the excitement over the completed draft sequence of the human genome—
certainly a grand accomplishment—it's easy to forget that this is
just the prologue. Much about the genome remains a mystery. Which
parts of it are actual genes? What do individual genes do, and how
do they do it? (See the box below.) A small, four-footed mammal—the
mouse—is helping to answer these questions. By comparing the human
and mouse genomes piece by piece, bioresearchers such as Lawrence
Livermore's Lisa Stubbs are uncovering clues to genomic mysteries.
After the draft sequence
for the human genome was completed last June (see the box below
entitled "Tools of the Comparative Trade"), the Department
of Energy's Joint Genome Institute (JGI) turned to sequencing pieces
of mouse DNA that correspond to human chromosome 19. "We focused
on this particular human chromosome because the Laboratory has created
an extremely thorough gene map for it over many years of research,"
says Stubbs. "The sequence is not finished yet, but its working
draft is easier to read than the draft sequence of many other human
chromosomes. Because of the careful way the map was constructed,
we know the sizes of the gaps in the chromosome and the way the
pieces fit together."
Since last October, when
the mouse sequencing was completed, Stubbs and her team have been
analyzing the mouse and human DNA sequences, examining both similarities
and differences to discover what the sequences reveal about our
genes and our genetic evolution.
Comparing the two sets helps
the scientists track down genes—which are not always easy to spot—and
provides information about the nongene portions of DNA that make
up nearly 99 percent of our genome. Beyond that, having an understanding
of why and how mouse and human genomes are different provides critical
information to the bioscience and medical research communities.
Stubbs explains, "If we're going to use the mouse as a model for
the human, which everybody is doing, we'd better know how the two
species differ and try to answer questions such as: How often do
human and mouse contain the same genes? How similar are the genes?
Are there exceptions to the rule of similarity? We must know these
things on a gene-by-gene basis because while some genes are very
similar, others are not. Knowing all this will help us understand
whether it's right to use mice for drug testing and as disease or
drug models. And if it's not right, why not? Even the 'why nots'
reveal something about the human gene and how it works."
Out the Human Genome
February, the International Human Genome Sequencing
Consortium—of which the Department of Energy's Joint
Genome Institute (JGI) is a part—and the commercial
company Celera simultaneously published papers in the
scientific journals Nature and Science describing the
draft sequencing of the human genome. The initial analysis
of this draft sequence held a number of surprises. All
in all, there appears to be only about 30,000 genes,
equaling about 1 to 1.5 percent of the sequence. In
other words, in the nearly 2-meter-long strand of DNA
that appears in each and every cell of our bodies, about
15 centimeters of it contain genes. The number of genes
is about a third to a half of what most scientists had
believed would be the case. As Trevor Hawkins, JGI director,
noted, "It puts us humans at something like about twice
as many genes as your average fruit fly, which, I think,
is quite a humbling thought."
Most of the leftover
99 percent of our DNA appears to be junk, or at least
DNA whose functions remain unknown.
in the junk are long sequences similar to those found
in viruses and bacteria. These sequences appear to have
taken up residence in the genome as far back as 700
million years ago, when life was composed of a single
cell. "These sequences clearly have the structure of
viral DNA," explains bioresearcher Lisa Stubbs, "but
they've lost the ability to turn into a virus particle."
Human Genome Sequencing Consortium includes 20 groups
from the United States, the United Kingdom, Japan, France,
Germany, and China. Among those groups is the JGI, a
virtual institute that integrates the sequencing activities
of the human genome centers at Lawrence Livermore, Lawrence
Berkeley, and Los Alamos national laboratories. For
more information about the initial analysis and sequencing
of the human genome by the International Human Genome
Sequencing Consortium, see www.nature.com/genomics/human/.
Paramvir Dehal sits in front of a computer screen showing results
of a genome sequence analysis tool he developed (see the box
below for more details). Part of the results show a comparison
of chromosome 19 from the human genome and a related piece from
the mouse genome sequenced by Livermore bioresearchers. Only
about 7 percent of the two genomes appear related, and within
that 7 percent, only about 60 percent are genes. Comparative
sequencing helps researchers zoom in on the important, conserved
(unchanged) sections of genomes.
Shattered Genes, and a Twist
intriguing elements of the human genome came to light as a direct
result of this comparative genomics: the different sizes of some
related human and mouse regions and the composition of "junk" between
the genes. Two pieces of related DNA for mouse and human show more
or less the same genes in more or less the same order. But when
Stubbs and her team spread out the two sequences and laid them side
by side—the first time this has been done on a chromosome-wide scale—they
discovered that many human regions are significantly larger and
less compact than the mouse regions. So what's the filler in the
human sequence? Scientists refer to it as junk, but not just any
"For instance," Stubbs says,
"there is a particular kind of junk sequence called the Alu sequence.
