High prevalence of autoantibodies to RNA helicase A in Mexican patients with systemic lupus erythematosus

Abstract

Introduction: Autoantibodies to RNA helicase A (RHA) were reported as a new serological marker of systemic lupus erythematosus (SLE) associated with early stage of the disease. Anti-RHA and other autoantibodies in Mexican SLE patients and their correlation with clinical and immunological features were examined.

Methods: Autoantibodies in sera from 62 Mexican SLE patients were tested by immunoprecipitation of 35S-labeled K562 cell extract and enzyme-linked immunosorbent assay (anti-U1RNP/Sm, ribosomal P, beta2GPI, and dsDNA). Anti-RHA was screened based on the immunoprecipitation of the 140-kDa protein, the identity of which was verified by Western blot using rabbit anti-RHA serum. Clinical and immunological characteristics of anti-RHA-positive patients were analyzed.

Results: Anti-RHA was detected in 23% (14/62) of patients, a prevalence higher than that of anti-Sm (13%, 8/62). Prevalence and levels of various autoantibodies were not clearly different between anti-RHA (+) vs. (-) cases, although there was a trend of higher levels of anti-RHA antibodies in patients without anti-U1RNP/Sm (P = 0.07). Both anti-RHA and -Sm were common in cases within one year of diagnosis; however, the prevalence and levels of anti-RHA in patients years after diagnosis did not reduce dramatically, unlike a previous report in American patients. This suggests that the high prevalence of anti-RHA in Mexican patients may be due to relatively stable production of anti-RHA.

Conclusions: Anti-RHA was detected at high prevalence in Mexican SLE patients. Detection of anti-RHA in races in which anti-Sm is not common should be clinically useful. Racial difference in the clinical significance of anti-RHA should be clarified in future studies.

Figure 1

Analysis of autoantibodies to RNA helicase A (RHA). (a) Immunoprecipitation using anti-RHA-positive sera. Immunoprecipitation of ³⁵S-methionine-labeled K562 cell extract by anti-RHA-positive sera from Mexican patients with systemic lupus erythematosus (SLE) (n = 14), anti-RHA prototype serum (lane RHA), and a normal human serum (NHS) is shown. Number of years between initial diagnosis and anti-RHA test of each patient is indicated below the lanes. Positions of RHA, UsnRNP components A, B'/B, U5-200k doublet, Ku (p70 and p80), Ro 60k, and ribosomal P P0, and molecular weight (MW) are indicated. Positivity of anti-Sm and U1RNP is indicated at the top. White arrowheads indicate major degradation products of RHA. (b) Immunoprecipitation and Western blot confirmation of anti-RHA. K562 cell extract was immunoprecipitated by sera positive for the 140-kDa protein that co-migrated with RHA. Identity of the 140-kDa protein as RHA was validated by Western blot using a rabbit anti-RHA serum. Lane RHA, anti-RHA prototype serum; lanes 1 to 6, anti-RHA-positive sera screened by immunoprecipitation; lanes 7 to 9, NHS.

Figure 2

Age at diagnosis, age at anti-RNA helicase A (anti-RHA) test, and years between diagnosis and anti-RHA test. Demographic data of anti-RHA-positive (n = 14) and -negative (n = 48) systemic lupus erythematosus (SLE) patients were compared. (a) Age at diagnosis. (b) Age at anti-RHA test. (c) Years from diagnosis to anti-RHA test. (d) Years from diagnosis to anti-RHA test versus levels of anti-RHA. (e) Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) in anti-RHA (+) versus (-) patients. (f) Correlation of SLEDAI and levels of anti-RHA. Anti-RHA levels were measured as integrated density of RHA protein band using phosphorimager as described in Materials and methods. y, years.

Figure 3

Levels of anti-RNA helicase A (anti-RHA) versus other autoantibodies. IgG anti-U1RNP/Sm, ribosomal P (P peptide), double-stranded DNA (dsDNA), and β2 glycoprotein I (β2GPI) were determined by enzyme-linked immunosorbent assay. Serum dilutions used were 1:2,500 for anti-U1RNP/Sm and 1:500 for all others. Anti-RHA levels were semiquantified from immunoprecipitation using phosphorimager. (a) Anti-U1RNP/Sm antibodies. (b) Anti-P peptide antibodies. (c) Anti-dsDNA. (d) Anti-β2GPI. (e) Anti-RHA levels in anti-U1RNP/Sm-positive versus -negative sera. (f) Correlation of anti-RHA versus anti-U1RNP/Sm.