The pathogenesis of Alport syndrome involves type IV collagen molecules containing the α3(IV) chain: Evidence from anti-GBM nephritis after renal transplantation

Abstract

Mutations in the COL4A5 collagen gene have been implicated as the primary defect in Alport syndrome, a heritable disorder characterized by sensorineural deafness and glomerulonephritis that progresses to end-stage renal failure. In the present study, the molecular nature of the defect in Alport glomerular basement membrane (GBM) was explored using anti-GBM alloantibodies (tissue-bound and circulating) produced in three Alport patients subsequent to renal transplantation. The alloantibodies bound to the alpha 3(IV)NC1 domain of type IV collagen and not to any other basement membrane component. In tissue sections, the alloantibodies bound specifically to peripheral GBM in normal kidney and the affected renal transplant but not to that of Alport kidney. These results establish that: the alpha 3 chain in type IV collagen molecules, the Goodpasture autoantigen, is the target alloantigen in post-transplant anti-GBM nephritis in patients with Alport syndrome, and that a molecular commonality exists in the pathogenesis of anti-GBM nephritis causing loss of renal allografts in patients with Alport syndrome and renal failure in patients with Goodpasture syndrome. These findings implicate: (1) defective assembly of type IV collagen molecules containing the alpha 3(IV) chain in Alport GBM; and (2) the existence of a mechanism linking the assembly of molecules containing the alpha 3(IV) chain with those containing the alpha 5(IV) chain.

The Molecular Basis of Goodpasture and Alport Syndromes

“…Endogenous oxidants can also open this immunologically privileged site in the GBM (109,110). In contrast, the epitopes of the alloantibodies, produced in some renal transplant recipients with Alport syndrome, are fully accessible on the hexamer surface, reside on the NC1 domains of the ␣3 and ␣5 chains, and seem to be distinct from the E A and E B epitopes (95,110) (Figure 12). This difference in structure between the epitopes for Alport alloantibodies and GP auto-antibodies may reflect crucial features of the mechanisms underlying the cause of autoimmune GP syndrome and, thus, warrant the crafting of questions for future explorations.…”

“…Consequently, because the Alport GBM lacks the ␣3.␣4.␣5 network, the GP antigen would be absent as well. The absence of this network in the Alport GBM also explained why sometimes a renal allograft loses its function; in this case, the immune system of the Alport patient recognizes the ␣3.␣4.␣5 network as foreign, which elicits alloantibodies against the ␣3 and ␣5 NC1 domains present in the allograft (95).…”

Isoform switching of type IV collagen is developmentally arrested in X-linked Alport syndrome leading to increased susceptibility of renal basement membranes to endoproteolysis.

“…Rabbit antibodies were prepared and characterized to all six ␣ -chains of type IV collagen as recombinant molecules (7,25,26). Alport alloantibodies and Goodpasture autoantibodies were also described previously (7,27).…”

“…These antibody specificities are of particular importance as ‫ف‬ 10% of Alport patients that undergo renal transplantation develop posttransplant anti-GBM disease, with both circulating and kidney bound anti-␣ 3(IV) NC1 and/or -␣ 5(IV) NC1 antibodies (27,42,43). The two-dimensional electrophoresis studies shown in Fig.…”

“…To further analyze the content of basement membrane collagens from XAS and normal kidneys, high resolution twodimensional gel electrophoresis and immunoblotting was performed using posttransplant Alport alloantibodies reactive to ␣ 5(IV) and ␣ 3(IV) NC1 domains, and a Goodpasture autoantibody reactive to ␣ 3(IV) NC1 domain (27,32). These antibody specificities are of particular importance as ‫ف‬ 10% of Alport patients that undergo renal transplantation develop posttransplant anti-GBM disease, with both circulating and kidney bound anti-␣ 3(IV) NC1 and/or -␣ 5(IV) NC1 antibodies (27,42,43).…”