Failure of 'imprinting' may explain clone deaths

The high level of fetal and neo-natal deaths in cloned animals may be linked to the disruption of a genetic mechanism known as 'imprinting,' according to American researchers interviewed last week for The Washington Post. Worrying rates of placental abnormalities, abnormal swelling and severe immunological deficiencies have been reported by numerous research teams world-wide.

At the Oregon Regional Primate Research Center - the only facility in the US trying to clone a primate - no cloned embryo has survived long enough to lead to an actual pregnancy. Imprinting, first identified over a decade ago, operates on the molecular level inside sperm and eggs and is thought to be a mechanism that labels genes as either maternal or paternal in origin. It is thought to work as 'on' or 'off' switches dictating which genes will be active in the different parts of an embryo.

As a clone is 'an embryo with only one parent', as described by Judith Hall, chief of paediatrics at the University of British Columbia, it would make sense if imprinting was at the root of at least some of the abnormalities. However, as some cloned animals such as Dolly the sheep seem to be entirely healthy, some cells may naturally carry the right 'maternal' and 'paternal' balance for normal embryonic development.

Some scientists also argue that the abnormalities in cloned animals may also be due to damage in the laboratory, the effects of the electric shock that starts embryo development or deficiencies in the culture medium in which embryos are initially left to grow.