Epigenetics Beyond Mendel part II MCB 140 2 14 07 1 Broadly speaking An epigenetic effect on the genome changes the phenotype without changing the genotype The power of the environment and of life history MCB 140 2 14 07 2 Technically A mitotically or meiotically heritable change in gene expression state or genome functional state that is not associated with a change in the primary sequence of DNA MCB 140 2 14 07 3 In other words Genetics Organism or a cell with a phenotype Mutation change in DNA Different phenotype Epigenetics Organism or a cell with a phenotype Something happens but not a change in the DNA Different phenotype MCB 140 2 14 07 4 Cloning hello Dolly and hello again Dolly MCB 140 2 14 07 5 King and Briggs 1956 Serial transplantation of embryonic nuclei Cold Spring Harbor Symp Quant Biol 21 271 289 MCB 140 2 14 07 6 McKinnell R G 1978 Cloning Nuclear Transplantation in Amphibia University of Minnesota Press Minneapolis MCB 140 2 14 07 7 MCB 140 2 14 07 8 How can one explain the fact that cloning works so much better if one use a cell from an early embryo as the donor of the nucleus MCB 140 2 14 07 9 Two explanations 1 Alteration of the actual DNA of the cells as the embryo develops 2 Something else MCB 140 2 14 07 10 MCB 140 2 14 07 11 MCB 140 2 14 07 12 MCB 140 2 14 07 13 MCB 140 2 14 07 14 Reya Clarke and Weissman Nature 2001 MCB 140 2 14 07 15 King and Briggs 1956 Serial transplantation of embryonic nuclei Cold Spring Harbor Symp Quant Biol 21 271 289 MCB 140 2 14 07 16 Dolly MCB 140 2 14 07 17 Bill Ritchie Ian Wilmut Dolly MCB 140 2 14 07 18 This cannot be except it is Maybe maybe the cell that gave the nucleus that Dolly came from was not an actual mammary gland cell but rather an adult stem cell MCB 140 2 14 07 19 Proof How oh how can one prove that the cell that one uses as a donor in nuclear transfer is a mature cell type Well in an ideal scenario the nucleus would carry a signature of the fact that it came from a mature cell But but all cells in the body have identical DNA Or DO THEY MCB 140 2 14 07 20 Monoclonal mice generated by nuclear transfer from mature B and T donor cells Hochedlinger K Jaenisch R Nature 2002 415 1035 1038 MCB 140 2 14 07 21 Embryonic stem cells MCB 140 2 14 07 22 Annu Rev Cell Dev Biol 2001 17 435 462 EMBRYO DERIVED STEM CELLS Of Mice and Men Austin G Smith MCB 140 2 14 07 23 MCB 140 2 14 07 24 MCB 140 2 14 07 25 MCB 140 2 14 07 26 What they did 1 Take mouse B cell and mouse T cell 2 Take its nucleus out 3 Make an ES line from that nucleus by somatic cell nuclear transfer put nucleus into enucleated egg let this develop into embryo disperse the embryo to make ES cells 4 Make a mouse from that ES line by tetraploid complementation MCB 140 2 14 07 27 The result A monoclonal mouse MCB 140 2 14 07 28 Important point The difficulty of reprogramming differentiated B and T cells is consistent with the notion that many previously cloned animals may actually be derived from the nuclei of rare somatic stem cells present in heterogeneous donor cell populations rather than from differentiated cells as has been assumed MCB 140 2 14 07 29 Semantics 1 Reproductive cloning make new organisms 2 Therapeutic cloning aka somatic cell nuclear transfer no organism made MCB 140 2 14 07 30 Extensive abnormalities in cloned animals Lung failure Liver failure Obesity Etc etc Two problems 1 Cloning is incredibly inefficient 2 Of the animals that are born many have severe defects MCB 140 2 14 07 31 Proof that these abnormalities are entirely epigenetic Dolly s lambs and the offspring of all cloned animals are normal MCB 140 2 14 07 32 Note Not all epigenetic effects vanish during meiosis In plants exposure to radiation increases overall DNA damage repair levels epigenetically and heritably up to 4 generations Transgeneration memory of stress in plants Molinier J Ries G Zipfel C Hohn B Nature 2006 Aug 31 442 7106 1046 9 In a very limited sense this represents an inheritance of acquired characteristics as per Lamarck This is a very rare phenomenon MCB 140 2 14 07 33 Prof Jay Hollick MCB MCB 140 2 14 07 34 Pl Pl MCB 140 2 14 07 35 Pl is Changed to Pl paramutation MCB 140 2 14 07 36 Mice cloned from olfactory sensory neurons Eggan et al Rudolf Jaenisch and Richard Axel Nature 2004 428 6978 44 9 MCB 140 2 14 07 37 Natalie Angier Unnatural Obsessions The adjective that scientists use to describe a well wrought experiment is elegant which means not only that it worked but it worked in style MCB 140 2 14 07 38 MCB 140 2 14 07 39 Sense of smell One particularly clear example of neuronal diversity is provided by the olfactory sensory epithelium In the mouse each of the 2 000 000 cells in the olfactory epithelium expresses only one of about 1 500 odorant receptor genes such that the functional identity of a neuron is defined by the nature of the receptor it expresses Thus the sensory epithelium consists of at least 1 500 neuronal types The pattern of receptor expression is apparently random within one of four zones in the epithelium suggesting that the choice of receptor gene may be stochastic One mechanism to permit the stochastic choice of a single receptor could involve DNA rearrangements Eggan et al Nature 2004 Mar 4 428 6978 44 9 MCB 140 2 14 07 40 Allelic inactivation regulates olfactory receptor gene expression We suggest a model in which a hierarchy of controls is exerted on the family of odorant receptor genes to assure that a sensory neuron expresses a single receptor from a family of 1000 genes We propose that a cis regulatory element directs the stochastic expression of only one gene from a large array of linked receptor genes Moreover only one allelic array encoding multiple receptor genes is active in an individual neuron We demonstrate that in a neuron expressing a given receptor expression derives exclusively from one allele Chess et al Cell 1994 Sep 9 78 5 823 34 MCB 140 2 14 07 41 Eggan et al Nature 2004 Mar 4 428 6978 44 9 MCB 140 2 14 07 42 Eggan et al Nature 2004 Mar 4 428 6978 44 9 MCB 140 2 14 07 43 MCB 140 2 14 07 44 Question answer The regulation of gene expression by DNA rearrangement is rare but this mechanism has nonetheless been suggested to explain the diversity inherent in complex nervous systems Well we now know that it is …
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