Somatic cell nuclear transfer and transcription-factor-based reprogramming revert adult cells to an embryonic state, and yield pluripotent stem cells that can generate all tissues. Through different mechanisms and kinetics, these two reprogramming methods reset genomic methylation, an epigenetic modification of DNA that influences gene expression, leading us to hypothesize that the resulting pluripotent stem cells might have different properties. Here we observe that low-passage induced pluripotent stem cells (iPSCs) derived by factor-based reprogramming of adult murine tissues harbour residual DNA methylation signatures characteristic of their somatic tissue of origin, which favours their differentiation along lineages related to the donor cell, while restricting alternative cell fates. Such an 'epigenetic memory' of the donor tissue could be reset by differentiation and serial reprogramming, or by treatment of iPSCs with chromatin-modifying drugs. In contrast, the differentiation and methylation of nuclear-transfer-derived pluripotent stem cells were more similar to classical embryonic stem cells than were iPSCs. Our data indicate that nuclear transfer is more effective at establishing the ground state of pluripotency than factor-based reprogramming, which can leave an epigenetic memory of the tissue of origin that may influence efforts at directed differentiation for applications in disease modelling or treatment.

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Epigenetic memory in induced pluripotent stem cells

Procedures have now been developed for inserting functional genes into the bone marrow of mice. The most effective delivery system at present uses retroviral-based vectors to transfer a gene into murine bone marrow cells in culture. The genetically altered bone marrow is then implanted into recipient animals. These somatic cell gene therapy techniques are becoming increasingly efficient. Their future application in humans should result in at least partial correction of a number of genetic disorders. However, the safety of the procedures must still be established by further animal studies before human clinical trials would be ethical.

https://science.sciencemag.org/content/sci/226/4673/401.full.pdf

Prospects for human gene therapy

We studied the efficiency with which two chemical mutagens, ethyl methanesulfonate (EMS) and N-ethyl-N-nitrosourea (ENU) can induce mutations at different stages of spermatogenesis in zebrafish (Brachydanio rerio). Both EMS and ENU induced mutations at high rates in post-meiotic germ cells, as indicated by the incidence of F1 progeny mosaic for the albino mutation. For pre-meiotic germ cells, however, only ENU was found to be an effective mutagen, as indicated by the frequencies of non-mosaic mutant progeny at four different pigmentation loci. Several mutagenic regimens that varied in either the number of treatments or the concentration of ENU were studied to achieve an optimal ratio between the mutagenicity and toxicity. For the two most mutagenic regimens: 4 x 1 hr in 3 mM ENU and 6 x 1 hr in 3 mM ENU, the minimum estimate of frequencies of independent mutations per locus per gamete was 0.9-1.3 X 10(-3). We demonstrate that embryonic lethal mutations induced with ENU were transmitted to offspring and that they could be recovered in an F2 screen. An average frequency of specific-locus mutations of 1.1 X 10(-3) corresponded to approximately 1.7 embryonic lethal mutations per single mutagenized genome. The high rates of mutations achievable with ENU allow for rapid identification of large numbers of genes involved in a variety of aspects of zebrafish development.

/pdf/efficient-recovery-of-enu-induced-mutations-from-the-tqcrsx1z0q.pdf

Efficient recovery of ENU-induced mutations from the zebrafish germline.

Publisher Summary This chapter summarizes the current knowledge about the structure–function relationships of the globin gene system as derived from studies at the level of DNA and RNA. It discusses some of the sequence comparisons that relate to the evolution of this gene system and reviews the status of the applications of this molecular genetic knowledge to the understanding of the human hemoglobinopathies. The bulk of literatureon globin precludes anything approaching exhaustive coverage. Hemoglobin is a metalloprotein tetramer consisting of two α-like globin polypeptides and two β-like globin chains. The striking achievements of the past five years has been the rapid definition at the DNA level of the heterogeneous collection of mutations that lead to α- and β-thalassemia, which have themselves provided crucial information about the control of gene expression in vivo. The recognition of linked polymorphisms, the development of mutation-specific oligonucleotide probes, and the possibility of first-trimester chrionic villus biopsy have made the prenatal diagnosis of sickle-cell anemia and β-thalassemias of immediate applicability, and appropriate resources must be marshaled to offer this technology to the areas of the world where it is most needed.

L. B. Barnett

Papers

A mouse model for β-Thalassemia

Mosaic mutants induced by ethylnitrosourea in late germ cells of mice.