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Boris Ephrussi and the discovery of mitochondrial genetics

May 7, 2012

Work in progress – by James Barnett

Mitochondria are very small organelles present in the cytoplasm of cells, and in these mitochondria most cellular respiration and energy production occurs. Mitochondria contain their own DNA, and studies of mitochondrial genetics have revealed that a remarkable number of human diseases, such as muscle disorders, are associated with related mitochondrial DNA deletions. So research on mitochondrial genetics is both medically and scientifically important.

Astonishingly, the development of mitochondrial genetics arose from the chance observation of a curious characteristic of yeast growth. Mitochondria had already been described in the 19th century but their role in the life of the cell was unknown; although their respiratory activity was worked out in 1949, they were not then known to contain DNA, which was thought to be solely in the cell’s nucleus. However, that same year, Boris Ephrussi (1901–1979) observed a peculiarity of yeast growth that led to the discovery of mitochondrial genetics.

Three mitochondria. Dr David Furness/Wellcome Images

Three mitochondria. Dr David Furness/Wellcome Images

Ephrussi’s cousin, the potter Edmund de Waal, recently wrote an outstanding book, The Hare with Amber Eyes, describing the history of the Ephrussis, a hugely wealthy Viennese banking family, originally from Odessa. During World War II, Boris Ephrussi was a refugee in the USA at Johns Hopkins University. On his return to Europe afterwards and working in Paris, he made his remarkable observation on the growth of colonies of baker’s yeast that he had spread on the surface of an agar medium. Most of these colonies grew to about the same size, but the cells of 1 or 2 per cent of the colonies grew much more slowly, forming colonies that were only a fraction of the size. When regrown, the cells of these little colonies, ‘petites’, produced only more petites, not the larger, standard colonies (‘grandes’). So the cells of the petites had changed genetically: they were stable mutants.

Biochemical studies showed that the slow growth of the petites was owing to the loss in those cells of the ability to respire aerobically, such yeast cells lacking several of the respiratory enzymes that occur in the mitochondria. If deprived of oxygen, the growth of all the yeasts was slowed to that of the petites. When petites were crossed with normal yeast, the progeny and subsequent generations were normal, save the 1 or 2 per cent of petites that normally occurred. In addition, Ephrussi and his colleagues found that growing the yeast in the presence of acriflavine, already known to react with DNA, greatly increased the mutation rate, producing a much greater proportion of petite colonies. The petites bred true and were found to lack two essential respiratory enzymes. What was truly remarkable was that genetic experiments showed these mutations to be cytoplasmically inherited and not a feature of nuclear genes. At that time, DNA was thought to be entirely nuclear, so its presence in the cytoplasm was quite unexpected; this new aspect of yeast genetics led ultimately to the discovery of mitochondrial DNA and to the important field of mitochondrial genetics in other eukaryotes, including humans.

Fred Sanger (1918–), twice a Nobel Prize winner, and his colleagues sequenced the human mitochondrial DNA genome in the early 1980s; the subsequent discovery of disease- causing mutations in mitochondrial DNA led to an explosion of research, which generated rapid advances in the understanding and diagnosis of mitochondrial genetic diseases. The accumulation of mitochondrial DNA mutations during life appears to contribute both to normal deteriorations associated with ageing and to several degenerative diseases. Another important discovery about mitochondrial genes is that they are passed on to children mainly from the mother, rather than the father, because after fertilisation, most of the sperm mitochondrial DNA is degraded. Its maternal transmission makes the pattern of inheritance of mitochondrial DNA completely different from the way nuclear DNA is inherited. A human mitochondrial genome database was set up in the year 2000 as a resource for population genetics and medical sciences. But these studies are of even wider interest: in the 1990s a completely different use of studies of human mitochondrial DNA was in establishing the authenticity of human remains, such as those of Tsar Nicholas II by sequencing the mitochondrial DNA of skeletal remains excavated from a mass grave in central Russia.

James Barnett is Honorary Research Fellow in the School of Biological Sciences, University of East Anglia, and author of Yeast Research: A historical overview.

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