Serotonin transporter polyadenylation polymorphism
modulates the retention of fear extinction memory
Catherine A. Hartleya,1, Morgan C. McKennab, Rabia Salmana, Andrew Holmesc, B. J. Caseyd, Elizabeth A. Phelpsa,e,
and Charles E. Glattb,1
aDepartment of Psychology andeCenter for Neural Science, New York University, New York, NY 10003;bDepartment of Psychiatry anddThe Sackler Institute
for Developmental Psychobiology, Weill Cornell Medical College, New York, NY 10065; andcLaboratory of Behavioral and Genomic Neuroscience, National
Institute on Alcohol Abuse and Alcoholism, Bethesda, MD 20852-9411
Edited by Michael Posner, University of Oregon, Eugene, OR, and approved February 21, 2012 (received for review February 3, 2012)
Growing evidence suggests serotonin’s role in anxiety and depres-
sion is mediated by its effects on learned fear associations. Pharma-
cological and genetic manipulations of serotonin signaling in mice
alter the retention of fear extinction learning, which is inversely
associated with anxious temperament in mice and humans. Here,
we test whether genetic variation in serotonin signaling in the form
of a common human serotonin transporter polyadenylation poly-
morphism (STPP/rs3813034) is associated with spontaneous fear
recovery after extinction. We show that the risk allele of this poly-
morphism is associated with impaired retention of fear extinction
memory and heightened anxiety and depressive symptoms. These
STPP associations in humans mirror the phenotypic effects of sero-
tonin transporter knockout in mice, highlighting the STPP as a
potential genetic locus underlying interindividual differences in se-
rotonin transporter function in humans. Furthermore, we show
that the serotonin transporter polyadenylation profile associated
with the STPP risk allele is altered through the chronic administra-
tion of fluoxetine, a treatment that also facilitates retention of
extinction learning. The propensity to form persistent fear associ-
ations due to poor extinction recall may be an intermediate pheno-
type mediating the effects of genetic variation in serotonergic
function on anxiety and depression. The consistency and specificity
of these data across species providerobust support for this hypoth-
esis and suggest that the little-studied STPP may be an important
risk factor for mood and anxiety disorders in humans.
An early “chemical hypothesis” that depression stems from a se-
rotonin deficiency was belied by observations that reducing se-
rotonin levels does not induce depression in healthy individuals
(1) and that inhibition of serotonin reuptake fails to yield rapid
antidepressant effects (2). These findings suggest that necessary
intervening processes mediate serotonin’s role in anxiety and
depression. The “network hypothesis” proposes that serotonin
fosters neural plasticity that supports adaptive processing of af-
fective information (3). Under this view, anxiety and depression
stem from aberrant integration of salient environmental in-
formation due to altered serotonergic function. Consistent with
fear associations as a central underlying mechanism by which
serotonin contributes to mood and anxiety disorders (4).
Extinction learning is a primary means of regulating conditioned
fear responses (5). During extinction, fear expression decreases,
reflecting new learning that a once-threatening stimulus now sig-
nals safety. The efficacy of extinction at reducing fear depends on
the ability to retrieve an extinction memory upon subsequent
encounters with a conditioned stimulus. However, failure to recall
extinction learning may occur with the passage of time, resulting in
“spontaneous recovery” of the original fear memory (6). Research
suggests that the degree of extinction retention is a relatively stable
individual trait with corresponding neurobiological substrates (7–
9). Recent reports in humans that spontaneous recovery of fear is
erotonin is implicated in the etiology of anxiety and de-
of extinction memory may underlie serotonin’s effects on anxiety
Research in animal models provides strong evidence for this
hypothesis. Genetic knockout of the serotonin transporter (5-
HTT) increases anxiety and depression-related behavior in the
mouse(12).Furthermore, duringfearconditioning,5-HTT knock-
out mice show normal acquisition and initial extinction learning,
but exhibit a selective deficit in extinction recall (13, 14) accom-
panied by abnormal neuronal morphology in regions that support
extinction retention (9, 13). These data suggest that 5-HTT down-
regulation impairs extinction retention, resulting in persistent fear
no study has yet examined whether normal variation in seroto-
nergic function modulates extinction memory in humans.
The 5-HTT is expressed as two mRNA species that differ in
the use of two polyadenylation signals (15, 16). Polydenylation
is a posttranscriptional modification of the 3′ end of the tran-
script that occurs in the majority of protein-coding mRNAs.
Alternative polyadenylation forms are common in the brain and
can lead to diversity in the regulation of gene expression (17).
The two reported 5-HTT polyadenylation forms differ by a 123-
bp element, and the human gene contains a common T/G single
nucleotide polymorphism (rs3813034) in the more distal of the
two polyadenylation signals. We have termed rs3813034 the se-
rotonin transporter polyadenylation polymorphism (STPP) be-
cause it alters the use of the polyadenylation signal in which it
occurs, influencing the balance of the two polyadenylation forms
in the brain (15, 16). G-allele carriers have a reduced fraction of
5-HTT mRNA containing the distal polyadenylation sequence
(distal polyadenylation fraction; DPF), and also exhibit increased
risk for panic disorder (15). Of functional interest, the distal
polyadenylation sequence element is positively correlated with
the steady-state level of 5-HTT mRNA (15) and occurs adjacent
to a microRNA (miR-16) binding site (18), suggesting that
binding of regulatory proteins to the distal sequence element
may modulate miR-16 binding (19) and alter 5-HTT translation
(18), as has been described in miR-16–mediated regulation of
cyclooxygenase 2 gene expression.
Here, we genotyped participants (SI Appendix, Table S1) in
a two-day fear conditioning paradigm (Fig. 1A) for the STPP to
influence extinction retention. During fear acquisition, a colored
square (the conditioned stimulus +, or CS+) was paired with
Author contributions: C.A.H., B.J.C., E.A.P., and C.E.G. designed research; C.A.H., M.C.M.,
R.S., and C.E.G. performed research; C.A.H. and C.E.G. analyzed data; A.H. contributed
previously published data in the 5-HTT knockout mouse; and C.A.H., A.H., B.J.C., E.A.P.,
and C.E.G. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
1To whom correspondence may be addressed. E-mail: firstname.lastname@example.org or ceg2004@med.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
| April 3, 2012
| vol. 109
| no. 14
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