of psoriatic spondyloarthritis [11,12]. Very few studies have investigated whether there are differences in the clinical and genetic risk profiles between men and women with PsA [13].
The present study analyzes the differences in clinical expression of PsA between men and women. We have also evaluated the possible existence of gender-linked differences in the distribution of genes and polymorphisms within the MHC and whether patients' age at the onset of psoriasis establishes any differences in these aspects.

Patients.
One-hundred ten consecutive patients who fulfilled the Classification criteria for psoriatic arthritis (CAS-PAR) criteria [14] were randomly selected from the rheumatology outpatient clinic of a tertiary care hospital. There were 55 men and 55 women with a mean age of 48 ± 12 years. The mean disease duration for psoriasis was 19 ± 10 years and 13 ± 8 years for arthritis. Psoriasis preceded the arthritis in 75% of cases and a family history of psoriasis was recorded in 43% of patients. The joint patterns on the baseline visits were those described by Moll and Wright [15], though in the final analysis increased emphasis was placed on the dominant joint pattern over the last 5 years of followup. Accordingly, 39% had oligoarthritis, 29% polyarthritis, and 32% presented a predominantly axial disease. Distal interphalangeal joint disease was seen in 32% of patients, whereas nail disease was seen in 43.6%. Dactylitis was noted in 30% of patients. Clinical enthesitis was present in 30% of subjects. All the disease variables, including laboratory, clinical and exploratory tests, disease activity, physical function, joint counts, and radiological studies were investigated on a standard basis in both men and women. The population was stratified by age at the time of onset of psoriasis, adopting a cutoff point of 40 years for distinguishing between early and late psoriasis. The study was carried out with the approval of the ethics committee of our hospital. All patients gave written informed consent before enrolling in the study.

HLA Typing.
Human leukocyte antigen, HLA-C, was typed using the polymerase chain reaction with sequencespecific primers (PCR-SSP) [7]. The polymorphisms of the octamer transcription factor 3 gene (OTF3), the corneodesmosin gene (CDSN), and the a-helix coiled-coil rod homologue (HCR) gene were analyzed as described previously [7]. The microsatellites C1 2 5 and C1 4 4 were amplified using the PCR primers described by Tamiya et al. [16]. HLA-B typing was carried out using the Dynal RELI SSO HLA-B test following the manufacturer's instructions. HLA-DRB1 alleles were typed and subtyped by performing polymerase chain reaction using specific primers (PCR-SSP). For the analysis of microsatellite repeat polymorphism in the MICA gene, PCR was carried out using primers labeled at the 5 end with the fluorescent reagent Cy5 as previously described [7]. For MICB typing, a dinucleotide microsatellite polymorphism in the first intron was analyzed by PCR analysis and TNF-polymorphism at positions −238 and −308 was typed by PCR as previously described [7]. The distribution of these markers was analyzed in the whole group as well as in both men and women separately. This distribution was also analyzed according to patients' age at the onset of psoriasis, using the previously mentioned cutoff point. All previous typing was also performed in a control population of 110 (55 men and 55 women) random blood donors matched ethnically and geographically.

Statistical Analysis.
The differences between the frequencies of these allelic markers in patients and controls, as well as the differences found in accordance with sex and age at disease onset, were assessed by univariate analysis using a Fisher's exact test, chi-squared test with Yates' correction and a Student's t-test. The differences between genders corresponding to the categorical variables were assessed with Fisher's exact test, while Student's t-test was used in the case of the continuous variables. The odds ratio (OR) was calculated by the cross-product ratio and the 95% confidence intervals by the cornfield method. The P significant values were corrected ( ) by multiplying them by the number of alleles at each locus. The extent of linkage disequilibrium between two loci is expressed as the observed disequilibrium value ( s ), that is, a proportion of the theoretical maximum disequilibrium value ( max ) achievable for this combination of alleles. The s were calculated using the formula:

Differences in the Clinical Variables of Disease between
Men and Women. Compared to PsA males, females tended to be affected more frequently by polyarthritis as the main joint pattern during followup (40% versus 20%, < 0.05), higher HAQ values (1.10 ± 0.53 versus 0.62 ± 0.42, < 0.01), and higher swelling joint count (5.2 ± 4.6 versus 3.2 ± 4.2 < 0.05). There were no gender differences in age at the onset of psoriasis or arthritis, family history of disease, distal interphalangeal involvement, dactylitis, nail disease, the presence of erosive disease in the radiological study, or severity of psoriasis (Table 1).

Differences in Disease Variables between Men and Women according to Age at the Onset of Psoriasis (Cutoff Point 40
Years). Most patients developed psoriasis before 40 years of age (45 men and 40 women). These individuals showed a family history of psoriasis in 47 cases (40%), while the rest of the patients with psoriasis first manifesting after 40 years of age presented antecedents of the disease in only three cases (12%), < 0.05. In general, the psoriasis-arthritis latency period was significantly shorter in patients with late psoriasis (3.5 ± 3.4 years) than in subjects with early psoriasis (7.3 ± 5.4 years) ( < 0.01). The only discordant data between genders in the patients with psoriasis developing before 40 years of age were a shorter psoriasis-arthritis latency period in men (5.5 ± 7.1 years) than in women (9.3 ± 6.6 years) ( < 0.01), and a greater frequency of polyarthritis during followup in women (37.5% versus 17.7%) ( < 0.05). Women with early psoriasis showed greater nail involvement than men, though the difference was not statistically significant. In women with type II psoriasis, polyarthritis likewise was the dominant PsA pattern (53.3% versus 30%), though the differences failed to reach statistical significance, due to the small number of cases in this group. In the same way, in men with type II psoriasis, the dominant joint pattern was axial involvement (70% versus 33.3%), though once again the differences failed to reach statistical significance, due to the small number of cases in this group (Table 2).

