High Mutation Levels are Compatible with Normal Embryonic Development in Mlh1-Deficient Mice

Why Tumor Genetic Heterogeneity May Require Rethinking Cancer Genesis and Treatment

Tumor genetic heterogeneity, in which individual tumors contain both multiple variant cancer-associated and normal genes, has been widely reported, although its significance has yet to be fully understood. We propose a genetic heterogeneity-based selection-centric hypothesis in which genetic heterogeneity, caused by the temporary reduction of DNA repair efficiency, occurs very early in human development, resulting in a small minority of cells in normal tissues acquiring cancer-associated genes that remain dormant. Cancer develops when precancer cells are selected for by altered tissue microenvironments; similar scenarios occur with development of metastases and therapeutic resistance in established cancer. This suggests that a normal cell selection treatment approach based on preferentially selecting normal cells within tumors may be effective in treating cancer.

Preserving Genome Integrity During the Early Embryonic DNA Replication Cycles

During the very early stages of embryonic development chromosome replication occurs under rather challenging conditions, including a very short cell cycle, absence of transcription, a relaxed DNA damage response and, in certain animal species, a highly contracted S-phase. This raises the puzzling question of how the genome can be faithfully replicated in such a peculiar metabolic context. Recent studies have provided new insights into this issue, and unveiled that embryos are prone to accumulate genetic and genomic alterations, most likely due to restricted cellular functions, in particular reduced DNA synthesis quality control. These findings may explain the low rate of successful development in mammals and the occurrence of diseases, such as abnormal developmental features and cancer. In this review, we will discuss recent findings in this field and put forward perspectives to further study this fascinating question.

Mlh1 deficiency increases the risk of hematopoietic malignancy after simulated space radiation exposure

Cancer-causing genome instability is a major concern during space travel due to exposure of astronauts to potent sources of high-linear energy transfer (LET) ionizing radiation. Hematopoietic stem cells (HSCs) are particularly susceptible to genotoxic stress, and accumulation of damage can lead to HSC dysfunction and oncogenesis. Our group recently demonstrated that aging human HSCs accumulate microsatellite instability coincident with loss of MLH1, a DNA Mismatch Repair (MMR) protein, which could reasonably predispose to radiation-induced HSC malignancies. Therefore, in an effort to reduce risk uncertainty for cancer development during deep space travel, we employed an Mlh1^+/− mouse model to study the effects high-LET ⁵⁶Fe ion space-like radiation. Irradiated Mlh1^+/− mice showed a significantly higher incidence of lymphomagenesis with ⁵⁶Fe ions compared to γ-rays and unirradiated mice, and malignancy correlated with increased MSI in the tumors. In addition, whole exome sequencing analysis revealed high SNVs and INDELs in lymphomas being driven by loss of Mlh1 and frequently mutated genes had a strong correlation with human leukemias. Therefore, the data suggest that age-related MMR deficiencies could lead to HSC malignancies after space radiation, and that countermeasure strategies will be required to adequately protect the astronaut population on the journey to Mars.