In one of the largest and most widely used genetic databases for cancer research, the genomes of white patients are often better read than those of black patients.

People with a non-Western background are underrepresented in genetic research, it has been known for some time. Databases simply contain much more genomes from people of European descent than from, say, people of Asian or African descent. This is worrisome, because that can mean that the treatments that have been developed on the basis of these databases work less well for people from the latter groups.

Now researchers from the Mayo Clinic in the American city of Rochester another problem. The genomes of black patients who are in a large genetic database appear to be average less well read than that of patients with a European background.

Less deep

This concerns the so-called sequencing depth, or the ‘depth’ of sequencing. “That is, the number of times a particular spot on a genome has been read,” explains Yan Asmann one of the researchers involved. “The more times that has happened, the more certain you can be of the result.”

Asmann and colleagues looked at this sequence depth for seven cancers in The Cancer Genome Atlas, one of the largest and most widely used genetic databases for cancer research. Why only seven species? Because of the other 26 cancers, there were less than 50 genomes of black people in the database; too few to perform a reliable analysis.

Of the seven cancers the scientists were able to analyze—including breast cancer, colon cancer and prostate cancer—six had on average the genomes of African-American patients read less “deeply” than those of the white patients. Only at one form of kidney cancer there was no difference.

Less sure of mutations

It is also known what the difference is in three types of cancer. “The genomes of patients sequenced longer ago were read more deeply than the genomes sequenced later,” Asmann says. “And those early patients were mainly of European background.”

For the other three cancers it is not clear what caused the difference, says Asmann. “We did look for causes, but found none.”

Whatever the cause, the consequence is that the detection of mutations in the genomes of patients with an African background is less reliable, says Asmann.

Re-sequencing

To remedy this lack, the researchers suggest resequencing the DNA of the black cancer patients from the database – if that DNA is still available. “Also, more patients of African background could be sequenced with high coverage to compensate for the current disparity.”