the many genetic studies performed in recent years showed that genes such as interleukin 23 receptor ( il23r ) and il12b and tumor necrosis factor alpha ( tnf ) are closely associated with psoriasis and related diseases such as rheumatoid arthritis , psoriatic arthritis , and crohn 's disease .
human leukocyte antigen c ( hla - c)0602 is the allele most closely associated with this disease .
the age at onset of psoriasis follows a bimodal distribution : type i psoriasis appears before the age of 40 years ( early - onset ) , with a peak at 1622 years ; type ii psoriasis appears after the age of 40 years ( late - onset ) , with a peak at 5760 years .
type i psoriasis has been associated with several single - nucleotide polymorphisms ( snps ) in genes associated with the immune response ( table 1 ) .
for example , hla - c0602 is more strongly associated with type i psoriasis than with type ii psoriasis .
although several association studies have already been performed in psoriasis in both populations ( type i or type ii psoriasis patients ) , knowledge of age at onset remains limited and controversial ( table 1 ) .
therefore , we performed a candidate gene study , where we evaluated genetic susceptibility to type i or type ii psoriasis in patients with moderate - to - severe chronic plaque psoriasis . this approach may help us to identify snps previously associated with psoriasis or other autoimmune diseases   that are specific to type i or type ii psoriasis .
furthermore , our genetic study could improve our understanding of psoriasis and of its etiology and pathogenesis .
we recruited 198 caucasian patients with moderate - to - severe plaque type psoriasis ( psoriasis area and severity index > 10 ) who attended the department of dermatology in four university hospitals in madrid between 16/10/2007 and 17/12/2012 .
five samples did not fulfill the quality criteria of the human genotyping unit - cegen ( cegen , spanish national cancer research centre , madrid , spain ) , and 2 samples had insufficient volume .
we also included 197 healthy volunteers ( controls ) recruited between 10/01/2011 and 14/12/2012 from the clinical pharmacology service ( hospital universitario de la princesa , madrid , spain ) .
all the volunteers were caucasian and had no personal or family history of psoriasis ( at least 2 generations ) .
the protocol fulfilled spanish law on biomedical research and was approved by the ethics committee for clinical investigation of hospital universitario de la princesa .
all controls and patients gave their written informed consent to donate a sample for investigation .
we preselected 320 snps based on an extensive review of 449 articles describing the association between polymorphisms and psoriasis and response to biological drugs and psoriasis and related inflammatory diseases ( rheumatoid arthritis , psoriatic arthritis , and crohn 's disease ) .
we finally selected 192 snps based on minor allele frequency ( 0.05 ) and on the results of studies performed in caucasians and psoriatic patients .
information on the 173 snps analyzed can be found in supplementary table s1 , which is published in .
dna was obtained from samples using an automatic dna extractor ( magna pure system , roche applied science , usa ) and its concentration was quantified in nanodrop nd-1000 spectrophotometer ( wilmington , usa ) . the extracted dna was stored at 80c in the clinical pharmacology service until use .
a total of 196 samples from patients ( 2 samples of 198 cases had insufficient volume ) and 197 samples from controls were sent to the human genotyping unit - cegen to genotype 192 snps .
the analysis was performed using the illumina veracode genotyping platform . if fluorescence was low or the genotype clusters were undifferentiated , the snps were removed .
in addition , if the call rate was less than 95% of the average of the 192 snps analyzed , the samples were removed .
since cegen quality criteria were not met in 19 snps and 5 patients , we finally analyzed 173 snps in 191 patients and 197 controls .
the statistical analysis was performed to compare the following stratified populations : patients with type i psoriasis ( n = 155 ) or type ii psoriasis ( n = 36 ) versus controls ( n = 197 ) and patients with type i psoriasis versus cases with type ii psoriasis .
hardy - weinberg equilibrium was tested for all the snps analyzed using the snpstats program .
snps that were not in hardy - weinberg equilibrium in controls were removed from the subsequent analysis .
we constructed various logistic regression models depending on the main types of inheritance ( codominant , dominant , recessive , and additive ) . in the additive model
, the presence of 2 mutant alleles confers double the risk of 1 mutant allele .
the results were adjusted for rs12191877 ( snp that is strongly associated with the hla - c0602 allele and is highly prevalent in our population ) [ 3 , 35 ] .
subsequently , snps with p < 0.1 in the univariate analysis ( adjusted for rs12191877 ) were included in a multivariate logistic regression model to adjust for relevant confounding factors ( spss 15.0 ) .
the results of the univariate analysis were adjusted for rs12191877 , except when we compared patients with type i psoriasis and patients with type ii psoriasis ( the influence of rs12191877 was not very relevant ) .
we expressed the results as the odds ratio ( or ) , 95% confidence interval , and p value .
the study population included 155 patients with moderate - to - severe chronic plaque type i psoriasis ( 92 men and 63 women ) , 36 patients with type ii psoriasis ( 19 men and 17 women ) , and 197 controls ( 98 men and 99 women ) .
