PRESS RELEASE
Parasite or partner? Study suggests new role for junk DNA
Public release date: 12-May-2002
ANN ARBOR, Mich. Junk DNA is the Rodney Dangerfield of the
genetics world. It makes up nearly half of all human DNA, but many
scientists dismiss it as useless gibberish. A new study published online
today from the June 2002 issue of Nature Genetics, however, suggests
that segments of junk DNA called LINE-1 elements deserve more respect.
Conducted by scientists from the University of Michigan Medical
School and Louisiana State University, the study is the first to show in
mammalian cells that some human LINE-1, or L1, elements can jump to
chromosomes with broken strands of DNA, slip into the break and repair
the damage.
Transposable L1 elements make up 17 percent of our DNA, but very
little is known about them, says John V. Moran, Ph.D., an assistant
professor of human genetics and internal medicine in the U-M Medical
School, who developed the first assay to identify mobile L1s in the
human and mouse genomes. Until now, everyone thought L1s were just
intracellular parasites in our DNA leftovers from the distant
evolutionary past. The big question in the field is: Are they still there
because we canąt get rid of them or do they have a function?
L1 s reproduce by using RNA and a process called reverse
transcription to make complementary DNA copies of themselves, which
can jump into other DNA sequences. Normally, L1s use an enzyme called
endonuclease to cut the genetic DNA and create a space, so they can
plug themselves into the genome.
We knew about the endonuclease pathway, says Tammy A. Morrish, a
U-M graduate student in human genetics and first author of the paper.
But we didnąt know there was another mechanism that didnąt require
endonuclease, or that L1s could jump into existing breaks in DNA.
Morrish tested human L1 s ability to repair DNA breaks in several normal
and DNA-repair mutant cell lines derived from Chinese hamster ovary
cells. Other researchers had demonstrated the ability of human L1s to
repair DNA breaks in yeast cells, but Morrish is the first to show the
effect can occur in mammalian cells.
Since DNA damage may lead to cell death unless it is repaired, the
existence of an alternate repair pathway could be a good thing for the
host cell. The question is, whatąs in it for the L1?
This study brings up the question of whether L1s are just taking
advantage of DNA breaks to plug themselves into these sites or are they
are being used by the host cell to mediate the repair, says Moran.
From the L1 s point of view, this gives it an alternate way of integrating
into the DNA.
Because L1s are so ancient and because they sometimes carry segments
of genes with them when they jump to a new location, Moran believes
they have played an important role in human evolution by increasing
genetic diversity. He is one of only a few scientists to study L1s in the
human genome.
We have more transposable L1s in the human genome than any other
species, but we know the least about where and how they move in
humans, says Moran. We are here today either because of, or in spite
of, L1s.
In future research, Moran s research team will examine whether it is
possible to direct L1s to repair specific breaks in DNA, whether L1s can
be used as vectors to deliver genetic material to specific DNA locations,
and the impact of an L1 insertion on genes.
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U-M researchers in the study were supported by the William M. Keck Foundation and the National Institutes of Health. Nicholas Gilbert, Ph.D.,
a U-M post-doctoral fellow in human genetics, collaborated in the study. Other collaborators included Mark A. Batzer, Jeremy S. Myers, and
Bethaney J. Vincent from Louisiana State University; Thomas D. Stamato from Lankenau Institute for Medical Research in Wynnewood, Penn.;
and Guillermo E. Taccioli from Boston University s School of Medicine.
DNA repair mediated by endonuclease-independent LINE-1 retrotransposition Tammy A. Morrish, Nicolas Gilbert, Jeremy S. Myers, Bethaney J.
Vincent, Thomas D. Stamato, Guillermo E. Taccioli, Mark A. Batzer & John V. Moran.
Nature Genetics: June 2002, 31:2, 159-165.
Long interspersed elements (LINE-1s) are abundant retrotransposons in mammalian genomes that probably retrotranspose by target site-primed
reverse transcription (TPRT)1, 2. During TPRT, the LINE-1 endonuclease cleaves genomic DNA3, freeing a 3' hydroxyl that serves as a primer for
reverse transcription of LINE-1 RNA by LINE-1 reverse transcriptase. The nascent LINE-1 cDNA joins to genomic DNA, generating LINE-1
structural hallmarks such as frequent 5' truncations, a 3' poly(A)+ tail and
variable-length target site duplications (TSDs)2. Here we describe a
pathway for LINE-1 retrotransposition in Chinese hamster ovary (CHO) cells that acts independently of endonuclease but is dependent upon
reverse transcriptase. We show that endonuclease-independent LINE-1 retrotransposition occurs at near-wildtype levels in two mutant cell lines
that are deficient in nonhomologous end-joining (NHEJ). Analysis of the pre- and post-integration sites revealed that endonuclease-independent
retrotransposition results in unusual structures because the LINE-1s integrate at atypical target sequences, are truncated predominantly at
their 3' ends and lack TSDs. Moreover, two of nine endonuclease- independent retrotranspositions contained cDNA fragments at their 3'
ends that are probably derived from the reverse transcription of endogenous mRNA. Thus, our results suggest that LINE-1s can
integrate into DNA lesions, resulting in retrotransposon-mediated DNA repair in mammalian cells.
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