Geneticists determine the lack of chromosomes

A healthy person’s genome contains 46 chromosomes, but an individual cell can contain less. Contemporary methods allow to determine the value of losses rather accurately. The Tomsk researchers’ effort has been supported by the Russian Foundation for Basic Research.

An average statistical cell of a healthy person contains 46 chromosomes. But a specific cell can turn out to have one chromosome less or more. Even a hundred years ago, some researchers accounted for the cancer origin by the lack of chromosomes. However, to solve the issue, it is necessary first to determine very accurately the permissible level of chromosome deviations in a healthy body.

The researchers failed to do that for a long time as the traditional chromosome calculation method is an extremely labour-intensive and fraugh with errors. They succeeded to solve the problem only about fifteen years ago, when it became possible to mark every chromosome in non-divisible cells. With the help of this method, specialists of the Research Institute of Medical Genetics (Tomsk Research Center, Russian Academy of Medical Sciences) have determined the frequency of spontaneous loss of four “ordinary” chromosomes and two sex chromosomes. Having specified the notion of the standard, the researchers got an opportunity to thoroughly investigate the deviations.

Applying a new method, the researchers scanned more than 70 thousand lymphocyte nuclei of peripheral blood. The blood-donors were 10 healthy non-smoking middle-aged men. The researchers have calculated that the frequency of the lack of one of ordinary chromosomes makes about 0.12 percent in the cells of a healthy person. The same is the frequency of occurrence of an extra chromosomes in the cell. Apparently, these deviations occur during the cell division, when chromosomes disperse incorrectly. As per sex chromosomes Y and ?, the cells that do not contain either of them occur with frequency of 0.15 percent. It is known that the loss of Y-chromosome is compatible with standard cell division, however, the cells deprived of ?-chromosome will necessarily perish. Probably, the researchers observed the cells which had just lost this chromosome and were already “one leg in a grave”, i.e. on the way to apoptosis.

If the cells are made to be divided, then the frequency of chromosome lacks will grow up after each division. Apparently, in the course of cell division, the probability of chromosome divergence errors increases. Similar processes also happen in the course of natural ageing of the organism. Its cells go on dividing, and with age, the share of cells with irregular number of chromosomes grows by several times.

The Tomsk geneticists have calculated that one of chromosomes is lacked by 5 to 6 percent of all somatic (non-sex) cells of a healthy person. This frequency is sufficiently high, if we take into account the overall diversity of mechanisms urged to ensure constant number of chromosomes in the cell and to limit the reproduction of “irregular” cells. Apparently, the findings can be considered the “standard level of abnormality”, and it can be applied for comparison with sensitivity of various methods of molecular cytogenetics, impact of various environment factors on the genome and the level of genetic instability of different persons.