Background:
Pregnancy-induced hypertension (PIH) complicates up to 8% of pregnancies with development
of high blood pressure usually after the 20th week of gestation. PIH is subclassified as gestational
hypertension when the hypertension occurs alone, or preeclampsia when it is accompanied by
proteinuria; both conditions exist along a continuum of hypertensive diseases potentially arising
during pregnancy, most likely caused by placental stress and/or inadequate blood flow from an
inadeqaute connection to the maternal uterus. In addition to reduced estrogen and increased
circulating androgen levels, changes in angiogenic and other metabolic factors have been
reported in women with PIH. Most relevant to our current hypothesis, however, is the wellestablished association between PIH and low circulating levels of insulin-like growth factor-1
(IGF-1) possibly due to an increase in protein-bound IGF-1, resulting in a reduction in
systemically available free IGF-1 activity during this critical phase of breast tissue maturation.
A 2002 study of over 900,000 Norwegian women revealed that a history of PIH may reduce the
subsequent risk of developing breast cancer, and a decade later it was shown that breast cancer
risk is significantly reduced in these participants both in women with gestational hypertension
(Hazard Ratio = 0.78) and preeclampsia (Hazard Ratio = 0.86), relative to women with children
that have not had any history of PIH. Most other epidemiologic studies including several metaanalyses have confirmed the breast cancer protective benefit of PIH. However, not all cohort
studies have confirmed this association, although we now have evidence suggesting that there
may be traits (or genotypes) variably present within a population that can offset the breast cancer
protective benefit of PIH. Importantly, the mechanistic basis for the breast tissue protective
benefit of PIH has remained obscure until now despite the fact that PIH-associated hormonal
changes have been implicated. Two relevant insights recently emerged from the Marin Women’s
Study (MWS), first with its finding that a history of PIH during first pregnancy strongly
associates with later life reduction in mammographic breast density, an established surrogate for
reduced breast cancer risk. In addition, this MWS study showed that only women inheriting the
more common T allele (occurring as either the GT or TT genotype) within the IGF1R Single
Nucleotide Polymorphism (SNP), rs2016347, showed reduced mammographic density with their
history of PIH, while parous women inheriting both G alleles of this IGF1R SNP showed no PIH
reduction in mammographic density, suggesting that PIH might not provide any breast cancer
risk protection for the 23% of all women who carry this GG genotype. Of note, one previous
study looking at various polymorphisms in the Insulin-like growth factor pathway (IGF-1, IGF-2,
IGFBP3, IGF1R, IGF2R) in relation to mammographic density by itself, found significant
associations between 3 different IGF1R SNPs, including rs2016347, with both the percent and
absolute levels of mammographic density. While experimentally and clinically unproven, the
IGF1R polymorphism at rs2016347 is believed to be functional since it encodes a sequence
variation within the 3’UTR of the IGF1R gene (3129T>G) potentially affecting transcript
stability and expression levels of the encoded IGF1R protein (http://compbio.cs.queensu.ca/FSNP/). Coincidentally, this same IGF1R polymorphism was recently found associated with
hormone receptor (HR)-positive breast cancer aggressiveness, predicting earlier disease
recurrence and patient death despite adjuvant tamoxifen treatment (outcome Hazard Ratio = 2.01
for the GG genotype relative to either the GT or TT genotypes), independent of other
clinicopathological determinants and adjusted for multiple comparisons. This same IGF1R SNP
(rs2016347) has also been linked to more aggressive forms of colorectal cancer and to an
increased risk of developing brain tumors, indicating that its functional consequences extend
beyond breast cancer.
IGF-1 is a well-studied growth hormone that has been pathogenically implicated in both PIH and
breast tumorigenesis, although the IGF growth factor axis has not been previously studied as a
common mechanistic link between PIH, mammographic density and breast cancer development.
Several studies have found that circulating levels of free IGF-1 in serum and cord blood are
lower in women with preeclampsia, while other studies suggest that this reduced free IGF-1 may
result from an increased protein-bound fraction of IGF-1 in women with either preeclampsia or
gestational hypertension, consistent with additional evidence that levels of IGF binding protein-1
(IGFBP-1) are higher in women with PIH. Whereas circulating levels of free IGF-1 ligand are
decreased following PIH, increased activation of the cellular IGF-1 pathway appears to drive
both breast cancer risk and tumor progression. Free IGF-1 ligand binds to and activates (by
tyrosine phosphorylation) membrane dimerized IGF1R receptors, resulting in cell growth and
survival responses essential for both normal mammary gland development (when this pathway is
properly regulated) or breast cancer growth and aggressiveness (when this pathway is
dysregulated). Consequently, the IGF growth factor axis is now being targeted (with both
antibodies and small molecule kinase inhibitors) as a breast cancer treatment approach and is
also being considered as a potential strategy for breast cancer prevention. While challenges
continue to slow the clinical development of this important therapeutic approach, preclinical
studies showing that dysregulation of the IGF growth factor axis can drive both HR-positive and
HR-negative/triple-negative breast cancers, including their stem and progenitor cell fractions,
fuel greater enthusiasm toward the targeting of this critical growth factor pathway as an effective
strategy for both the treatment and prevention of all breast cancer subtypes.
