Myelogenous leukemia in the rat.

T HE fundamental problems involving the etiology, pathogenesis and control of leukemia remain unsolved. In man, research has been greatly restricted by the rarity cf the disease and the impossibility of carrying out adequate experiments. Because of biochemical, genetic and cytologic clissimilarities, objecticns have been raised to the use of data obtained from animal studies. Nevertheless, valuable information has been gathered from research on mice, especially in regard to the leukemogenic effect of radiation and chemical carcinogens, screening of various chemotherapeutic agents, and the virus transmission of leukemia. While leukemia commonly occurs in some strains of mice and in fowl, it is a rare disease in the dog, rabbit, hamster and guinea pig. in the rat, leukemia was not reported until 1936, but in recent years it has been noted more frequently in this species.’ Myelogenous leukemia has been produced in the rat by chemical carcinogens, radioactivity amid other agents. The chronic and subacute forms are easily identified cytologically, and in the rat the myeloid cells contain readily detectable enzyme activities. Moreover, chronic myelogenous leukemia in the rat has many similarities to the human disorder and the disease has other characteristics of special research interest. In 1951, Shay et al. reported on the induction of chloroleukemia in the rat by the use of 20-methylcholanthrene (MCA) and the successful transfer of the disease to rat pups.2 5 Recently, Zipf and his co-workers produced a transplantable radiation-induced chloroleukemia in Sprague-Dawley rats.6 Both the MCA and radiation-induced disease have been carried on in subtransfer by a number of investigators.75 l)uring the past three years in our laboratory the Shay chloroleukemia has undergone 23 transplants and more recently (since June 1960) the Mound Laboratory chloroleukemia has also been studied. Results of observations on the factors influencing growth of these chloroleukemias and studies on the cytologic and enzymatic characteristics of the leukemic cells will be reported. However, transplants to non-inbred animals pose immunologic problems which invalidate observations on growth and other aspects of these neoplasms. Nevertheless, evidence will be presented that induced myelogenous leukemia in the rat offers a unique opportunity to investigate important aspects of leukemogenesis.