RESEARCH ARTICLE
Analysis of CHRNA7 Rare Variants in
Autism Spectrum Disorder Susceptibility
Elena Bacchelli,1 Agatino Battaglia,2 Cinzia Cameli,1 Silvia Lomartire,1 Raffaella Tancredi,2
Susanne Thomson,3 James S Sutcliffe,3 and Elena Maestrini1*
1
Department of Pharmacy and Biotechnology, University of Bologna, Bologna, Italy
2
Stella Maris Clinical Research Institute for Child and Adolescent Neuropsychiatry, Calambrone (Pisa), Italy
Department of Molecular Physiology & Biophysics and Psychiatry and Vanderbilt Brain Institute, Vanderbilt University,
Nashville, Tennessee
3
Manuscript Received: 16 May 2014; Manuscript Accepted: 30 September 2014
Chromosome 15q13.3 recurrent microdeletions are causally
associated with a wide range of phenotypes, including autism
spectrum disorder (ASD), seizures, intellectual disability, and
other psychiatric conditions. Whether the reciprocal microduplication is pathogenic is less certain. CHRNA7, encoding for the
alpha7 subunit of the neuronal nicotinic acetylcholine receptor,
is considered the likely culprit gene in mediating neurological
phenotypes in 15q13.3 deletion cases. To assess if CHRNA7 rare
variants confer risk to ASD, we performed copy number variant
analysis and Sanger sequencing of the CHRNA7 coding sequence
in a sample of 135 ASD cases. Sequence variation in this
gene remains largely unexplored, given the existence of a fusion
gene, CHRFAM7A, which includes a nearly identical partial
duplication of CHRNA7. Hence, attempts to sequence coding
exons must distinguish between CHRNA7 and CHRFAM7A,
making next-generation sequencing approaches unreliable for
this purpose. A CHRNA7 microduplication was detected in a
patient with autism and moderate cognitive impairment; while
no rare damaging variants were identified in the coding region,
we detected rare variants in the promoter region, previously
described to functionally reduce transcription. This study represents the first sequence variant analysis of CHRNA7 in a sample
of idiopathic autism. Ó 2015 Wiley Periodicals, Inc.
Key words: 15q13.3; neurodevelopmental disorders; copy
number variants; sequence variants
INTRODUCTION
Autism spectrum disorder (ASD) comprises a group of neurodevelopmental traits characterized by repetitive and stereotypic
behaviors and impairments in communication and social interactions, with an onset within the first three years of age. Family
studies indicate a significant genetic basis for ASD susceptibility,
but the underlying genetic architecture is highly complex and
heterogeneous. Recent genome-wide studies have documented
that common variants exert only small individual main effects
on risk, although when common variation (CV) across the genome
Ó 2015 Wiley Periodicals, Inc.
How to Cite this Article:
Bacchelli E, Battaglia A, Cameli C,
Lomartire S, Tancredi R, Thomson S,
Sutcliffe JS, Maestrini E. 2015. Analysis of
CHRNA7 rare variants in Autism spectrum
disorder susceptibility.
Am J Med Genet Part A 9999:1–9.
is considered in aggregate, CV is found to contribute measurably to
ASD risk [Klei et al., 2012]. In addition, rare inherited and de novo
copy number variants (CNVs) and single nucleotide variants
(SNVs) of large effect size have a major role in the etiology of
ASD, contributing in as many as 5–10% of idiopathic cases examined [Devlin and Scherer, 2012].
In spite of the overall relevance of CNVs in autism, each
individual risk CNVs is very rare. Recurrent CNVs, still infrequent or rare, are typically flanked by segmental duplications and
are often implicated in multiple developmental and/or neurological disorders, perhaps not surprisingly given the overlap in
phenotype across conditions. Among recurrent CNVs, the
15q13.3 microdeletion is highly but not always fully penetrant,
and it is significantly enriched in cases of intellectual disability,
autism, epilepsy, schizophrenia, and bipolar disorder [Sharp
et al., 2008; Stefansson et al., 2008; Ben-Shachar et al., 2009
Dibbens et al., 2009; Helbig et al., 2009; Miller et al., 2009;
Pagnamenta et al., 2009; van Bon et al., 2009; Cooper et al., 2011].
This 15q13.3 recurrent microdeletion, resulting in the loss of a
1.5 Mb region between low-copy repeat (LCR) sequences
Conflict of interest: The authors declare no conflict of interest.
