The NYTimes has a good story on a family in Colombia that is affected with multiple generations of severe dementia--Alzheimer's Disease. This family is a large, localized and hence accessible family affected by what they refer to as 'La Bobera', a term generally referring to a kind of foolishness, which this affliction certainly is.
The locals have attributed this to various mystical sources, but the scientifically determined cause is known. The family is transmitting a specific genetic change in a gene called Presenillin1, which codes for a neural membrane protein, and has been shown in other studies to be responsible for cases of AD. The Times story is about attempts to impose medical preventive measures for those who have inherited the mutation. Hopefully, it will go well.
This family resembles another famous South American story, that of Huntington's Disease in extended families in the Maracaibo region of Venezuela. There, too, the genetic cause is known but there is no effective treatment yet available.
HD is always, or at least almost always, due to a particular kind of mutational change in a specific single gene. This has been known for about 20 years. The mutation can easily be detected, the disease strikes later in life as a rule, and yet there is still no effective treatment and, importantly, no gene-therapy kind of treatment.
AD is more complex in that many different genes are known that can affect AD risk, but not all cases can be accounted for in terms of specific genes, much less specific mutations. Some of the molecular and physiological changes in the brain have been identified but, as in HD, there's no miracle preventive or treatment.
Single-gene diseases, or clear-cut single-gene subsets of more complex diseases should provide optimal material in which to show that gene-based knowledge can lead to prevention (other than by selective abortion) or treatment. They are only subsets of the whole, usually small subsets, but they are at least truly genetic, unlike many complex disorders (as we've posted about many times).
It is sobering to realize that we've known about another single-mutation disease, that is very common, and affects very accessible tissues, for about 60 years. That's sickle cell anemia. The sobering aspect is that there is still no reliable and no gene-based treatment or preventive.
Does this mean that gene therapies are vain hopes? Nobody knows, but technology is powerful when there is a clear problem to be solved, so we think this is a place for heavy research investment in genetics--unlike the massive investment being made to enumerate the hundreds of genes that contribute to risk of complex, chronic diseases like heart disease, where environmental prevention could already be highly effective.
Hopefully, gene based therapies will result from the study of the clearly genetic subset of devastating diseases like AD and HD.