Why do we age? Why do humans live longer than mice? What determines life span? How is cellular senescence related to aging? Scientists met at the Molecular Gerontology FASEB Summer Research Conference in Copper Mountain, Colorado, July 4-9, 1999, to discuss these and other questions, and to review progress in cloning genes that are responsible for diseases of aging, premature aging syndromes, and life span extension.

Kaare Christensen at Odense University in Denmark analyzed the influence of genes on health and life span in over 3,000 Danish twins aged 75 years and older, with a mean age of 81 years. Genetic factors explain one quarter of the variation in both life span and general health, and account for 31% of the variation in hospitalization. Jan Vijg at the University of Texas Health Science Center discussed the influence of genes on life span. Chances of living to 100 are greater if you had unusually old parents or siblings. Extreme longevity is clustered in families, with the percent of very old siblings greater than expected. Vijg suggested that once the genome is known, we can look for genetic variants associated with longevity.

"When people get diseases at 50 or 60, everyone assumes it's normal," Vijg said. "But it's not. Some centenarians remained healthy and never had problems or saw a doctor." The world's oldest woman, Jeanne Calment, lived to 122 and remained healthy her entire life.

Longevity Assurance Genes

Specific genes that increase life span were first found in the nematode roundworm Caenorhabditis elegans. Tom Johnson at the University of Colorado at Boulder cloned a mutant longevity gene, age-1. This mutation results in a 65% extension of mean life span and a 110% extension of maximum life span.

Longevity assurance genes in nematodes comprise: age-1, which encodes a phosphatidyl inositol-3' kinase and confers increased heat tolerance; daf-2, which encodes a tyrosine kinase and results in a more than twofold extension of life span; and daf-16, which encodes a transcription factor. The genes daf-2 and daf-12 together result in an almost fourfold increase in life span. Another gene, tkr-1, which encodes a tyrosine kinase receptor, increases longevity 40% to 100% and increases resistance to heat and UV radiation. During starvation, the tkr-1 gene product is increased.

"There is no reason to assume in [worms] that aging is inevitable," Johnson said. The screening system now available could help locate genes that confer human longevity.

Michael Jazwinsky of Louisiana State University Medical Center reviewed genes that affect life span in yeast. Each gene that increases life span is in a different metabolic pathway.

Aging in Cell Culture

In humans, progeroid (premature aging) syndromes mimic many symptoms associated with aging and result in early death. Werner syndrome, Bloom syndrome, Cockayne syndrome, and xeroderma pigmentosum are model systems used to study human aging.

Cells from patients with hereditary premature aging syndromes senesce prematurely. According to Judith Campisi at the Lawrence Berkeley National Lab, "Cell culture is the preferred 'test tube' for testing molecular hypotheses in human cells because we've learned how to manipulate cells, including gene insertions and inactivations." Campisi continued, "Werner's gene [WRN] is a longevity assurance gene. If WRN is missing, then we see accelerated aging."

Junko Oshima, at the University of Washington School of Medicine, characterized 19 mutations in the WRN gene from over 100 patients, including DNA base substitutions, small deletions or insertions, and large rearrangements or insertions. The WRN gene codes for a multifunctional protein with exonuclease activity, and has helicase activity. Helicase enzymes guard the genome and maintain stability. Mutations in WRN lead to several progeroid syndromes.

"We don't know the exact pathway," Campisi said. "Does it work in recombination, or replication? Without the protein, you get children that appear normal until they are early teenagers. It takes a long time to see the effects if the protein is missing. The first sign is, there's no growth spurt at puberty. The WRN protein may be needed for a normal hormone response, or it might cause damage that prevents cells from responding to hormones."

Campisi continued, "Another model system for the 2000s is the mouse." Transgenic, or knockout, mice are used in the study of premature aging syndromes. Campisi found that the mouse WRN protein has the same five amino acids that make up the active sites of the exonuclease as in the human gene. The mouse WRN protein is an exonuclease, like the human protein.

"Why does WRN only postpone aging for three years in the mouse, but for fifty years in humans?" Campisi asked.

"In a knockout mouse [WRN inactivated], there is no phenotype. The human knockout shows premature aging. Although the mouse and human genes have similar DNA sequences, they may operate in different pathways."

(One explanation for conservation of the active site, with such different phenotypes, is that the two genes are in different pathways.)

