How does the body know what shape to adopt when we are formed? How does it know where to put the arms, the liver, the brain? This complex operation is controlled by our genes, our DNA, which oversees and regulates all the functions within the body.
The winner of this year's Royal Irish Academy biochemis- try science writing competition, co sponsored by The Irish Times and Yamanouchi Ireland Co, Ltd, is Mr Gareth Brady, a fourth year biochemistry student at Trinity College, Dublin. His award included a John Coen bronze sculpture, a cheque for £800, both sponsored by Yamanouchi, and an opportunity to have his essay published in this newspaper.
The title of his presentation, "Hox genes: The Molecular Architects" is reproduced in full here.
"The secrets that en- gage me - that sweep me away - are generally secrets of inheritance: how a pear seed becomes a pear tree, for instance, rather than a polar bear." Cynthia Ozick (1989)
WHEN Watson and Crick unleashed the structure of our genetic blueprint the DNA double helix upon the world in 1953, a new age had begun. This was so not only in the mind of the few who had begun to understand its biochemical implications but also in the mind of those content to bask in the enthusiasm of the former with the assurance that scientists held all the answers to what made us what we are.
The double helix became a kind of cultural archetype for all that was wonderful about science, civilisation and all things complex in our collective understanding of life and limb.
For most, the understanding of DNA stopped there with the erroneous notion that simply characterising the static form which nature takes at the molecular level in any way readily translates to the diverse forms which fly, scuttle and swim about us.
At face value, DNA contains a very simple language. It is a code built from different arrangements of just four types of bases named adenine, guanine, thymine and cytosine. The entire DNA code, or genome, spans approximately three billion base units in the human. The central guiding principle is that three of these bases code for one amino acid and that these amino acids become chemically linked together to form proteins. The DNA component required to produce one protein is termed a gene and the process of producing a protein from one of the 100,000 human genes is termed gene expression.
The resulting expressed proteins in turn perform all the chemical processes or reactions which characterise life, delineate the overall cellular structure and determine the form in which other biochemical constituents such as carbohydrates, fats, vitamins and minerals participate in our complex biological systems. But in what manner does DNA, in its capacity to produce proteins, code for the diverse variety of biological body forms which shape our natural world?
In 1978, EB Lewis, an American geneticist studying mutations in the fruit fly Drosophila, proposed a remarkable concept which was to be crucial in answering this question. These mutations took the form of gross alterations of the fly's body plan, such as Antennapedia, where legs are formed in place of antennae. He conceptually divided the body plan into eight distinct sections based on characteristic developmental mutations in these areas, arising randomly from time to time in large fly populations. From this, Lewis concluded that these developmental errors must be caused by a mix-up in the functioning of master control genes that instigated the development of the fruit fly's body in defined segments.
To prove his theory, workers in the field quickly set upon isolating the genes which Lewis had proposed existed in the fruit fly. Employing newly developed genetic tools which allowed sensitive detection of genes expressed in the developing body, these eight master regulators were soon discovered. When a more broad spectrum of life was probed for similar control genes, a remarkable and unprecedented discovery was made.
THE same eight genes, with only minor variations in the genetic code, were present all across the animal world. Literally, from fish to fowl, we share the same master control genes that sculpt the basic body plan.
An even more remarkable discovery followed from a lab in Switzerland where the geneticist Walter Gehring developed a technique to express the master control gene for eye development in various areas of the embryo that do not normally develop eyes. This resulted in the development of an insect exhibiting eyes on its wings, legs and other areas.
Furthermore, when this lab isolated the same eye development gene from the mouse and inserted it into the fruit fly, the resulting embryo developed an extra fly eye instead of a mouse eye. It was clear that these genes acted as a common molecular currency that could perform the same job in completely different animals. The experiment firmly established the role of this gene family as master control genes.
These genes became known as Hoxeobox, or Hox genes (derived from the term homeosis, meaning the developmental transformation of a body segment). It was subsequently discovered that mammals possess four sets, or clusters, of Hox genes as opposed to the single set controlling development in the fruit fly. By studying these gene clusters in other species it has become clear that their overriding mechanism, as well as their basic genetic codes, have been highly conserved across evolution and time suggesting an early development in the history of life.
Plants too possess similar genes which, whilst being coded and activated differently, perform the same basic role as developmental architects. It was a level of conservation that nobody had ever dreamt existed between species separated by hundreds of millions of years of evolution.
Hox genes act by producing proteins in the developing embryo. These proteins act at the tip of a developmental cascade, turning on their target genes by directly binding to very specific DNA sequences preceding the other gene codes, thus causing these target genes to produce new proteins themselves.
Like a set of molecular dominoes they recruit a host of protein messengers that lay down the pattern of the basic body plan. All these developmental molecules are expressed in highly specific concentrations that percolate across regions of the embryo in a gradient which lets every cell know exactly where it is in relation to its neighbours and, more importantly, exactly what type of cell it is to become if it is to effectively participate in the overall scheme of things. BUT one must again ask a question: if all animals utilise this common conserved mechanism with the same or similar genes for development, why don't all animals look exactly alike? The key determining factors are (1) concentration; (2) location; (3) timing and (4) target gene specificity.
Since common species possess these highly similar developmental genes, differences of body shape are generated by evolutionary changes in these three areas - the concentration or amounts of the Hox proteins produced, the location of their production in the developing embryo and the timing with which they become active in the body plan.
The fourth factor, target gene specificity, crucially affects the former three, for if a given Hox gene sequence is altered by mutation, the resulting Hox protein that is expressed may not bind to its target genes and therefore will not attain the required concentration in a given location or time. Alternately, it may bind to a different target gene than it does in other members of the species.
Any such changes result in alterations in the body form. It is rather like cutting a new groove into the surface of an old key, that whilst it may not open the same door it may fit perfectly into an entirely different lock.
Such subtle changes in any of these factors may result in acute, catastrophic mutations, such as antennapedia, or slight, subtle alterations with no overt consequences for the animal as a whole.
Over time, natural selection makes the final determination: changes that occur in the body plan of a species allowing adaptation are eventually conserved, whilst changes detrimental to the animals ability to survive in its native ecosystem are rapidly removed. But, with such complex messages and timing, this system can, from time to time, go horribly wrong.
White cells of the blood, in a constant process of turn-over, are renewed by the rapid division of stem cells from the bone in a distinct and highly controlled developmental process regulated, in part, by Hox genes.
Leukaemia can occur when this system malfunctions, resulting in the uncontrolled growth of rapidly dividing cells at an early, non-functional stage of white cell development.
Professor Terry Lappin and co-workers at Queens University, Belfast, have been characterising the role of Hox genes in this process. With funding from the Leukaemia Research Fund (LRF) and the Northern Ireland Leukaemia Research Fund (NILRF), this group has been working on this problem since 1993 and has isolated and characterised a number of Hox genes which malfunction along the pathway of malignant transformation to leukaemia.
Hox gene research has opened up exciting new prospects not only for a novel perspective on evolution at the molecular level but also for potential medical advances. As understanding precedes advancement, it is hoped to gain a thorough knowledge of how this system functions as a whole in order to generate new strategies for treating some forms of leukaemia and for developmental abnormalities, in which Hox genes have also been implicated.
The discovery of these genes has opened a new and beautifully complex window into the development of new life, in the process revealing an undeniable molecular kinship between all species. In the words of the philosopher Paul Weiss: "But nature is not atomised. Its patterning is inherent and primary, and order underlying beauty is demonstrably there; what is more, the human mind can perceive it only because it is itself part and parcel of that order."