Geneticists determine the lack of chromosomes

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Geneticists determine the lack of chromosomes News-Medical.Net

15-Jun-2004

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.