Another progeroid syndrome, Bloom's, results from a mutation in a helicase (BLM) in the RecQ family. Cockayne syndrome, a progeroid syndrome that results in abnormal UV sensitivity and neurological problems, is due to mutations in genes called CSA and CSB, which are necessary for DNA repair. In a related syndrome, COFS (cerebro-oculo-facial-skeletal syndrome), a deletion in the CSB gene was found by Errol Friedberg at the University of Texas Southwestern Medical School. The deletion results in a truncated gene product. Two more DNA repair diseases, xeroderma pigmentosum and TTD (trichothiodystrophy), result from gene mutations that result in loss of DNA transcription. Mutations in a gene essential for DNA replication or repair can cause severe illness and death.

"Gene therapy will come someday," said Friedberg. "But it's a long way from cloning a gene to finding out what it does. The TCA cycle and glycolysis were the easy biochemical pathways. Now we have multi-protein complexes - the DNA repair and translation machines. It's a whole new level of complexity."

Life Span Extension

One known way to increase life span, caloric restriction, was first observed in the 1930s in rats. Total calories were restricted to about 70% of normal soon after weaning. Underfed rats lived up to twice as long as rats fed ad libidum. Arlan Richardson at the University of Texas Health Sciences Center looked at diet and survival in genetically altered rats (p53 knockout). Rats lacking a functional p53 gene develop cancers earlier, and more of them, and they die earlier. A restricted diet increased the number surviving, and decreased age-related diseases in these rats. Only total calories were found to be important, not composition of the diet. Studies are now underway on the effect of caloric restriction on primates.

Jazwinski found a related effect in yeast. Yeast were switched from their normal diet of glucose to the sugar raffinose, which forced them to utilize acetate. This has less calories than glucose and induces a shift in the metabolic pathway. Yeast grown on raffinose live longer and have increased resistance to UV and heat. If we learn how caloric restriction affects metabolic regulation, it may be possible to mimic the effect in humans with drugs.

Meanwhile, Calvin Harley from Geron viewed the new techniques as bringing about a gradual extension of maximum life span. Some people, treated in their eighties to postpone or prevent certain diseases, might live on to extend the maximum life span. "Although the most dramatic effect of medicine will be on improving healthspan," Harley said. "It is possible that if we combine telomerase and histocompatible tissue transplants, we could increase maximum longevity."

Vijg again asked: What is the relation of cellular to organismal aging, and what is the genetic control of life span and aging? He looked at polymorphisms in the alpha-2 macroglobulin gene (A2M-2), common in late Alzheimer's disease, and the BRCA-1 gene, a breast cancer susceptibility gene. Vijg is studying gene frequencies in centenarians and the general population to see whether centenarians have different gene frequencies. There are six variants of the BRCA-1 gene, which Vijg can quickly distinguish using new instruments for 2-D gel scanning that he developed in collaboration with CBS Scientific in Del Mar, California. He can find mutations present in as little as 1% of the DNA. Vijg's goal is to find disease avoidance genes.

"The human population is the key to aging," he said. "Once the genome is sequenced, we should analyze every one of our genes in both centenarians and controls." Until then, his goals include screening of genes important in aging and longevity, such as DNA repair genes. One potential application would be screening for Werner syndrome (WS) carriers. WS is most common in Japan (of about 1,100 cases worldwide, 810 are found in Japan). Yasuhiro Furuichi of Agene Research Institute estimated that there are about one million carriers of the WS gene in Japan, out of a total population of 120 million.

"In principle, we have the technology to screen everyone for the WS gene," Vijg said. The biggest obstacle is not technology, but fear. After Hiroshima, in Japan there is a serious stigma associated with genetic defects. In view of the consequences to children (25% of the offspring of two recessive carriers are afflicted), is it a price that parents can refuse to pay?

"People now in their sixties and seventies have a reasonable expectation [to live] to be 120," Vijg said. " Biotechnology has fancy drugs. It is not so fanciful that in the next years something [will be] developed that allows us to live to be 150."

"What about in fifty or a hundred years?" I asked after the banquet. "Will we live to be 200 or 300?"

"Yes, after drinking two glasses of wine, it looks like it will be feasible," he replied. "Biotechnology developments are now exponential - so many new nutraceuticals, pharmaceuticals - yes, I don't see why not. People now in their twenties have a real serious chance of reaching this 200-year-old level, with the right choice of diet, exercise, vitamins, and drugs."

Sibylle Hechtel is a freelance writer whose articles' topics include science and rock climbing.

Andrzej Krauze is an illustrator, poster maker, cartoonist, and painter who illustrates regularly for HMS Beagle, The Guardian, The Sunday Telegraph, Bookseller, and New Statesman.