David S. Thaler, Ph.D.
The Rockefeller University

Mitochondrial Mutation and Aging.

Mitochondria have been proposed by others to be both initiators and targets of cellular degeneration associated with aging. The proposed work is designed to test specific hypotheses in which cellular degradation and aging are related to mitochondrial mutation. Measurement and implications of reversible intermediates in mitochondrial mutation.

This proposal builds on a concept originally put forward by FW Stahl. Most mutation and recombination occur, via reversible intermediates.

Consider a gene, G, mutating to a new allele, G', via an intermediate state, G*. Suppose state G* is a reversible state. In fact it usually is: The double helix has two copies of genetic information, so a modification of one chain leaves intact the information on the other chain and allows for conservative reversal of G*. In addition to the non-canonical base pair, which is a substrate for repair, many G* states are due to chemical modification of bases into non-standard forms that are recognized as substrates for restorative repair. G -k1---> G*-k3---> G' and G<--k2--- G*---k5-->cell dies

Suppose further that G*, the reversible intermediate state, is responsible for (a) phenotype(s). A particular example of interest is to suppose G* encodes mRNA of the new allele, i.e. G*-k4>mRNAG'. This would follow if a particular lesion codes in an informationally similar way, whether the template containing it is a substrate for transcription or for replication. Several reversible mutational intermediates have this property of informational miscoding. These include DNA-DNA mismatches, DNA-ribonucleotide mismatches (my own work, part published and part in progress), alkylated bases (especially O-6methyl Guanine), and oxidized bases (especially 8-Oxy Guanine). The phenotypes of G*s also may engender repair. A particular G* may thus make its own fixation either more or less probable depending on its own phenotypes.

Consider the consequences if the phenotype(s) of intermediates- e.g. mRNAG'- alters the rate of allele fixation, k3. k3 becomes a function of the phenotype of G* (e.g. mRNAG'). The [mRNAG'] is a function of k4, therefore, k3 is a function of k4 and k1, and an inversely related function of k2 and k5. Further complications exist. For example, G* may transit to other states G** each with its own vectors of transition. A particular G* or G** allele or set of G* alleles in a single cell or in the population may alter the global intracellular environment and the k's of other cells through cell-cell communication.

Mitochondria are an especially likely site for the modulation of mutation fixation inherent in this model. Mitochondrial information is multicopy and potentially selectable at two levels not available to the nuclear genome: within each organelle, and by multiple organelles in each cell.

We will develop both the theory and the experimental analysis of this Reversible-Intermediate model with reference to mitochondria. This work and related studies of mutation in mitochondria, and elsewhere in the cell but initiated by mitochondria 'gone bad' will be undertaken in collaboration with Dr. Marcelo Magnasco (Institute for the Study of Physics and Biology at Rockefeller University) and Dr. Zeena Nackerdian who is joining our group.

Contact Dr. Thaler.