Correspondence:
Elena Maestrini, Dept. of Pharmacy and Biotechnology, University of
Bologna, via Selmi 3, Bologna 40126, Italy.
E-mail: elena.maestrini@unibo.it
Article first published online in Wiley Online Library
(wileyonlinelibrary.com): 00 Month 2015
DOI 10.1002/ajmg.a.36847
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AMERICAN JOURNAL OF MEDICAL GENETICS PART A
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designated as breakpoints 4 and 5 (BP4 and BP5), contains six
genes (MTMR15, MTMR10, TRPM1, KLF13, OTUD7A, and
CHRNA7) and an miRNA gene (hsa-mir-211).
Strong evidence supporting CHRNA7 as responsible for the
majority of neurodevelopmental phenotypes resulting from deletion, comes from the identification of individuals carrying smaller
deletions which encompass the entire CHRNA7 gene and the first
exon of OTUD7A [Shinawi et al., 2009] or even smaller deletions,
including only CHRNA7 [Masurel-Paulet et al., 2010; Mikhail et al.,
2011; Hoppman-Chaney et al., 2013], and conveying most or all of
the phenotypic abnormalities associated with the larger 15q13.3
recurrent deletions.
Genomic imbalance at chromosome 15q13.3 also includes the
reciprocal microduplication, which has a less certain clinical significance in comparison with the deletions. Nevertheless, de novo
and inherited 15q13.3 duplications are associated with a wide
spectrum of neuropsychiatric disorders, including ASD. The clinical uncertainty of the 15q13.3 duplications could be due to the fact
that a larger sample size is necessary to detect a low penetrant effect
[Szafranski et al., 2010; Moreno-De-Luca et al., 2013].
CHRNA7 encodes the a7 subunit of the neuronal nicotinic
acetylcholine receptor, which is the only subunit able to form a
homopentameric chloride channel receptor, and is highly expressed
in the brain. Receptors containing a7 are localized both pre- and
post-synaptically and regulate the release of both the inhibitory
neurotransmitter GABA and the excitatory neurotransmitter glutamate in the hippocampal formation [Albuquerque et al., 2009].
Alpha-7 nicotinic acetylcholine receptor mediated signaling causes
an influx of Ca2þ into the cell [Vijayaraghavan et al., 1992].
Mobilization of intracellular Ca2þ plays a critical role in synaptic
plasticity and immediate early gene expression associated with
learning and memory [Benfenati, 2007], thus supporting CHRNA7
involvement in the cognitive deficits apparent in neuropsychiatric
disorders. Furthermore, a recent study reported a significantly
reduced CHRNA7 expression in the frontal cortex of individuals
with Rett syndrome or with typical ASD [Yasui et al., 2011]. The
binding of the methyl CpG binding protein 2 (MeCP2), encoded by
MECP2, influences the chromatin loop organization of the much
larger 15q11.2–13.3 region that includes the Prader-Willi/Angelman syndrome region, and is required for optimal expression of AS/
PWS region genes implicated in the ASD phenotype
[Yasui et al., 2011]. Therefore these discoveries suggest that transcription of CHRNA7 is modulated by these regulatory elements
and is involved in ASD-like phenotypes [Yasui et al., 2011].
Based on the evidence that CHRNA7 may be responsible for the
majority of the spectrum of abnormal phenotypic features of
patients with 15q13.3 CNVs, we decided to focus our studies on
this candidate gene. In order to capture the entire spectrum of
genetic variation in CHRNA7 contributing to ASD risk, it is
essential to integrate both CNV and sequence data. However,
mutation screening of CHRNA7 is complicated by the existence
of a 300 kb duplication, which contains exons 5–10 and the 30 end of
CHRNA7; the duplicated portion of CHRNA7 is fused to exons A–E
of FAM7A resulting in a hybrid gene known as CHRFAM7A [Gault
et al., 1998]. Hence, any attempts at sequencing to detect mutations
must distinguish the nearly identical sequence for the interval
containing exons 5–10 from CHRNA7 and CHRFAM7A. This
renders next-generation sequencing approaches unreliable for
this purpose.
In this study, we have performed CNV analysis and sequence
mutation screening of the coding sequence of the CHRNA7 gene
in a sample of 135 Italian ASD probands, with the aim of
investigating if rare variants in CHRNA7 could play an important
role in ASD.
