and understanding of congenital heart block (CHB) came in the l970s with the observation that mothers of affected infants frequently had autoimmune diseases and in particular many maternal sera contained antibodies to SSA/Ro and SSB/La ribonucleoproteins. Autoimmune mediated-CHB is considered to result from the transplacental passage of autoantibodies into the fetal circulation resulting in damage to the otherwise normally developing heart. CHB carries a substantial morbidity and mortality with over 60% of the affected children requiring pacemaker therapy. Moreover, neonatal deaths ranged from 20-30% and further deaths may occur secondary to pacemaker failure. Other neonatal abnormalities affecting the skin, liver, and blood elements were also reported to be associated with anti-SSA/Ro antibodies in the maternal and fetal circulation and are now grouped under the heading of Neonatal Lupus Syndromes. An inflammatory phase of cardiac injury has been noted in utero with decreased ventricular function and pericarditis. This may resolve or the newborn may suffer from congestive heart failure in addition to CHB. While extensive work from others has resulted in the molecular characterization of the maternal autoantibody responses and the cloning of genes expressing the cognate antigens, little is known about the pathophysiologic mechanism whereby these antibodies might directly contribute to the conduction defects. Using patch-clamp technique on single ventricular myocytes from human fetal and adult rat hearts, our preliminary data showed that sera and IgG fractions containing anti-SSA/Ro antibody and anti-La/SSB antibodies from mothers whose children have CHB inhibited L-type Ca current. In addition, using microelectrode technique on multicellular AV nodal preparation, we showed that these same IgG fractions caused electrical dissociation and conduction abnormalities between the atria and AV node. Also, affinity purified anti-SSA/52kD Ro caused complete AV block in human fetal heart perfused by the Langendorff technique. Finally, immunization of pregnant mice with SSA/52kD Ro recombinant protein resulted in high anti-SSA/52kD Ro specific antibody titers and offsprings with various degrees of AV conduction abnormalities including complete AV block. These preliminary data provide strong evidence supporting an etiologic role of antibodies involvement in the pathogenesis of CHB. Accordingly, the hypothesis being tested is that anti-SSA/Ro and/or anti-La/SSB antibodies alter conduction in the heart by affecting ion channels and specifically L-type Ca channels that are responsible for action potential conduction at the AV node. We propose to define the role of these putative maternal autoantibodies in the electrogenesis of heart block by 1) Characterizing the electrophysiologic effects of these antibodies on transmembrane ion currents with special emphasis on whole-cell and single channel L-type Ca current, 2) investigating the possible interaction of the autoantibodies with sarcolemma receptors, 3) determining the site of block induced by maternal antibodies in in vitro/AV node preparation using multiple microelectrode recordings of action potentials, 4) establishing an animal model for CHB. The findings from this proposal will provide important insights into the pathogenesis and etiology of CNB and could help in the development of new therapeutic strategies in a disease generally considered to be irreversible.
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