Role of Pitx1 Upstream of Tbx4 in Specification of Hindlimb Identity

Abstract

In spite of recent breakthroughs in understanding limb patterning, the genetic factors determining the differences between the forelimb and the hindlimb have not been understood. The genes Pitx1 and Tbx4 encode transcription factors that are expressed throughout the developing hindlimb but not forelimb buds. Misexpression of Pitx1 in the chick wing bud induced distal expression of Tbx4, as well as HoxC10 and HoxC11, which are normally restricted to hindlimb expression domains. Wing buds in which Pitx1 was misexpressed developed into limbs with some morphological characteristics of hindlimbs: the flexure was altered to that normally observed in legs, the digits were more toe-like in their relative size and shape, and the muscle pattern was transformed to that of a leg.

Deletions in PITX1 cause a spectrum of lower-limb malformations including mirror-image polydactyly

“…[11][12][13][14] The hypothesis was supported by studies performed in chick and mouse showing that misexpression of Pitx1 in forelimbs results in the adoption of hindlimb characteristics. 14,15 In fish, pelvic loss in different natural populations of stickleback fish has occurred through regulatory mutations deleting a tissue-specific enhancer of Pitx1 and demonstrates an evolutionary change in vertebrates via this genomic region. 16 In humans, a missense mutation (p.E130K) in PITX1 has been shown to segregate with dominantly inherited clubfoot with reduced penetrance in a large kindred, together with unilateral tibial hemimelia and preaxial polydactyly in one individual in this family.…”

Mouse Ror2 receptor tyrosine kinase is required for the heart development and limb formation

“…Intriguingly, our observed phenotypes of mRor2 À/À mice are somewhat reminiscent of those of Holt±Oram syndrome, which is characterized by upper limb malformations and cardiac septation defects, in which the TBX5 gene is mutated (Basson et al 1997;Li et al 1997;Basson et al 1999). It has been reported that the TBX5 gene is expressed in the upper (fore) limbs and another T-box family gene, TBX4, is expressed dominantly in the lower (hind) limbs (Chapman et al 1996), and that TBX4 (and PITX1) and TBX5 determine the identities of the upper (fore) limbs (or wings) and lower (hind) limbs (or legs), respectively (Ohuchi et al 1998;Logan & Tabin 1999;Vogel 1999). However, the characteristic features of the upper (fore) and lower (hind) limbs were apparent in mRor2 À/À mice, suggesting that mRor2 is not involved in the speci®cation of the upper (fore) and lower (hind) limb identities.…”

Finger or toe: the molecular basis of limb identity

“…If allowed to develop to later stages, the resulting limbs produce elements with morphological characteristics of a wing, such as wing digits and feather buds. In the converse experiment, hindlimb-specific genes Tbx4 or Pitx1 misexpressed in the forelimb can transform the wing to a more hindlimb character (Logan and Tabin, 1999;Takeuchi et al, 1999). Similarly, Tbx4 can transform the identity of Fgf-induced ectopic limbs induced in the interlimb region of the LPM (RodriguezEsteban et al, 1999;Takeuchi et al, 1999).…”

“…This can be explained in part by the observation that, following the misexpression of Tbx4 or Pitx1, the endogenous expression of Tbx5 in the forelimb is unchanged (Logan and Tabin, 1999;Rodriguez-Esteban et al, 1999;Takeuchi et al, 1999). This observation indicates that in these misexpression experiments, cells in the limb are presented with two competing programmes; in some cases one programme may dominate, whereas in others the cells may assume a mixed or intermediate fate.…”

“…Misexpression of Pitx1 in the developing chick wing can, at least partially, transform both muscle and skeletal elements of the forelimb and give them a more hindlimb-like character (Logan and Tabin, 1999;Takeuchi et al, 1999). In Pitx1 null mice, the hindlimb does form, but features of hindlimb identity are lost and the limb has a more forelimb-like character (Lanctot et al, 1999;Szeto et al, 1999).…”