August 10, 2001
Researchers Discover New Route to High Blood Pressure
A composite confocal image of cells of the
distal convoluted tubule of the kidney is shown
stained with antibodies to WNK4 (red) and the tight
junction protein ZO-1 (green). The two proteins
co-localize, establishing WNK4 as a component of the
tight junction.
After years of detailed genetic analysis, researchers have
discovered two genes that underlie a new metabolic pathway that governs
blood pressure in humans. These findings could offer novel molecular
targets for new blood pressure medicines.
High blood pressure affects about one-quarter of all adults
worldwide and is an important risk factor for death from stroke, heart
disease, and congestive heart and kidney failure. In an article
published in the August 10, 2001, issue of the journal Science,
an international research team led by Howard Hughes Medical Institute
investigator Richard
P. Lifton at Yale University School of Medicine reported
identifying two genes that cause pseudohypoaldosteronism type II
(PHAII). This disorder leads to hypertension by causing increased
reabsorption of salt by the kidneys and impaired secretion of potassium
and hydrogen ions.
Although the disorder seemed to point to an unknown cause of
hypertension, said Lifton, tracing its genetic roots in affected
families proved difficult. "In contrast to the other single-gene forms
of high blood pressure we have studied, PHAII was complicated," he
said. "Patients with the disorder get hypertension as adults — rather
than as children — like the majority of people with hypertension, and
the abnormal potassium and acidity levels are variable. This
complicated unraveling the genetics."
After attempting to trace the genetics of the disease in numerous
affected families, Lifton and his colleagues identified two types of
families — one with a gene mutation on chromosome 12, and the other
with a mutation on chromosome 17. This provided researchers with the
information they needed to begin to zero in on the genomic location of
the mutated genes.
In analyzing genetic data from a family with a mutation on
chromosome 12, the researchers determined that the disorder appeared to
be due to a deletion of a segment of DNA in a large region of the
chromosome. Fortunately, said Lifton, analysis by Frederick H. Wilson
at Yale, the lead author of the Science article, yielded a
critical clue that helped the researchers pinpoint the gene.
"One of the genetic markers he was using was completely absent in
chromosome 12 of the disease gene, but present in normal genes," said
Lifton. "That was an incredibly fortunate stroke of luck, without which
we would still be searching."
A search of a human genome database revealed that the
disease-causing deletion lay within a gene called WNK1, which is
expressed at high levels in the kidney, heart and skeletal muscle. The
WNK1 gene codes for a type of enzyme, called a serine-threonine
kinase, that often acts as a metabolic activating switch in cells.
WNK1’s role in PHAII was confirmed when the researchers
discovered an overlapping but different WNK1 deletion in members
of another family with chromosome 12-related PHAII. To understand how
the deletion might cause PHAII, the scientists next studied the
function of the mutated WNK1 gene in affected family members.
They found that the mutated gene was expressed at a five-fold higher
level in affected family members.
"Thus, we believe that PHAII in these families is caused by an
overexpression of WNK1," said Lifton. The discovery of the
WNK1 gene defect gave the scientists a vital clue to the
possible identity of the disease-causing gene in families whose
affected members had a gene mutation on chromosome 17.
"Although we had mapped the gene for PHAII to chromosome 17 in those
families, we had not been able to narrow down the segment containing
the gene," said Lifton. "So we were still swimming in a very large sea
of about fifteen million base pairs of genomic DNA."
By searching the human genome database for WNK1-related genes
on chromosome 17, the scientists found one called WNK4 that was
right in the middle of the region containing the PHAII gene. After
screening affected families for mutations in WNK4, the
scientists found four families whose affected members showed different
but closely related "missense" mutations in WNK4. The scientists
theorized that the WNK4 mutations also increased the activity of
the gene or its enzyme, said Lifton.
Using antibody markers, the researchers traced the localization of
both the WNK1 and WNK4 enzymes in the kidney. They found that both
enzymes appeared in regions of the kidney involved in regulating the
reabsorption or secretion of salt, potassium and hydrogen ions. While
the WNK1 enzyme appeared in the cytoplasm within kidney cells, WNK4
appeared in "tight junctions" — interfaces between cells that are
thought to be important in regulating the passage of ions, such as
chloride, in the kidney and other tissues. According to Lifton, the
localization studies suggest that the enzymes might be important in a
regulatory pathway by which the kidney "decides" about reabsorption of
sodium, chloride, potassium and hydrogen ions. Overactivity of the
enzymes could increase reabsorption, expand blood volume and raise
blood pressure.
Further exploration of the new blood-pressure-regulating pathway
revealed by the WNK1 and WNK4 mutations could lead to new
anti-hypertensive drugs, said Lifton. Intriguingly, he said, the
WNK4 gene maps to the same region as the gene linked to blood
pressure regulation in the long-term Framingham Heart Study, which has
followed the health of a large group of people over many decades.
"Since this is a new pathway proven to affect blood pressure,
antagonists of this pathway might prove to be useful new
medications,” Lifton said.
Image: Keith Choate and Richard P. Lifton
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