A Reevaluation of X-Irradiation Induced Phocomelia and Proximodistal Limb Patterning

Phocomelia is a devastating, rare congenital limb malformation in which the long bones are shorter than normal, with the upper portion of the limb being most severely affected. In extreme cases, the hands or fingers are attached directly to the shoulder and the most proximal elements (those closest to the shoulder) are entirely missing. This disorder, previously known in both autosomal recessive and sporadic forms, showed a dramatic increase in incidence in the early 1960’s due to the tragic toxicological effects of the drug thalidomide, which had been prescribed as a mild sedative1, 2. This human birth defect is mimicked in developing chick limb buds exposed to X-irradiation3-5. Both X-irradiation5 and thalidomide-induced phocomelia5, 6 have been interpreted as patterning defects in the context of the Progress Zone Model, which states that a cell’s proximodistal (PD) identity is determined by the length of time spent in a distal limb region termed the “Progress Zone” 7. Indeed, studies of X-irradiation induced phocomelia have served as one of the two major experimental lines of evidence supporting the validity of the Progress Zone Model. Here, using a combination of molecular analysis and lineage tracing, we show that X-irradiation-induced phocomelia is fundamentally not a patterning defect, but rather results from a time-dependent loss of skeletal progenitors. As skeletal condensation proceeds from the shoulder to fingers (in a proximal to distal direction), the proximal elements are differentially affected in limb buds exposed to radiation at early stages. This conclusion changes the framework for considering the effect of thalidomide and other forms of phocomelia, suggesting the possibility that the etiology lies not in a defect in the patterning process, but rather in progenitor cell survival and differentiation. Moreover, molecular evidence that PD patterning is unaffected following X-irradiation does not support the predictions of the Progress Zone Model.