斯坦福大学研究发现小鼠年老的干细胞可能与老年性疾病相关

随着血液中的干细胞逐渐衰老，遗传学上的突变可能是老年性血液疾病的根源，以上结论是由于斯坦佛大学医学院的研究人员在小鼠上发现的。

“此项研究以及先前的研究表明为什么老年人容易得一些血液疾病，诸如，白血病或贫血，并且不能产生有效的抗体以防止流感类疾病的感染。”第一作者干细胞生物学和再生性药物专家Irving Weissman博士解释道。此项研究刊登在今年六月的自然杂志上。 

前人的研究表明，小鼠骨髓中的造血干细胞随着年龄的增大分裂活性和再生能力逐渐下降。现在有很多理论是关于细胞是怎样衰老的，其中有一条认为是由于遗传学上的突变造成的。此研究的另一作者Derrick Rossi博士说，“理想的解释是随着年龄的增长，DNA破坏过程逐渐破坏了细胞的正常功能。” 

然而，研究人员认为突变可能不是干细胞衰老的主要原因，因为干细胞是很少分裂的，而大多数突变是在分裂的过程中不经意发生的。频率不高的分裂可能保护细胞使其避免新的突变。 

研究人员为了证明这个观点，使用两种不同的实验手段。第一项实验中，研究人员首先在造血干细胞中引入单个突变，使这些细胞具有获得更多遗传错误的倾向。在三种不同种类的突变小鼠中，每一种类型中，干细胞表现都十分正常并能产生新的血液细胞。 

然而，将三种类型小鼠任意一种的造血干细胞移植入照射后的小鼠，这个过程类似于接受充分化疗后的病人接受供体的骨髓，以达到骨髓重建的目的。观察移植骨髓后小鼠的再生情况。 

结果发现，正常情况下，只需很少的干细胞就能完全使受体骨髓再生并产生正常量的血液和免疫细胞。然而取自突变小鼠的骨髓不能有效克隆，使得受体骨髓再生不完全，如果骨髓取自年老的的突变小鼠，受体骨髓再生效率会更低。 

Rossi说，这些结果表明，随着年龄的增长，干细胞中的突变会增加，并阻碍干细胞产生新血液和免疫系统细胞的功能。然而，这些结果仅是从突变小鼠中获得的。Rossi想要知道，干细胞在正常健康的条件下会是否会产生相同的结果。 

第二种实验方案中，Rossi从正常壮年和年老小鼠骨髓中分别分离得到干细胞，随后使用一种化学试剂将这些细胞染色，该试剂可特异性吸附于与DNA损伤相关的蛋白质。这种蛋白质可作为附近有DNA受损的旗标。研究发现，来自壮年正常小鼠的干细胞染色阴性，显示很小甚至没有DNA损坏。而年老的干细胞，通过染色显示存在DNA受损。 

所有的研究结果都表明，造血干细胞随着年龄增长，DNA损伤会增加，虽然这些细胞较少分裂，这些损伤可影响其随后所产生的血液和免疫系统细胞。Weissman说这些发现可以解释血癌（白血病）和免疫功能受损为什么会频繁发生于老年病人。 

下一步研究需要证明从小鼠身上得到的结果是否适用于人类造血干细胞。“如果这项研究研究可外延至人类，那么绝对可以证明干细胞是前-白血病突变的滋生地。”Weissman教授解释道。他是肿瘤研究方面的专家。


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.