METHODS
Patients
A total of 135 Italian individuals with ASD from 133 families, were
recruited at the Stella Maris Clinical Research Institute for Child
and Adolescent Neuropsychiatry (Calambrone, Pisa, Italy). ASD
diagnosis was based on the Autism Diagnostic Interview-Revised
(ADI-R) and the Autism Diagnostic Observation Schedule
(ADOS); and a clinical evaluation was undertaken in order to
exclude known syndromes associated with autism. Standard karyotyping, fragile-X testing, EEG, and array-based comparative
genomic hybridization (aCGH) were obtained for all probands.
The main clinical features of the 135 Italian ASD cases included in
this study are reported in Table I. The control sample consists of 174
unrelated Italian individuals with no psychiatric disorders.
TABLE I. Description of the Italian Cohort of ASD Patients Included in CHRNA7 Mutation Screening
Sex
Autism
Atypical
autism
Asperger
syndrome
Total
Intellectual disability
Female
22
6
Epilepsy
2
0
Severe
(20<IQ<34)
0
2
Moderate
(35<IQ<49)
11
3
Mild
(50<IQ<69)
27
11
BCI
(70<IQ<85)
20
3
Normal
(IQ>85)
17
7
Unknown
23
8
2
1
0
0
0
0
1
1
1
106
29
2
2
14
38
24
25
32
Total
98
34
Male
76
28
3
135
BCI, borderline cognitive impairments.
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All data from either affected patients or their parents and controls,
including informed consent, were handled in accordance with the
local ethical committee’s approved protocols and in compliance
with the Helsinki declaration.
Clinical Assessment of Proband 3474_3 With
CHRNA7 Duplication
The propositus of family 3474 was the only child of healthy
nonconsanguineous parents. The paternal grandfather’s sister was
affected by ID, and the paternal grandfather’s brother had speech
delay. During pregnancy there was no exposure or history of
alcohol, tobacco, or drug abuse. The child was born at term by
caesarean. Birth weight was 3550 g (50th centile), and length
50 cm (50th centile). He was breast-fed with good suction. He
sat unaided at age eight months, and started walking independently at 16 months. Babbling was noted at age nine months, and
at age 12 months he pronounced his first words. At 18 months, his
parents observed a regression of his language skills, associated
with refusal of physical contact, selective feeding, and hyperactivity. When seen by us, at age five years five months, he exhibited
moderate cognitive impairment; was able to pronounce only
single words with a reduced gesture repertoire, and limited
comprehension. On examination, there were bilateral epicanthal
folds, broad-hypoplasic nasal bridge, and prominent digit pads.
Weight was 26 kg (97th centile), height 114 cm (50th–75th centile), and OFC 55.5 cm (>90th centile). Metabolic work-up, high
resolution karyotype, molecular analyses of the FRAXA/E loci,
thyroid function, Brainstem Auditory Evoked Potentials (BAEPs),
Electrocardiogram (EKG), Electroencephalography (EEG), and
Magnetic Resonance brain Imaging (MRI) were reported to be
normal. ADIR, and ADOSG scores met criteria for a diagnosis of
autism. He was not available for a follow-up visit. However, his
mother reported, on a phone call, that he had developed complex
partial seizures with secondary generalization at age 13 years. His
father, who transmitted the microdeletion, as well as his mother
are both apparently normal.
CNV Analysis
All ASD probands were analyzed by Quantitative Real Time PCR
(qPCR) using Fast SYBR-green (Life Technologies) to amplify a
region of 111 bp partially overlapping exon 3, specific for CHRNA7
(Primer forward: 50 -GGCTGCAAATGGTAAGTTAAGAG-3’ and
primer reverse: 50 -AACAGGACCTCTCAGAAGCAAG-30 ). The
data were normalized to the reference gene FOXP2. Each assay
was conducted in three replicates for the target region and for the
control region. Relative levels of region dosage were determined
using the comparative CT method assuming that there were two
copies of DNA in the control region. The relative copy number for
each target region was calculated as 2-DDCT with confidence interval
as 2-(DDCTSD).
The validation and the resolution of the extension of the
CHRNA7 duplication identified in proband 3474_3, was carried
out using Genome Wide SNP arrays data (Illumina 1M-duo array)
generated as part of a large genome-wide scan for CNVs carried out
by the Autism Genome Project (AGP)[Pinto et al., 2014].
