Association analysis using refined microsatellite markers localizes a susceptibility locus for psoriasis vulgaris within a 111 kb segment telomeric to the HLA-C gene

Family-based analysis using a dense single-nucleotide polymorphism-based map defines genetic variation at PSORS1, the major psoriasis-susceptibility locus

Psoriasis is a common skin disorder of multifactorial origin. Genomewide scans for disease susceptibility have repeatedly demonstrated the existence of a major locus, PSORS1 (psoriasis susceptibility 1), contained within the major histocompatibility complex (MHC), on chromosome 6p21. Subsequent refinement studies have highlighted linkage disequilibrium (LD) with psoriasis, along a 150-kb segment that includes at least three candidate genes (encoding human leukocyte antigen-C [HLA-C], alpha-helix-coiled-coil-rod homologue, and corneodesmosin), each of which has been shown to harbor disease-associated alleles. However, the boundaries of the minimal PSORS1 region remain poorly defined. Moreover, interpretations of allelic association with psoriasis are compounded by limited insight of LD conservation within MHC class I interval. To address these issues, we have pursued a high-resolution genetic characterization of the PSORS1 locus. We resequenced genomic segments along a 220-kb region at chromosome 6p21 and identified a total of 119 high-frequency SNPs. Using 59 SNPs (18 coding and 41 noncoding SNPs) whose position was representative of the overall marker distribution, we genotyped a data set of 171 independently ascertained parent-affected offspring trios. Family-based association analysis of this cohort highlighted two SNPs (n.7 and n.9) respectively lying 7 and 4 kb proximal to HLA-C. These markers generated highly significant evidence of disease association (P<10-9), several orders of magnitude greater than the observed significance displayed by any other SNP that has previously been associated with disease susceptibility. This observation was replicated in a Gujarati Indian case/control data set. Haplotype-based analysis detected overtransmission of a cluster of chromosomes, which probably originated by ancestral mutation of a common disease-bearing haplotype. The only markers exclusive to the overtransmitted chromosomes are SNPs n.7 and n.9, which define a 10-kb PSORS1 core risk haplotype. These data demonstrate the power of SNP haplotype-based association analyses and provide high-resolution dissection of genetic variation across the PSORS1 interval, the major susceptibility locus for psoriasis.

Visualizing human leukocyte antigen class II risk haplotypes in human systemic lupus erythematosus

Human leukocyte antigen (HLA) class I and class II alleles are implicated as genetic risk factors for many autoimmune diseases. However, the role of the HLA loci in human systemic lupus erythematosus (SLE) remains unclear. Using a dense map of polymorphic microsatellites across the HLA region in a large collection of families with SLE, we identified three distinct haplotypes that encompassed the class II region and exhibited transmission distortion. DRB1 and DQB1 typing of founders showed that the three haplotypes contained DRB1*1501/ DQB1*0602, DRB1*0801/ DQB1*0402, and DRB1*0301/DQB1*0201 alleles, respectively. By visualizing ancestral recombinants, we narrowed the disease-associated haplotypes containing DRB1*1501 and DRB1*0801 to an approximately 500-kb region. We conclude that HLA class II haplotypes containing DRB1 and DQB1 alleles are strong risk factors for human SLE.

Genetic approaches to the molecular understanding of type 2 diabetes

The appreciation that individual susceptibility to type 2 diabetes (T2D) and related components of the dysmetabolic syndrome has a strong inherited component provides a coherent framework within which to develop a molecular understanding of the pathogenesis of T2D. This review focuses on the main approaches currently adopted by researchers seeking to identify the inherited basis of T2D and the present state of our knowledge. One central theme that emerges is that progress in defining the genetic basis of the common, multifactorial forms of T2D is hindered by etiological heterogeneity: T2D is likely to represent the final common pathway of diverse interacting primary disturbances. Such heterogeneity equally compromises efforts to understand the basis for T2D by use of other approaches, such as cellular biochemistry and classical physiology. Analyses that seek to ally sophisticated physiological characterization with measures of genomic variation are likely to provide powerful tools for redressing the loss of power associated with such heterogeneity.

