Mutations in Six3 have been previously identified in patients with holoprosencephaly, a disease that can cause the part of the brain called the cerebrum to fail to divide normally into two lobes. Holoprosencephaly is the most common abnormality of the development of the forebrain (front part of the brain) in humans. A few years ago the St. Jude team demonstrated that Six3 activity is critical for the normal development of the forebrain in mice.

St. Jude researchers have now extended these results by showing in the developing eye that Six3 normally exerts its effect by directly activating Pax6, a gene considered the "master regulator of eye development." In the absence of Six3, Pax6 fails to coordinate the activity of a series of additional genes that cooperate to form the lens. A report on this work appears in the prepublication online issue of The EMBO Journal.

Previously, the researchers were not able to address the gene's possible role in mouse eye formation because inactivation of Six3 significantly disrupted development of the area of the brain where the eye normally forms. The St. Jude team overcame this problem by taking advantage of Cre/loxP-technology, which allowed them to choose the time and place in which to remove Six3 function from specific cells. This permitted the investigators to remove Six3 activity from the presumptive lens ectoderm (PLE)--the area of the developing head where the lens will ultimately form in response to a series of biochemical signals. Following this systematic approach, the St. Jude team demonstrated that Six3 plays its important role in the PLE. The investigators also showed that a key consequence of removing Six3 during early development is that the PLE fails to undergo its normal thickening, an initial critical step in lens formation.

"Our discovery helps to better unravel the regulatory pathway that controls normal lens formation," said the paper's senior author, Guillermo Oliver, Ph.D., a member of the St. Jude Genetics and Tumor Cell Biology department. "Specifically, it puts the Six3 gene at the top of the genetic cascade that controls the development of the lens. Understanding the early steps leading to lens formation will help us determine what goes wrong in disorders in which the lens does not form, or forms abnormally. That kind of information is often the first step in designing both preventive and treatment strategies for congenital diseases."

The St. Jude team further showed that the proteins coded for by Six3 are present in the PLE before those coded by Pax6. This was evidence that production of Pax6 proteins in that region of the head starts after Six3 proteins are made, further supporting the idea that the Six3 gene controls the Pax6 gene. The researchers also demonstrated that the Six3 protein directly controls the expression of the Pax6 gene by binding to some of the DNA regions that regulate Pax6 activity.

"Our work confirms the early and vital role Six3 plays in the overall development of the eye, and gives us important insights into the interplay of genes during this time," said Wei Liu, Ph.D., a postdoctoral fellow in Oliver's laboratory. Lui is first author of the paper.

Other authors of this study include Oleg V. Lagutin (St. Jude); and Michael Mende and Andrea Streit, King's College, London.

This work was supported in part by the National Institutes of Health, a Cancer Center Support Grant, the Biotechnology and Biological Sciences Research Council and ALSAC.

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