NATURE MEDICINE • VOLUME 6 • NUMBER 5 • MAY 2000 507

NEWS & VIEWS

UNDERSTANDING THE MECHANISMS underly-ing the aging process remains one of

the main unsolved problems of modernbiology. Aging is a complex processinvolving many cell types, changingcell–cell interactions and in humans,long periods of study. Of necessity, mostresearch on human aging has focused onmodel systems that age in a shorter timeperiod and for which the environmentis, more or less, under the investigator’scontrol.

Some of the earliest and most influen-tial experiments in models of humanaging showed that serially cultured nor-mal human fibroblasts had a decreasingreplicative activity and were capable ofundergoing only 50–60 rounds of cell di-vision1. In addition, during their replica-tive life span, the cells maintained anormal karyotype and underwent mor-phologic and physiologic changes thatwere reminiscent of the changes thatoccur in some cells in the bodies of olderpeople. Hayflick proposed that this senes-cence in culture is a model for humanaging. The model has the advantages of ashort lifetime (typically 3–6 months), afull complement of human genes and theinvestigator’s ability to control the cellu-lar environment. This approach has beensomewhat controversial, however, be-cause of a lack of definitive evidence relat-ing replicative aging in culture to cellularaging in vivo.

In a recent Science article, Ly et al. ex-amined the transcriptional profiles of fi-broblast cultures taken from different agedonors2. Using high-density oligonucle-tode arrays to measure messenger RNAlevels, they compared gene expression incell cultures derived from a 7-year-oldand a 9-year old, two 37-year-olds andthree individuals in their 90s. The studyalso included three individuals withprogeria (Hutchinson-Guildford syn-drome, a so-called ‘disease of precociousaging’): two 8-year-olds and one 9-year-old. They found that about 1% of thegenes monitored showed reproducibleexpression level differences between dif-ferent age samples, and that most ofthese genes were involved in mitosis andin extracellular matrix remodeling. Theauthors concluded that a central under-

lying process of aging involves errors inthe mitotic machinery that lead to chro-mosomal pathologies and ultimatelymisregulation of essential structural, sig-naling and metabolic genes.

This study demonstrates the dazzlingpower of DNA microarray analysis to de-tect specific processes involved in thecomplex process of aging. However, de-spite the power of ‘high through-put’technologies to show general patterns ofgene expression, variations in gene ex-pression among individuals must also beconsidered. The study of Ly et al. in-cluded only seven individuals as donors,two or three from each of three agegroups, overlooking the enormous indi-vidual variability in humans and of cul-tured human cells2. The studies should beconfirmed through the evaluation of alarger number of subjects of definedhealth status. Alternatively, one poten-tial approach might be to ‘pool’ mRNAfrom 5–10 people from each age group tonormalize for individual variation.Differences between age groups shouldalso be compared with the individual dif-ferences within an age group.

The gene expression differences seen inpatients with Hutchinson-Guilford syn-drome should also be interpreted withcaution. Individuals with this syndromepossess some characteristics common tonormal aging, but not others. Thus, thedisease cannot be considered a simple caseof accelerated senescence. The controlgroup of normal 7-year-old and 9-year oldsubjects, presumably chosen to match theages of the progeroid donors, is also inap-propriate for comparison with older indi-viduals. In this case, it is difficult todetermine whether differences in gene ex-pression are associated with aging, or withspecific events that occur during develop-ment. For example, the differences be-tween 37-year olds and 90-year olds arenot as great as the differences betweenadults and children.

Ly et al. sub-cultured actively dividingfibroblasts from human biopsies andcompared gene expression between thedifferent groups2. However amplification

of cells in culture places them under im-mediate selection for those that are mostactively replicating. Recent studies haveshown that in cell cultures derived fromhealthy humans there is no correlationbetween donor age and replicative capac-ity, presumably reflecting this selectionfor the most actively replicating cells4.Thus, it is difficult to determine whethertranscriptional differences in fibroblastcultures accurately reflect differences inaging humans.

It is surprising, then, that the most im-pressive of the gene expression differencesreported by Ly et al. were associated withproliferative regulation3. It is essential inmaking comparisons of gene expression infibroblasts that the proliferation status of in-dividual cultures is well-controlled. It willalso be necessary to analyze differences ingene expression of dermal cells taken di-rectly from skin samples that have not beenamplified in culture.

We now have at our disposal powerfultools that allow us to examine and dis-sect complex processes such as aging.Studies such as those by Ly et al. are pio-neering efforts that will lead to a betterunderstanding of the complex biology ofaging as well as the limitations of themethodologies we use. Thoughtful use ofthese powerful techniques will lead us tobetter insights to the mechanisms ofaging and the increasing vulnerability todisease which aging brings. Progress,however, will depend on a thorough un-derstanding of the criteria of aging andthe basic biology of the model systems,and an acknowledgement of strengthsand limitations of these models.

1. Hayflick, L. & Moorhead, P. The serial cultivation ofhuman diploid cell strains. Exp. Cell Res. 25, 585–621(1961).

2. Hayflick, L. The limited in vitro lifetime of humandiploid cell strains. Exp. Cell Res. 37, 611–636 (1965).

3. Ly, et al. Mitotic misregulation and human aging.Science 287, 2486–2492 (2000).

4. Cristofalo, V.J. et al. Relationship between donor ageand the replicative lifespan of human cells in culture:a reevaluation. Proc. Natl. Acad. Sci. USA 95,10614–10619 (1998).

Lankenau Institute for Medical Research and Thomas Jefferson University100 Lancaster AvenueWynnewood, Pennsylvania 19096, USAEmail: cristofalov@mlhs.org

VINCENT J. CRISTOFALO

A DNA chip off the aging blockMicroarray analysis of gene expression differences between young and aging fibroblasts may provide insight

into the complex biological process of aging. But do changes in cultured cells from a few individuals accurately reflect overall changes in aging people?

© 2000 Nature America Inc. • http://medicine.nature.com©

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