**19. Giant cell arteritis**

GCA is a form of vasculitis. It is very common in elderly people and may cause blindness and stroke [263]. The environmental, infectious, and genetic risk factors have been associated with GCA development; however, the pathogenesis is not clear yet. PTPN22 is a gene of interest which is proposed to be an "archetypal non-HLA autoimmunity gene" [251, 264]. The T-allele of a functional PTPN22 C1858T polymorphism has been reported to be associated with biopsy-proven GCA in Spanish patients (**Table 17**), with supporting data from three replicate Northern European studies [265]. Recently, this observation has been extended with additional patients and controls and studies encompassing European, Scandinavian, UK, and American patients [266], though an earlier report from Spanish patients does not support the potential involvement of PTPN22 gene polymorphism in the susceptibility or clinical expression of GCA [263].

Though Lester et al. [267] could not find any significant difference in the distribution of alleles and genotypes of PTPN22 C1858T polymorphism between patients and control groups, they suggested that there is a significant association between the minor allele of PTPN22 C1858T polymorphism and GCA.
