Alport Syndrome and Thin Glomerular Basement Membrane Nephropathy: A Practical Approach to Diagnosis

Abstract Context.—Alport syndrome and thin glomerular basement membrane nephropathy (TBMN) are genetically heterogenous conditions characterized by structural abnormalities in the glomerular basement membrane and an initial presentation that usually involves hematuria. Approximately 40% of patients with TBMN are heterozygous carriers for autosomal recessive Alport syndrome, with mutations at the genetic locus encoding type IV collagen α3 α3(IV) and α4 chains. However, although the clinical course of TBMN is usually benign, Alport syndrome, particularly the X-linked form with mutations in the locus encoding the α5 chain of type IV collagen α5(IV), typically results in end-stage renal disease. Electron microscopy is essential to diagnosis of TBMN and Alport syndrome on renal biopsy, although electron microscopy alone is of limited value in distinguishing between TBMN, the heterozygous carrier state of X-linked Alport syndrome, autosomal recessive Alport syndrome, and even early stages of X-linked Alport syndrome. Objectives.—To review diagnostic pathologic features of each of the above conditions, emphasizing the need for immunohistology for α3(IV) and α5(IV) in addition to electron microscopy to resolve this differential diagnosis on a renal biopsy. The diagnostic value of immunofluorescence studies for α5(IV) on a skin biopsy in family members of patients with Alport syndrome also is reviewed. Data Sources.—Original and comprehensive review articles on the diagnosis of Alport syndrome and TBMN from the past 35 years, primarily the past 2 decades, and experience in our own renal pathology laboratory. Conclusions.—Although Alport syndrome variants and TBMN do not show characteristic light microscopic findings and can be difficult to differentiate from each other even by electron microscopy, using a combination of electron microscopy and immunohistology for α3(IV) and α5(IV) enables pathologists to definitively diagnose these disorders on renal biopsy in most cases.