The initial aim of this work was to investigate whether the two dermatological
conditions, psoriasis and atopic dermatitis (AD), can co-exist in an individual, either concurrently or consecutively over time, in contrast to the widely believed and taught idea that the two are mutually exclusive. A study was initiated and the relevant details of 428 psoriasis patients, 224 atopic dermatitis patients, and 286 control patients were recorded. The main result of finding 45 patients with both conditions was supported by the family history of both psoriasis and atopic conditions in the families of the probands. A hypothesis for the concomitance is presented.
Questions arose from the initial study on the possibility of two types of
psoriasis existing, one of early onset and the other of late onset. Ages of onset were formally analysed by the method of maximum likelihood and showed the presence of two distributions, both normal, divided at the age of 40 years. Two negative binomial distributions were found in the number of affected relatives associated with the age of onset in the probands, divided at the age of 40 years. These results raised questions on the nature of the inheritance of the two onset groups in psoriasis.
Segregation analysis was applied to both early and late onset groups as well
as overall. A single dominant or recessive gene model was found to be
inconsistent with the data in all groupings. However, a double recessive model
indicating the presence of two gene loci, fitted the data for late onset. A
multifactorial model of inheritance was a good fit to all the data. Heritabilities and the associated genetic correlations were calculated by the methods of Falconer (1965).
These indicated that early onset has a polygenic mode of inheritance mainly due to additive gene effects, while late onset has a more complex multifactorial inheritance pattern with environmental factors and non-additive gene effects playing an important role. Psoriasis patients who have 1 parent with psoriasis have early onset, which also appears to be associated with the HLA-B locus.
It is suggested that any reporting of simple modes of inheritance in psoriasis
are specific to certain families or populations, where. the multifactorial model of
inheritance allows for the possibility of certain genes becoming predominant.