Preliminary Results:
Funded by the Centers for Disease Control and Prevention (CDC) and the Avon Foundation, the
MWS enrolled Marin women at multiple mammography screening centers between 2006-2009;
the 13,344 study participants represented 21.5% of all Marin County women of mammography
age. The MWS collected detailed risk factor information and saliva biospecimens from women
obtaining mammograms at area facilities, as well as mammographic breast density (both BIRADS and SXA compositional density). All biospecimens were shipped to the Buck Institute for
Research on Aging where they were cryobanked (-80C) and processed into cell free solute (for
chemical analysis) and cellular DNA (for SNP genotyping) fractions. Pathology and case status
data were obtained from the San Francisco Mammography Registry, which collected data from
the same women during the same time period. All women enrolled in the MWS were asked to
complete an in-depth 87-item (20 page) questionnaire. The richness of the MWS questionnaire
data alone, which includes history of hypertension in pregnancy (PIH) and other reproductive
factors, has already led to several reports; and also enabled detailed correlation of this data with
SNP genotyping and mammographic density, resulting in our most recent and provocative report
showing the protective effective of first pregnancy PIH on later life mammographic density, and
the dependence of this protective effect on the specific IGF1R SNP, rs2016347. In additional
preliminary studies, we have examined the association of this IGF1R SNP with breast cancer
case status among the 374 MWS women who reported a history of PIH and provided a
biospecimen for DNA extraction and SNP genotyping. As shown below and despite this small
sample size, there was a statistically significant decrease in breast cancer risk (case status) for
those MWS women reporting a history of PIH that were also found to carry the same IGF1R
SNP genotype (TT) as previously associated with the greatest reduction in mammographic
density:
Breast cancer cases in women with PIH by IGF1R genotype (rs2016347)
IGF1R Genotype
Number with genotype
GG
91
GT
195
TT
88
Fisher's exact = 0.008
Number breast cancer cases
8
14
0
Percent breast cancer cases
8.79%
7.18%
0.00%
Of note, when not stratified by history of PIH, we did not observe any statistical association
between the IGF1R rs2016347 TT genotype and breast cancer case status, with 27.1% of breast
cancer cases possessing the TT genotype (108/398) relative to 27.0% of controls (382/1415).
From these reported and preliminary MWS observations, as well as the wealth of general
background information showing lower IGF-1 levels following PIH, involvement of this growth
factor and the IGF growth factor axis in determining both mammographic density and driving
breast tumorigenesis, we now propose to further validate our population-based findings and test
our novel hypothesis implicating reduced mammary gland IGF1R expression and responsiveness
as the long term breast protective mechanism minimizing both breast density and risk of
developing breast cancer risk in women with a history of PIH who also carry the appropriate
IGF1R genotype.
Our first project objective is to validate and extend our MWS findings using the California
Teachers Study (CTS), a prospective study of 133,479 current and former public school teachers
and administrators from across the entire state of California. The CTS contains all the necessary
data to accomplish this objective including questionnaire and case outcome data, and genotyping
data covering >200,000 SNPs, including most known IGF growth factor axis variants. The
second objective is to analyze normal breast tissue from the Komen Tissue Bank (KTB) donated
by parous women, with and without a history of PIH, that have never been diagnosed with breast
cancer. To prove our mechanistic hypothesis, these normal breast biopsy samples will be
evaluated histologically for tissue constituents relating to breast density and
immunohistochemically for cellular levels of IGF1R expression and its activation; we will also
measure transcriptional changes in mammary gland IGF axis signaling. Blood samples from
these same donors will be used for IGF1R SNP genotyping to identify those with PIH history
who we believe have a significantly reduced lifelong risk of developing breast cancer. The goal
of this objective is to indentify the changes that occur in the breast tissue of women with breast
cancer protection so that these changes can be duplicated in all women.