Corneodesmosin DNA polymorphisms in MHC haplotypes and Japanese patients with psoriasis

In order to examine the relationship between corneodesmosin (CDSN) and psoriasis we have determined the presence of CDSN polymorphisms by DNA sequencing in (a) nine B-LCL cell lines of major histocompatibility complex ancestral haplotypes known to be associated with psoriasis vulgaris including 13.1AH, 46.1AH, 46.2 and 57.1AH, and in (b) a group of 267 unrelated individuals comprising Japanese psoriasis patients (n = 101) and Japanese subjects without the disease (n = 166). Three novel CDSN gene sequences were identified. In addition, we have classified the 18 alleles into seven main groups based on phylogeny of non-synonymous substitutions. However, we have found no statistically significant differences between the patients and the unaffected individuals in any of these groups. These findings indicate that CDSN is not a major psoriasis susceptibility gene.

Susceptibility locus for non-obstructive azoospermia is localized within the HLA-DR/DQ subregion: primary role of DQB1*0604

Non-obstructive azoospermia is a male infertility characterized by no or little sperm in semen as a result of a congenital dysfunction in spermatogenesis. Previous studies have reported a higher prevalence of particular human leukocyte antigen (HLA) antigens in non-obstructive azoospermia. As the expression of the RING3 gene located in the HLA class II region was predominant in the testis, mainly around spermatids and pachytene spermatocytes, it is tempting to speculate that RING3 is one of the strong candidate genes responsible for the pathogenesis of the disease. In this study, the genetic polymorphism in the RING3 gene was investigated by the direct sequencing technique. As a result, a total of 14 single nucleotide polymorphisms were identified. Among them, six were localized in the coding region but none of them was accompanied by an amino-acid substitution. No significant difference in the allelic distribution at these 14 polymorphic sites was observed between the patients and healthy controls, suggesting that the susceptible gene for non-obstructive azoospermia is not the RING3 gene. Then, in order to map the susceptibility locus for non-obstructive azoospermia precisely within the HLA region, 11 polymorphic microsatellite markers distributed from the SACM2L gene just outside the HLA class II region (187 kb telomeric of the DPB1 gene) to the OTF3 gene in the HLA class I region were subjected to association analysis in the patients. Statistical analysis of distribution in the allelic frequency at each microsatellite locus demonstrated that the pathogenic gene for non-obstructive azoospermia is located within the HLA-DR/DQ subregion. In fact, DRB1*1302 and DQB1*0604 were found to be strongly associated with non-obstructive azoospermia by polymerase chain reaction-based DNA typing. Further, haplotype analysis suggested that the DQB1*0604 allele may play a decisive role in the pathogenesis of non-obstructive azoospermia.

Haplotype associations of the major histocompatibility complex with psoriasis in Northeastern Thais

BACKGROUND

To evaluate the distributions of the human leukocyte antigen (HLA) at class I and II loci that may contribute to the genetic susceptibility to psoriasis patients in the north-eastern Thai population.

MATERIALS AND METHODS

We analyzed the allelic frequencies of HLA class I and II by using the polymerase chain reaction-amplification refractory mutation system (PCR-ARMS) technique and polymerase chain reaction-single stranded conformation polymorphism (PCR-SSCP), respectively, in 140 north-eastern Thais with psoriasis that were sudivided into two groups: one with age at onset < 40 years (type I psoriasis; 95 cases) and the other with age at onset > 40 years (type II psoriasis; 45 cases). Three hundred healthy unrelated north-eastern Thais were used as controls.

RESULTS

HLA-A*01, -A*0207, -A*30, -B*08, -B*13, -B*4601, -B*57, -Cw*01, -Cw*0602, and -DRB1*07 were positively associated with type I psoriasis, whereas HLA-A*24, -A*33, and -Cw*04 were negatively associated with type I psoriasis with statistical significance when compared to the controls. The Cw*0602 allele showed the strongest correlation with this type. In addition, the frequencies of HLA-A*0207, -A*30, -Cw*01, and -DRB1*1401 were significantly increased in type II psoriasis. HLA-A*207, -B*4601, -Cw*01, -DRB1*09, -DQB1*0303 (AH46.1), HLA-A*01-B*57-Cw*0602-DRB1*07-DQB1*0303 (AH57.1), and HLA-A*30, -B*13, -Cw*0602, -DRB1*07, and -DQB1*02 (AH13.1) were identified as high-risk major histocompatibility complex (MHC) halotypes for psoriasis patients in the early onset group in north-eastern Thais.

