The Unravelling of Chromosome 21
A Therapeutic Breakthrough?
The results of an international research group comprising Americans, Europeans and Japanese, were published in Nature (18thMay, 2000) reporting the successful unravelling of the structure of chromosome 21. Since Professor Jerome Lejeune discovered in 1959 that Down's Syndrome resulted when an extra chromosome became attached to the two existing chromosomes of chromosome 21, its structure has been intensively studied. It is the smallest of all human chromosomes. Although chromosome 22 has been completely unravelled and comprises 545 genes, chromosome 21 contains only 225 genes. Of these, 127 were already known and 98 corresponded to genes not yet isolated. It contains also 59 pseudo genes; derived from archaic genes incapable of creating a protein and whose actual purpose is not really known at present. It represents 1-1.5% of the human genome.
The First Stage
This study opens the way to a better understanding of the pathologies bound up in chromosome 21. Trisomy 21 is without doubt the most frequent chromosomal anomaly and a leading cause of mental handicap in the world, and possibly with certain forms of Alzheimer's disease, certain leukaemias and neoplasms.
As the map of this chromosome has now been unravelled, it raises the exciting possibility that all its activities may be revealed. By this is meant its production of specific proteins with direct actions on the cells with ultimate consequences for the individual. The research will take at least a dozen years. It will however, manifest great possibilities both for future genetic knowledge and the families of these children.
A New Break-through
The poverty of chromosome 21 is a quite unexpected result of sequencing. The fact that chromosome 21 only contains a relatively small number of genes might explain why trisomy 21 with its chromosomal defects is reasonably well tolerated in the human species. Trisomy 21 is compatible with life whereas one does not know whether an infant afflicted with Trisomy 22 is capable of living The discovery is then a new reason for hope among families. The research must be continued in all areas; both fundamentally, and in its psychological and social settings. Its therapeutic aim will be geared to the service of patients and their families. Let us hope that this important discovery will renew the interest of public authorities for a chromosomal anomaly which affects one newly born in 700. It will have important consequences on the knowledge of the mechanisms underlying many other genetic abnormalities.
Translated from a French submission to the Lejeune Clinic.