An article from yesterday's newspaper commented on the progress (or the lack of progress) in reaping the rewards of having sequenced the human genome. According to the article, although the sequencing (determination of the order of As, Ts, Cs and Gs of your genome) of human genome was completed 10 years ago and scientists have since further their understanding on human diseases such as diabetes and cancers. However, this has yet to translate to clinical applications, with the most advanced therapies under still under clinical trials.
Perhaps it was over optimism or lack of sufficient knowledge in those days (inevitable because science is always progressing), the people involved in the sequencing projects were hawking the potential of having the genome sequences: cancer therapies that targets precisely the genomic abnormalities and treatment of other genetic diseases such as osteoporosis and autoimmune diseases such as lupus.
Obtaining the human genome sequence is, no doubt, a big achievement. Knowing the sequence will allow us to predict outcome of cancer treatments (genetic markers as prognostic factor) for example (if someone has a particular mutation in a gene or multiple mutations in multiple genes, what how good will be his response to anti-cancer therapy). Having your genomic sequence will also allow detection of diseases caused by the absence of certain genes or by having too many copies of certain genes.
However, knowing having the genome sequence is merely the start of a long journey. Having a particular gene that codes for a disease does not mean you will have the disease. A disease can involve single gene or multiple genes. If it is a single gene, then dominant and recessive alleles will come into play when determining if you will have the disease. Humans have two copies of a gene in their genome (in general). If one of the copy is "defective" and this copy is dominant, then having one single "defective" copy means you will have the disease (if we ignore other pathways that can compensate for this defect). However, if the "defective" copy is recessive, then you will need two of such "defective" copies to have the disease -- you are safe as long as you have a normal copy which is dominant. In this regards, the genome sequence will tell you that you have a defective copy but not whether it is recessive or dominant (which will require genetic studies to elucidate). Therefore, having a "defective" copy of a gene does not mean you will the disease.
Furthermore, for the effect of the "defective" gene to manifest, it has to be expressed. Not all genes are expressed at all times. Some are expressed at specific points of time (such as during fetal development or during puberty) or under specific conditions (insulin is expressed when there is high blood glucose level). Therefore, even if your genome sequence shows that you have a "defective" gene, you may not have the disease if the gene is not expressed. In addition, there is the epigenetics factor to consider. Modification of proteins bound to your DNA may alter the accessibility to factors needed to express the gene. Therefore, again, the correlation between having a gene and disease is not so straightforward.
The problem of correlating a gene and a disease becomes much more complex when it is a multiple gene system. A mutation of one or more genes may alter the system and the interactions between these genes may or may not result in you having the disease. Treatment targeting a "defective" gene may perturb these interactions, resulting in exacerbating the disease or improving the patient's conditions. Having the genome sequence does not tell you which gene interacts with which gene(s) or even what are the function(s) of each individual gene.
To put it simply, sequencing the genome is merely analogous to deciphering the spelling of each word. However, we do not know what the word means; we do not know what it meaning will be conveyed when we put several words together into a sentence. A lot more work, a lot more time, a lot money is needed if we are to realize the potential of human genome sequence.
