Digenic Inheritance Of Mutations In FOXC1 And PITX2 Causes Severe Anterior Segment Dysgenesis

Abstract

Purpose: : Disease-causing mutations affecting either one of the developmental transcription factor genes PITX2 or FOXC1 have been identified in approximately 40% of patients with Axenfeld-Rieger syndrome (ARS). We identified a family with highly variable anterior segment phenotypes and investigated the role of mutations in PITX2 and FOXC1.

Methods: : All four members of the family were screened for mutations in the FOXC1 and PITX2 genes. Transactivation and bandshift assays were performed to test the functional effect of mutations identified in these genes, both independently and in combination.

Results: : The family were shown to segregate novel mutations in both PITX2 (c.698C>T) and FOXC1 (c.607delC). The most severely affected individual inherited mutations in both genes whereas the single heterozygous mutations caused a milder phenotype. This is the first report of such digenic inheritance of mutations in both genes in a single individual. Our in vitro analyses showed that both FOXC1 and PITX2 are capable of regulating the promoters of the same putative target genes. Mutations in either gene caused reduced transcriptional activation to different extents on different promoters, whereas both mutations in combination showed the lowest level of activation.

Conclusions: : Tight control of both FOXC1 and PITX2 activity levels appears to be critical for normal anterior segment development, as reduced activity of both transcription factors in the same patient correlated with increased disease severity. Our data suggests an under-reported mechanism for phenotypic variability in ARS by which mutation of either gene can cause similar phenotypes, whilst digenic inheritance of mutations in both genes increases phenotypic severity.

Keywords: anterior segment • genetics • transcription factors