Neural epidermal growth factor-like 1 protein (NELL1)-associated membranous nephropathy with heterogeneous underlying diseases: a case report.

Neural epidermal growth factor-like 1 protein (NELL1) is a target antigen of membranous nephropathy (MN). NELL1-associated MN (NELL1-MN) was originally described as a primary form but has subsequently been associated with other diseases, including malignancies, pre-exposure to certain drugs, hepatitis B virus (HBV) and hepatitis C virus (HCV) infections, and rheumatoid arthritis (RA). We present a case of a 78-year-old woman with long-standing RA who developed persistent proteinuria and was diagnosed with MN. Evaluation of the underlying cause revealed chronic active HCV infection and past HBV infection. The underlying cause was less likely to be drug-related; however, there was no evidence of malignancy. The patient was diagnosed with HCV-associated MN. At 4 years after the diagnosis of MN, the patient died of breast cancer with multiple metastases. Subsequent immunohistological analysis revealed that she had NELL1-MN, and her breast cancer tissue stained positive for NELL1. Our case illustrates the difficulty in establishing the underlying cause of NELL1-MN, even after diagnosis. However, the incidence of malignancies, particularly breast and prostate cancers, is higher in NELL1-MN than in MN with other target antigens. Therefore, malignancies are considered a priority for investigation because of their frequency and prognosis among patients with NELL1-MN.