Label-retaining epithelial cells in mouse mammary gland divide asymmetrically and retain their template DNA strands

Abstract

It has been postulated that the stem cells of somatic tissues protect themselves from mutation and cancer risk by selective segregation of their template DNA strands. Self-renewing mammary epithelial stem cells that were originated during allometric growth of the mammary ducts in pubertal females were labeled using [3H]-thymidine (3HTdR). After a prolonged chase during which much of the branching duct morphogenesis was completed, 3HTdR-label retaining epithelial cells (LREC) were detected among the epithelium of the maturing glands. Labeling newly synthesized DNA in these glands with a different marker, 5-bromodeoxyuridine(5BrdU), resulted in the appearance of doubly labeled nuclei in a large percentage of the LREC. By contrast, label-retaining cells within the stroma did not incorporate 5BrdU during the pulse, indicating that they were not traversing the cell cycle. Upon chase, the second label (5BrdU) was distributed from the double-labeled LREC to unlabeled mammary cells while 3HTdR was retained. These results demonstrate that mammary LREC selectively retain their 3HTdR-labeled template DNA strands and pass newly synthesized 5BrdU-labeled DNA to their progeny during asymmetric divisions. Similar results were obtained in mammary transplants containing self-renewing, lacZ-positive epithelial cells suggesting that cells capable of expansive self-renewal may repopulate new mammary stem cell niches during the allometric growth of new mammary ducts.

Unbiased segregation of fission yeast chromosome 2 strands to daughter cells

“…It was proposed that stem cells might always inherit template-containing DNA strands allowing them to escape cancer-causing mutations. In support of the genome-wide directedsegregation mechanism, nonrandom segregation of sister chromatids was observed in some studies that followed the distribution of bromo-deoxyuridine-or radio-labeled chromosomes (Lark et al 1966;Rosenberger and Kessel 1968;Potten et al 1978;Merok et al 2002;Smith 2005), while the random pattern was found in other studies (Ito and McGhee 1987;Yadlapalli et al 2011). Another interesting case of selective segregation of chromatids of only the paternal set of chromosomes in the apical cells of shoots and roots of the water fern Marsilea vestita has been reported (Tourte et al 1980).…”

From milk to malignancy: the role of mammary stem cells in development, pregnancy and breast cancer

“…By performing pulse-chase experiments with DNA labels, Smith et al showed there was a population of cells within the mammary gland which retained their DNA label through asymmetric segregation of DNA strands. These cells were still actively dividing and featured stem cell characteristics [14]. Roughly 30-40% of the cells that retained their DNA label also expressed receptors for the reproductive hormones estrogen and progesterone [15].…”

Attributes of adult stem cells

“…Thus, any errors in replication are readily transferred (within one generation) to TACs that are soon lost from the population. Such a mechanism will produce LRCs after injection of DNA labels when stem cells are being formed, but validation of the hypothesis requires the observation that LRCs go through further rounds of DNA synthesis, yet still segregating the immortal DNA strands from sister chromatids into the daughter cells that remain as stem cells; this has been demonstrated in the small intestine and breast [9,10], and in some muscle satellite cells [11]. The existence of LRCs during normal stem cell renewal is based upon the random segregation of sister chromatids; however, this is somewhat incompatible with the 'immortal strand hypothesis', which predicts loss of a newly incorporated DNA synthesis marker within two cycles [12].…”