The Human Genome

The Human Genome2016-11-30T11:43:43+00:00

Implications of Knowledge

What Is The Human Genome?

It is no accident that offspring resemble their parents. Our genetic code is stored within our cells. Every cell nucleus (the “brain” of the cell) contains a special chemical called deoxyribonucleic acid, or DNA. Each of our genes is actually a segment of the DNA spiral that, under a microscope, looks like a long, twisted ladder. Base pairs make up the rungs of the ladder. There are four different substances that make up the base pairs: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). The base pairs always match up the same way: Adenine always connects to Thymine, and Cytosine always connects to Guanine. This pairing is the key to how DNA works. When we talk about DNA, we are referring to all of the genetic information in the nucleus. The DNA is broken up into different segments called chromosomes. The human standard is to have 23 pairs of chromosomes (46 in total). One of each pair comes from your mother and the other one from your father. Your genetic information is unique (unless you have an identical twin). Each chromosome is broken up into genes. Genes are specific segments of DNA. Genes, called the blueprint of life, carry the instructions that manage the way our bodies grow, develop, and decay.

Traits are the physical representations of your genes. In all, there are about three billion bits of information contained in our genes that determine our traits, which are everything including the colour of our hair, the shape of our bodies, our intelligence potential, behaviour predispositions, and the diseases we inherit. Of course, more than our genes contribute to what we eventually become. Factors such as nurturing and environment are also of extreme importance.

Deciphering the human genome will help bioscientists learn more about human diseases and develop appropriate treatments. The ultimate altruistic hope is that once we understand the genome, we will be able to prevent human disease. However, many thorny ethical issues associated with this hope remain unresolved.

Are we ready to use this technology wisely?

 

Some Goals Are Met, Some May Never Be

The goal of the International Human Genome Project (HGP) was to have 90% of the genome mapped by the end of 2001. Although two drafts of the human genome were published in February 2001, well ahead of schedule, it was a case of, “The more we know, the more we know we don’t know.” HGP researchers are striving to accomplish their ultimate goal to finish mapping the human genome sequence by the end of 2003, when funding runs out. However, many scientists are doubtful that this will ever be completely accomplished because significant “unidentifiable” gaps exist within the sequence of the human genome.

According to researchers, we can group these gaps into two broad categories – those that represent limitations of present molecular biology techniques, and those that represent a lack of insight into the sequences underlying the gaps.

Scientists from the UK announced in October 2001 that they have finished the first draft of the genome of an exotically dangerous fish, which may hold a key that will assist in decoding the human genome. Puffer fish is only the second vertebrate animal genome we have sequenced and, like the mouse, has approximately the same number of genes as a human. A particular gene in the fish may decide where the fins are, and what they look like, while a very similar gene sequence may be present in humans, but acts slightly differently to produce a leg. The great advantage of the puffer fish genome is that it is a staggering eight times more compact than the human genome. When compared to the human sequence, active regions are immediately obvious. Therefore, with comparisons, it becomes easier for scientists to identify mechanisms of genes and this knowledge will have direct application in unravelling the secrets of the human genome.

We must continue to develop new reagents and techniques in order to aid in the sequencing and characterization of gaps. It is important to note, however, that even with such advances, characterizing and sequencing these regions will be time consuming and labour intensive. It is unlikely that, even by comparison to genome sequencing of other species, we will complete the total sequencing of the human genome any time soon.

 

Disease and Genetic Screening

The human race carries 3,000 to 4,000 diseases in its genes. A faulty gene may be one of many healthy thousands. There are anywhere from 100,000 to 200,000 genes in each cell. To help pinpoint the location of a faulty gene, scientists search for variations in larger pieces of DNA called markers. These subunits lie nearby on the DNA chain, and form the basis of genetic testing. With such markers, it becomes theoretically possible to screen individuals of every age, from infants to adults, and even in fetuses before birth. Markers found so far include those for Huntington’s disease, cystic fibrosis, Duchenne muscular dystrophy, Parkinson’s disease, hemophilia, thalassaemia, and some rare cancers.

Connections to genes are recognized for Alzheimer’s disease, diabetes, epilepsy, certain cancers, heart disease, and inflammatory bowel disease. Scientists also search for any genetic connection with more common disorders where genes plus other factors play roles. For example, someone may have a genetic predisposition to type II diabetes but with proper diet and exercise, they might never manifest the disease. We reported in The Inside Tract® – Issue #126 regarding a genetic breakthrough for Crohn’s disease with the identification of the NOD2, a gene on chromosome 16 that is associated with susceptibility to Crohn’s disease. In this case, the section of the chromosome that is implicated is large, containing about 20 million base pairs, and around 250 genes!

Researchers in the US are using a genetic technology called DNA micro array to look for differences in the expression of 7,306 genes in tissues taken from the large intestines of six patients with Crohn’s disease, six with ulcerative colitis, and six healthy subjects. They published their results in the March 2001 issue of Human Molecular Genetics. A total of 170 genes showed significant differences in their expression levels – the degree to which they were turned on – in Crohn’s and colitis. Although a number were clustered in known areas, most of the genes had not previously been associated with IBD.

For some illnesses, the offspring must inherit a rogue gene from both parents, thereby having two copies of the faulty gene, in order to develop the disease. Those with only one faulty gene stay well but may unknowingly pass it on to their children. We call these people carriers.

