MONOZYGOTIC TWINS DISCORDANT FOR CONGENITAL COMPLETE HEART-BLOCK

Objective. Isolated congenital complete heart block (CCHB) occurs in 1 /20,000 live births. More than 85% of mothers giving birth to affected infants are anti-Ro antibody positive, but only similar to 1% of babi es with anti-Ro-positive mothers develop CCHB. We studied 2 sets of mo nozygotic twins discordant for CCHB. Methods. Monozygosity was deter;m ined using placental examination and DNA microsatellite analysis. HLA typing was performed, Autoantibody studies were performed using counte rimmunoelectrophoresis, immunoblotting, Ro 52 and Ro 60 enzyme-linked immunosorbent assay (ELISA), and indirect immuno-fluorescence (IIF) on Ro 60- and Ro 52-transfected HEp-2 cells. Results. Both sets of twins were monozygotic. They had similar birth weights. Twin 2 in the secon d set required a pacemaker at age 2 months, Both mothers were positive for anti-Po 52 and anti-Re 60 antibody, and neither had anti-La antib ody on immunoblot, One set of twins was studied at birth, Similar tite rs of anti-Re 52 and anti-Re 60 antibody were found by IIF and ELISA. Conclusion. There are no previous well-documented reports of monozygot ic twins discordant for CCHB, These cases demonstrate that there is st ill discordance in the development of CCHB despite identical genetics and environmental exposure to anti-Ro antibody.