Editors: Norio Fujiki, M.D. & Darryl R.J. Macer, Ph.D.
Professor, Department of Medical Genetics, Institute of Basic Medical Sciences, University of Tsukuba, JAPAN
Example: Coronary artery disease
(1) Familial hypercholesterolemia due to the mutant LDL receptor gene
Large effects on the development of coronary artery disease are not common (frequency 0.2%)
(2) High plasma Lp(a) levels due to apo(a) isoforms and variability in gene sequence
Significant effects on the development of coronary artery disease are common (frequency 5-10%)
We have been detecting individuals with familial hypercholesterolemia from the family members of the patients with this disease who are under medical treatment, using clinical genetic methods. Then we lead or treat them for the prevention of coronary artery disease. Thus far we have detected 40 families with familial hypercholesterolemia. We also have detected various types of mutations in the LDL receptor gene among these families (1-4).
We have been carrying out genetic screening for familial hypercholesterolemia utilizing the school survey which is carried out by Ibaraki Prefectural Service Association (5,6). The subjects are 9 year old and 12 year old children. Prior to this genetic screening, we have obtained the approval from the P.T.A., school principals, and the local medical associations. After screening of hypercholesterolemic children by the Prefectural Health Service Association, we have explained the cause, preventive measures and therapy of hypercholesterolemia to the parents of hypercholesterolemic children. Then we have done voluntary genetic analysis of families of hypercholesterolemic children. We have obtained informed consent from the parents before conducting the tests for families.
We have detected 242 hypercholesterolemic children from 3453 screened. About half of the parents of the hypercholesterolemic children participated in our genetic screening and five families with familial hypercholesterolemia have been detected (Table 2). Almost all adults with familial hypercholesterolemia are under medical care. The mothers of children with familial hypercholesterolemia are eager to implement and maintain dietary therapy for their children. The social acceptance for this genetic screening is considerably good in this local community.
The reasons why our genetic screening for familial hypercholesterolemia is so well accepted, with good consequences locally include the following:
1) Availability of preventive measures and therapy for hypercholesterolemia, especially medication in the case of Japanese
2) No prejudice against hypercholesterolemia
3) The spread of education among the public for hypercholesterolemia
4) Positive cooperation of staff of the Prefectural Health Service Association, public health nurses and school nurses
School age children screened 3457
Hypercholesterolemic children 242 (7%)
Parents participating in the test 230
Familial hypercholesterolemia 5 families
The frequency distribution of plasma Lp(a) levels in Japanese are very similar to that in most Caucasian populations. About 10% of individuals have high plasma Lp(a) levels above 30 mg/dl. There are at least 25 kinds of apo(a) which are genetically different in molecular weight (7). This means the presence of extreme differences in the apo(a) gene size among different individuals. There is a tendency that the lower the molecular weight of apo(a), the higher the Lp(a) levels. According to our data, about 70% of healthy subjects whose apo(a) molecular weights are smaller than 680 kDa have high plasma Lp(a) levels above 30 mg/dl (8).
In our studies on 369 Japanese patients with coronary artery disease and 389 healthy controls, the odds ratio for the disease was 2.7 in the case of high plasma Lp(a) levels and 2.8 in the case of apo(a) with molecular weights smaller than 680 kDa, respectively (9). This level of the risk seems to be similar to that of smoking and moderate hypercholesterolemia. Therefore I think genetic tests for plasma Lp(a) levels are important as a presymptomatic diagnosis for premature coronary artery disease on an individual and family basis. Genetic screening for Lp(a), however, might be premature, mainly because dietary therapy and medication are not useful and because the risk for the disease is not large.
Genetic screening for presymptomatic diagnosis of common diseases where medical intervention is possible will become more and more important, because Human Genome Project will promote the discovery of more and more disease susceptibility genes. In the genetic screening for common diseases, the following points should be considered:
1) Preventive measures and therapy
2) Prejudice against the disease in question
3) Levels of effects of the gene on the disease
4) Education of the public
5) Voluntary genetic screening coupled with genetic counseling
6) Informed consent
7) The duty of confidentiality to protect individual privacy
This year it was revealed that a common allele at the apolipoprotein E locus, E4 is an important susceptibility gene for common late-onset Alzheimer's disease. In many Caucasian populations, the frequencies of heterozygotes for E4 and homozygotes for E4 are 20 to 30% and 2 to 3%, respectively. According to our data, the frequencies of heterozygotes for E4 and homozygotes for E4 in the Japanese population are about 18% and about 1%, respectively (10). For part of a rare autosomal dominant early-onset form of Alzheimer's disease, the responsible gene has already been identified. Since no preventive measures and therapy for Alzheimer's disease are available at present, presymptomatic DNA analysis for this disease can not be coupled with medical intervention. The development of preventive measures and therapy are the prerequisite for genetic screening for common diseases. The development of preventive measures and therapy are also necessary to get rid of the prejudice against some common diseases such as schizophrenia.
1. Yamakawa, K. et al. (1989) Three novel partial deletions of the LDL receptor gene in familial hypercholesterolemia. Human Genetics 82: 317.
2. Yamakawa, K. et al. (1991) Family studies of the LDL receptor gene of relatively severe hereditary hypercholesterolemia associated with Achilles tendon xanthomas. Human Genetics 86: 445.
3. Yamakawa, K. et al. (1993) Two novel frameshift mutations associated with the presence of direct repeats of the LDL receptor gene in familial hypercholesterolemia. Human Genetics 92: 331.
4. Yamakawa-Kobayashi, K. et al., A novel complex mutation in the LDL receptor gene probably caused by the simultaneous occurrence of deletion and insertion in the same region. Human Genetics (in Press)
5. Yanagi, H. et al. (1990) Family studies of primary hypercholesterolemia in school children: The Tsukuba study. Jpn. J. Public Health 37: 585 (in Japanese with English abstract).
6. Yanagi, H. et al., Cardiovascular risk factors among Japanese school-age children: A screening system for children with high risk for atherosclerosis in Ibaraki, Japan. Jpn. J. Public Health (in Press), (in Japanese with English abstract)
7. Kikuchi, S. et al. (1993) High degree of genetic polymorphism in apolipoprotein(a) associated with plasma lipoprotein(a) levels in Japanese and Chinese populations. Human Genetics 92: 537.
8. Yanagi, H. et al. (1993) Frequent occurrence of familial aggregations of high lipoprotein(a) levels associated with small apolipoprotein(a) isoforms. Human Genetics 92: 547.
9. Kikuchi, H. et al., Role of apolipoprotein(a) size polymorphism and plasma lipoprotein(a) levels in coronary heart disease in Japanese. (submitted)
10. Hamaguchi, H. et al., Genetic risk factors for coronary heart disease in the Japanese population, pp. 110-126 in Berg, K. et al. (eds) Genetic approaches to coronary heart disease and hypertension (Springer-Verlag, Berlin-Heidelberg, 1991).