HuR, a novel target of anti-Hu antibodies, is expressed in non-neural tissues

Paraneoplastic encephalomyelitis (PEM) is characterized by a diverse set of clinical signs that are limited to the nervous system. The serologic hallmark of PEM is the presence of circulating autoantibodies, collectively referred to as `anti-Hu,' which immunoreact specifically with members of the Elav protein family. Until recently, the ELAV antigens were only detected in neurons, thus strongly supporting a role for anti-Hu antibodies in the selective neural tissue injury in PEM. The identification of HuR, however, a new member with a broad, non-neural pattern of RNA expression, raises several fundamental questions regarding PEM. First, why are non-neural tissues spared in PEM? Second, why is PEM predominantly associated with neuroendocrine tumors? To begin addressing these questions, we sought to determine whether the antibody response to HuR differs from the neural-specific counterparts in patients with PEM, and to characterize the protein expression pattern of this novel antigen in peripheral tissues and tumors. Using sera from 11 patients with Hu-positive PEM, we found that the majority of samples (73%) were weakly or non-reactive for recombinant HuR on Western blot, in contrast to consistently strong immunoreactivity with the neural-specific members HuD and Hel-N1. We also demonstrate that HuR is expressed at the protein level in both non-neural tissues and non-neuroendocrine tumors. These findings suggest that immunoreactive differences among Elav family members may contribute to the neural-restrictive pattern of tissue injury in patients with PEM.