It's a repetitive DNA sequence that, in the human, has made lots
and lots of copies of itself and has infected our DNA to a much
greater extent than anything we see in mouse. It's just one of many
DNA junk elements that make copies of themselves and litter the
human genome in the millions."
Repetitive sequences like
Alus are essentially DNA parasites. Their duplication generally
does not appear to have serious functional consequences, although
Alu copies that get inserted into genes have been shown to cause
human disease. Stubbs notes that this sort of litter is also seen
in mouse DNA. However, the Alu sequence invasion shows up more recently
in the evolution of DNA and appears to have occurred more dramatically
in the primate than the rodent lineages. Because mouse and human
evolution haven't been separated all that many years, the difference
in overall size and amount of junk is remarkable. "This is something
we wouldn't have seen if we hadn't been able to lay out the pieces
of sequence and compare them," she said. Why junk sequences happen
and what they mean remain to be seen.
of the Comparative Trade
such as the Joint Genome Institute (JGI) are extremely
proficient in sequencing DNA, turning a task that used
to be done painstakingly by hand a quarter century ago
into an industrial procedure. However, analysis of that
sequence—particularly comparing the sequence of two species—remains
in the domain of human interpretation. Livermore bioresearcher
Lisa Stubbs notes that there are computer programs to
help scientists align the DNA sections of interest and
to visualize similarities and differences. Computer algorithms
can also identify a piece of DNA as a probable gene. But
these tools are right only about 60 to 70 percent of the
time and require human confirmation. Among the few computer
tools available to help scientists visualize the differences
and similarities between the sequence of two species are
percent identity plots (PIPs) and dot plots.
The PIP, developed
by Webb Miller at Pennsylvania State University, is often
used to find genes and regulatory elements. A scientist
sends a file representing the bases of a piece of human
sequence to the computer, followed by the piece of mouse
DNA that corresponds to it. The program plots out the
matches within the sections, marking matches with a dot
and plotting them on a scale showing how similar the two
sections are. Scientists can look along a stretch of DNA
and quickly see that one piece is conserved—that is, hasn't
changed during evolution—and then there's another little
stretch of DNA that is somewhat less conserved and so
on. The PIP program allows them to see how far apart those
matches are. The program also can plot out positions of
repetitive elements and find stretches of DNA that are
rich in C and G bases. "We call these CpG islands," Stubbs
explains, "Often, for some reason we don't yet understand,
these islands are associated with control sequences. If
you find an area rich in CpGs in both human and mouse,
fairly close to a gene, it's a good candidate for a control
Dot plots are another
tool that can be used to plot mouse DNA against the related
piece of human DNA. In dot plots, the order of matching
sequences of human and mouse DNA can be compared. Where
the two aligned sequences match, a little mark is added
to the graph. "This helps us see how the genomes align,
where the similarities and differences in structure occur.
For example, dot plots help us pinpoint the spots where
the mouse chromosome has shattered, and half of it matches
chromosome 19 and half matches
human chromosome," Stubbs says. "It helps us find those
Stubbs notes that tools such as PIPs and dot plots are
slow and are better suited for looking at small pieces
of sequence. At the JGI, Paramvir Dehal, a bioinformaticist
and Ph.D. candidate in the Department of Genetics at the
University of California at Davis, is working with Stubbs,
computer scientist Art Kobayashi, and others to develop
tools for examining and analyzing larger pieces of sequence.
The tools they develop will be specifically designed as
aids for comparative genomics. One sequence analysis tool
being developed by Dehal uses a color code to show areas
of similarity among various types of sequence, whether
human, mouse, Drosophila (fruit fly), flatworm, yeast,
or expressed sequence tags. A yellow bar along the chromosome
map means the human DNA at that site has similarity to
DNA from another species or to a recognized, previously
studied human gene. Clicking on the bar brings up another
screen that shows details of the sequence matches at that
site and the degree of similarity between the matches,
which is indicated by its colors. Red means an almost
identical match; pink indicates a related sequence, but
not a perfect match; and green or blue indicates that
the matching sequence has few similarities to the human
Scientists can use this tool to find out which areas of
the sequence are conserved among species. Areas of conservation
usually indicate an important function, whether the area
is a gene, regulatory sequence, or something else. "A
pink match to Drosophila is truly significant because
flies and humans are so far removed from each other in
evolution. The likelihood is high that such a highly conserved
piece of DNA is coding for a protein," Stubbs notes.
