The Mysterious World of the Human Genome. Frank RyanЧитать онлайн книгу.
a cohort of laboratory mice.
Griffith’s experimental findings galvanised bacteriologists and immunologists around the world. His discovery was confirmed in several different research centres, including the Robert Koch Institute in Berlin, where the pneumococcal types had first been classified. The news was inevitably a hot topic of discussion in Avery’s department, as Dubos would recount: ‘but we did not even try to repeat them at first, as if we had been stunned and almost paralysed intellectually by the shocking nature of the findings’.
At first Avery simply couldn’t believe that bacterial types could be transformed. Indeed, he had been one of the authoritative figures who had settled the fixity of bacterial reproduction being true to type years before. But from 1926 Avery encouraged a young Canadian physician working in the Rockefeller Laboratory, M. H. Dawson, to investigate the situation. According to Dubos, Dawson, unlike Avery, was convinced from the start that Griffith’s conclusion must be correct because he believed that ‘work done in the British Ministry of Health had to be right’.
Dawson began by confirming Griffith’s findings in laboratory mice. His results suggested that the majority of non-virulent bacteria – the R types – had the ability in certain circumstances to revert to the virulent S type. By 1930 the young Canadian was joined by a Chinese colleague, Richard P. Sia, and between them they took the experimental observations further by confirming that the hereditary transformation could be brought about in culture media, without the need for passage through mice. At this stage, Dawson left the department and Avery encouraged another young physician, J. L. Alloway, to take the investigation further. Alloway discovered that all he needed to bring about the transformation was a soluble fraction derived from the S pneumococci by dissolving the living cells in sodium deoxycholate, then passing the resultant solution through filters to remove the bits of broken-up cells. When he added alcohol to the filtered solution, the active material precipitated out as sticky syrup. Throughout the laboratory this sticky syrup was referred to as the ‘transforming principle’. So the work continued, experiment following experiment, year by year.
When Alloway left the department, in 1932, Avery began to devote some of his own time to the pneumococcus transformation, in particular aiming to improve the extraction and preparation of the transforming substance. Frustration followed frustration. He focused on its chemical nature. Discussion took place with other members of the department, ranging from the ‘plamagene’ that was thought to induce cancer in chickens (now known to be a retrovirus), or to the genetic alterations in bacteria that were thought to be caused by viruses. According to Dubos, Alloway suggested the transforming agent might be a protein-polysaccharide complex. But by 1935 Avery was beginning to think along other lines. In his annual departmental report that year he indicated that he had obtained the transforming material in a form that was essentially clear of any capsular polysaccharide. In 1936, Rollin Hotchkiss, a biochemist who had now arrived to work in the department, wrote a historic comment in his personal notes:
‘Avery outlined to me that the transforming agent could hardly be a carbohydrate, did not match very well with a protein and wistfully suggested it might be a nucleic acid!’ At this stage, Dubos, who many years later would write a book about Avery and his work, dismissed this as no more than a surmise. There were good reasons for his caution.
That year few researchers throughout the world believed that the answer to heredity lay with nucleic acids. These chemical entities had been discovered by a Swiss biochemist, Johann Friedrich Miescher, back in the late 1800s. Fascinated by the chemistry of the nucleus, Miescher had broken open the nuclei of white blood cells in pus, and subsequently the heads of salmon sperm, to discover a new chemical compound which was acidic to pH testing, rich in phosphorus and comprised of enormously large molecules. After a lifetime of experimentation on the discovery, Miescher’s pupil, Richard Altmann, would introduce the term nucleic acid to describe Miescher’s discovery. By the 1920s, biochemists and geneticists were aware that there were two kinds of nucleic acids. One was called ribonucleic acid, or RNA, which contained four structural chemicals: guanine, adenine, cytosine and uracil, or GACU. The other was called ‘desoxyribonucleic acid’, or DNA, which was a major component of the chromosomes. They had deciphered its four bases – three identical to RNA, guanine, adenine and cytosine, but with the uracil replaced by thymine – making the acronym GACT. They knew that these four bases consisted of two different pairs of organic chemicals; adenine and guanine being purines, and cytosine and thymine being pyrimidines. They also knew that they were strung together to form very long molecules. At first they thought that RNA was confined to plants while DNA was confined to animals, but by the early thirties this was dismissed when it was found that both RNA and DNA were universally distributed throughout the animal and plant kingdoms. Still they had no knowledge of what nucleic acids actually did in the nuclei of cells.
A distinguished organic chemist based at the Rockefeller Institute, Phoebus Aaron Levene, proposed that the structures of DNA and RNA were exceedingly boring – they formed groups of four bases that repeated themselves in the identical repetitive formation throughout the molecule, like a four-letter word, repeated ad nauseam. This was called ‘the tetranucleotide hypothesis’. Such a banal molecule couldn’t possibly underlie the exceedingly complex basis of heredity. In the words of Horace Freeland Judson, ‘the belief was held with dogmatic tenacity that DNA could only be some sort of structural stiffening, the laundry cardboard in the shirt, the wooden stretcher behind the Rembrandt, since the genetic material would have to be protein’.
Proteins are lengthy molecules made up of smaller organic chemical units known as amino acids. There are 20 amino acids in the make-up of proteins, reminiscent of the number of letters that make up alphabets. If genes were the hereditary equivalents of words, only the complexity of proteins could fashion the words capable of spelling out the narratives. Chemists, and through extrapolation geneticists, not unnaturally assumed that only this level of complexity could possibly accommodate the incredible memory template that the complexity of heredity demanded – a line of thought that Judson labelled ‘The Protein Version of the Central Dogma’.
This was the contentious zeitgeist that Avery now confronted. As early as 1935, in his annual reports to the Board of the Institute, he indicated that he had growing evidence that the ‘transforming substance’ appeared free of capsular polysaccharide and it did not appear to be a protein.
Further progress on this line of research appeared to drag. In part this was because Dubos, working in the same department, had made a breakthrough in his search for antibiotic drugs. In 1925, Alexander Fleming, at St Mary’s Hospital in London, had discovered a potential antibiotic, penicillin, but he had been unable to take his work to the stage of useful production for medical purposes. Now, working on the philosophical principle encapsulated by the biblical saying ‘dust to dust’, Dubos had pioneered the search for microbes in soil that would potentially attack the polysaccharide coat of the pneumococcus. By the early 1930s he was making progress. From a cranberry bog in New Jersey he found a bacillus that dissolved the thick polysaccharide capsule that coated the pneumococcus with its armour-like outer covering. Dubos went on to extract the enzyme that the Cranberry Bog bacillus produced. He and Avery had reported their discovery in a paper in the journal, Science, in 1930. In a further series of papers the two scientists would report further experiments, all aimed at extrapolating the discovery to human trials of the Cranberry Bog enzyme in treating the potentially fatal pneumonia and meningitis caused by the pneumococcus.
But their researches encountered difficulty after difficulty. In part these arose from a predictable ignorance in a field of such pioneering research. A more personal, and devastating, problem arose when, under the stress of it all, Avery developed thyrotoxicosis – a debilitating autoimmune illness in which his thyroid gland became overactive.
Thyrotoxicosis causes the system to be flooded by thyroid hormones, which would have inappropriately switched his metabolism into a dangerous overdrive. He would have felt shaky, agitated, physically and mentally restless, suffering difficulties with relaxation and sleep – an impossible situation for a creative person. Avery had to spend time away from the lab undergoing surgery to remove the bulk of the ‘toxic goitre’, a procedure that carried risk of side-effects, even fatality in a minority of cases. His surgeon advised him against any early activity, physical or mental, that provoked stress. Dubos later