CONCLUSIONS

This study demonstrates not only the differential association between HLA markers and types of psoriasis according to age at onset, but also a newly found high-risk and a protective haplotype in Thai psoriasis patients.

Searching for the major histocompatibility complex psoriasis susceptibility gene

Psoriasis, a common skin disorder, is widely regarded to be multifactorial in origin including gene-gene and gene-environment interactions. Genetic and allelic heterogeneity, multifactorial inheritance, and low penetrance of susceptibility alleles substantially complicate both study design and interpretation of results. Notwithstanding these difficulties, genome-wide scans for psoriasis susceptibility have generated robust evidence for a major locus lying within the major histocompatibility complex (PSORS1, Psoriasis Susceptibility 1), on the short arm of chromosome 6. Subsequent studies have sought to refine the PSORS1 boundaries by means of linkage disequilibrium fine mapping. Studies of positional candidate genes have also been undertaken, focusing on HLA-C, corneodesmosin, and alpha-helix coiled-coil rod homolog genes. Methodologic approaches, results, and interpretations of these studies are discussed, as well as future research objectives. In particular, we emphasize the importance of characterizing PSORS1 linkage disequilibrium patterns and developing functional assays for disease-associated alleles.

Combined segregation and linkage analysis of HLA markers in familial psoriasis

Marker-based segregation analysis (MBSA) is a modification of a published method of combined linkage and segregation analysis (Am J Hum Genet 51: 1111-1126, 1992), to determine whether a candidate gene known to be associated with the disease of interest is truly segregating with the disease in families. Here we outline the conceptual basis of MBSA and present a Monte Carlo method for significance testing. The method is applied to PSORS1, a locus within the major histocompatibility complex (MHC) for which linkage and linkage disequilibrium with psoriasis has already been demonstrated. The results are very consistent with our current knowledge of PSORS1, and suggest that MBSA can provide useful information on genotype-phenotype relationships such as penetrance and allelic heterogeneity.

The human major histocompatability complex: lessons from the DNA sequence

The entire 3.6-MbpDNA sequence of a human major histocompatibility complex derived from a composite of DNA clones from different haplotypes, was completed in 1999, primarily through the work of four main groups. At that time, it was the longest contiguous human DNA sequence to have been determined. The sequence is of extremely high quality and accuracy. In this review, we discuss how the DNA sequence has facilitated our understanding of the biology and genetics of the major histocompatibility complex. We suggest some ways in which the sequence may be exploited in the future to explore the relationship between the extraordinary polymorphism of the region and its association with both autoimmune and infectious diseases.

Fine mapping of the PSORS4 psoriasis susceptibility region on chromosome 1q21

Psoriasis is a chronic skin disorder affecting approximately 2% of the Caucasian population. Family clustering of the disease is well established and nonparametric linkage analyzes have mapped disease susceptibility loci on chromosomes 6p (PSORS1) and 17q (PSORS2). Nonconfirmed evidence for linkage is also available for chromosomes 2q 3q, 4q (PSORS3), 8q, 16q, and 20p. We mapped an additional susceptibility locus on chromosome 1q21 (PSORS4). In this study, we have carried out a linkage disequilibrium analysis, in order to achieve a finer localization. We recruited 79 triads from continental Italy and typed them at five loci spanning the 1.6 Mb region generating the highest multipoint LOD scores in our previous linkage study. We observed significant evidence for association with D1S2346 marker (p = 0.004). Results consistent with this data were obtained by typing an independent sample that included 28 patients and 56 controls, originating from Sardinia. In fact, p values of 0.02 were observed with both D1S2346 and D1S2715 markers. We sought further confirmation of our results by typing both samples with two novel markers (140J1C and 140J1D) flanking D1S2346. Marker 140J1D generated a p value of 0.003 in the continental Italy sample where a D1S2346/140J1D haplotype was found with a higher frequency among patients' chromosomes. Altogether our data indicate that the 1q21 susceptibility gene may be localized in the genomic interval spanned by D1S2346 and 140J1D. This report provides evidence supporting the refinement of a non-HLA psoriasis susceptibility locus.

Psoriatic arthritis: clinical aspects, genetics, and the role of T cells

In the last 2 years there has been considerable progress in investigating the genetic and immunologic background of psoriasis and psoriatic arthritis. This review focuses on genetics and the role of T-cells in the immunopathogenesis of the disease, with particular reference to psoriatic arthritis.

