Hematopoietic Stem Cell Transplant-Membranous Nephropathy Is Associated with Protocadherin FAT1. Issue 5 (May 2022) - Sethi, Sanjeev
Significance Statement: Hematopoietic stem cell transplant (HSCT) is a treatment for certain hematologic malignancies and immune disorders. A complication of HSCT is membranous nephropathy (MN), which results from antibodies targeting an antigen in the glomerular basement membrane (GBM). The antigen in most cases of HSCT-associated MN is not known. Laser microdissection and mass spectrometry identified a novel protein, protocadherin FAT1 (FAT1), in HSCT-associated MN. Kidney biopsy specimens showed granular staining for FAT1 along the GBM in HSCT-associated MN. Antibodies to FAT1 were detected in serum and in kidney biopsy tissue in HSCT-associated MN, but not in controls. FAT1-associated MN appears to be a unique type of MN associated with HSCT. FAT1-associated MN represents the majority of MN associated with HSCT. Visual Abstract: Abstract : 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 from 250 patients with PLA2R-negative MN to detect novel antigens in MN. This was followed by immunohistochemical (IHC)/immunofluorescence (IF) microscopy studies to localize the novel antigen. Western blot analyses using serum and IgG eluted from frozen biopsy specimen to detect binding of IgG to new 'antigen'. Results: MS/MS detected a novel protein, protocadherin FAT1 (FAT1), in nine patients with PLA2R-negative MN. In all nine patients, MN developed after allogeneic HSCT (Mayo Clinic discovery cohort). Next, we performed MS/MS in five patients known to have allogeneic HSCT-associated MN (Cedar Sinai validation cohort). FAT1 was detected in all five patients by MS/MS. The total spectral counts for FAT1 ranged from 8 to 39 (mean±SD, 20.9±10.1). All 14 patients were negative for known antigens of MN, including PLA2R, THSD7A, NELL1, PCDH7, NCAM1, SEMA3B, and HTRA1. Kidney biopsy specimens showed IgG (2 to 3+) with mild C3 (0 to 1+) along the GBM; IgG4 was the dominant IgG subclass. IHC after 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 those with FAT1-asssociated MN, but not from those with 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.