At Last, A Complete Human Genome
Plus: CRISPR’d cat cells & a supercharged prime editor to swap DNA bases.
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Hello. A complete human genome sequence adds a full chromosome’s worth of data to prior versions. An allergy-causing gene in cats is modified by CRISPR. And a tweak to prime editing boosts its efficiency by 353x, on average.
The Human Genome Sequence is Complete
About 8 percent of the human genome was “missing” from prior sequences. That’s about 200 million base pairs of DNA or roughly the length of an entire chromosome. Those gaps have now been filled in.
For a set of papers in this week’s issue of Science, DNA was extracted from CHM13hTERT human cells, first taken from a woman who conceived at Magee-Women’s Hospital in Pittsburgh, Pennsylvania. This cell line is homozygous, which means that both copies of its various chromosomes are the same, and thus easier to sequence. These cells do not carry a Y chromosome.
The completed genome sequence contains 3.055 billion base pairs of DNA and adds more than 1900 predicted genes to the prior human reference genome. About 100 of these ‘new’ genes are likely to encode proteins.
The new studies were spearheaded by the Telomere-to-Telomere (T2T) consortium, an open-source community co-chaired by Adam Phillippy at the National Institutes of Health and Karen Miga at the University of California, Santa Cruz.
Members of the consortia spent years filling in the genome sequence; mainly, these missing pieces are highly repetitive sequences or transposable elements that are difficult to sequence using current technologies, or else are difficult to ‘align’ using computational tools. The researchers used a mixture of technologies to complete the work, including sequencing machines from PacBio, Oxford Nanopore, and 10X Genomics.
Read more at Science.
Cats have a protein, called Fel d 1, that is present in their saliva and causes allergies in about 15 percent of people. For a new study, researchers used the CRISPR gene-editing tool to knock out the genes encoding Fel d 1 in cells taken from cats — but not in living, breathing animals.
Disrupting the genes did not cause the cells to die, which means those genes are likely nonessential for cats and could be a target for people who suffer from cat allergies in the future.
Read more at The CRISPR Journal.
Prime editors are modified proteins that can substitute bases in DNA (like swap a C for a T). They are not very efficient, though. For a new study, researchers modified the RNA polymer that guides a prime editor to a specific site in the genome. With the new guide, base-editing efficiency was increased by a maximum of nearly 5000-fold, or by an average of 353-fold.
Read more at Nature Communications.