Corneodesmosin expression in psoriasis vulgaris differs from normal skin and other inflammatory skin disorders

SUMMARY

Corneodesmosin (Cdsn) is a late differentiation epidermal glycoprotein putatively involved in keratinocyte adhesion. The Cdsn gene lies within the susceptibility region on chromosome 6p21.3 (PSORS1) for psoriasis, a common chronic disfiguring skin disease. A particular allelic variant of Cdsn has a strong association with psoriasis. Therefore, genetically and biologically, Cdsn is a possible candidate gene for psoriasis susceptibility. To investigate a potential role for Cdsn in psoriasis pathogenesis, protein expression studies were performed by quantitative immunohistochemistry on normal skin, psoriatic skin (lesional and nonlesional), and other skin disorders using monoclonal antibodies (G36-19 and F28-27). In normal and nonlesional skin, Cdsn was expressed in stratum corneum and one or two layers of superficial stratum granulosum. In lesional psoriasis, there was a significant increase in Cdsn expression, which was observed in multiple layers of stratum spinosum and in stratum corneum. The expression pattern varied from granular, cytoplasmic immunoreactivity to cell surface labeling with weakly immunoreactive cytoplasm. In chronic atopic dermatitis, lichen planus, mycosis fungoides, and pityriasis rubra pilaris, Cdsn immunoreactivity was confined to stratum corneum and upper stratum granulosum with no stratum spinosum immunoreactivity. Immunoelectron microscopy of normal and lesional psoriatic skin demonstrated Cdsn release concomitant with involucrin incorporation into cell envelopes and completed before mature envelope formation. Extracellular release of Cdsn occurred at a lower level of the epidermis in psoriasis than normal skin. These protein expression studies provide evidence of altered Cdsn expression in psoriasis consistent with a role of Cdsn in disease pathogenesis. Further functional and genetic studies of Cdsn are justified to determine its role as a potential psoriasis-susceptibility factor.

Genetic aspects of psoriasis

Epidemiological studies indicate that genes play an important role in the pathogenesis of psoriasis. Multiple genes are likely to be involved, interacting not only with each other but also with the environment to cause disease expression. Molecular genetic studies indicate that there are multiple susceptibility loci present throughout the human genome. It is clear that a gene or genes of major impact on psoriasis is present on chromosome 6 within the major histocompatibility complex (MHC). Linkage disequilibrium studies indicate this gene to reside within a 300 kb interval centred around the centromeric end of class I MHC. Known candidate genes in this region are HLA-C, corneodesmosin and HCR, although novel genes, as yet unknown, may also exist. There is accumulating evidence that HLA-C is not itself the causative gene but rather a marker for it. Identification of the genes involved in psoriasis susceptibility will represent a step forward in our understanding of the disease and our future ability to help patients with psoriasis.

New polymorphic microsatellite markers in the human MHC class III region

The human major histocompatibility complex (MHC) class III region spanning approximately 760 kb is characterized by a remarkably high gene density with 59 expressed genes (one gene every 12.9 kb). Recently, susceptibility loci to numerous diseases, such as Graves disease, Crohn disease, and SLE have been suggested to be localized to this region, as assessed by associations mainly with genetic polymorphisms of TNF and TNF-linked microsatellite loci. However, it has been difficult to precisely localize these susceptibility loci to a single gene due to a paucity to date of polymorphic markers in the HLA class III region. To facilitate disease mapping within this region, we have analyzed 2 approximately 5 bases short tandem repeats (microsatellites) in this region. A total of 297 microsatellites were identified from the genomic sequence, consisting of 69 di-, 62 tri-, 107 tetra-, and 59 penta-nucleotide repeats. It was noted that among them as many as 17 microsatellites were located within the coding sequence of expressed genes (NOTCH4, PBX2, RAGE, G16, LPAAT, PPT2, TNXB, P450-CYP21B, G9a, HSP70-2, HSP70-1, HSP-hom, MuTSH5 and BAT2). Eight microsatellite repeats were collected as polymorphic markers due to their high number of alleles (11.9 on average) as well as their high polymorphic content value (PIC) (0.63). By combining the 38 and the 22 polymorphic microsatellites we have previously collected in the HLA class I and class II regions, respectively, we have now established a total of 68 novel genetic markers which are uniformly interspersed with a high density of one every 63.3 kb throughout the HLA region. This collection of polymorphic microsatellites will enable us to search for the location of any disease susceptible loci within the HLA region by association analysis.