 

Testing Versus Screening

While much of the literature appears to use the terms “testing” and “screening” interchangeably, there is a difference in intent or purpose.

We use the terms genetic test, genetic assay, and genetic analysis interchangeably to mean the actual laboratory examination of samples. In contrast, genetic screening usually uses the same assays employed for genetic testing but is distinguished from genetic testing by its target population. The US National Academy of Sciences defines screening as the “systematic search of populations for persons with latent, early, or asymptomatic disease.”

 

Nutrigenomics – The Future Of Medicine And Nutrition

A merging of nutrition, medicine, and genomics, may mean that the old adage “you are what you eat” becomes, “you eat what you are,” as consumers of the future adapt their grocery lists to their DNA.

The recent mapping of the human genome has also triggered an explosion in research into how drugs might be customized to capitalize on an individual patient’s unique genetic code. We call this new area of study Pharmacogenetics. In the future, doctors may prescribe the population drugs that best suit their individual genetics, offering the most benefit with the least side effects for that individual. A common name for this new class of medication is smart drugs.

Nutritional science consultant Dr. Nancy Fogg-Johnson of Life Sciences Alliance in Pleasanton, California presented “A New Nutritional Paradigm – The Genetic Era of Nutrition” at the annual meeting of the American Chemical Society in Chicago this past August. Fogg-Johnson believes the line between food and drug is blurring in the era of the genome.

Consumers already make dietary choices based on genetics – including switching from cow’s milk to soy when lactose-intolerant, decreasing dietary fibre when diagnosed with inflammatory bowel disease, increasing fibre intake with irritable bowel syndrome, and choosing cholesterol-lowering spreads when diagnosed with a propensity to heart and blood vessel disease. The next step is for industry to provide consumers with quick, cheap methods of assessing their gene profiles so that they can use them to make informed decisions about what they eat.

As an example, Fogg-Johnson noted that individuals with just one mutation in a specific gene might need a higher-than-average intake of folate-rich foods, such as fortified breads, so that they could fully protect themselves from heart disease. Having a copy of one’s genetic profile would alert consumers to these types of important aberrations and help them plan to circumvent the consequences.

Nutrigenomics could become a focus every step of the way through life. Imagine that in infancy, the DNA is scanned and a diet assigned to start a child on the road to lifelong health. Adjustments could be made along the way as environmental factors influence the genetic blueprint.

Although it may seem like a scene from a science-fiction movie, Fogg-Johnson envisages the day when shoppers routinely hand over a copy of their DNA sequence during every trip to the pharmacy or grocery store. We could also tailor food production and delivery in a “crop-to-fork” way, with specific products aimed at groups of consumers sharing similar genetic makeup. Medicine will be custom-designed to treat specific diseases by re-sequencing DNA in the patient.

Already, the “scientific continuums of nutrition and genomics have absolutely merged,” Fogg-Johnson said, noting that a few small US companies now offer consumers genetic profiling. “The technology to accomplish this in an economically feasible, consumer-relevant way is becoming a reality,” she stated.

 

Who Has A Right To Your Genetic Code?

Should insurance companies, banks, the government, your employer, and other businesses have access to your genetic information? It is already hard enough for patients with chronic disease to purchase life insurance. Imagine having insurance coverage denied just because your DNA suggests that you have a susceptibility to disease. How might we protect employees from discrimination, if employers make genetic screening mandatory?

 

What about Language?

Should our propensity for politically correct language extend to the human genome? Will words like defects, flaws, deleterious genes, and disorders, be inflammatory, and should we be using the more neutral, non-accusatory words such as conditions or characteristics?

Is using words such as normal and its opposite, abnormal, likely to foster stigmatization and discrimination?

 

More Issues of Genetic Screening

Some of the ethical questions under debate include:

  • Will genetic screening lead to the ultimate “Big Brother” scrutiny that George Orwell introduced to us in his book, 1984?
  • Genetic screening will be expensive: In the face of severe healthcare cuts nation-wide how can this extra cost be justified? Finding a faulty gene does not predict where or when an individual will develop a condition, nor its severity, only that disease might occur. Screening programs will also need to provide education and counselling for all those tested, thereby further increasing the associated costs of testing.
  • Prenatal screening may encourage parents to select for perfection rather than disease prevention, opening the door to social engineering. Desirable traits such as grace, athleticism, intelligence, and beauty, involve many genes interacting with environmental factors. It is impossible for anyone to make reliable decisions based on genetic analysis alone, but some may want to try. Already with pre-screening for gender, many people around the world are selectively aborting their children. Proper safeguards and more education may help avoid this.
  • With a drop of blood or a cheek swab, we can do these tests. How can we safeguard against these being done without our knowledge, perhaps as part of another medical examination?

 

Conclusion

The prospects of accumulating information on the human genome are exciting and open the door for so many positive changes to the human condition. As science races ahead though, it is important for society to envision where this is leading. Appropriate social debate must take place and laws created to control the use of the ultimate private information – the blueprint of our individual lives. As DNA modification is rampant in food supplies and now growing in the pharmaceutical industry, it might be too late to stop the influence of imbalance in a world designed for balance.


Gail Attara, President & Chief Executive Officer, Canadian Society of Intestinal Research
First published in the Inside Tract® newsletter issue 128 – November/December 2001