Genetic and Environmental Causes of Disease

[Guest guest]

The following interesting article on the various causes of disease featured

recently on the WebMD site. It discusses the contributions of genetics and

environment to different diseases in humans.

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Does Nature or Nurture cause disease?

<http://my.webmd.com/dna_basics/article/3204.126>

Here are some extracts from this article.

In the last century, researchers have learned that many diseases arise from

variations in the genes we inherit from our parents. And in the last three

decades, an explosion of genetic information has implicated genes in nearly

every disease, and even in behaviors.

Do scientists now think that all diseases are genetic in origin? No. Instead,

medical researchers see diseases as arising from both genes and the

environment. In some cases, genes predominate. In other cases, the

environment does. But most diseases arise from a complex combination of

genetic influences and environmental influences. You could line up all

diseases on a scale from purely genetic to purely environmental. The result

would be a curve, with most diseases in the middle, arising from both genetic

and environmental causes.

Extremes of the spectrum -- purely Environmental disease

Some diseases lie at the extreme ends of this spectrum. Imagine that an

entirely random event causes you an injury -- say a meteorite crashes through

your roof. Clearly, genetics has nothing to do with your injury. But that

doesn't mean that genetics plays no role in accidents. Scientists suspect

that the genes you inherit may influence how likely you are to take up risky

activities, such as mountain climbing or scuba diving.

Alcohol is involved in many accidents, and alcoholism has a significant

genetic component. What's more, how long it takes you to recover from that

meteorite injury may well depend on your genes. But the point is clear: there

are some medical problems that arise by chance.

Extremes of the spectrum -- purely genetic disease

At the other extreme, if you inherit a gene for sickle hemoglobin from each

of your parents, you will have sickle cell anemia, a life-threatening

illness. Here, the environment plays no part in your getting the disease --

if you get the genes, you get the disease. Your own behavior may affect when

and how severely the disease strikes, however. Sickle cell attacks are more

common when the body isn't getting enough oxygen, such as when exercising

heavily, or living at high altitudes. Scientists believe that some 2,500

diseases are simply genetic in origin.

The middle of the spectrum -- genetic and environmental disease

Most diseases require contributions of genetics and environment. One example

is a disease called PKU or phenylketonuria. A child cannot have PKU unless he

or she inherits two copies of the defective gene, one from each parent.

Before this genetic disease was well understood, children who inherited these

gene variants developed severe mental retardation. Then, a few decades ago,

scientists realized that the child will suffer no illness, unless he or she

eats a chemical called phenylalanine.

So, the key to avoiding problems in this genetic disease is to carefully

control the child's environment -- specifically, food. Phenylalanine is

present in many, but not all, foods, and it takes knowledge and vigilance to

avoid them all. (Look at a can of Diet Coke, for example. Notice it has a

warning in bold letters that it contains phenylalanine.) Because the worst

aspects of PKU can be avoided by avoiding foods with phenylalanine, most

states require screening for PKU at birth.

The majority of diseases, however, result from a more complex interaction

between genetics and environment. Several genes are often involved, and there

are several environmental factors that contribute to these diseases. We

showed asthma as an example in the figure above.

Cancer is a special case. It arises from a combination of many genetic

defects, some caused by the environment and some potentially inherited. The

number of inherited defects determines a person's susceptibility to cancer.

We'll talk more about this later.

What do we mean by " environment " ?

To a geneticist, the environment is everything that is not genetic! Some

aspects of the environment that influence health and disease are listed

below:

1. Diet -- food, preservatives, coloring, method of preparation (smoked

foods, for example), composition of diet (fats, carbohydrates, protein), and

amount.

2. Air -- clean air, smog, pollution, tobacco, chemical fumes in the

workplace, dust (coal, cotton, etc.), humidity, temperature.

3. Water -- everything we drink, cook, or bathe in. Also, fluoride,

pesticides, minerals.

4. Radiation -- sunlight, tanning lights, radiation (X rays, microwaves,

radio waves).

5. Infection -- bacteria, viruses, fungi, parasites. Also includes

infection-related factors, such as sanitation and proximity to people,

animals, or insects.

Genetic susceptibility

The genes you inherit can affect how likely you are to get a disease, and how

that disease may affect you. If a person is genetically susceptible to a

particular disease, that person's risk of getting the disease is higher.

Genetic susceptibility combines with environmental input to produce disease.

But the combination need not be half and half. Genes can cause a slight

susceptibility or a strong susceptibility. If the genetic contribution is

weak, the environmental influence must be strong to produce disease, as we

show in the diagram below.

Here are a few examples of diseases that have some genetic contribution and

some environmental contribution. Genetic susceptibility is part of a spectrum

of factors, including age and general health, that contribute to disease or

protect against it........

Conclusion

We are in the early stages of a genetic revolution in medicine. Many of the

genetic diseases that we understand arise from large health effects of

mutations in a single gene. But most diseases probably arise from the effects

of variations in several genes, combined with a variety of environmental

influences. As we learn more about genetics, we will begin to understand

these complex interactions. We will no doubt learn that genetics plays a role

in nearly all diseases. Also, we will learn that the genes influencing the

development of a disease are often different from the genes influencing the

progression of the disease. Each step along this path is likely to yield new

approaches to treating or preventing disease.

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Dr Mel C Siff

Denver, USA