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[|Although I agree that in different families with psoriasis the genes
involved are different, form the main point of the genetics in
psoriasis should be made more clear: psoriasis is a polygenic disease,
i.e. within one patient there is a set of genes responsible for the
predisposition to psoriasis, which is different from that of another
patient, even in the same family.|auteur195]

[|The genetic aspects of psoriasis are well reviewed. From a personal perspective, having had the pleasure of working with Dr. Anne Bowcock, the discoverer of PSORS2 gene on chromosome 17 in 1994, for the past 14 years, I have always felt that the various phenotypic manifestations of psoriasis need to be carefully genotyped. Time needs to be spent in evaluating the genetics of these various psoriatic subtypes. In addition, there are obviously genetic associations with other immune-mediated diseases such as Crohn’s disease. Finally, is psoriatic arthritis truly a variation of psoriasis vulgaris as to date the genetics of the various forms of psoriatic arthritis remains to be fully elucidated. With the complexities of the phenotypic expressions in the skin, as well as the multiple variations of psoriatic arthritis, it is likely that the full genetic spectrum of psoriasis may take years to unravel.|auteur215]

Psoriasis is associated with familial involvement in 30 to 40% of cases. In patients whose illness starts during childhood, a strong linkage is noted between the disorder and certain HLA groups (CW6, CW7 and, less so, HLA B13, B17, B37, B38, B57, DR4 and DR7). In homozygous twins, one twin being affected goes hand in hand with the other being affected in 70% of cases. This concordance illustrates at once the existence of genetic predispositions and the triggering role of the environment.

By studying large families with psoriasis, it has been possible to detect chromosomal regions associated with the disease. These regions vary from one family to another, but three regions have been identified with some frequency by different teams: PSOR S1 on chromosome 6p, PSOR S2 on chromosome 17q and PSOR S3 on chromosome 4. Other regions have been identified: 1q21, 3q21, 4qter, 14q31-q32, 17q24-q25, 19p13.3 and 20p. Some of these regions represent genes of interest, like corneodesmosine, the key protein in desquamation, or a gene associated with the Crohn disease. Some of these chromosomal regions, particularly region 17q24-q25 (PSORS2), are currently arousing a quite special interest following the work of A.M. Bowcock. The genes in this region encode for a set of genes of the superfamily of immunoglobulins. Recently, in one psoriatic family, this researcher identified a gene and the corresponding protein that might be associated with the expression of the disease.

Whatever may be the interest of this research, one must not forget that the genes involved are probably different from one family to another, that most psoriasis are not familial and that there is no psoriasis gene, as some unscrupulous researchers have made believe to some families. Psoriasis must be clearly explained as being greatly different from the monogenic genetic diseases. In fact, given certain environmental conditions, it is likely that all the genotypes associated directly or indirectly with a significant increase of cytokinic expression in response to aggressions are capable of expressing themselves through different forms of psoriasis. It would be particularly interesting to be able to define groups of patients that belong to homogenous clinical phenotypes.

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