the mean age was 46.01  13.11 years in patients with type i psoriasis ( 45.72  11.69 in men and 46.43  15.04 in women ) , 67.72  11.85 years in patients with type ii psoriasis ( 65.95  11.18 in men and 69.71  12.59 in women ) , and 24.51  4.29 years in the controls ( 25.07  4.94 in men and 23.95  3.46 in women ) .
the mean age at onset of psoriasis was 23.31  8.52 in patients with type i psoriasis and 52.58  10.45 in patients with type ii psoriasis .
a total of 192 snps were analyzed ( see supplementary table s1 published in ) .
one snp was monomorphic ( rs165161 in the junb gene ) and was excluded from the statistical analysis .
all the minor allele frequencies were in hardy - weinberg equilibrium except 9 snps in the controls and 12 snps in the patients ( see supplementary table s1 published in ) .
the 9 snps which were not in hardy - weinberg equilibrium in controls were removed from the statistical analysis .
our findings showed an association between type i psoriasis and 10 snps ( n = 155 versus n = 197 controls ) : rs1634517 ( ccl4l ) , rs1975974 ( c17orf51 ) , rs12720356 ( tyk2 ) , rs1800925 ( il13 ) , and rs6908425 ( cdkal1 ) decreased the risk of psoriasis 2.94-fold , 2.08-fold , 10-fold , 100-fold , and 2.44-fold , respectively ; and rs2282276 ( clmn ) , rs10782001 ( fbxl19 ) , rs3792876 ( slc22a4 ) , rs12191877 ( hla - c ) , and rs13437088 ( hla - b / mica ) increased the risk of psoriasis 3.90-fold , 2.10-fold , 3.75-fold , 30.54-fold , and 2.52-fold , respectively ( table 2 ) .
however , comparison of 36 patients with type ii psoriasis and 197 controls revealed no significant association ( results not shown ) .
four snps were associated with significant decreases in the risk of type i psoriasis ( n = 155 ) compared with type ii psoriasis ( n = 36 ) , namely , rs191190 ( tnfr1 ; 126.08-fold ) , rs361525 ( tnf- ; 190.76-fold ) , and rs10499194 and rs6920220 ( tnfaip3 ; 155.02-fold and 19.14-fold , resp . ) .
we also found 5 snps that were associated with a significant increase in the risk of type i psoriasis , namely , rs1801274
( fcgr2a ; 5.26-fold ) , rs763361 ( cd226 ; 33.3-fold ) , rs12459358 ( psors6 ; 11.11-fold ) , rs12191877 ( hla - c ; 12.5-fold ) , and rs1576 ( cchcr1 ; 166.66-fold ) ( table 3 ) .
about 75% of patients with chronic plaque psoriasis have type i psoriasis before age 40 , whereas a lower number of patients develop psoriasis at around 5060 years .
our results are consistent with these findings , since 79.06% of our patients developed psoriasis before the age of 40 .
when we compared patients with type i psoriasis and controls , we found 10 significant snps in clmn , fbxl19 , ccl4l , c17orf51 , tyk2 , il13 , slc22a4 , cdkal1 , hla - c , and hla - b / mica . the hla - c0602 allele is
a risk factor for psoriasis   and has been associated with both type i [ 69 ] and type ii psoriasis . in one study ,
85.3% of patients with type i psoriasis had this allele , whereas only 14.7% of patients with type ii psoriasis were carriers .
other authors found an association between rs10484554 ( hla - c ) and type i psoriasis compared with type ii psoriasis ( or = 3.24 in type i ) .
the hla - c gene was associated with type i psoriasis ( p = 2.97e  18 for rs1265181 , p = 2.58e  15 for rs12191877 , p = 1.84e  15 for rs4406273 , and p = 1.10e  07 for rs2395029 ) , but not with type ii psoriasis after application of the bonferroni correction .
in addition , our results showed significant differences in rs12191877 ( hla - c ) in patients with type i psoriasis ( p = 2.50e  19 ) .
however , we did not find this association in patients with type ii psoriasis , probably owing to the small sample size in this group ( n = 36 ) .
found an association between rs1295685 in the il13 gene and type i psoriasis ( p = 2.47e  03 ) .
our results showed an association between another snp in il13 ( rs1800925 ) and type i psoriasis ( p = 0.034 ) .
in addition , munir et al . did not obtain significant results when they compared controls with type ii psoriasis or type i psoriasis with type ii psoriasis .
both snps in il13 have been associated with predisposition to psoriasis [ 36 , 37 ] .
our comparison of patients with type i psoriasis and controls is the first to obtain significant results for a series of snps in type i psoriasis , although the snps have already been associated with the risk of psoriasis .
