title: The MICA-A9 triplet repeat polymorphism in the transmembrane region confers additional susceptibility to the development of psoriatic arthritis and is independent of the association of Cw*0602 in psoriasis.
abstract: OBJECTIVE
  To investigate the relative contribution of HLA antigens in the susceptibility to psoriasis and to localize additional genetic factors involved in psoriatic arthritis (PsA).
  
  
  METHODS
  DNA from 45 patients with psoriasis, 65 with PsA, and 177 healthy control subjects was examined by polymerase chain reaction (PCR) using sequence-specific oligonucleotide probes to determine HLA-C. To examine whether MICA (class I major histocompatibility complex chain-related gene A) confers additional susceptibility, trinucleotide repeat polymorphism in the transmembrane region of the MICA gene was investigated by radioactive PCR. Further analysis of MICA was made by PCR-single-strand conformational polymorphism to determine the allelic variant corresponding to MICA transmembrane polymorphism.
  
  
  RESULTS
  Our results reveal new findings: 1) the frequency of the Cw*0602 allele was significantly increased in both patient groups: psoriasis (corrected P [Pcorr] < 10(-5), relative risk [RR] 6.2), PsA (Pcorr < 10(-6), RR 6.3), 2) the trinucleotide repeat polymorphism MICA-A9 was present at a significantly higher frequency in PsA patients (Pcorr < 0.00035, RR 3.2), whereas a similar distribution was found in both the control and psoriasis population, 3) this polymorphism corresponds to the MICA-002 allele and was found to be overrepresented in patients with the polyarticular form (Pcorr < 0.0008, RR 9.35), 4) the increase in MICA-A9 in PsA patients is independent of linkage disequilibrium with Cw*0602, 5) this allele confers additional relative risk (RR 3.27, etiologic fraction 0.44; etiologic fraction is the proportion of disease cases among the total population that are attributable to 1 allele when the relative risk is > 1) in PsA patients who carry Cw*0602.
  
  
  CONCLUSIONS
  The data obtained in this study are consistent with the polygenic inheritance of psoriasis. Cw*0602 appears to be the stronger genetic susceptibility factor for psoriasis. Independent of the HLA-C association, MICA-A9 polymorphism corresponding to the MICA-002 allele is a possible candidate gene for the development of PsA.
mesh_terms: D000328:Adult; D000483:Alleles; D015535:Arthritis, Psoriatic / Q000150:complications / Q000235:genetics*; D005260:Female; D008040:Genetic Linkage; D020022:Genetic Predisposition to Disease / Q000235:genetics*; D015235:HLA-B Antigens / Q000032:analysis; D015236:HLA-C Antigens / Q000235:genetics*; D015395:Histocompatibility Antigens Class I / Q000235:genetics* / Q000276:immunology; D006801:Humans; D008297:Male; D011110:Polymorphism, Genetic; D011565:Psoriasis / Q000150:complications / Q000235:genetics*