X-linked, COL4A5 hypomorphic Alport mutations such as G624D and P628L may only exhibit thin basement membrane nephropathy with microhematuria and late onset kidney failure

Abstract

Alport syndrome (ATS) results from X-linked, COL4A5 mutations (85%) or from autosomal recessive homozygous or compound heterozygous COL4A3/A4 mutations (15%), associated with alternate thinning and thickening as well as splitting and lamellation of the glomerular basement membranes. In contrast, familial microhematuria with thin basement membranes is thought to result from heterozygous COL4A3/A4 mutations. This absolute separation may not always be true. Renal biopsies and molecular genetics were used to study microhematuric families in the Hellenic population we serve. The COL4A5 gene was studied by PCR and direct re-sequencing for new mutations, while PCR-RFLP was used to identify more carriers of known COL4A5 and COL4A3/A4 mutations. Molecular genetics in two undiagnosed microhematuric Cypriot families, revealed COL4A5 mutation P628L indicating X-linked ATS. Of nine males, seven developed end stage kidney disease (ESKD) between 31 and 56, while two are well at 51 and 57, exhibiting microhematuria and thin basement membrane nephropathy (TBMN). COL4A5 mutation G624D was also identified in six Greek families. Seventy five members had DNA tests and 37 proved positive. Four positive males developed ESKD at 61, 51, 50 and 39 years, while the remaining and all females showed only microhematuria. A literature search revealed eight papers with six similar hypomorphic COL4A5 mutations presenting as phenocopies of TBMN. In conclusion, X-linked COL4A5 ATS mutations produce a phenotypic spectrum with a) classical ATS with early onset ESKD, neurosensory deafness and ocular defects b) males with only ESKD and late deafness and c) males due to missense mutations, such as G624D and P628L that may only exhibit microhematuria, TBMN, mild chronic renal failure (CRF) or late onset ESKD. Consequently when investigating "benign familial hematuria" these and other similar X-linked COL4A5 mutations should also be searched for.