The tool is also handy for hunting down regulatory or
control sequences. A piece of human sequence is a good
candidate for a regulatory sequence if it matches mouse
DNA, but not a cDNA sequence, and does not appear to be
encoding a protein. Experiments must be done to verify
the function of a conserved sequence because scientists
presently cannot really predict a piece of DNA's function
just by looking at its sequence. However, conservation
does tell them which sequences are important and points
them to the 1 to 5 percent of the genome they should focus
on, which is an important first step.
pip plot comparing the human APLP1 gene with its mouse
counterpart. A high degree of similarity is shown between
regions of human and mouse exons—the protein—coding DNA
sequence of a gene. The exons are indicated by the black
boxes at the top of the plot that are numbered from 1
to 16. The matches between human and mouse exons are marked
by dots or lines. They indicate similarity generally over
When the mouse and human
sequences are compared, other broad similarities and differences
quickly become apparent. Of the small percentage of the parts that
make up genes, about 85 percent appear to be the same in sequence
for both species. In addition, both mouse and human have basically
the same number of genes generating more or less the same kinds
of proteins. However, the genes lying on human chromosome 19 show
up on several different mouse chromosomes. It's as if someone shattered
the human chromosomes and rearranged blocks of 20 to 200 genes into
different orders to produce the mouse genome.
"This sort of rearrangement
happens in evolution," says Stubbs, "but when we look at the genomes
of other mammals that are just as far removed in evolution from
the human as the mouse—the cat, dog, or cow—their chromosomes are
much more similar to ours than the chromosomes from the rodent family.
So what drives the breakup of mouse chromosomes? There are several
theories, most concerning the short generation time and breeding
habits of rodents, but what it comes down to is, we don't know yet."
In another interesting twist,
when mouse and human genes were compared, quite a number of human-specific
and mouse-specific genes were found. These species-specific genes
are altogether a small fraction of our 30,000 genes, but still a
significant number, probably several hundred genome-wide. "We—and
nearly everyone else—expected to find a nearly one-to-one correspondence
between mouse and human genes," says Stubbs. "The species-specific
genes are of several different types, but the largest number of
them appear to make or express regulatory proteins that do the actual
business of turning genes on and off."
These proteins, continues
Stubbs, are probably not critical, meaning that gaining or losing
them will probably not result in disaster to the organism. Instead,
they probably are involved in fine-tuning traits. "These species-specific
genes are very likely to be a major source of subtle diversity and
keys to the subtle differences in gene expression between species,"
she says. Although the effects of changing a single gene are probably
small, the combined effects of hundreds of changes are likely to
human cell contains 23 pairs of chromosomes in its nucleus.
Each chromosome contains two tightly coiled strands
of DNA (deoxyribonucleic acid), with each DNA strand
composed of "base pairs" of chemical bases, normally
abbreviated A, C, T, and G (for adenine, cytosine, thymine,
and guanine). Scientists estimate that about
3 billion base characters comprise the human genome,
with about 1.5 percent of those characters forming genes.
Genes are special stretches of DNA that carry a code
for making proteins, which are critical to helping our
cells function. The process for making proteins is exact.
Each cell contains complex proteins called transcription
machinery. When it is time for a protein to be made,
these machines go into the nucleus, find the control
sequences that signal a particular gene to start, and
bind to them.
The transcription machinery then makes a mirror copy,
or transcript, of the gene's sequence, as indicated
by the control. The transcript, referred to as RNA (ribonucleic
acid), then moves out of the nucleus and into the cell's
cytoplasm where it encounters another biological machine,
the ribosome. The ribosome, using the RNA as a set of
instructions, assembles a protein from amino acids.
One way scientists
identify genes is to capture RNA sequences in the cytoplasm
and analyze them to determine which DNA sequences correspond
RNA sequences. These captured RNA sequences are called
complementary DNA (cDNA) sequences, and numerous collections
of cDNA sequence snippets, called expressed sequence
tags, are available in public databases. "A cDNA is
a copy of the gene," explains
bioresearcher Lisa Stubbs. "Bioscientists have
ways to take RNA out of the cells, 'reverse transcribe'
them into cDNA copies, clone them into bacteria, and
sequence them. From the reverse transcription, we
get a snapshot of the sequences in a particular cell
that are being turned on and turned into proteins
at a particular time."
Makes Humans Human?
Whether a gene resides on
chromosome 2 or 20 usually does not affect its function. (The main
exceptions to this rule are the genes on the sex-linked chromosomes
X and Y.) That being said, scientists have to question why, with
mice and humans having almost identical sets of 30,000 genes, they
aren't more alike. Part of the answer is that a 15 percent difference
in the sequence of a gene can change its function dramatically.
