Stress-induced reversible cell-cycle arrest requires PRC2/PRC1-mediated control of mitophagy in Drosophila germline stem cells and human iPSCs

Following acute genotoxic stress, both normal and tumorous stem cells can undergo cell-cycle arrest to avoid apoptosis and later re-enter the cell cycle to regenerate daughter cells. However, the mechanism of protective, reversible proliferative arrest, "quiescence," remains unresolved. Here, we show that mitophagy is a prerequisite for reversible quiescence in both irradiated Drosophila germline stem cells (GSCs) and human induced pluripotent stem cells (hiPSCs). In GSCs, mitofission (Drp1) or mitophagy (Pink1/Parkin) genes are essential to enter quiescence, whereas mitochondrial biogenesis (PGC1α) or fusion (Mfn2) genes are crucial for exiting quiescence. Furthermore, mitophagy-dependent quiescence lies downstream of mTOR- and PRC2-mediated repression and relies on the mitochondrial pool of cyclin E. Mitophagy-dependent reduction of cyclin E in GSCs and in hiPSCs during mTOR inhibition prevents the usual G1/S transition, pushing the cells toward reversible quiescence (G0). This alternative method of G1/S control may present new opportunities for therapeutic purposes.

The Human Genome Project and eugenic concerns

The U.S. Human Genome project is the largest scientific project funded by the federal government since the Apollo Moon Project. The overall effect from this project should be of great benefit to humankind because it will provide a better understanding both of single gene defects and multifactorial or familial diseases such as diabetes, arteriosclerosis, and cancer. At first this will lead to more exact ways of screening and diagnosing genetic disease, and later it will lead, in many if not most instances, to specific genetic cures. However, in the past, in both the U.S. and German eugenic movements genetic information has been misused. Hopefully, by remembering and understanding the past injustices and inhumanity of negative eugenics, further misuse of scientific information can be avoided.

Eugenics: past, present, and the future

During the past 20 years there has been a resurgence of interest in the history of the eugenics movements, particularly those of the United States and Germany. Unfortunately, most of these accounts have been published in nonmedical and nongenetic journals, so they are not readily available to geneticists or physicians. The authors of this article are concerned about the lack of information that geneticists, physicians, and students have concerning the origin and progress of these movements. This article provides a short history of the American and German eugenics programs and concludes with a review of their possible relations to our current practices. It is hoped that this will encourage institutions to include, in master's Ph.D., and M.D. programs in human genetics, lectures, seminars, and journal clubs on the topic of eugenics.

Hirschsprung disease: a genetic study

This study examines the association of Hirschsprung disease with Down syndrome and calculates the recurrence risk for families of Hirschsprung patients. Information was collected from 134 histologically diagnosed patients with Hirschsprung disease, from Children's Hospital in Pittsburgh, PA between 1950 and 1977. One hundred and three patients had short segment Hirschsprung disease which is defined as involvement up to and including the sigmoid colon. Thirty-one patients had the long segment type with aganglionosis extending in some cases to the small intestine. As in other studies, we found a significant association between Hirschsprung disease and Down syndrome in that 5.9% of probands had both. Mean maternal age of cases with both Hirschsprung disease and Down syndrome (33.5 years) was significantly different from controls (26.7 years) and non-Down syndrome Hirschsprung patients (26.6 years). The overall sex ratio for Hirschsprung disease was 3.6. Recurrence risks were dependent on proband sex and the degree of aganglionic involvement.