The achievements of genetics have created a misleading image, as if most diseases are a destiny dictated to us by genes
In the late XNUMXs, researchers announced that they had identified a gene that causes manic-depression. The researchers reported the discovery in the prestigious scientific journal "Nature". Reports of the study made headlines around the world. It was the first time that scientists were able to prove that a certain genetic defect could cause a mental illness. The discovery sparked a huge wave of hopes, doctors who dreamed out loud then said that DNA scans would be able to identify the defect, and locate the people who are at an increased risk of developing the serious disease, and perhaps even start treating them before they get sick.
Two years later, it turned out that the joy was premature. A new analysis of the data, including new testimonies from a number of people who participated in the study and have been sick since its inception, cast serious doubt on the results. More than a decade has passed since the study, but they still have not been able to identify with certainty a gene that causes manic-depression, or schizophrenia or depression - or any other mental illness - although no one doubts that heredity plays a large role in the development of these diseases.
And it's not because they didn't try. Today, it is clear to scientists that there is no single gene that controls complex diseases such as heart disease, cancer, or mental illness, but that they are caused by several mutations in several genes, and to truly understand how these defects cause diseases, one must also understand the interrelationships between these genes.
But to understand this complex genetic picture, researchers not only need to identify the genes involved in the diseases, but also exactly which DNA units have been damaged in the sequence that makes up the gene. Sometimes, different mutations in the same gene may cause a different risk of developing the disease in the future.
The tremendous achievements of genetics in recent years have created a misleading image, as if most diseases are a destiny dictated to us by our genes. But most diseases are caused by mutations that happen during life, not by inherited mutations.
In a study that analyzed the medical files of twins, it was discovered that hereditary genetic defects cause only a quarter of the three common types of cancer - this is more than previously thought, but not enough for those who thought that deciphering the human genome would lead to the elimination of diseases.
In a report on the research, published this month in the medical journal The New England Journal of Medicine, Dr. Paul Lichtenstein and his colleagues from the Karolinska Institute in Stockholm wrote that hereditary defects account for 42% of the risk of developing prostate cancer, 35% for colon and rectal cancer, and 27 in breast cancer. The rest of the risk of getting cancer is influenced by environmental factors.
In a commentary article that accompanied the study, it was stated that this was the largest study conducted to date, involving almost 45 thousand pairs of twins. However, two main factors may harm the quality of the research results: the researchers did not have data about the lifestyle habits of the subjects (such as smoking, diet, exposure to industrial pollution), and an unknown percentage of them are not identical twins (therefore they share only 25% of their genetic load, like normal siblings).
However, all of this does not change the main message of the study: most people will get cancer not because of a mutation that their parents bequeathed to them, but because of their lifestyle habits, which will cause defects in the sequence of their DNA units during their lifetime.
The researchers have difficulty answering the question to what extent heredity is responsible for the diseases also in other disorders such as blood pressure, diabetes or stroke. One of the biggest obstacles is that because many genes are involved in the disease, the contribution of each of them is relatively small, so it is difficult to identify them with the methods accepted today. With these methods, the genetic structure of sick family members is compared. They were very effective in identifying the genes that cause cystic fibrosis, Tay-Sachs or Huntington's disease, because these are diseases caused by a mutation in a single gene, but it is difficult to use them to identify the complex set of genes that causes schizophrenia, for example. It is possible that the gene, which in the eighties was thought to cause manic-depression, does indeed contribute to the development of one of the stages of the disease, but its effect is small and to characterize it a large number of patients must be tested.
Decoding the genome, in which the sequence of the DNA units is identified, does not guarantee that all the genes that cause depression or obesity will ever be identified, but it will provide for the first time the possibility to simultaneously analyze how groups of genes work together, and to better understand how changes in the sequence -N-A may cause suffering and diseases.
{Appeared in Haaretz newspaper, 31/7/2000}
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