Leiden scientists sequence first female DNA

Geneticists of Leiden University Medical Centre (LUMC) are the first to determine the DNA sequence of a woman. She is also the first European whose DNA sequence has been determined. This has been announced by the researchers this morning, during a special press conference at ‘Bessensap', a yearly meeting of scientists and the press in the Netherlands.

Following in-depth analysis, the sequence will be made public, except incidental privacy-sensitive findings. The results will contribute to insights into human genetic diversity.

DNA of geneticist Marjolein Kriek

The DNA is that of dr Marjolein Kriek, a clinical geneticist at LUMC. "If anyone could properly consider the ramifications of knowing his or her sequence, it is a clinical geneticist," says professor Gert-Jan B van Ommen, leader of the LUMC team and director of the ‘Centre for Medical Systems Biology' (CMSB), a centre of the Netherlands Genomics Initiative.

Van Ommen continues: "Moreover, while women don't have a Y-chromosome, they have two X-chromosomes. As the X-chromosome is present as a single copy in half the population, the males, it has undergone a harsher selection in human evolution. This has made it less variable. We considered that sequencing only males, for ‘completeness', slows insight into X-chromosome variability. So it was time, after sequencing four males, to balance the genders a bit. And after Watson we also felt that it was okay to do Kriek".

Eight times coverage

The DNA sequencing was done with the ‘Illumina 1G' equipment, installed in January 2007 in the Leiden Genome Technology Centre, the genomics facility of LUMC and CMSB. In total, approx. 22 billion base pairs (the ‘letters' of the DNA language) were read; almost eight times the size of the human genome.

Dr. Johan den Dunnen, project leader at the Leiden Genome Technology Centre said: "This high coverage is needed to prevent mistakes, connect the separate reads and reduces the chance of occasional uncovered gaps. The sequencing itself took about six months. Partly since it was run as a ‘side operation' filling the empty positions on the machine while running other projects. Done in one go, it would take just ten weeks".

The cost of the project was approximately €40.000 excluding further in-depth bioinformatics analysis. This is estimated to take another six months.