Hematopoietic Stem Cell Transplant-Membranous Nephropathy is Associated with Protocadherin FAT1.

Background: Membranous nephropathy (MN) is a common cause of proteinuria in patients receiving a hematopoietic stem cell transplant (HSCT). The target antigen in HSCT-associated MN is unknown. Methods: We performed laser microdissection and tandem mass spectrometry (MS/MS) of glomeruli of 250 cases of PLA2R-negative MN to detect novel antigens in MN. This was followed by immunohistochemical (IHC)/immunofluorescence microscopy (IF) studies to localize the novel antigen. Western blot analyses were performed using serum and IgG eluted from frozen biopsy tissue to demonstrate IgG binding to the antigen. Results: MS/MS detected a novel protein, protocadherin FAT1 (FAT1), in 9 cases of PLA2R-negative MN. All 9 MN cases developed following allogeneic HSCT (Mayo Clinic discovery cohort). Next, we performed MS/MS in 5 known cases of allogeneic HSCT-associated MN (Cedar Sinai validation cohort). FAT1 was detected in all 5 cases by MS/MS. The total spectral counts for FAT1 ranged from 8 to 39 (mean 20.9 ± 10.1). All 14 cases were negative for known antigens of MN, including PLA2R, THSD7A, NELL1, PCDH7, NCAM1, SEMA3B, and HTRA1. Kidney biopsy showed IgG (2-3+) with mild C3 (0-1+) along the GBM; IgG4 was the dominant IgG subclass. IHC following protease digestion and confocal IF confirmed granular FAT1 deposits along the GBM. Lastly, Western blot analyses detected anti-FAT1 IgG and IgG4 in the eluate obtained from pooled frozen kidney biopsy tissue and in the serum of FAT1-asssociated MN, but not from PLA2R-associated MN. Conclusions: FAT1-associated MN appears to be a unique type of MN associated with HSCT. FAT1-associated MN represents a majority of MN associated with HSCT.