A Muscle-specific Promoter Directs Pitx3 Gene Expression in Skeletal Muscle Cells

Abstract

The Pitx homeobox transcription factor genes have been implicated in different developmental processes, including determination of hind limb identity for Pitx1, left-right asymmetry for Pitx2, and eye development and survival of midbrain dopaminergic neurons for Pitx3. Pitx1 and Pitx2 have partly redundant activities in craniofacial development, including in pituitary organogenesis, as indicated by their names. These genes also exhibit redundant activities in the control of hind limb bud growth. Recent studies have shown expression of the three Pitx genes in muscle, with Pitx3 being the most widely expressed in all skeletal muscles. We now report the identification of a muscle-specific promoter within the Pitx3 gene that is situated between the first exon for eye and brain expression and exon 2 that contains the initiator ATG codon. Sequences proximal to this muscle-specific exon 1 are essential and sufficient to confer muscle-specific expression in transgenic mice, they are responsive to myogenic basic helix-loop-helix regulatory factors, and they recruit these factors in vivo. In agreement with exclusive use of the muscle-specific promoter in aphakia mice that are deleted of the brain promoter, the trimethyl-lysine 4 histone H3 promoter signature shifts to this promoter in embryonic day 13 ak limb bud muscle cells. Myogenic basic helix-loophelix regulatory factor activation of Pitx3 transcription may be part of a positive feedback loop contributing to establishment of the myogenic program.

Distinct and dynamic myogenic populations in the vertebrate embryo

“…Pitx2 mutants fail to form body wall muscles and show abnormalities in limb myogenesis [68], suggesting a role in hypaxial dermomyotome derivatives, although in the trunk, defects in body wall closure complicate the interpretation. Pitx3 is expressed later than Pitx2 in all skeletal muscles [64,67] and has a muscle-specific promoter [69]. In the absence of Pitx3, Pitx2 is not downregulated, probably compensating since muscle defects have not been detected in the Pitx3 À/À mutant [67].…”

A regulatory toolbox of MiniPromoters to drive selective expression in the brain

“…The expression of the lacZ reporter is observed at E11.5 in the ventricular region at the mesencephalic flexure, but not in neural tissue at E10.5, delineating the onset of Ple162 MiniP expression. In a recent study, several mouse Pitx3 promoter constructs, one overlapping in sequence with Ple162, were analyzed in E12.5 mouse transgenic embryos but the authors do not report a similar brain expression pattern (18).…”

The Use of Autozygosity Mapping and Next-Generation Sequencing in Understanding Anterior Segment Defects Caused by an Abnormal Development of the Lens

“…Interestingly, two siblings from a consanguineous union within the same family were found to be homozygous for the same deletion and had a more severe phenotype of PPC with microphthalmia and corneal opacification resulting in blindness [120] . They were also developmentally delayed and suffered from neurological abnormalities including increased muscle tone, choreiform movements, deep tendon reflexes and mental retardation likely resulting from the loss of PITX3 expression in dopaminergic neurons of the midbrain [111] . More recently, Aldahmesh et al [17] found a homozygous deletion in PITX3 (c.640_656del) in a female child born to first-cousin parents who presented with severe microphthalmia and ASD including sclerocornea.…”

“…It is expressed at high levels in the eye, muscles and brain [111][112][113] . Pitx3 is highly expressed in the early developing lens with a well-defined expression pattern first realised in the lens vesicle, then later in the developing lens epithelium and at the equator where lens fibre differentiation occurs [86] .…”