Editors: Norio Fujiki, M.D. & Darryl R.J. Macer, Ph.D.
Professor, Dept. of Neurology, University of Tokyo, Tokyo, JAPAN
Huntington's disease (HD) is a neurodegenerative disorder characterised clinically by choreic involuntary movements and psychiatric problems such as change of character, dementia or schizophrenic episodes. Neuropathologically, a loss of small neurons in the striatum is the most prominent feature. HD is an autosomal dominant trait characterised by an extremely low mutation rate, a very high penetrance rate and a very similar age at onset for twins. These features strongly support the view that HD is regulated almost exclusively by a single gene (1).
Until 1983, a number of studies had been performed searching for the gene among the 22 autosomal chromosomes using classical genetic markers which show genetic polymorphism. However, none revealed the genetic linkage between HD and any classical marker. Exactly ten years ago, James F. Gusella in Massachusetts General Hospital in Boston succeeded in finding a tight linkage between HD and one of the eight DNA markers available at that time (2). The blood samples of HD families used for this monumental study were from those in the USA and Venezuela. The latter family is probably the biggest affected in the world and has been looked after by Nancy S. Wexler over a long time. Stimulated by this epoch making event, genetic linkage studies were performed in a world collaboration with Gusella's group. The first part of this paper looks at the dilemmas arising during the course of such a genetic linkage study.
In early 1993, Gusella and his colleagues reported that a hitherto unknown gene (IT15) located in the HD region had less than 24 CAG repeat sequences that were found in unaffected people, whereas there were more than 40 repeats in HD patients. IT15 is, therefore, believed to be the gene responsible for HD (3). Since the abnormal expansion of CAG repeats is easily detected by the PCR, it has become possible to diagnose HD patients accurately and to determine whether an at risk person is definitely a carrier of the abnormal gene or not. The second part of this paper will, therefore, focus on the dilemmas of genetic counseling.
Dilemmas in Genetic Linkage Studies
The DNA marker probe tightly linked to HD was G8, which originated from a genomic clone at the short arm of chromosome 4 (4p16.3). The calculated LOD score, which is an index of the closeness of the correlation between the DNA marker and the disease, was high enough for us to expect that the HD gene is located on the short arm of chromosome 4 (4).
Until 1987, four years after the first report of linkage to G8, LOD scores reached +80 using more than 60 families from around the world, including Japan, and the locus named D4S10 was found to be localised approximately 4cM distant to the HD gene. Therefore, many efforts concentrated on developing new DNA probes flanking the HD gene. Next, more informative probes in the 4p16.3 region were found and then proved to be much closer to the HD gene, for example D4S127, D4S43, D4S95.
In 1984 we began collaboration with Gusella's group on the linkage between G8 and the Japanese HD families (5). Since the frequency of HD in Japan is less than one-tenth of that in Western countries, the purpose of the collaborative study was to clarify whether or not the gene responsible for HD in Japan is the same as that in other countries. For this purpose we collected blood samples not only from those affected but also from unaffected but at-risk persons. The results of these genetic linkage studies were not always informative, therefore, our study was at the laboratory level rather than the diagnostic level for presymptomatic persons. In fact, the first step of this research was to seek the understanding of neurologists and psychiatrists to provide us with family charts and blood samples of key individuals. At the end of this laboratory study in March 1993 the number of clinically diagnosed HD families from which blood samples had been collected and registered in our "cell bank" had reached 50.
There were several problems in performing the linkage study, as follow. The first was how to protect the family confidentiality. In order to do this, as head of the study I kept all the original data in my own hands without copying. The second was how to ask at-risk persons to join in the study. Sometimes at-risk persons do not want to make contact with doctors. If these were older persons, for whom, the possibility of affection may be less than 50%, we carefully explained the importance of the study and asked them to join. On the other hand, we did not further push the younger persons to join the study, but accepted their spontaneous proposals to join us.
The third problem was how to tell the result to the family members. To help solve this, from the start of the study we told the family members that the results of the linkage would not always be informative. If all at-risk persons belonging to the family had normal haplotypes we decided to tell the results clearly to the family members through their own doctor. This was the case with my very first family. On the other hand, if even one at-risk person might have a linked haplotype, i.e., be a candidate for being affected, we decided to tell the family that the results were non-informative. Of course, we reported the real results to the family doctors and asked them to keep watching the candidates.
Dilemmas in direct gene analysis
In March 1993, the International Huntington's Disease Collaborative Group reported the novel gene, IT15, to be responsible for HD. The IT15 gene of normal individuals has 10-24 CAG repeats at the 5' end and the number of repeats expands to 40-100 in HD patients. Thus clinical diagnosis of HD has become possible without analysis of other family members. Direct gene analysis using the PCR has made presymptomatic diagnosis possible. We are, therefore, confronted with a dilemma as to how to respond to the requests of at-risk persons who really want to know whether they have the abnormal gene or not.
We are now planning to perform DNA analysis in response to the requests of at-risk persons under the following conditions:
1) a person must be a member of a definite HD family
2) a person must be over 20 years old
3) a person must know precisely about the genetic transmission of the disease
4) their request to receive the predictive test must be of their own free will, not from coercion by other persons
5) the person must be supported physically, mentally, socially and spiritually, should the result be positive.
I was asked on several occasions to perform predictive testing. We have asked the University of Tokyo Medical School Research Ethics Committee to commence screening and counseling under these careful conditions, and we expect these criteria to be approved very soon.