Inhibition of Myo6 gene expression by co-expression of a mutant of transcription factor POU4F3 (BRN-3C) in hair cells

An eight-base pair (bp) deletion in the Pou4f3 gene in hair cells is associated with DFNA15, a hereditary form of hearing loss. To explore the pathological mechanisms underlying the development of DFNA15, the effect of the mutation in Pou4f3 on the activity of the myosin VI (Myo6) promoter, was investigated. The upstream regulatory sequence of Myo6 (2625 bp), consisting of an 1899 bp upstream sequence and a 727 bp intron 1 sequence, was amplified using polymerase chain reaction and subcloned into the pGL3-Basic vector expressing firefly luciferase. For verification of inserted fragments, plasmids were subjected to restriction analysis and then sequenced. HEK293T human embryonic kidney cells were transiently transfected with renilla luciferase-thymidine kinase vectors expressing Renilla luciferase and the Myo6 promoter-driven firefly luciferase expressing vectors along with pIRES2-enhanced green fluorescent protein (EGFP)-Pou4f3 (expressing wild-type Pou4f3) or pIRES2-EGFP-Pou4f3 (expressing the truncation mutant of Pou4f3). The relative luciferase activities were measured to determine the activity of the Myo6 promoter. The Myo6 promoter activity was not affected by co-expression of wild-type Pou4f3, as indicated by the comparable relative luciferase activities in the presence of the pIRES2-EGFP-Pou4f3 and the empty control vectors. However, co-expression of mutated Pou4f3 significantly inhibited the activity of the Myo6 promoter to almost half of that of the control (P<0.001). The data suggests that mutated Pou4f3 has a negative role in the promoter activity of Myo6, and by extension, the expression of myosin VI, and this may be an underlying mechanism of DFNA15 hearing loss.