The evolution of Fox genes and their role in development and disease

Abstract

The forkhead box (Fox) family of transcription factors, which originated in unicellular eukaryotes, has expanded over time through multiple duplication events, and sometimes through gene loss, to over 40 members in mammals. Fox genes have evolved to acquire a specialized function in many key biological processes. Mutations in Fox genes have a profound effect on human disease, causing phenotypes as varied as cancer, glaucoma and language disorders. We summarize the salient features of the evolution of the Fox gene family and highlight the diverse contribution of various Fox subfamilies to developmental processes, from organogenesis to speech acquisition.

Figure 1. Evolutionary tree of mouse forkhead box (Fox) genes

A neighbourjoining tree is shown that is based on the protein sequence of the forkhead domain. The relationships shown in the tree are based on multiple alignment (FIG. 2), using ClustalX as the alignment tool. Each branch is annotated with a bootstrap value that was based on 1,000 samples. The branch lengths are proportional to the mutation rate. For a recent phylogenetic analysis of Fox genes, see REFS ,,.

Figure 2. Alignment of forkhead box (Fox) genes in mice

All protein sequences that contain the forkhead (FKH) domain (Pfam identification number PF00250) were extracted from ENSEMBL v50 (see Further information), and the longest isoform for each gene was used. The Foxl1 sequence (RefSeq identification number NP_032050) was obtained from the National Center for Biotechnology Information (NCBI). The protein sequences were aligned using the multiple alignment tool T-Coffee. The alignment was viewed using the alignment editor Jalview. The bottom panel shows the FKH domain as recorded in the Pfam database. The FKH domain is the only consistently conserved portion of the protein across all members of the family, whereas there are limited similarities in other regions among Fox subfamilies. The colour coding of the amino acids is based on the physicochemical properties provided in Jalview.

Figure 3. The insulin, Akt and FoxO pathway

a | In the absence of insulin binding, FoxO is a transcriptional activator of multiple insulin-responsive (IR) genes. b | Engagement of the phosphatidylinositol 3-kinase (PI3K) pathway in response to insulin or other factors leads to the activation of Akt and to the phosphorylation (P) of FoxO proteins on crucial serine and threonine residues (Thr24, Ser256 and Ser319 in FOXO1)–. Phosphorylation seems to be sequential and Ser256 is phosphorylated first. Ser256 phosphorylation is sufficient for reduced DNA- binding affinity, presumably because this residue is located in the basic region of the DNA-binding domain, but nuclear exclusion also requires phosphorylation of the other two residues, Thr24 and Ser319 (REF. 54). Therefore, an elegant mechanism for insulin-dependent gene inhibition exists: insulin binding to its receptor on the cell surface initiates PI3K and Akt activation, followed by FoxO phosphorylation, nuclear exclusion and loss of transcriptional activation.