A novel gain of function of the IRX1 and IRX2 genes disrupts axis elongation in the Araucana rumpless chicken

Caudal dysplasia describes a range of developmental disorders that affect normal development of the lumbar spinal column, sacrum and pelvis. An important goal of the congenital malformation field is to identify the genetic mechanisms leading to caudal deformities.;To identify the genetic cause(s) and subsequent molecular mechanisms I turned to an animal model, the rumpless Araucana chicken breed. Araucana fail to form vertebrae beyond the level of the hips. I performed a genome wide association study to identify candidate genomic regions associated with the rumpless phenotype, compared to tailed Araucana. A candidate region of chromosome 2 containing just two genes, IRX1 and IRX2, was identified. In situ hybridization analysis showed that a gain-of-function mutation resulted in both genes being misexpressed at the onset of secondary neurulation in the caudal organizer progenitor population. The caudal progenitor population has a bipotential fate, contributing cells to both mesoderm and neural lineages. This finding is significant because it is the first identified instance of a gain-of-function mutation resulting in axial truncation.;The main question that arises from this novel finding is what is the functional mechanism leading to axial truncation? Possibilities include: the effect on the balance of cell fates within the progenitor population, on proliferation and apoptosis, on cell ingression, and the effect on molecular signaling within caudal tissues. Whereas none of these is mutually exclusive, I wanted to identify the single molecular event that triggers the cascade of downstream changes that results in axial truncation. I functionally examined each potential to determine the sequence of events in affected Araucana embryos.;Based on the results of this study, I propose a model of development where initial misexpression of the two proneural Iroquois gene family members directs the bipotential progenitor population toward the neural lineage. This results in premature reduction of the progenitor population due to 1) the withdrawal of neuralized cells from the cell cycle, 2) reduced ingression of new progenitor cells via the ventral ectodermal ridge 3) reduced proliferation rates resulting in a failure to extend the axis that then results in 4) early termination of axial elongation and widespread apoptosis.;In conclusion, I have identified a novel genetic basis for axial truncation that sheds light on the molecular mechanisms operating during secondary neurulation and axial elongation.