Comparative association analysis reveals that corneodesmosin is more closely associated with psoriasis than HLA-Cw*0602-B*5701 in German families

HLA antigens are associated with psoriasis vulgaris across populations with different ethnic background. We have previously shown that in Caucasians this association is primarily based on the class I alleles of the extended HLA haplotype 57.1 (EH57.1/I), HLA-Cw6-HLA-B57. However, it remained unclear whether HLA-Cw6 itself or a closely linked locus predisposes to the disease. An interesting candidate for this presumed locus is corneodesmosin, which is exclusively synthesized in keratinocytes. The corneodesmosin gene locus (CDSN) is only 160 kb telomeric to HLA-C and tightly associated with psoriasis. In order to find out whether EH57.1/I or a corneodesmosin variant are the susceptibility determinants on 6p, HLA class I alleles and single-nucleotide polymorphisms (SNPs) of corneodesmosin were investigated at the sequence level and analyzed by comparative association tests. Transmission disequilibrium tests (TDT) were performed in 52 nuclear families, of which 36 were fully informative for a joint comparison of HLA and CDSN with regard to association to psoriasis. The extended TDT according to Wilson was employed to test for locus interaction. Using the HLA haplotype EH57.1/I and the CDSN haplotype formed by three intragenic variant sites at nt=619 (T), 1236 (T), and 1243 (C), we obtained the best resolution of parental transmission to index cases in the trio families. On direct comparison of the contributions of the HLA and the CDSN haplotypes, there was a markedly stronger association of the corneodesmosin TTC haplotype, which is not apparent in single locus analysis. We show furthermore that there is no higher order interaction between psoriasis, HLA, and CDSN. This lack of three-locus interaction is suggestive of two independent genetic contributions to psoriasis within the major histocompatibility complex (MHC).

A second susceptibility gene for developing rheumatoid arthritis in the human MHC is localized within a 70-kb interval telomeric of the TNF genes in the HLA class III region

Rheumatoid arthritis (RA) is a chronic inflammatory joint disease with a multifactorial genetic basis. However, pathogenic genes for RA other than the human leukocyte antigen (HLA)-DRB1 gene have yet to be identified. Here, we investigated whether there is a second susceptibility locus for RA within the human major histocompatibility complex using 18 microsatellite markers distributed from the centromeric (HSET) to the telomeric end (P5-15) of the 3.6-Mb HLA region. Statistical studies of associated alleles on each microsatellite locus showed that one pathogenic gene for RA in the HLA region is localized in the DRB1 gene, as expected. Further, a second susceptibility gene of RA was suggested to be present in the HLA class III region, narrowed to 70 kb, that is just telomeric of the TNF gene cluster (TNFA and LTA) and that is located between the microsatellites TNFa and C1-2-A. In this critical segment, four expressed genes have been thus far identified, NFKBIL1 (IkappaBL), ATP6G, BAT1, and MICB, all of which are candidate genes for determining susceptibility to RA. These results exclude the possibility of involvement of the TNFA genes (TNF-alpha) in the development of RA, which was suggested previously to be a strong candidate for RA in the class III region.

Identification of six novel polymorphisms in the human corneodesmosin gene

Psoriatic epidermis is characterised by a defective differentiation program leading to an abnormal permeability barrier and impaired desquamation. The corneodesmosin gene (CDSN) or "S" gene is a strong candidate in psoriasis susceptibility, due first to its genomic position ("S" gene, 160 kb telomeric to HLA-C) and second to its expression and function in the epidermis. Moreover, an association between CDSN and psoriasis vulgaris was recently shown in Caucasian populations. In order to pursue the CDSN polymorphism analysis, we determined the sequence of its alleles in 14 HLA-Cw6-positive individuals. A 4.6 kb genomic fragment encompassing the first exon, the unique intron and the coding sequence of the second exon was amplified from 8 psoriatic patients and 6 controls. Allelic discrimination was performed by restriction fragment length polymorphism analysis. The entire coding sequence and the intron boundaries of 27 alleles were sequenced. A total of 26 dimorphic sites were found, 23 consisting in single nucleotide polymorphisms (SNPs) and 3 in triplet modifications. Five out of the 23 SNPs have not been previously reported, and among them, one causes amino-acid exchange leading to the suppression of a potential chymotrypsin site. Among the triplet modifications, one leads to deletion of one out of five consecutive valines in the protein. The high polymorphism of the gene allowed the identification of 13 different alleles. These haplotypes will permit additional family-based studies that could provide new genetic support for the involvement of CDSN in psoriasis susceptibility. Moreover, the establishment of an extensive catalogue of CDSN alleles will allow functional analyses of the different protein isoforms.

