Nov. 17, 1998 Cutting and pasting enormous chunks of DNA into the chromosomes of mice, scientists have uncovered at least one genetic cause for a spectrum of urinary tract disorders, the most common survivable congenital defects in humans. The finding, published in the November 16 issue of EMBO Journal, links a previously studied gene, called GATA-2, with ensuring the proper development of the genitourinary tract. From earlier work, scientists already knew that GATA-2 is critical for the formation of blood.
The discovery paves the way for researchers to identify diagnostic and/or treatment strategies to combat a variety of bladder and kidney disorders in newborns, most notably a condition called hydroureteronephrosis, a physiological plumbing problem in which urine cannot make its way to the bladder because of an anatomical defect in the tubes (ureters) that connect the kidney to the bladder.
"This is a great example of how a basic research project can serendipitously lead to a better understanding of the cause of a common human birth defect," said Dr. Judith Greenberg, director of the Division of Genetics and Developmental Biology at the National Institute of General Medical Sciences, which provided funding for the study.
Despite its unfamiliar-sounding name, hydroureteronephrosis is a prevalent malady. One in 200 babies has the disorder, which is usually diagnosed by prenatal ultrasound screening and in many cases can be surgically repaired. Nevertheless, scientists don't understand the underlying cause of the problem.
But while the disease-related implications of the finding may be readily apparent, Dr. Douglas Engel of Northwestern University, who led the study, is excited about the more general, but profound, impact his team's discovery will have on the larger task of ascribing genes to functions in the developing mouse--and almost certainly human--embryo.
Until now, no one would have suspected that the GATA-2 protein did anything besides helping the developing organism make blood, since when researchers first created mice that lacked the GATA-2 gene, the inability of the mutant mice to make blood caused them to die in utero. Using a new genetic technique, Dr. Engel and his group have for the first time succeeded in rescuing mutant GATA-2-less mice from death. This technical feat revealed the GATA-2 protein's second, but equally important, function in programming the correct development of the kidney and bladder.
"This is a very powerful strategy to identify the more subtle genetic abnormalities that lead to disease," said Dr. Engel.
The task of assigning genes to functions is indeed a daunting one that is currently limited by the fact that researchers are stuck in a virtual "Catch-22:" they must work backwards from an outwardly expressed trait or defect (called a phenotype) to its root genetic cause. But in doing so, only the earliest activity of a specific developmental gene can be identified, since deleting it often has fatal consequences, ruling out further studies. If the same protein is required in a different tissue later in development, the activities of these so-called intermediary genes--as Dr. Engel describes them, "things that signal other things"--get lost in the shuffle, and thereby are extremely difficult to study via standard mammalian genetic experiments.
According to Dr. Engel, his team's new results stemmed from the fact that such standard--and usually effective--genetic strategies, in which relatively small stretches of DNA containing a gene of interest are studied, were not answering basic questions the team kept asking about the GATA-2 gene, such as why the protein is so plentiful in tissues all over the body if its only role is in blood formation, a process that takes place in the fetal liver and spleen.
Indeed, Dr. Engel recalled, "We were going about dissecting this problem the wrong way."
Changing tack, the team began a series of experiments using molecules called YACs (yeast artificial chromosomes) that can package extraordinarily large amounts of DNA--containing not only the gene of interest but also thousands of base pairs that "flank," or surround, the gene in its usual environment within the chromosomes inside the mouse cell's nucleus. With the help of YACs, Dr. Engel and his group hoped to replace not only the missing GATA-2 gene in mouse embryos lacking it, but also all of the gene's "control elements"--the coded information specifying where, when, and how much of a particular gene is made into RNA, and then into protein, in all of the tissues where it is needed.
In fact, the team was lucky in the sense that despite the large amount of DNA inserted into the YACs along with the GATA-2 gene--which did contain the elements specifying GATA-2 production in blood-forming tissues and "rescued" the mice from death--the DNA sequences accounting for expression of the gene in the kidneys, ureters, and bladder were still missing, thus unveiling the protein's new role in genitourinary development.
Scientists generally assume that a gene's control elements reside near the gene itself, usually within a few hundred base pairs from a gene's beginning or end. Thus, Dr. Engel and his colleagues at Northwestern University, Harvard Medical School, and Tsukuba University School of Medicine in Tsukuba, Japan, were particularly surprised to discover that some of GATA-2's control elements--specifically, those governing the expression of the gene in tissues such as the genitourinary system--must reside hundreds of thousands of base pairs away from the gene itself.
Notwithstanding the power of the new method, Dr. Engel was quick to note that it does not preclude the need for other, more routine genetic probing strategies. Rather, he said, using large DNA vectors such as YACs offers genetic researchers a second chance, as it were, to peer into later developmental windows, thereby exposing previously unknown functions of a variety of genes.
In addition to NIGMS, other funders of the study included the Japanese Ministry of Education, Science, Sports and Culture, and the Japanese Society for Promotion of Science.
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