Role of Cytoskeletal Diaphanous-Related Formins in Hearing Loss

Abstract

Hearing relies on the proper functioning of auditory hair cells and on actin-based cytoskeletal structures. Diaphanous-related formins (DRFs) are evolutionarily conserved cytoskeletal proteins that regulate the nucleation of linear unbranched actin filaments. They play key roles during metazoan development, and they seem particularly pivotal for the correct physiology of the reproductive and auditory systems. Indeed, in Drosophila melanogaster, a single diaphanous (dia) gene is present, and mutants show sterility and impaired response to sound. Vertebrates, instead, have three orthologs of the diaphanous gene: DIAPH1, DIAPH2, and DIAPH3. In humans, defects in DIAPH1 and DIAPH3 have been associated with different types of hearing loss. In particular, heterozygous mutations in DIAPH1 are responsible for autosomal dominant deafness with or without thrombocytopenia (DFNA1, MIM #124900), whereas regulatory mutations inducing the overexpression of DIAPH3 cause autosomal dominant auditory neuropathy 1 (AUNA1, MIM #609129). Here, we provide an overview of the expression and function of DRFs in normal hearing and deafness.

Keywords: DIAPH1; DIAPH2; DIAPH3; autosomal dominant auditory neuropathy; diaphanous-related formins; inherited sensorineural hearing loss; mouse mutants.

Figure 1

The Diaphanous-Related Formin (DRF) family. (a) Cladogram of Diaphanous-Related Formins in metazoa, based on the UniProt sequences and generated using Clustal Omega. The UniProt accession numbers of the selected sequences are: DIAPH1_hsa (O60610), DIAPH2_hsa (O60879), DIAPH3_hsa (Q9NSV4), Diaph1_mmu (O08808), Diaph2_mmu (O70566), Diaph3_mmu (Q9Z207), Diaph1_oan (A0A6I8NW07), Diaph2_oan (A0A6I8NX37), Diaph3_oan (F7CXN1), Diaph1_ola (H2LGS4), Diaph2_ola (A0A3B3I776), Diaph3_ola (A0A3P9MFT1), and dia_dme (P48608). hsa, Homo sapiens; mmu, Mus musculus; oan, Ornithorhynchus anatinus (platypus); ola, Oryzias latipes (medaka fish); dme, Drosophila melanogaster (fruit fly). (b) Schematic representation of the multi-domain structure of DRF proteins in the inactive autoinhibited form and in the Rho-activated dimeric form. Domains responsible for the interaction with actin and microtubules are indicated. CC: coiled coil domain; DAD: diaphanous autoregulatory domain; DD: dimerization domain; DID: diaphanous inhibitory domain; FH1: formin homology 1 domain; FH2: formin homology 2 domain; RBD: Rho Binding Domain. (c) Tridimensional structure of a diaphanous protein (monomeric), based on the AlphaFold prediction (AF-O08808-F1) for mouse Diaph1.

Figure 2

Mutations in DRFs and hearing loss. Lollipop plots showing genetic variants within human DIAPH1 (a) and DIAPH3 (b) that are associated with hearing phenotypes (see list in Table S1) and mapped on the corresponding protein. Mutations are color-coded based on the predicted functional effect (green, missense; black, truncating; orange, splice; brown, in-frame deletion/insertion; purple, regulatory). *, stop codon; fs, frame-shift. Plots were generated using Mutation Mapper, available at cBioPortal.