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Fatal mutations

Team from Rio details the action of the genes responsible for malformation of the head

ELY BUENOResearchers from the Federal University of Rio de Janeiro (UFRJ) have identified in mice three regions of the main gene responsible for the development of the head and of other basic structures of the embryo. Mutations in this gene, called MSX 1, may lead to problems of malformation in a series of organs and tissues, such as arms, legs, heart valves, neural tube, teeth and cheekbones, besides causing mental retardation.

The three regions which the team from Rio has characterized are called Mad 1, Mad 2 and Mad 3 and are involves in the process of regulation the expression (workings) of the MSX 1. They may, therefore, be vital for the proper functioning of this gene, widely distributed among animals – it is found all the way from flies to man. The results of this work, which was also able to count on a scientist from the Pasteur Institute in France, yielded an article in the March 3 issue of the Biochemical and Biophysical Research Communications magazine.

The team from UFRJ believes that mutations in these regions may get in the way or even prevent the correct interaction with these proteins and thus cause various kinds of damage to the fetus in formation. “The next step is to see if these regions interact in vivo with the embryo’s proteins”, says Eliana Abdelhay, from the Carlos Chagas Filho Institute of Biophysics, at UFRJ, who is coordinating the study. To do so, the researchers are planning to produce, shortly, genetically modified mice that carry specific mutations in the three regions.

This way, they are going to try to determine what role these regions play during the various stages of the expression of this important gene that is connected with the development of the embryo. One of the most serious clinical conditions associated with defects in MSX 1 is craniosynostoses, a congenital abnormality that can increase the pressure on the brain and is characterized by the premature closure of one or more sutures of the skull.

To find the three regions apparently connected with regulating the expression of MSX 1 in rodents, the team from UFRJ had to turn to molecular analyses and comparisons. The researchers pored over all the regions of the segment that regulates this gene in mice, a stretch that comprises 4,900 base pairs (chemical units of DNA), in search of a given sequence of seven bases: GCCGnCG (in the n position, either a G or a C may appear). The chemical units that codify a gene – or even the whole genome – are, in any organism, a succession of four kinds of bases: adenine (A), cytosine (C), guanine (G) and thymine (T).

Why is it that Eliana and her colleagues decided to look specifically for regions of the genome that had the GCCGnCG sequence? Because in the fruit fly (Drosophila melanogaster), an organism in which the MSX 1 gene was identified at the end of the 80’s, the proteins from the BMP family interact precisely with this sequence of seven bases, later called Mad. If in the fruit fly the regulation of the process of expressing MSX 1 seems to be related to the region containing this sequence called Mad, it makes sense to suppose that the same may occur in the version of this gene present in other organisms, such as in mice or men. “In terms of evolution, MSX 1 is a gene that is preserved a lot in the species”, explains Eliana. The result of this search was the identification in rodents of three regions of MSX 1 – Mad 1, Mad 2 e Mad 3 – that showed this sequence of seven bases.

Eliana has been studying MSX 1 for ten years. To start with, scientists thought that the gene was related above all to the development of muscles, but later studies expanded its range of action to other tissues and organs. It is believed that different MSX 1 regulating regions act at different stages of the formation of the embryo. According to the stage at which its action is needed, the gene appears to avail itself of a given stretch of its regulatory sequence, activated by the presence of proteins from the BMP family, to produce localized effects on the development of one or more organs or tissues.

The conclusion that MSX 1 is the main gene responsible for the formation of the head and other structures of the embryo is based on the finding that genetically modified mice, whose two copies of MSX 1 had been totally disabled, failed to originate only these two structures, but were capable of forming all the other tissues and organs. This does not mean that MSX 1 is not related to the development of these other tissues and organs.

It means that when MSX 1 shows some defect that changes its normal functioning, other genes – provided that these are not affected by mutations – may take on a good deal of its role, though not in its entirety. “We think that, with the exception of the teeth and the bones of the skull, the other functions of MSX 1 can be carried out by other genes”, Eliana comments. The candidates for being replacements for MSX 1 are MSX 2 and MSX 3, which show a similar pattern of expression. “If one day we come to understand well the expression pattern of this group of genes, we may be able to interfere in the workings of MSX 1 to avoid problems of malformation in the embryo”, says Eliana.

In the case of the formation of the head, one case in which the action of MSX 1 is decisive, the researchers from Rio believe they have taken an important step in this direction: in transgenic mice obtained in the laboratory at UFRJ in 1999, they pinpointed the region of the gene that controls the growth of the upper part of the skull. It is a stretch made up of six base pairs that interacts with proteins codified by the OTX-2 gene, whose pattern of expression is similar to the MSX 1. When there is a mutation in this region, there is no interaction and the upper part of the rodents’ skull is not formed.