The Central Role of Cytogenetics in Radiation Biology.

Radiation cytogenetics has a rich history seldom appreciated by those outside the field. Early radiobiology was dominated by physics and biophysical concepts that borrowed heavily from the study of radiation-induced chromosome aberrations. From such studies, quantitative relationships between biological effect and changes in absorbed dose, dose rate and ionization density were codified into key concepts of radiobiological theory that have persisted for nearly a century. This review aims to provide a historical perspective of some of these concepts, including evidence supporting the contention that chromosome aberrations underlie development of many, if not most, of the biological effects of concern for humans exposed to ionizing radiations including cancer induction, on the one hand, and tumor eradication on the other. The significance of discoveries originating from these studies has widened and extended far beyond their original scope. Chromosome structural rearrangements viewed in mitotic cells were first attributed to the production of breaks by the radiations during interphase, followed by the rejoining or mis-rejoining among ends of other nearby breaks. These relatively modest beginnings eventually led to the discovery and characterization of DNA repair of double-strand breaks by non-homologous end joining, whose importance to various biological processes is now widely appreciated. Two examples, among many, are V(D)J recombination and speciation. Rapid technological advancements in cytogenetics, the burgeoning fields of molecular radiobiology and third-generation sequencing served as a point of confluence between the old and new. As a result, the emergent field of "cytogenomics" now becomes uniquely positioned for the purpose of more fully understanding mechanisms underlying the biological effects of ionizing radiation exposure.

Dose-dependent Transmissibility of Chromosome Aberrations at First Mitosis after Exposure to Gamma Rays. I. Modeling and Implications Related to Risk Assessment.

The relationship between certain chromosomal aberration (CA) types and cell lethality is well established. On that basis we used multi-fluor in situ hybridization (mFISH) to tally the number of mitotic human lymphocytes exposed to graded doses of gamma rays that carried either lethal or nonlethal CA types. Despite the fact that a number of nonlethal complex exchanges were observed, the cells containing them were seldom deemed viable, due to coincident lethal chromosome damage. We considered two model variants for describing the dose responses. The first assumes independent linear-quadratic (LQ) dose response shapes for the yields of both lethal and nonlethal CAs. The second (simplified) variant assumes that the mean number of nonlethal CAs per cell is proportional to the mean number of lethal CAs per cell, meaning that the shapes and magnitudes of both aberration types differ only by a multiplicative proportionality constant. Using these models allowed us to assemble dose response curves for the frequency of aberration-bearing cells that would be expected to survive. This took the form of a joint probability distribution for cells containing ≥1 nonlethal CAs but having zero lethal CAs. The simplified second model variant turned out to be marginally better supported than the first, and the joint probability distribution based on this model yielded a crescent-shaped dose response reminiscent of those observed for mutagenesis and transformation for cells "at risk" (i.e. not corrected for survival). Among the implications of these findings is the suggestion that similarly shaped curves form the basis for deriving metrics associated with radiation risk models.

Part I: the development of the catalytic Wittig reaction.

We have developed the first catalytic (in phosphane) Wittig reaction (CWR). The utilization of an organosilane was pivotal for success as it allowed for the chemoselective reduction of a phosphane oxide. Protocol optimization evaluated the phosphane oxide precatalyst structure, loading, organosilane, temperature, solvent, and base. These studies demonstrated that to maintain viable catalytic performance it was necessary to employ cyclic phosphane oxide precatalysts of type 1. Initial substrate studies utilized sodium carbonate as a base, and further experimentation identified N,N-diisopropylethylamine (DIPEA) as a soluble alternative. The use of DIPEA improved the ease of use, broadened the substrate scope, and decreased the precatalyst loading. The optimized protocols were compatible with alkyl, aryl, and heterocyclic (furyl, indolyl, pyridyl, pyrrolyl, and thienyl) aldehydes to produce both di- and trisubstituted olefins in moderate-to-high yields (60-96%) by using a precatalyst loading of 4-10 mol%. Kinetic E/Z selectivity was generally 66:34; complete E selectivity for disubstituted α,ß-unsaturated products was achieved through a phosphane-mediated isomerization event. The CWR was applied to the synthesis of 54, a known precursor to the anti-Alzheimer drug donepezil hydrochloride, on a multigram scale (12.2 g, 74% yield). In addition, to our knowledge, the described CWR is the only transition-/heavy-metal-free catalytic olefination process, excluding proton-catalyzed elimination reactions.