October 27, 1998
Mutant Zebrafish Provide Clues About Human Anemia
The names sauternes, chardonnay, chianti, weissherbst, and zinfandel
usually connote varieties of fine wine. For HHMI researcher Leonard
Zon, however, these names describe mutant zebrafish that represent
one of the first fish models of human disease.
Like wine, these fish come in two varieties: red and white. The
colors refer not to the shade of the fish's skin, which is transparent,
but rather to the color of their blood cells. "The sauternes
mutation gives the fish pale blood cells, so they're a white wine,"
says Zon, an HHMI investigator at Harvard Medical School and Children's
Hospital in Boston.
“Soon every major institute and school will have a zebrafish project.”
Leonard I. Zon
Zon isn't breeding fish to complement wines at the dinner
table—heis using them to investigate the genetics of red blood
cell diseases. In the November 1998 issue of the journal Nature
Genetics, Zon's research group reports that they've cloned a gene
that when mutated causes zebrafish to develop a disease that mimics
congenital sideroblastic anemia (CSA) in humans. In the same issue of
Nature Genetics another research group unveils a zebrafish model
of another form of congenital anemia. These reports describe the first
fish models of any human disease.
Zebrafish with the sauternes mutation develop a disease
identical to CSA, a condition that reduces the number of red blood
cells and lowers levels of hemoglobin in both fish and humans. Symptoms
of CSA include fatigue, dizziness, and weakness. "CSA is a rare
disease, but it can be very severe," says Zon.
In humans, CSA arises from a mutation in the gene alas2,
which codes for an enzyme that helps produce heme, the oxygen-carrying
component of hemoglobin. People who have the alas2 mutation
don't produce functional heme, which leads to hemoglobin deficiency.
Sauternes mutants also have reduced levels of the protein
beta-globin, another component of hemoglobin.
Fish may seem an unlikely model for human disease, but Zon had
compelling reasons to study CSA in fish. "The amazing thing about the
zebrafish is that they're completely transparent, so you can see the
circulating blood cells," Zon says. Even though adult zebrafish are
only one inch long, researchers can tell whether the fish's blood cells
are making hemoglobin by examining the live fish under a microscope.
Blood cells with a red hue carry hemoglobin, while pale blood cells
lack the crimson molecule.
Zon's research group nailed down the fish equivalent of the
alas2 gene using a technique called positional cloning. First
they mated sauternes mutants with a strain of zebrafish known to
have high genetic variability. By tracking swatches of DNA shared by
progeny carrying the sauternes mutation, the researchers mapped
the gene to chromosome 8 in the zebrafish. Next, the scientists
identified genetic markers that flanked the sauternes mutation
and used a medley of genetic techniques to "walk" along the chromosome
from the markers to the gene.
Like the human CSA gene, the sauternes gene encodes the
enzyme ALAS2. Many researchers had hypothesized that if a mutation
prevented proper expression of ALAS2, a similar enzyme called ALAS1
would be ramped up to compensate. But Zon's experiments showed that
ALAS1 levels weren't boosted in the sauternes mutants. "That
really threw out one of the models," Zon says.
Further experiments revealed that a transcription factor called
GATA-1, normally abundant in early embryos and diminishing as blood
cells begin to form, remains high in the sauternes mutants.
"There's something wrong with the way these cells are maturing, and it
may be that regulation of this GATA-1 is heme dependent," says Zon, who
hopes future genetic studies on zebrafish will clarify the mechanisms
behind the disease.
A few years ago, zebrafish were considered useful only in the realm
of developmental studies. Now the future of zebrafish genetics studies
looks bright. "When I first started working on the fish, there were
concerns about whether this would be a relevant model system to study,"
says Zon. "By finding an instance where the fish and the human have the
same disease, we've illuminated the relevance," he says.
While a lack of infrastructure had stalled zebrafish genetics
studies, a recent $5 million grant from the National Institutes of
Health to fund the Zebrafish Genome Initiative should encourage more
work in the field. "I really think more people are going to jump in,"
says Zon. "Soon every major institute and school will have a zebrafish
project," he predicts.
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