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Why are somatic mutations in the adrenal gland a common cause of human
hypertension?
Supervisor: Professor Morris Brown, Clinical Pharmacology Unit.
Project code for applications: SCM041
Background:
Hypertension is frequently due to aldosterone-producing adenomas (APA) of the adrenal. Most
APAs have appeared to arise not in the physiological site of aldosterone production, the zona
glomerulosa (ZG), but in the adjacent zona fasciculata (ZF). This paradox was resolved with our
discovery that APAs of the ZG do indeed occur, but are usually much smaller than ZF-APAs, and
easily missed on CT scans.1 However, our finding of frequent, specific somatic mutations in ZG,
distinct from those causing ZF-APAs, raises the further puzzle of how mutation can commonly
arise in a non-proliferative tissue, and be selected in small, non-proliferative tumours.2 We have
found a probable solution in the transcriptome of the human adrenal, together with
immunohistochemical and functional analysis of key genes, upregulated in ZG vs ZF. Among these
were the stem-cell marker and Wnt-signalling gene, LGR5 (>50-fold), DACH1 (23-fold), and
several other Wnt signalling genes, including the LGR5-ligand, RSPO3. Transfection and silencing
of LGR5 or DACH1, in human adrenal cells, and responses to RSPO3, show each of these to
regulate aldosterone production, in part through regulation of cell turnover. ZG of human
adrenal shows significant positivity for markers of apoptosis and proliferation. These findings are
all unique to human adrenal.
Hypothesis:
Because [i] deletion of the gene encoding aldosterone synthesis (CYP11B2) in mice, or treatment
of other mammals with a CYP11B2 inhibitor, leads to proliferation, migration and apoptosis of ZG
cells, and [ii] salt excess suppresses aldosterone production, we have developed the hypothesis
that: ZG cells are driven by Wnt to switch between synthetic and ‘proliferative’ modes; that, high
salt intake by humans generally suppresses aldosterone production, hence favouring
proliferation etc; and the proliferation permits frequent mutations, with those leading to
constitutive aldosterone production being selected because protective against apoptosis. The
hypothesis would explain why human ZG looks so much thinner, with only patchy CYP11B2
expression, compared to other species; and why several genes which are upregulated many-fold
in normal ZG, but not in ZG-like APAs, have a role in suppressing aldosterone production, in part
by directing the cells into either apoptosis or migration to the inner adrenal zones. The
hypothesis would also explain why ZG-like APAs are so small, namely the cells have a survival
advantage not through proliferation, but by switching on genes that maintain constitutive
aldosterone production.
Project and techniques:
The PhD student will be part of a vibrant group studying genes expressed in the human adrenal,
which are of interest either because frequently mutated, or because upregulated in ZG. Most of
the upregulated genes have yet to be studied. The aim is to test our hypothesis above, and to
explain why APAs are one of the commonest tumours in Medicine. Several of the findings are
being introduced into the routine diagnosis of APAs at an earlier stage e.g. through detection of
somatic mutations in adrenal vein blood of patients with an adrenal adenoma on CT or MR scan,
or of upregulated genes in ultrasound-guided aspirations from adenomas. The student will have
the opportunity to be part of this clinical innovation, while learning techniques that include: highthroughput DNA sequencing, RNA analyses, immunohistochemistry, 2- and 3-D cell culture,
transfection, silencing RNA, radioimmunoassay. Elena Azizan undertook the studies cited below
during her PhD, and won Young Investigator prizes at the International Society of Hypertension,
and American Endocrine Society.
1.
Azizan EA, Lam BY, Newhouse SJ, et al. Microarray, qPCR, and KCNJ5 sequencing of
aldosterone-producing adenomas reveal differences in genotype and phenotype between zona
glomerulosa- and zona fasciculata-like tumors. J Clin Endocrinol Metab. 2012; 97(5): E819-29.
2.
Azizan EA, Poulsen H, Tuluc P, et al. Somatic mutations in ATP1A1 and CACNA1D underlie
a common subtype of adrenal hypertension. Nat Genet. 2013; 45(9): 1055-60.