rs10782001 in fbxl19 , rs1975974 in c17orf51 , rs12720356 in tyk2 [ 3 , 39 ] , rs3792876 in slc22a4 , rs6908425 in cdkal1 , and rs13437088 in hla - b / mica   have previously been associated with psoriasis , but not with type i psoriasis .
furthermore , snps in clmn ( rs2282276 ) and ccl4l ( rs1634517 ) have not been associated with psoriasis or age at onset .
we found no significant differences between patients with type ii psoriasis and controls owing to the small sample size ( n = 36 ) .
comparison between patients with type i psoriasis and patients with type ii psoriasis revealed significant associations for the following genes : fcgr2a , tnfr1 , cd226 , psors6 , tnfaip3 , hla - c , tnf- , and cchcr1 .
polymorphisms in cchcr1 ( 386 and 404 , cchcr1ww allele ) have been associated with type i psoriasis [ 9 , 31 ] .
we found significant differences between rs1576 in cchcr1 and age at onset . in a study comparing controls ( 54.8% ) and patients with psoriasis type ii ( 66.0% ) , allen et al
analyzed rs763361 in cd226 in patients with early - onset psoriasis and patients with late - onset psoriasis , although they found no associations .
rs12459358 in psors6 has been associated with type i psoriasis ( g risk allele , or = 1.47 and p = 0.005 ) .
in contrast , our data showed an association between the t allele and type i psoriasis ( or = 11.11 ; p = 0.026 ) .
rs361525 ( 238 ) in the tnf gene has been associated with susceptibility to psoriasis , and the a allele was more frequent in male patients with type i psoriasis ( p = 2e  07 ) [ 15 , 22 ] .
we found significant results in rs361525 ( tnf- ) when we compared patients with type i psoriasis and patients with type ii psoriasis , although we found no gender differences .
other authors confirmed our association with type i psoriasis in caucasian [ 20 , 23 ] and mongolian patients .
baran et al . found no significant differences between rs1800629 in the 308 promoter ( tnf ) and type i or type ii psoriasis .
likewise , rs12191877 in hla - c has been associated with increased risk of psoriasis .
compared patients with type i psoriasis and patients with type ii psoriasis and obtained significant results for rs1265181 , rs4406273 , and rs12191877 in hla - c .
we replicated these results in rs12191877 ( t allele risk ; p = 0.045 ) .
rs191190 in tnfr1   and rs10499194 in tnfaip3   have been associated with psoriasis , but not with age of onset .
moreover , rs1801274 in fcgr2a and rs6920220 in tnfaip3 have not been studied in patients with psoriasis according to age of onset .
given the small sample size in the group with type ii psoriasis in our study , our results should be interpreted with caution .
our results highlight the role of the immune system in psoriasis and enhance our understanding of pathogenic mechanisms .
second , the sample size was limited by the number of study patients treated in the dermatology department , thus making it difficult to detect snps with a low probability of causing psoriasis .
third , since the snps were selected based on a literature review , several major snps may not yet have been investigated . in conclusion ,
our study confirmed an association between rs12191877 ( hla - c ) and type i psoriasis and between type i and type ii psoriasis patients .
ours is the first study to show an association between clmn , fbxl19 , ccl4l , c17orf51 , tyk2 , il13 , slc22a4 , cdkal1 , and hla - b / mica and type i psoriasis .
in addition , psors6 and tnf have been described as more prevalent genes in type i psoriasis and we showed a significant association when we compared type i psoriasis and type ii psoriasis .
ours is the first study to identify an association between fcgr2a , tnfr1 , cd226 , tnfaip3 , and cchcr1 and age at onset of psoriasis .
 psoriasis is a chronic skin disease in which genetics play a major role . 
 although many genome - wide association studies have been performed in psoriasis , knowledge of the age at onset remains limited . 
 therefore , we analyzed 173 single - nucleotide polymorphisms in genes associated with psoriasis and other autoimmune diseases in patients with moderate - to - severe plaque psoriasis type i ( early - onset , < 40 years ) or type ii ( late - onset , 40 years ) and healthy controls . 
 moreover , we performed a comparison between patients with type i psoriasis and patients with type ii psoriasis . 
 our comparison of a stratified population with type i psoriasis ( n = 155 ) and healthy controls ( n = 197 ) is the first to reveal a relationship between the clmn , fbxl19 , ccl4l , c17orf51 , tyk2 , il13 , slc22a4 , cdkal1 , and hla - b / mica genes . when we compared type i psoriasis with type ii psoriasis ( n = 36 ) , we found a significant association between age at onset and the genes psors6 , tnf- , fcgr2a , tnfr1 , cd226 , hla - c , tnfaip3 , and cchcr1 . 
 moreover , we replicated the association between rs12191877 ( hla - c ) and type i psoriasis and between type i and type ii psoriasis . 
 our findings highlight the role of genetics in age of onset of psoriasis . 
1. Introduction
2. Material and Methods
3. Results
4. Discussion