For example, many human genes that cause disease differ from their
normal counterparts by a single nucleotide. For most genes, this
nucleotide change would constitute less than a 0.1-percent sequence
change, but the result is a devastating functional difference.
Take the PEG3 gene, which
is shared by mouse and human. It plays an important role in embryonic
mouse development and an even more important role in mouse maternal
behavior. Research shows that when the PEG3 gene is removed from
mice, the mothers ignore their young to the point that their babies
die. A similar protein is expressed in the human brain, says Stubbs,
so the maternal caring function is probably conserved—unchanged
during evolution—to some extent. "However, the levels of expression
differ—the protein is expressed like gangbusters in the mouse brain,
not so highly in the human. Even more intriguing, it's highly expressed
in human ovaries and placentas, but not at all in mouse ovaries.
It seems likely that this gene has taken on a role in humans that
it isn't playing in mice."
Stubbs notes that many similar
mouse and human genes have differing behavior: activated in one
kind of tissue in mouse but not in human, or perhaps appearing in
the same tissue in both, but at different times or with different
intensities. "In other words, the same genes are not necessarily
regulated or controlled in the same way in both species. The dissimilarities
may be part of the answer as to why mice are mice and humans are
So what controls the on–off
switch in genes and the timing of gene expression? Here again, rodents
provide some clues. When researchers compare human and mouse sequences,
they find small sections that are similar between the species but
are not genes or junk such as Alus or other identifiable repetitive
elements. Stubbs explains, "We can look at a piece of sequence and
see that it isn't making part of a protein—so it isn't part of a
gene. These mystery pieces, like genes, stand out as conserved DNA
against a nearly 95-percent background of totally dissimilar sequence
and are good candidates for a control sequence." Researchers know
little about these types of sequences except that they are extremely
important, hard to detect, and have been conserved because their
sequence is linked to function. Many researchers are beginning to
explore control sequences now that there is a way to find them through
their conservation (because human and mouse genome sequences are
known). Gene regulation, Stubbs says, is turning out be one of the
most exciting areas of current research in the field.
human and mouse genomes are both similar and different. The
long arm of human chromosome 19 has a close counterpart in mouse
chromosome 7—the human and mouse versions of the same genes
(see middle column) are found in them in roughly the same order.
However, genes in human chromosome 19's short arm correspond
to mouse versions that are located in many different mouse chromosomes,
as indicated by the colored bars to the right, labeled by chromosome
Section by Section
more about control sequences and other regulatory elements in gene
expression is one of the next genomic frontiers. One technique used
by the biomedical research community is tissue-section analysis,
which is related to a standard hospital biopsy technique. The technique
involves slicing 10-micrometer-thick sections of tissue (about the
thickness of a single cell). It permits single cells to be viewed
in their native context using microscopy and standard pathological
Adopting this technique,
Stubbs and her team place thin slices of fetal or adult mouse tissue
on a slide and add a gene probe, which is a specific gene sequence
to which a fluorescent dye has been added. The probe binds to the
unique RNA sequence produced by the gene under study. (The RNA—ribonucleic
acid—is a mirror image of the DNA sequence of a gene and an intermediate
in the process of protein coding.) When the tissue is observed under
a microscope, the fluorescent probe can be seen binding to and highlighting
the cells in which the particular RNA has been expressed. This technique
of highlighting cells is called in situ hybridization.
Because a mouse fetus in
even the latest stages of development is only about l centimeter
long, its entirety can fit on a slide to give researchers a whole-
body picture of where a particular gene is expressed. Stubbs explains,
"Our pathologist Xiaochen Lu can look at a single specimen and tell
us what cells are activated and what the purpose of those cells
is. So if that gene is turned on in the heart, brain, and skin cells,
we'll see the fluorescence in all those areas, in the exact cells
that are activated. Finding out the exact cell type is important,
because two cells that carry out the same function—say, secretion—
may be more similar to each other than two different adjacent cells
in the same tissue. For example, when we want to know what a gene
does, it is much more important to know that the gene is expressed
in a Purkinje cell, which helps regulate movement, than to know
it's expressed somewhere in the thousands of different cell types
that make up the brain."
slices of mouse tissue are placed on a slide, and a gene probe—a
specific gene sequence with a fluorescent dye—is added. When
the tissue is observed under a microscope, the fluorescent probe
can be seen binding to and highlighting the unique gene sequence
being studied. Here, this highlighting is shown for gene sequences
in the cerebellum, cerebral cortex, epidermis, and pancreas.
