Uncovering Modifier Genes of X-Linked Alport Syndrome Using a Novel Multiparent Mouse Model

Significance Statement Kidney disease severity is partly determined by modifier genes. These genes can be important therapeutic targets but are difficult to identify in patient populations. Our study demonstrates a novel mouse genetic approach using Diversity Outbred mice to identify modifier genes for X-linked Alport Syndrome. We identify several candidate modifier genes and validate the candidacy of Fmn1 . We show that a decrease in Fmn1 expression in Col4a5 knockout mice leads to a decrease in albuminuria and fewer podocyte protrusions in the glomerular basement membrane. Our approach can be easily adapted to identify modifier genes for other forms of kidney disease. Background Mutations in COL4A5 are responsible for 80% of cases of X-linked Alport Syndrome (XLAS). Although genes that cause AS are well characterized, people with AS who have similar genetic mutations present with a wide variation in the extent of kidney impairment and age of onset, suggesting the activities of modifier genes. Methods We created a cohort of genetically diverse XLAS male and female mice using the Diversity Outbred mouse resource and measured albuminuria, GFR, and gene expression. Using a quantitative trait locus approach, we mapped modifier genes that can best explain the underlying phenotypic variation measured in our diverse population. Results Genetic analysis identified several loci associated with the variation in albuminuria and GFR, including a locus on the X chromosome associated with X inactivation and a locus on chromosome 2 containing Fmn1 . Subsequent analysis of genetically reduced Fmn1 expression in Col4a5 knockout mice showed a decrease in albuminuria, podocyte effacement, and podocyte protrusions in the glomerular basement membrane, which support the candidacy of Fmn1 as a modifier gene for AS. Conclusion With this novel approach, we emulated the variability in the severity of kidney phenotypes found in human patients with Alport Syndrome through albuminuria and GFR measurements. This approach can identify modifier genes in kidney disease that can be used as novel therapeutic targets.