CHRNA7 Sequencing
The genomic sequence comprising CHRNA7 exons 5–10 is duplicated and nearly identical (>99%) in the CHRFAM7A gene,
complicating the mutation screening [Gault et al., 1998]. The first
four exons, which are specific for CHRNA7, were amplified using
exon specific primers corresponding to flanking intronic sequences
and then subjected to Sanger sequencing. We aligned the sequences
of the duplicated segment of CHRNA7 as reported in human
genome reference sequence (GRCh37, hg19) and we identified
three small insertions present in CHRNA7 and absent in
CHRFAM7A , namely: 36 bp at the 30 end of CHRNA7 (chr15:
32,462,638–32,462,673); 4 bp in intron 5 (chr15: 32,448,389–
32,448,392); 5 bp in intron 9 (chr15: 32,458,725–32,458,729)
(Fig. S1). To selectively amplify CHRNA7 exons 5–10, we took
advantage of the 3’UTR 36 bp insertion to design a primer specific
for CHRNA7. Two long range PCRs (LR-PCRs) were performed to
amplify: a) a segment encompassing exon 5 to exon 8 (x5–x8 LRPCR) using primer 50 -CACCTGCAGTTCAGTCATTCAA-30 , outside the duplication, and 50 -AAAGTCAAACCTCAAAGCTGAA30 , in the duplicated region; b) a segment encompassing exon 9 to
exon 10 (x9–x10 LR-PCR) using primer 50 -AGTGCATGGAAGTGCAATGA-30 , in the duplicated region, and 50 -CACTTCTACTTGTTTCTAAAGACACTG-30 , in the 36 bp CHRNA7
specific region (Fig. 1). LR-PCR was performed using KAPA
HiFi HotStart DNA Polymerase (Resnova), following the manufacturer’s protocol, and using a ‘touch-down’ protocol: 95˚C for
5 min, followed by 10 cycles of 20 sec at 98˚C, 15 sec at 65˚C 0.5˚C
per cycle, 7 min at 72˚C, followed by 25 cycles of 20 sec at 98˚C, 15
sec at 60˚C, 7 min at 72˚C. All LR-PCR products were purified using
a Millipore Multi Screen PCR 96 well and then they were directly
sequenced by Sanger method using specific primers for each exon.
Primers, used to perform PCR and sequencing, were designed using
Primer 3 software (http://bioinfo.ut.ee/primer3–0.4.0/). All PCR
and sequence primers are listed in Supplementary Table S1 (see
supporting information online).
To verify the specificity of our two LR-PCR reactions, we checked
for the presence of the two CHRNA7 specific sequences in intron 5
and intron 9 (Fig. S1-in supporting information online). Primers
flanking these two regions were used to amplify the genomic DNA
of 30 patients, thus leading to co-amplification of both CHRNA7
and CHRFAM7A. For each patient, we sequenced and compared
the co-amplification product and the LR-PCR product.
For the exon 5-exon 8 fragment, sequencing of the LR-PCR
product shows the 4 bp CHRNA7 specific insertion, while sequencing of the coamplification products leads to overlapping
sequence peaks, as expected (Figs. S2 A and B-in supporting
information online). This result confirmed that our x5-x8 LRPCR is specific for CHRNA7. We did notice, however, that in a
minority of individuals the LR-PCR sequence showed overlapping peaks, suggesting that the 4 bp insertion in intron 5 is
actually polymorphic in CHRNA7. This hypothesis is supported
by comparison of the coamplification sequence in these heterozygote individuals, which is compatible with the presence of one
allele with the 4 bp insertion and three alleles without the
insertion, thus still confirming the specificity of this LR-PCR
(Figs. S2C and D-in supporting information online).
AMERICAN JOURNAL OF MEDICAL GENETICS PART A
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FIG. 1. Genomic structure of the human CHRNA7 gene, with the position of the primers used for the x5–x8 and x9–x10 LR-PCRs and the qPCR
probe. The dashed rectangle indicates the region duplicated in CHRFAM7A. [Color figure can be viewed in the online issue, which is available at
http://onlinelibrary.wiley.com/journal/10.1002/(ISSN) 1552–4833.]
