COL4A5 and LAMA5 variants co-inherited in familial hematuria: digenic inheritance or genetic modifier effect?

BACKGROUND: About 40-50% of patients with familial microscopic hematuria (FMH) caused by thin basement membrane nephropathy (TBMN) inherit heterozygous mutations in collagen IV genes (COL4A3, COL4A4). On long follow-up, the full phenotypic spectrum of these patients varies a lot, ranging from isolated MH or MH plus low-grade proteinuria to chronic renal failure of variable degree, including end-stage renal disease (ESRD). METHODS: Here, we performed Whole Exome Sequencing (WES) in patients of six families, presenting with autosomal dominant FMH, with or without progression to proteinuria and loss of renal function, all previously found negative for severe collagen IV mutations. Hierarchical filtering of the WES data was performed, followed by mutation prediction analysis, Sanger sequencing and genetic segregation analysis. RESULTS: In one family with four patients, we found evidence for the contribution of two co-inherited variants in two crucial genes expressed in the glomerular basement membrane (GBM); LAMA5-p.Pro1243Leu and COL4A5-p.Asp654Tyr. Mutations in COL4A5 cause classical X-linked Alport Syndrome, while rare mutations in the LAMA5 have been reported in patients with focal segmental glomerulosclerosis. The phenotypic spectrum of the patients includes hematuria, proteinuria, focal segmental glomerulosclerosis, loss of kidney function and renal cortical cysts. CONCLUSIONS: A modifier role of LAMA5 on the background of a hypomorphic Alport syndrome causing mutation is a possible explanation of our findings. Digenic inheritance is another scenario, following the concept that mutations at both loci more accurately explain the spectrum of symptoms, but further investigation is needed under this concept. This is the third report linking a LAMA5 variant with human renal disease and expanding the spectrum of genes involved in glomerular pathologies accompanied by familial hematurias. The cystic phenotype overlaps with that of a mouse model, which carried a Lama5 hypomorphic mutation that caused severely reduced Lama5 protein levels and produced kidney cysts.

Frequent COL4 mutations in familial microhematuria accompanied by later-onset Alport nephropathy due to focal segmental glomerulosclerosis.

Familial microscopic hematuria (FMH) is associated with a genetically heterogeneous group of conditions including the collagen-IV nephropathies, the heritable C3/CFHR5 nephropathy and the glomerulopathy with fibronectin deposits. The clinical course varies widely, ranging from isolated benign familial hematuria to end-stage renal disease (ESRD) later in life. We investigated 24 families using next generation sequencing (NGS) for 5 genes: COL4A3, COL4A4, COL4A5, CFHR5 and FN1. In 17 families (71%), we found 15 pathogenic mutations in COL4A3/A4/A5, 9 of them novel. In 5 families patients inherited classical AS with hemizygous X-linked COL4A5 mutations. Even more patients developed later-onset Alport-related nephropathy having inherited heterozygous COL4A3/A4 mutations that cause thin basement membranes. Amongst 62 heterozygous or hemizygous patients, 8 (13%) reached ESRD, while 25% of patients with heterozygous COL4A3/A4 mutations, aged >50-years, reached ESRD. In conclusion, COL4A mutations comprise a frequent cause of FMH. Heterozygous COL4A3/A4 mutations predispose to renal function impairment, supporting that thin basement membrane nephropathy is not always benign. The molecular diagnosis is essential for differentiating the X-linked from the autosomal recessive and dominant inheritance. Finally, NGS technology is established as the gold standard for the diagnosis of FMH and associated collagen-IV glomerulopathies, frequently averting the need for invasive renal biopsies.