Metabolic Diseases
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In some cases the actual manifestation of the disease will identify the defective gene. This is the case with a number of congenital metabolic disorders such as phenylketonuria. In this case the amino acid phenylalanine accumulates in the blood because it cannot be broken down. This is because the gene coding for the enzyme that breaks it down (phenylalanine hydroxylase) is defective.
In most cases it is extremely difficult to identify the genes responsible for a hereditary disease especially if many different defective genes are required to trigger a disease.
What is the candidate method?
The candidate method starts with an idea that certain proteins may not be functioning properly and may therefore be causing a problem. The method then involves hunting for defects in the genes coding for the suspect proteins.
Genes and depressive disorders
It is thought that the neurotransmitters serotonin, noradrenaline and dopamine may be involved in the biology of depressive disorders. Scientists have therefore looked at the genes coding for some of the enzymes used to produce these neurotransmitters. There have also been studies of their receptors and some of their transport proteins.
It has been found that some genes come in different variants. However, it has not yet been proven that one particular variant of one of these genes is found only in people who suffer from depression and another variant only in those who are healthy. Depression is probably due to a combination of genetic, environmental and psychological factors.
What is linkage analysis?
Another method called linkage analysis is used to see if a particular hereditary disorder occurs together with a specific genetic marker or DNA marker.
A DNA marker may itself be a hereditary characteristic but may also be an area of the DNA strand that shows some variation.
For example: A hereditary disease appears in family members with a particular ear shape. If others in the family never get the disease, this means that the gene for the disease and the gene governing this ear shape are passed down together. These two genes must therefore be close together on the same strand of DNA. If we know where the gene behind the ear shape is, we will be close to finding out where the gene responsible for the disease is.
Genes get mixed up from generation to generation due to the exchange of genes that takes place in the sex cells. This means that sooner or later the gene for the disease and the DNA marker will become separated, however close together they may be. Your child will inherit 50% of your DNA, your grandchildren 25% and your great-great-great-great-grandchildren only around 1%.
A number of linkage analyses have been carried out in a bid to link depressive disorders in particular families to various DNA markers. However, no genes have yet been found with a definite role in the development of depression.
One of the reasons is that the number of known DNA markers is limited. However, the ongoing research into the human gene pool is expected to result in the identification of many hundreds of thousands of markers over the next few years, and so we are expecting linkage analyses to be very rewarding in the future.
It will be really exciting to see what the future holds in terms of genes and depression.