Distribution of Genetic Markers according to Patient Age at Onset of Psoriasis.
There were no significant differences in age at the onset of psoriasis between men and women with early psoriasis and HLA-C * 06 positivity (21 ± 8.3 versus 18.1 ± 8.9 years, resp.). However, among the individuals with late psoriasis (over 40 years of age) and HLA-C * 06 positivity, the age at onset of psoriasis was significantly younger in men

Discussion
Spondyloarthritic disorders are generally diagnosed more often in men than in women [17]. In reference to the most representative form of these disorders, ankylosing spondylitis, three male cases are documented for every female case [17]. However, since the introduction of the new ASAS criteria for axial spondyloarthritis, these differences are no longer so apparent [18]. PsA appears to be more frequent in men than in women, particularly in its axial presentations [19]. Nevertheless, little is known of the differential clinical expression of PsA between males and females. The explanation for these differences is not clear, and the participation of multifactorial parameters cannot be discarded. In any case, these clinical expression tendencies in men and women with spondyloarthritis appear to persist in the more recently published series [9].
We confirmed the greater peripheral involvement in women with PsA (with more frequent polyarthritis), as well as greater physical functional impairment as measured Table 3: Distribution of most significant markers in men and women with PsA.   Clinical and Developmental Immunology with the HAQ. We do not know whether the same genetic conditioning factors are operative in both males and females with PsA, and the data found in the literature are very limited. Apart from the mentioned relationship between HLA-B27 and the risk of psoriatic spondyloarthritis in males, the only study of note corresponds to that published by Hüffmeier et al., who detected a greater frequency of the R620W mutation of the PTPN22 gene in males with PsA [20]. This study confirmed HLA-C * 06, B * 27, and MICA-A9 ( * 002) as three of the most relevant PsA risk genes within the MHC. All three were significantly overexpressed in men and women with PsA. Our data corroborate the growing importance of the PsA risk genes in the MHC. However, these genetic profiles revealed differential characteristics between men and women. Thus, women with psoriatic nail disease were seen to express the C * 06 allele significantly more often than men with the same disease. This finding contrasts with the data published by Gudjonsson et al., who reported greater nail involvement in their HLA-C * 06 negative psoriatic patients [21]. These same authors found C * 06 positive women to be younger at the time of onset of psoriasis than C * 06 positive men-an observation not recorded in our series [21]. In fact, among C * 06 positive men with an age of over 40 years at onset of the disease, onset proved significantly earlier than in C * 06 positive women with an age at onset of psoriasis of over 40 years. In turn, the HLA-B * 27 positivity was a marker of early onset disease in both sexes confirming recent observations [22].
Patient age at the onset of psoriasis has been a key stratification element in studies of psoriasis. In this sense, it has been known for decades that early psoriasis (manifesting before 40 years of age) is associated with a family history of disease, increased severity, and a stronger correlation to HLA-C * 06 positivity, while late psoriasis (manifesting after 40 years of age) appears to be milder, with no clear genetic correlations [23,24]. Our findings coincide with these observations. However, in our series the impact of the C * 06 allele upon disease risk was found to decrease more clearly with age among females (only 33% of the women with psoriasis onset above 40 years of age proved positive for this allele) than in men (50% positivity for this allele in individuals with late psoriasis onset). On the other hand, allele 384 of the C4 1 1 microsatellite was associated with late disease in men but not in women-though this allele was in linkage disequilibrium (LD) with C * 06 ( s : 0.6). We found that certain markers that are overexpressed in women, such as C * 07, TNF-308A and HLA-DR17 (DRB1 * 03), behave differently according to age at the onset of psoriasis. Specifically, C * 07 was seen to be elevated in women with early psoriasis, while the latter two markers were significantly overexpressed only in women with late psoriasis. In women there was a certain LD between TNF-308A, and DR17 ( s : 0.3). In any case, it is interesting to note that C * 07, TNF-308A, and DRB1 * 03 form part of the extended haplotype 8.1, which also contains an allele recently linked to PsA risk (HLA-B * 08); as a result, it is nowadays difficult to establish which of these alleles are the true disease risk markers [25].
The present work has not been designed as a genetic study, since the small number of patients involved clearly implies a lack of the statistical power needed for such studies. Rather, we aimed to determine whether there are genetic distinctions between men and women that could help explain the little known gender differences in the clinical expression of PsA. Overall, our genetic findings are supported by recent data confirming a good number of our observations referred to the susceptibility role of HLA-B * 27, C * 06, and C * 07 as risk alleles in PsA [22,25]. As a novelty, we have seen that the impact of these genetic risk elements differs according to patient's gender and age at the onset of psoriasis.

Conclusions
In sum, men and women with PsA show differences in the expression of the disease, and this is probably due in part to a differential overexpression of certain MHC genes between the two genders. We consider patient's gender and age at the onset of psoriasis to be key stratification factors that should be considered in future studies of the associations between MHC genes and the risk of PsA.