據美國媒體10月22日報導，美國科學家日前發現人體內部DNA生物時鐘，可用來衡量人體組織和器官的生物年齡。研究人員說，揭開這套機制的奧秘有助理解老化過程，或許有助找到可延緩老化的藥物和其它治療方法。
洛杉磯加州大學(UCLA)遺傳學和生物統計學教授霍瓦特(Steve Horvath)說：「如最後能發展出治療方法，重設DNA生物時鐘，讓我們保持年輕，將令人非常興奮。」
霍瓦特研究了51個不同的健康和癌症細胞和組織、近8000個DNA樣本，他特別檢視甲基化如何隨著年齡而有不同。甲基化是修正DNA的自然過程。
霍瓦特發現，353個DNA標記的甲基化過程隨年齡固定變化，可拿來當作生物時鐘。時鐘一直到約20歲前運轉得最快，隨後放慢至更穩定速率。至於是DNA的改變導致老化、或老化改變了DNA仍未知，是科學家目前極力要弄清楚的。
DNA生物時鐘透露一些令人感興趣的結果，如健康心臟組織的生物年齡約比預期年輕9歲。女性乳房組織較身體其它部位老得快，平均老2歲。
霍瓦特說：「即使是健康女性的乳房組織似乎都比人體其它組織老一些，鑑於乳癌是女性最普遍的癌症，這個現象令人感興趣。另外，年齡也是癌症主要風險因子之一，這可解釋乳癌為何如此普遍。」
霍瓦特在《基因組生物學》(Genome Biology)期刊發表此研究發現，他指出，取自成人身上的細胞重新設定返回幹細胞狀態時，生物時鐘也歸零。他說：「這證實能重設生物時鐘的概念。」(生物谷Bioon.com)
生物谷推薦的英文摘要
Genome Biology doi:10.1186/gb-2013-14-10-r115
DNA methylation age of human tissues and cell types
Steve Horvath et al.
Background
It is not yet known whether DNA methylation levels can be used to accurately predict age across a broad spectrum of human tissues and cell types, nor whether the resulting age prediction is a biologically meaningful measure.
Results
I developed a multi-tissue predictor of age that allows one to estimate the DNA methylation age of most tissues and cell types. The predictor, which is freely available, was developed using 8,000 samples from 82 Illumina DNA methylation array datasets, encompassing 51 healthy tissues and cell types. I found that DNA methylation age has the following properties: first, it is close to zero for embryonic and induced pluripotent stem cells; second, it correlates with cell passage number; third, it gives rise to a highly heritable measure of age acceleration; and, fourth, it is applicable to chimpanzee tissues. Analysis of 6,000 cancer samples from 32 datasets showed that all of the considered 20 cancer types exhibit significant age acceleration, with an average of 36 years. Low age-acceleration of cancer tissue is associated with a high number of somatic mutations and TP53 mutations, while mutations in steroid receptors greatly accelerate DNA methylation age in breast cancer. Finally, I characterize the 353 CpG sites that together form an aging clock in terms of chromatin states and tissue variance.
Conclusions
I propose that DNA methylation age measures the cumulative effect of an epigenetic maintenance system. This novel epigenetic clock can be used to address a host of questions in developmental biology, cancer and aging research.