Two experimental proposals clarifying the mechanism of HOX gene collinearity
Hox gene collinearity is a surprising phenomenon observed during axonal development of embryos. This phenomenon is not satisfactorily understood as yet. In the last years, a series of genetic engineering experiments on mice embryos were performed in Duboule’s laboratory (ref. 1-3). These interesting experiments reveal some unexpected facets of Hox gene collinearity. In order to describe their findings, Duboule and coworkers proposed a ‘two-phases model’ (T-P M). According to this model (ref. 1-3), in the early phase (up to about stage E9.5) two regulatory mechanisms come into play, one positive acting from the anterior side (3’) of the Hox cluster and a repressive one acting from the centromeric side of the cluster. In their latest paper (ref. 3) the authors claim that the repressive regulation is related to a ‘landscape effect’ from the extended region centromeric to the cluster.
I have proposed a quite different model, the biophysical model (BM), which explains Hox gene collinearity satisfactorily (ref. 4-6). The recent genetic engineering experiments are also well reproduced by the biophysical model which is based on the hypothesis that physical forces decondense the Hox cluster and pull the Hox genes sequentially from inside the chromosome territory toward the interchromosome domain. Thus the Hox cluster behaves like an elastic spring whose posterior end is fixed inside the chromosome territory and the anterior end (3’) is free and can be pulled toward the interchromosome domain (ref. 4-6). In this mechanical representation, the fixed end of the spring is essential. It is natural to assume that it lies in the small region (Ev-13) between Evx2 and the last gene Hoxd13 of the cluster.
In what follows two simple experiments are proposed in order to distinguish which of the two models (T-P M and BM), if any, is correct. For each of the proposed experiments the above models predict divergent results. It will be therefore interesting to compare the model expectations to the experimental findings.
1. Experiment A:
With the methods described in ref. 1-3, the small posterior region (Ev-13) between Evx2 and Hoxd13 is deleted.
a) According to T-P M, this small deletion should not affect substantially the spatial and temporal expressions of Hoxd13, Hoxd12, Hoxd11, …No difference between wild-type and mutant expressions is expected.
b) In contrast, according to the BM, this small posterior deletion destroys the spring character of the Hox cluster since its fixed end is removed and the spring remains loose and can be easily shifted by small forces as explained in ref. 5-6. Therefore, Hoxd13, Hoxd12, Hoxd11, … should be prematurely over-expressed in the early phase.
2. Experiment B: Following the method described in ref. 3 an inversion is performed centromeric to Evx2 (not including the intergenic region between Evx2 and Hoxd13).
a) The Hoxd13, Hoxd12, Hoxd11… expressions in the early phase should be comparable to the observed expressions of the experiment presented in Fig. 3 of ref. 3 since, in both experiments, the inverted centromeric landscape is almost the same. According to T-P M, this inverted centromeric region is responsible for the observed abnormal expressions. Therefore, we expect the mutant expressions to differ significantly from the wild-type embryo expressions.
b) The biophysical model predicts at the early phase a quite different behavior: the elastic spring with its fixed end remains intact, therefore its function is normal and this centromeric inversion does not affect the mutant Hoxd gene expressions. As a result, the mutant Hoxd13, Hoxd12, Hoxd11… expressions should not differ from the wild-type expressions.
I think the above experiments constitute crucial tests for both T-P M and BM.
References
1. B. Tarchini and D. Duboule (2006). Developmental Cell 10, 93-103.
2. P. Tschopp, B. Tarchini, F Spitz, J. Zakany and D. Duboule (2009). PloS Genet. 5
(3).
3. P. Tschopp and D. Duboule (2011). Dev. Biol. 351, 288-296.
4. S. Papageorgiou (2006). Int. J. Dev. Biol. 50, 301-308.
5. S. Papageorgiou (2009). Hum. Genomics 3, 275-280.
6. S. Papageorgiou (2011). Develop. Growth Differ. 53, 1-8.
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Spyros Papageorgiou
Email: spapage@bio.demokritos.gr
