Ability to produce unbroken DNA sequencing could eventually be used by medical laboratories to identify gene sequences that play significant roles in a variety of diseases and health conditions
While near 24/7 coronavirus coverage occupies much of the media, it is refreshing to report on important breakthroughs in clinical laboratory medicine and diagnostics that are unrelated to the COVID-19 pandemic. It wasn’t long ago that the top stories in advanced medicine revolved around whole-genome sequencing, so it’s nice to return to the topic, if just for a little while.
Using nanopore sequencing technology from multiple companies, researchers at the University of California Santa Cruz Genomics Institute (UCSC Genomics Institute) have produced what they say is the first telomere-to-telomere or end-to-end map of the human X chromosome. This could prove to be a major milestone for genomics research and help scientists gain a better understanding of certain genetic conditions.
The completely gapless DNA sequencing was produced by using new sequencing technologies that enable much longer reads of strings of DNA base pairs. In the past, most sequencing technologies produced relatively short reads of each sequence, which then had to be painstakingly pieced together to assemble the complete genome.
“With nanopore sequencing we get ultra-long reads of hundreds of thousands of base pairs that can span an entire repeat region, so that bypasses some of the challenges,” said Karen Miga, PhD, a post-doctoral research scientist at the UCSC Genomics Institute and lead author of the study, in a UCSC news release.
The UCSC researchers published their paper, titled, “Telomere-to-Telomere Assembly of a Complete Human X Chromosome,” in the multidisciplinary scientific journal Nature.
UCSC Researchers Find ‘Rich’ Information in the ‘Gaps’ in Reference Sequences
Nanopore sequencing technology from Oxford Nanopore Technologies was combined with sequencing technologies from Pacific Biosciences (PacBio) and Illumina, as well as with optical maps from Bionano Genomics, to produce the results of the research. The combination of these technologies allowed the UCSC team to produce a whole-genome sequence assembly with no gaps and with a previously unforeseen level of accuracy.
“These repeat-rich sequences were once deemed intractable, but now we’ve made leaps and bounds in sequencing technology,” Miga said.
According to the Oxford Nanopore Technologies website, “Nanopore sequencing is a unique, scalable technology that enables direct, real-time analysis of long DNA or RNA fragments. It works by monitoring changes to an electrical current as nucleic acids are passed through a protein nanopore. The resulting signal is decoded to provide the specific DNA or RNA sequence.”
By filling in gaps in the human genome, the UCSC researchers opened up new possibilities to finding clues and answers regarding important questions about our genes and how they may contribute to illnesses.
During their research, the team had to manually resolve several gaps in the sequence and identify variants within the repeat sequence to serve as markers. They were then able to align the long reads and connect them together to span the centromere of the X chromosome. The centromere is a difficult region of repetitive DNA that is found in every chromosome. It encompasses an area of very repetitive DNA that spans 3.1 million base pairs.
“For me, the idea that we can put together a 3-megabase-size [3-million base pairs] tandem repeat is just mind-blowing. We can now reach these repeat regions covering millions of bases that were previously thought intractable,” Miga said.
The researchers also had to employ a polishing strategy using data obtained from different sequencing technologies to ensure accuracy.
“We used an iterative process over three different sequencing platforms to polish the sequence and reach a high level of accuracy,” Miga explained. “The unique markers provide an anchoring system for the ultra-long reads, and once you anchor the reads, you can use multiple data sets to call each base.”
Linking Gene Variations to Specific Genetic Diseases
Besides the advantages of providing ultra-long reads, nanopore sequencing can also detect bases that have been modified by methylation, a biological process by which methyl groups are added to the DNA molecule. Methylation is an epigenetic change that can alter the activity of a DNA segment without changing the sequence, and can have important effects on the DNA structure and gene expression. When located in a gene promoter, DNA methylation typically acts to repress gene transcription.
The researchers were able to observe and map patterns of methylation on the X chromosome and found some interesting trends in methylation patterns within the centromere. By looking at this previously unmapped area of the genome, scientists may be able to search for potential links between these variations and genetic diseases.
“You could be turning a blind eye to some of the richest sequence diversity that exists in the human population, and some of that sequence diversity that you’re not looking at could be correlated with disease in a way we’ve never been able to study before,” Miga told OneZero.
The work performed by researchers at the UCSC Genomics Institute could provide genetic scientists with a road map for producing complete sequences in other human chromosomes. This may lead genomic researchers to identify gene sequences that play significant roles in a variety of diseases and health conditions. In turn, this would give clinical laboratories new biomarkers for diagnosing disease and other chronic conditions in patients.