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Decoding the genetics of
complement factor H
Drs. Dixon and Frazer-Abel’s quest to decode the practical
functions of complement factor H won’t be easy, says
microbiologist Alisa Gaskell, PhD, Scientific Director of the
Molecular Genetics Lab at Children’s Colorado. In fact, in many
ways, it will require radically revising the standard practice of
gene sequencing.
“First generation sequencing was akin to reading long stretches of
DNA kind of like you read a book,” she says. “We’ve since developed
new technologies where they chop the book into single words, then
we amplify those words, and a computer program rearranges them
back into order and gives a report — basically the Cliff’s Notes.”
It’s a much faster method. The Human Genome Project took just
under 13 years to complete. The current record, completed at The
Institute for Genomic Medicine in San Diego (see “Every Letter of
the Genome,” p. 31), is 19 hours.
But the current method is a problem when analyzing genes like
those of complement factor H, which closely resembles five other
genes. Sequestering and ferreting out the meaning of tiny changes
that lead to protein malfunctions depends on context.
The conventional method can’t carry that kind of context.
“We’re looking at it differently,” says Dr. Gaskell. “We’re saying,
‘How can we, within these constraints, leave sort of cookie
crumbs that give us clues to the sequence. We’re basically
breaking with doctrine.”
Essentially, Dr. Gaskell and her team are changing the way they
break up the DNA, a method that involves overlapping longer
strands of code.
On the wet lab side, where the DNA is extracted, that means using
more mature enzymes that will read a longer stretch of code
without error. On the dry lab side, where the DNA is processed, it’s
writing new algorithms that can not only arrange those stretches,
but accurately flag where something might have gone wrong.
“It’s reengineering both those components in a unified way that
will get us to the point where we’re only looking at words that come
from this gene and nothing else,” says Dr. Gaskell. “It will take us
a few cycles, but once we get to where we can resolve a sentence,
we can start pressing clinical samples through and doing blinded
studies that compare what we find to clinical results. Then we’ll see
if we have a system that works.”
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The right (but risky) choice
There was only one real way to find out: an
exacting version of a test called CH50. It’s the most
sensitive measure of complement — and the one
most difficult to perform. Because of the difficulty,
it can take weeks to get results. Only a handful of
labs in the country offer it.
One of them belongs to Dr. Frazer-Abel: Exsera
BioLabs, just across the street from Children’s
Colorado on the Anschutz Medical Campus.
“We’re friends with them,” says Dr. Goebel.
“We can draw blood in the morning and get a
result by sunset.”
The result proved what Drs. Dixon and Goebel
suspected: complement wasn’t blocked. The
trial drug wasn’t enough to suppress Lucy’s
complement system through the month, and
because the drug was experimental, there was no
way to know if they could safely adjust the amount
or frequency of the dose.
They also knew that a drug already on the market
called eculizumab, though it would require more
frequent infusions, would do the job.
“The recommended dosing of these complement-
blocking agents is, in Dr. Dixon’s and my
experience, not sufficient for children with rare
variants of this disease,” Dr. Goebel says. “We’ve
seen it repeatedly and often enough that we don’t
blink anymore when we say we need to treat this
child more aggressively.”
“At the end of the day, we pulled her out of the
study,” says Dr. Dixon.
It took courage and a sure hand — but it
worked. On a high dose of eculizumab every two
weeks, Lucy’s condition stabilized. A genetic
test eventually confirmed the diagnosis. With
vaccinations against meningococcus and
prophylactic antibiotics, along with several blood
pressure medications, Lucy lives the life of a
relatively normal 2-year-old.
“We have about an acre of property,” Brad
remarks, watching Lucy zip around it on a strider.
“She knows every inch of it.”
He and Cynthia, both of whom work as ski
instructors for the National Sports Center for the
Disabled, plan to take her down the bunny hills in a
backpack this year. They’ll wait until she’s 3 to put
her on skis.
“She’s doing great. She really is,” says Cynthia.
“She’s at home with me four days a week and goes
to day care one day. She likes the social part, but
I guess we’re still a bit terrified of the infection
piece. Maybe a little bit further down the track, we
could push it to two or three days. But right now
one day a week is as much as I can put up with.”
It’s not perfect, but it’s much better than the
alternative. It wasn’t so long ago, after all, that
aHUS wasn’t often diagnosed. Kids went through
fruitless kidney transplants, sometimes several,
to no avail. For now, Lucy’s team team is working
with a health center near Winter Park to try to
move Lucy’s bi-weekly infusions closer to home.
As it stands, that’s the best and only option,
perhaps for the rest of Lucy’s life. But there’s
hope the work of complement researchers like
We don’t blink anymore
when we say we need to treat
this child more aggressively.”
JENS GOEBEL, MD
Chief, Pediatric Nephrology
Drs. Dixon and Frazer-Abel may change that one
day. Currently, they’re working with the Molecular
Genetics Lab at Children’s Colorado to understand
the functional consequences of mutations in
complement factor H, the group of genes that
governs complement’s braking system.
“That’s the final piece we need for our existing
arsenal,” says Dr. Frazer-Abel. “We’re realistically
only five or six months out.” In the meantime, Brad
and Cynthia are taking it a day at a time.
“We’re stronger just knowing there’s a treatment,”
says Cynthia. “Lucy’s team is amazing. We trust
them with her life completely.” ●
Bursting blood cells: how CH50 works
A few tests out there measure complement system
activity, but none are as sensitive or accurate
as CH50. It’s also the most difficult to perform,
because it involves living cells.
“Basically you take sheep red blood cells and
decorate them with antibodies that prime them
for killing by complement,” says Dr. Dixon. “Then
you dump in patient serum and if there’s active
complement, it will punch holes in these cells.
That releases hemoglobin, which is a pigment,
so then you measure the color.”
bacterial meningitis, say — CH50 has been around
for nearly 100 years. But it’s not easy to pull off.
“Since they’re live cells, they’re more complex
to work with than a purified reagent you just
pull off the shelf,” says Dr. Frazer-Abel. “It’s also
more realistic, because our bodies are made of
cells. A sheep can get sick. You have to be able to
control for that. So it takes a lab that knows what
they’re doing.”
“You can actually see it,” adds Dr. Frazer-Abel. It’s also far from the only complement test Exsera
— one of the only labs in the nation dedicated
exclusively to complement — performs.
Historically used to measure complement in the
opposite direction — in the case of recurrent “There are 56 proteins in the system,” Dr. Frazer-
Abel says. “We can measure almost all of them.”
NEW CONSTELLATIONS
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