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.