HLA-C and guttate psoriasis

BACKGROUND

Psoriasis is a heterogeneous disease in its clinical expression. Both genetic and environmental factors are thought to contribute to the pathogenesis of the inflammatory and hyperproliferative components of the typical skin lesions. Predisposing genetic influences include associations with human leucocyte antigens (HLA) of which that with HLA-Cw6 is the strongest. Guttate psoriasis is a specific clinical manifestation of psoriasis frequently associated with group A beta-haemolytic streptococcal throat infection.

OBJECTIVES

We set out to determine whether further clinical subdivision of psoriasis is associated with tighter correlation with HLA-C alleles.

PATIENTS/METHODS

We determined the HLA-C locus genotype of 29 caucasian patients with guttate psoriasis presenting consecutively with guttate psoriasis associated with a history of a sore throat and/or an antistreptolysin O titre > 200 IU mL-1. Polymerase chain reaction typing using sequence-specific primers was used to detect all known HLA-C alleles. These data were compared with a control population of 604 random caucasian cadaver donors.

RESULTS

All patients (100%) with guttate psoriasis carried the Cw*0602 allele compared with 20% of the control population (odds ratio = infinity; 95% confidence limits 25.00-infinity; Pcorrected < 0.0000002).

CONCLUSIONS

This result is consistent with HLA-Cw*0602 playing a part directly in the pathogenesis of guttate psoriasis.

The OTF3 gene polymorphism confers susceptibility to psoriasis independent of the association of HLA-Cw*0602

Psoriasis has been strongly associated to HLA-Cw6, but it remains unclear whether Cw6 itself or a closely linked gene is associated with the disease. The aim of this study was to clarify whether the HLA-C itself determines disease susceptibility or whether it acts only as a marker for the susceptibility allele. We examined a sample of 95 type I psoriasis patients and 104 Spanish matched controls to investigate whether HLA-Cw*0602 or other closely related class I loci, such as HLA-B and MICA (which are centromeric to HLA-C), or corneodesmosin gene and octamer transcription factor-3 genes (which are telomeric to HLA-C), might play a part in disease development. DNA samples were genotyped by polymerase chain reaction/sequence-specific primers (HLA-C), polymerase chain reaction/sequence-specific primers (HLA-B), radioactive polymerase chain reaction (MICA-TM polymorphism in the transmembrane region), and polymerase chain reaction/restriction fragment length polymorphism (protein S and octamer transcription factor-3). Our results show a significant increase of Cw*0602 in psoriasis patients (odds ratio = 3.64; pc < 0.0006). A significant association between the beta allele of octamer transcription factor-3 (HindIII) and psoriasis was also detected (odds ratio = 3.76; pc < 0.0003). The allele octamer transcription factor-3B (etiologic fraction = 0.62) was found to be more strongly associated to psoriasis vulgaris than Cw*0602 (etiologic fraction = 0.35) and the increase of octamer transcription factor-3 B allele is independent of the linkage disequilibrium with Cw*0602 as this was also found in Cw*0602 negative patients (odds ratio = 3.63; pc < 0.015, etiologic fraction = 0.55). We did not detect an association between the corneodesmosin gene and psoriasis. This fact suggests that the psoriasis susceptibility gene is located within a critical region of 147 kb, telomeric to HLA-C and centromeric to the corneodesmosin gene, and the association of Cw6 to psoriasis may be secondary to linkage disequilibrium.

