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

Genetic heterozygosity is thought to enhance resistance of hosts to infectious diseases, but few tests of this idea exist. In particular, heterozygosity at the MHC, the highly polymorphic loci that control immunological recognition of pathogens, is suspected to confer a selective advantage by enhancing resistance to infectious diseases (the "heterozygote advantage" hypothesis). To test this hypothesis, we released mice into large population enclosures and challenged them with multiple strains of Salmonella and one of Listeria. We found that during Salmonella infections with three avirulent strains, MHC heterozygotes had greater survival and weight than homozygotes (unlike sham controls), and they were more likely to clear chronic Salmonella infection than homozygotes. In laboratory experiments, we found that MHC heterozygosity enhanced the clearance of multiple-strain Salmonella infections. Yet, contrary to what is widely assumed, the benefits of heterozygosity were due to resistance being dominant rather than overdominant, i.e., heterozygotes were more resistant than the average of parental homozygotes, but they were not more resistant than both. The fact that MHC heterozygotes were more resistant to infection and had higher fitness than homozygotes provides a functional explanation for MHC-disassortative mating preferences.

/pdf/mhc-heterozygosity-confers-a-selective-advantage-against-1sdxpnn5ym.pdf

MHC heterozygosity confers a selective advantage against multiple-strain infections.

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

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.

The molecular genetics of human hemoglobin.

A mouse model for β-Thalassemia

INHERITANCE OF SERUM ESTERASES HAVING DIFFERENT ELECTROPHORETIC PATTERNS: Among Inbred Strains of Mice

Mutations in the Murine Fitness 1 Gene Result in Defective Hematopoiesis

A Mouse Model for b-Thalassemia

Mice homozygous for a spontaneous mutation, in which the ..beta..-major globin gene is deleted, have clinical symptoms of ..beta..-thalassemia. These mice have a hypocellular, hypochromic, microcytic anemia that becomes more severe with increasing age. The defective red cell morphology, decreased osmotic fragility of erythrocytes and shortened red cell life span found in ..beta..-thalassemic mice are similar to those observed in human ..beta..-thalassemia. Synthesis of ..beta..-globin is depressed but not as much as might be expected because the expression of the..beta..-minor globin gene is enhanced to encode two to three times more globin than in normal mice. Splenomegaly, an enlarged pool of stem cells for erythropoiesis, and iron overloading occur in older mice. The fact that these mice remain moderately healthy makes them a very suitable animal model in which to develop and test alternative techniques of gene therapy that could be successfully applied to the treatment of human thalassemia. Homozygous ..beta..-thalassemic mice have large deposits of iron in their tissues, which might make these mice also useful for in vivo tests of the effectiveness and possible long-term side effects for newly developed iron chelators.

Diana M. Popp

Papers

A mouse model for β-Thalassemia

INHERITANCE OF SERUM ESTERASES HAVING DIFFERENT ELECTROPHORETIC PATTERNS: Among Inbred Strains of Mice

Mutations in the Murine Fitness 1 Gene Result in Defective Hematopoiesis

A Mouse Model for b-Thalassemia

Hematology of a murine beta-thalassemia: a longitudinal study.