Familial clustering of polymorphic light eruption in relatives of patients with lupus erythematosus: Evidence of a shared pathogenesis

Background: Abnormal photosensitivity is a common feature of many forms of lupus erythematosus (LE). Objectives: To examine the role of polymorphic light eruption (PLE) as a possible predisposing factor for cutaneous forms of LE. Methods: Eighty-five patients with well-characterized subacute cutaneous LE (SCLE) and discoid LE (DLE) were recruited from outpatient clinics, and the prevalence of PLE determined by detailed interview and clinical examination. Results: Symptoms consistent with PLE were reported in 61% and 55% of SCLE and DLE patients, respectively: this was significantly higher than the overall population prevalence of 13.6% (P < 0.001) giving a relative risk (RR) for PLE in SCLE patients of 3·37 (95% confidence interval, CI 2·46-4·28) and DLE patients of 3·11 (95% CI 2·31-3·91). PLE developed before the onset of LE in 61% of cases (median interval 12 years, range 1-40), concomitantly in 24%, and subsequently in a further 15% (median interval 3·5 years, range 1-25). To delineate the relationship between PLE and LE further, the prevalence of PLE was determined in 103 otherwise unaffected first-degree relatives of SCLE and DLE probands: we had previously demonstrated clustering of PLE in families, reflecting a strong genetic component. We found a significantly higher PLE prevalence in relatives of the LE probands than in the general population (P < 0.001), giving an RR for PLE of 2·29 (95% CI 1·55-3·03) and 2·61 (95% CI 1·32-3·89) for female and male relatives, respectively. Conclusions: The high prevalence of PLE in LE patients, together with clustering of PLE among first-degree relatives of SCLE and DLE probands, suggests that there may be a shared pathogenetic basis for PLE and cutaneous LE. We propose that predisposition to PLE may contribute to the LE phenotype in otherwise susceptible individuals.