Advances in the search for psoriasis susceptibility genes

Psoriasis (PS) is a common skin disorder affecting approximately 2% of the Caucasian population. Despite the established influence of several environmental factors, epidemiological data and twin studies have long demonstrated a genetic basis for psoriasis susceptibility. Moreover an association between PS and HLA-Cw6 has been reported in different ethnic groups. In recent years, the availability of statistical methods for complex disease linkage analysis has prompted many researchers to carry out genome-wide scans. Their results have been conflicting and linkage replication has seldom been documented. However, a few chromosome regions have been confirmed in independent studies. In particular, compelling evidence supports the existence of a susceptibility locus within the HLA region. Moreover, loci on chromosomes 17q and 1q have been reported in at least two independent genome scans. Several groups have undertaken the refinement of regions identified during genome scans, using linkage disequilibrium data. This approach has allowed the fine mapping of the 6p21 locus, now restricted to a 60-kb genomic segment. As critical regions get smaller, candidate gene analysis becomes an attractive approach. So far, three genes have been extensively investigated: S100A7 on chromosome 1q and CDSN and HCR on chromosome 6p21. Even though several SNPs have been identified within these genes, none of them seems to meet the requirement needed to prove an involvement in PS pathogenesis. These criteria include association replication in different populations and functional studies of SNP biological significance. Thus, only a collaborative and multidisciplinary approach will allow the identification of PS susceptibility genes.

Psoriatic arthritis

Psoriatic arthritis occurs in 5 - 42% of patients with psoriasis. It is an inflammatory arthritis distinct from rheumatoid, being usually sero-negative, asymmetrical and often affecting the spine, sacro-iliac and distal interphalangeal joints. It runs a very variable course, from a mild non-destructive disease to a severe rapidly progressive erosive arthropathy, producing an 'arthritis mutilans' with a combination of bone lysis and joint ankylosis. Its pathogenesis is not as well understood as rheumatoid arthritis, but is thought to be similarly immune driven, with a qualitatively similar immunomodulatory cascade and cytokine profile. Quantitatively, however, there are distinct differences in cell ratios and cytokine levels that may well impact on therapeutic strategies. Current therapies, such as methotrexate and sulphasalazine, have yet to be shown to be significantly more effective than placebo in delaying damage and produce only marginal improvements in symptoms. The newer specific biological agents, such as the anticytokine antibodies, interleukins and more specific anti-T-cell therapies, are starting to be studied in psoriatic arthritis. The rationale for their use comes mostly from extrapolation of their efficacy in rheumatoid arthritis. It has yet to be seen whether they will be efficacious in treating the osteolysis, fibrosis and new bone formation particular to psoriatic arthritis. Any treatment for the arthritis must also help the skin. Greater understanding of psoriatic arthritis, its pathogenesis and natural history is required if we are to target these exciting but expensive therapies effectively.

Localization of psoriasis-susceptibility locus PSORS1 to a 60-kb interval telomeric to HLA-C

Recent genome scans have established the presence of a major psoriasis-susceptibility locus in the human leukocyte antigen (HLA) complex on chromosome 6p21.3. To narrow the interval for candidate gene testing, we performed a linkage-disequilibrium analysis of 339 families, with the use of 62 physically mapped microsatellite markers spanning the major histocompatibility complex (MHC). As detected by use of the transmission/disequilibrium test (TDT), individual markers yielded significant linkage disequilibrium across most of the MHC. However, the strongest evidence for marker-trait disequilibrium was found in an approximately 300-kb region extending from the MICA gene to the corneodesmosin gene. Maximum-likelihood haplotypes were constructed across the entire MHC in the original sample and across a 1.2-Mb region of the central MHC in an expanded sample containing 139 additional families. Short (two- to five-marker) haplotypes were subjected to the TDT using a "moving-window" strategy that reduced the variability of TDT P values relative to the single-locus results. Furthermore, the expanded sample yielded a sharp peak of evidence for linkage disequilibrium that spanned approximately 170 kb and that was centered 100 kb telomeric to HLA-C. The 1.2-Mb interval was further dissected by means of recombinant ancestral haplotype analysis. This analysis identified risk haplotype 1 (RH1), which is a 60-kb fragment of ancestral haplotype 57.1, on all identifiable HLA risk haplotypes. One of these haplotypes exhibits significant linkage disequilibrium with psoriasis but does not carry Cw6, which is the HLA allele most strongly associated with the disease. These results demonstrate that RH1 is highly likely to carry the disease allele at PSORS1, and they exclude HLA-C and corneodesmosin with a high degree of confidence.