One gene that was examined
in this manner turns out to be activated in only a small section
of mouse sequence from a family line extensively studied by Stubbs,
where the mice are prone to both deafness and stomach cancer. "What
we found out about this gene through section in situ hybridization
makes perfect sense to us," says Stubbs. "The gene expresses a protein
that protects the epithelial cells lining the insides of body cavities,
for example, the stomach. The cells lining the inner ear are also
delicate and may require the same kind of protection. We theorize
that this same protein performs a similar protective function inside
the ear. We haven't proved it, but we think that's why our mice
are deaf and have stomach cancer."
Because a single specimen
provides 1,000 tissue slices, it can be used to test many genes.
Stubbs and her team can create a probe of any gene found on the
sequence—whether its purpose is known or unknown—and pinpoint where
it is expressed, down to type and location of a single cell, in
Elsewhere in the comparative
genomics community, researchers are focusing on using microarrays
to rapidly discover what genes express in tissues or tissue regions
and to examine many genes in parallel. However, microarrays do not
provide information about the type and location of a cell within
a tissue that is expressing a gene or what that cell's context is
in the living tissue. "With tissue-section-based techniques, we
see exactly where a gene is turned on and can correlate it with
the knowledge that pathologists have about what that particular
cell does. We can also begin to correlate the state of the gene—its
expression patterns in specific types of cells—with its regulatory
sequences. This is completely unknown territory."
Stubbs and her team are working
to industrialize this process. (See the box above entitled "Tools of the Comparative Trade.") With so many
genes to look at, they need to generate a huge amount of information
about gene expression to make generalizations about the genes and
their regulatory controls. The team is now going through the sequence,
looking and testing for candidate versions of these control sequences.
"We're beginning to develop some testing techniques that will help
us here. Ultimately, we want to go through the chromosome, find
these control elements, prove that they are control elements, and
then try to correlate expression patterns among them."
of the members of the mouse genomics group are, from left, Laura
Chittenden, Xiaojia Ren, Lisa Stubbs (team leader), Xiaochen
Lu, Paramvir Dehal, and Joomyeong Kim.
If nothing else, all the
questions and possibilities just show that, even with the progress
scientists have made in piecing together the story of life embedded
in the DNA code, complete understanding still eludes them. "The
human sequence means absolutely nothing when viewed by itself,"
notes Stubbs. "We can do very little with it. We can find some of
the genes from the expressed sequences we already know about. But
we can't read it. We can't figure out where the important sequences
are; we miss a lot of the genes; we miss all of the control sequences.
What comparative sequence analysis allows us to do is to 'light
up' the functional parts of the sequence. If a piece of DNA has
an important function, evolution won't let it change. That's the
important message in all this. But if we can't find the piece that
is doing something important, we won't get very far in our understanding."
does this matter? Consider the gene tied to muscular dystrophy.
When the gene is removed from the mouse, the mouse survives. It's
a bit uncoordinated, Stubbs says, but it can move around, get on
with its life, and reproduce. But when the gene is missing or malfunctioning
in humans, the result is a disease of devastating proportions. "Obviously,
this gene is much more important to humans than to mice," says Stubbs.
"And looking at the differences between the genes and the proteins
and how they are regulated in mouse and human will help us understand
what part of the human protein is most important. Now we'll be able
to do the same sort of analysis for an entire chromosome, thanks
to the mouse."
chromosome 19, comparative genetics, DNA, Human Genome Project (HGP),
gene expression, Joint Genome Institute (JGI), mouse genome, PEG3,
sequencing, section in situ hybridization.
For For further
information contact Lisa Stubbs (925) 422-8473 (email@example.com).
information about DOE-funded genetic research, see these Web sites:
received a B.S. in biology from the University of Puget Sound
in Tacoma, Washington, and a Ph.D. in biology from the University
of California (UC) at San Diego. She joined Lawrence Livermore
in 1997 as a senior staff scientist in the Genomics and Bioengineering
Research Division and in the DOE Joint Genome Institute, where
Livermore is one of three collaborating national laboratories.
She is currently also acting director of the Genomics and Bioengineering
Stubbs leads a team studying mouse genomics, specifically the
comparative analysis of structure, function, and evolution of
genes in related mouse and human chromosome regions. Her research
interests include the generation, biological characterization,
and molecular mapping of mouse mutants that provide useful models
for studying acquired and inherited human diseases. Stubbs has
published over 60 papers in professional journals and is on
the editorial board of Mutation Research Genomics. She serves
on several scientific committees, including the UC Davis Cancer
Center Internal Advisory Board, the DOE Biology and Environmental
Research Advisory Committee, and the National Institutes of
Health Human Genome Study Section.