Pitx1andPitx2are required for development of hindlimb buds

Abstract

Two closely related homeobox transcription factors, Pitx1 and Pitx2, have been implicated in patterning of lateral plate mesoderm derivatives: Pitx1 for specification of hindlimb identity and Pitx2 for determination of laterality. We show that, together, Pitx1 and Pitx2 are required for formation of hindlimb buds and, when present in limited doses, for development of proximal (femur) and anterior (tibia and digit 1) hindlimb structures. Although Pitx1 is expressed throughout developing hindlimb buds, Pitx2 is not expressed in limb bud mesenchyme itself, but is coexpressed with Pitx1 in the presumptive hindlimb field before bud growth. Thus, Pitx1 and Pitx2 genes are required for sustained hindlimb bud growth and formation of hindlimbs.

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

“…This and the finding that Pitx1 is expressed only in the hindlimb suggested a role in the determination of hindlimb morphology and identity. [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.…”

“…This hypothesis is corroborated by Shh expression analysis showing normal patterns of Shh expression in the ZPA of Pitx1 À/À mice. 12 Only a combined inactivation of Pitx1 and Pitx2 resulted in abnormal Shh expression and Pitx1 overexpression in the forelimb caused downregulation of Shh, 14 indicating some degree of interaction between the pathways. A similar discrepancy between mouse and human phenotypes has been observed for WNT7A.…”

Pitx2c overexpression promotes cell proliferation and arrests differentiation in myoblasts

“…Several authors have described Pitx2 and Pax3 coexpression in migratory precursor myoblasts as well as limb bud myoblasts (Kioussi et al, 2002;Marcil et al, 2003). The inhibitory effects of Pax3 on myogenic differentiation in myoblasts have been reported (Epstein et al, 1995).…”

“…Furthermore, Pitx2 expression has been detected in myotomes colocalizing with Pax3 (Kitamura et al, 1999). Pitx2 protein is present in myoblasts of the limb bud, displaying a similar pattern of expression to Pax3 and MyoD (Marcil et al, 2003), suggesting a putative muscle pattern involvement. However, the molecular mechanism by which Pitx2 acts in skeletal muscle cell lineages as well as Pitx2 isoform-specific functions remain unclear.…”

Finger or toe: the molecular basis of limb identity

“…During embryogeneis, the Pitx2 gene is asymmetrically expressed at many sites on the left side of the embryo and is an effector of L-R asymmetry (reviewed by Capdevila et al, 2000). Remarkably, analyses of the Pitx1 -/-;Pitx2 -/-double knockout and Pitx1 -/-;Pitx2 -/+ mouse embryos have confirmed this, and have established that Pitx1 and Pitx2 can co-operate during hindlimb formation (Marcil et al, 2003). Pitx1 and Pitx2 are co-expressed in the tailbud region, which includes a region that is destined to become hindlimb, although this co-expression is not maintained at later stages of development.…”