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Extensive contribution of Rb-deficient cells to adult chimeric mice with limited histopathological consequences
B O Williams1, E M Schmitt, L Remington
1Department of Biology, Massachusetts Institute of Technology, Cambridge 02139.
Abstract:
Homozygosity for a mutation in the Rb tumor suppressor gene causes mid-gestation embryonic lethality in the mouse. Using a two-step targeting protocol, we have constructed Rb homozygous mutant mouse embryonic stem cells and used them to create chimeric animals partially composed of Rb-deficient cells. Analysis of these chimeras demonstrates widespread contribution of the mutant cells to adult tissues, including the retina and mature erythrocytes. Despite the presence of large numbers of Rb-deficient cells in most tissues of these mice, they are remarkably normal but do exhibit certain histological defects including cataracts, hyperplasia of the adrenal medulla, and enlarged cells in the cerebellum and the liver. Like animals heterozygous for the Rb mutation, the chimeras develop tumors of the intermediate lobe of the pituitary, and the rate of pituitary tumorigenesis is greatly accelerated.
Insights
Homozygous Rb mutations cause embryonic lethality in mice. However, chimeric mice with Rb-deficient cells show widespread tissue contribution but develop pituitary tumors, accelerating cancer progression.
Area of Science:
- Developmental Biology
- Cancer Genetics
- Molecular Oncology
Background:
- The Retinoblastoma (Rb) tumor suppressor gene is crucial for cell cycle regulation.
- Homozygous mutations in Rb lead to embryonic lethality in mice, preventing in-depth study of Rb's role in adult tissues.
- Understanding Rb's function in adult tissues and tumorigenesis requires models that bypass embryonic lethality.
Purpose of the Study:
- To investigate the role of the Rb tumor suppressor gene in adult mouse tissues.
- To determine the consequences of Rb deficiency in a mosaic setting.
- To analyze the impact of Rb loss on tumor development, particularly in the pituitary gland.
Main Methods:
- Generation of Rb homozygous mutant mouse embryonic stem cells using a two-step targeting protocol.
- Creation of chimeric mice by introducing Rb-deficient cells into wild-type embryos.
- Histological analysis and tumor assessment in chimeric mice to evaluate Rb-deficient cell contribution and effects.
Main Results:
- Rb-deficient cells contributed widely to adult tissues, including the retina and erythrocytes.
- Chimeric mice exhibited normal development overall but showed specific defects like cataracts and adrenal medulla hyperplasia.
- Accelerated pituitary intermediate lobe tumorigenesis was observed in chimeras, similar to heterozygous Rb-mutant mice.
Conclusions:
- Partial Rb deficiency can be tolerated in adult tissues, though it leads to specific histological abnormalities.
- The Rb tumor suppressor plays a critical role in preventing pituitary tumorigenesis.
- Chimeric mouse models are valuable for studying the in vivo functions of essential genes that cause embryonic lethality when fully mutated.