隨著血液中的干細胞逐漸衰老，遺傳學上的突變可能是老年性血液疾病的根源，以上結論是由于斯坦佛大學醫學院的研究人員在小鼠上發現的。

“此項研究以及先前的研究表明為什么老年人容易得一些血液疾病，諸如，白血病或貧血，并且不能產生有效的抗體以防止流感類疾病的感染。”第一作者干細胞生物學和再生性藥物專家Irving Weissman博士解釋道。此項研究刊登在今年六月的自然雜志上。 

前人的研究表明，小鼠骨髓中的造血干細胞隨著年齡的增大分裂活性和再生能力逐漸下降。現在有很多理論是關于細胞是怎樣衰老的，其中有一條認為是由于遺傳學上的突變造成的。此研究的另一作者Derrick Rossi博士說，“理想的解釋是隨著年齡的增長，DNA破壞過程逐漸破壞了細胞的正常功能。” 

然而，研究人員認為突變可能不是干細胞衰老的主要原因，因為干細胞是很少分裂的，而大多數突變是在分裂的過程中不經意發生的。頻率不高的分裂可能保護細胞使其避免新的突變。 

研究人員為了證明這個觀點，使用兩種不同的實驗手段。第一項實驗中，研究人員首先在造血干細胞中引入單個突變，使這些細胞具有獲得更多遺傳錯誤的傾向。在三種不同種類的突變小鼠中，每一種類型中，干細胞表現都十分正常并能產生新的血液細胞。 

然而，將三種類型小鼠任意一種的造血干細胞移植入照射后的小鼠，這個過程類似于接受充分化療后的病人接受供體的骨髓，以達到骨髓重建的目的。觀察移植骨髓后小鼠的再生情況。 

結果發現，正常情況下，只需很少的干細胞就能完全使受體骨髓再生并產生正常量的血液和免疫細胞。然而取自突變小鼠的骨髓不能有效克隆，使得受體骨髓再生不完全，如果骨髓取自年老的的突變小鼠，受體骨髓再生效率會更低。 

Rossi說，這些結果表明，隨著年齡的增長，干細胞中的突變會增加，并阻礙干細胞產生新血液和免疫系統細胞的功能。然而，這些結果僅是從突變小鼠中獲得的。Rossi想要知道，干細胞在正常健康的條件下會是否會產生相同的結果。 

第二種實驗方案中，Rossi從正常壯年和年老小鼠骨髓中分別分離得到干細胞，隨后使用一種化學試劑將這些細胞染色，該試劑可特異性吸附于與DNA損傷相關的蛋白質。這種蛋白質可作為附近有DNA受損的旗標。研究發現，來自壯年正常小鼠的干細胞染色陰性，顯示很小甚至沒有DNA損壞。而年老的干細胞，通過染色顯示存在DNA受損。 

所有的研究結果都表明，造血干細胞隨著年齡增長，DNA損傷會增加，雖然這些細胞較少分裂，這些損傷可影響其隨后所產生的血液和免疫系統細胞。Weissman說這些發現可以解釋血癌（白血病）和免疫功能受損為什么會頻繁發生于老年病人。 

下一步研究需要證明從小鼠身上得到的結果是否適用于人類造血干細胞。“如果這項研究研究可外延至人類，那么絕對可以證明干細胞是前-白血病突變的滋生地。”Weissman教授解釋道。他是腫瘤研究方面的專家。


Aging stem cells in mice may hold answers to disease of the aged, Stanford study finds  

As stem cells in the blood grow older, genetic mutations accumulate that could be at the root of blood diseases that strike people as they age, according to work done in mice by researchers at the Stanford University School of Medicine.

“This and our previous work points out why older people are more likely to get blood diseases, such as leukemia or anemia, and are less likely to make new antibodies that would protect against infections like the flu,” said senior author Irving Weissman, MD, director of the Stanford Institute for Stem Cell Biology and Regenerative Medicine and of the Stanford Comprehensive Cancer Center. The work is published in the June 6 issue of Nature.

In past studies, this group of researchers had shown that blood-forming stem cells in the bone marrow of mice became less able to divide and replenish the supply of blood cells as they aged. The question was why.

Researchers have put forward many theories about how cells age, said Derrick Rossi, PhD, postdoctoral scholar and co-first author of the paper. One of those theories has to do with cells accumulating genetic mutations. “The idea is that, over time, accumulated DNA damage progressively diminishes the cell’s ability to perform its normal function,” he said.

However, researchers had thought that mutations were unlikely to underlie aging in blood-forming stem cells because they very rarely divide, and most mutations crop up during division. The infrequent divisions were believed to protect the cells from acquiring new mutations.

Rossi, Weissman and the other first author, postdoctoral scholar David Bryder, PhD, tested that idea in two different sets of experiments. In the first, they studied the blood-forming stem cells of mice engineered to have single mutations that make them especially prone to accumulating additional genetic errors. In each of the three different types of mutant mice they studied, the stem cells appeared to behave normally and to produce new blood cells.

However, the full truth came out when they took blood-forming stem cells from any of the three types of mice and used those cells to repopulate the bone marrow of irradiated mice. This type of experiment is much like using a bone marrow transplant to bring back the bone marrow in a person who has undergone extensive chemotherapy.

Normally, a few stem cells are enough to completely replenish the bone marrow of mice and produce normal amounts of blood and immune cells. However, error-filled blood-forming stem cells taken from the mutant mice were much less effective at colonizing the depleted bone marrow than normal stem cells, and became even less effective when taken from older mutant mice.

Rossi said these results suggest that mutations accumulating in stem cells as they age were preventing them from doing their normal job of producing new blood and immune system cells. However, these results were in mutant mice. Rossi wanted to know if the stem cells in normal, healthy mice also accumulate damage as they age.

To address this, in the second set of experiments, Rossi isolated stem cells from the bone marrow of normal young and old mice, then stained those cells with a chemical that clings to a protein that’s associated with DNA damage. This protein can act as a flag to highlight nearby DNA damage.

What he found is that young stem cells from normal mice contained no stain and therefore little or no DNA damage. Older stem cells, on the other hand, showed extensive staining.

All of this adds up to one thing: blood-forming stem cells do accumulate DNA damage with age even though they rarely divide, and that damage is passed on to the blood and immune system cells they make. Weissman said these findings could explain the origin of blood cancer (leukemia) and immune dysfunctions that occur as people age.

The next step is to show whether these results from mice hold true for human blood-forming stem cells. “If this work does extrapolate to humans, then it is absolutely consistent with the idea that blood-forming stem cells are the breeding ground for pre-leukemic mutations,” said, the Virginia and D.K. Ludwig Professor for Clinical Investigation in Cancer Research.

Additional Stanford researchers who contributed to this work include postdoctoral scholar Jun Seita, MD, PhD.

Funding for this study came from the National Cancer Institute’s Center for Cancer Research, the Damon Runyon Cancer Foundation, the California Institute of Regenerative Medicine, a Swedish Medical Research Council scholarship (STINT) and a Cancerfonden grant.