For the exon 9-exon 10 fragment, sequencing of the 5 bp specific
region in intron 9 did not reveal the expected differences between
the LR-PCR and the co-amplification products, as the 5 bp insertion was always present. However, using the same strategy, we
compared the genotypes of two common SNPs in exon 10
(rs1042724, rs2253967), by sequencing the x8-x9 LR-PCR versus
the coamplification product in all ASD individuals. Since we
obtained different genotypes (Fig S3-in supporting information
online), we concluded that our x9-x10 LR-PCR is indeed specific for
CHRNA7, while the 5 bp insertion in intron 9 as reported GRCh37
is actually present in both CHRNA7 and CHRFAM7A, at least in
most individuals.
RESULTS
CNV Analysis
All 135 ASD individuals were tested by qPCR to identify CNVs in
CHRNA7 using a probe mapping in exon 3, specific for CHRNA7
(Fig. 1). No deletions were identified, while a duplication was found
in a proband with ASD (3474_3). qPCR was then repeated in all
family members, showing that the duplication is inherited from the
unaffected father (Fig. 2A).
Illumina 1 M-Duo SNP arrays data for all members of family
3474 were used to validate and to define the extension and the
boundaries of the duplication. The presence of the duplication was
confirmed with high confidence by two algorithms (QuantiSNP
[Colella et al., 2007] and PennCNV [Wang et al., 2007]); it spans a
region of approximately 500 kb, including exon 1 of the longer
isoform of OTUD7A and the entire CHRNA7 gene (chr15:
32,005,348–32,515,973 NCBI build 37 coordinates). The presence
of the microduplication was also verified by visual inspection of the
1 M SNP data, by plotting intensities (logR ratio) and allelic ratios
(B allele frequency) of all members of the family 3474 for the region
encompassing the duplication (Fig. 2B).
CNV analysis of Family 3474 led also to the identification of a
55 kb paternal deletion of ARHGAP11B, which is known to
accompany the majority of CHRNA7
microduplications
[Szafranski et al., 2010], and a rare maternal gain of unknown
significance on chromosome 5q13.2(chr5:68,594,539–68,638,941).
This duplication involves the first seven exons of CCDC125
(Coiled-coil domain-containing protein 125), a gene with a putative role in controlling the cell motility of immune systems
[Araya et al., 2009].
Mutation Screening
All 10 exons of CHRNA7 were sequenced in the sample of 135
Italian ASD probands. CHRNA7 exons 5–10, along with a large
cassette of DNA (300 kb), are duplicated and map approximately
1.5 Mb proximal to the full-length CHRNA7 gene [Gault et al.,
1998]. CHRFAM7A is a chimeric gene derived by the fusion of
CHRNA7 (exons 5–10) to one of many copies of a novel gene called
FAM7A, that encodes exons A–E [Gault et al., 1998] [Riley et al.,
2002]. Exons C–A are the result of a partial duplication of a putative
kinase-like gene (ULK4) on chromosome 3p22.1, while exon D is of
unknown provenance. CHRNA7 DNA sequence in CHRFAM7A is
99.9% conserved. Therefore, in order to selectively analyze the
duplicated region of CHRNA7, long range PCR was used to amplify
fragments that contained exons 5–8 and 9–10 (see Methods for
details).
A total of four putatively functional rare (<1% minor allele
frequency) variants were identified in CHRNA7: a non-synonymous
variant in exon 10, and three variants located in the proximal
promoter region [Leonard et al., 2002] (Table IIa). The non-synonymous variant in exon 10 causes an amino acid substitution from
Glutamate to Lysine (p.E452K, rs199504752) and it has been identified in only one ASD proband who inherited it from the unaffected
mother. Since this variant is located in CHRNA7 exon10, which is
duplicated in the CHRFAM7A gene, it was not possible to estimate
the frequency of this non-synonymous variant from public databases. Therefore we have sequenced CHRNA7 exon 10 by LR-PCR in
a control sample consisting of 125 Italian unaffected individuals and
we detected the p.E452K variant in three unrelated individuals. As the
frequency of this non-synonymous variant is not different between
our ASD sample and the control sample (1/135 in ASD vs. 3/125 in
controls, two sided Fisher exact test P-value ¼ 0.35) and in silico
analysis using PolyPhen-2(http://genetics.bwh.harvard.edu/pph2/)
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BACCHELLI ET AL.
FIG. 2. A) qPCR results for the CHRNA7 duplication in family 3474. B) GenomeStudio screenshot showing B-allele frequency and log R ratio for
family 3474. The duplication results in an increase to the Log R ratio and deviation in the allelic ratio for heterozygote variants away from the
expected 0.5. The SNPs included in the duplication are boxed. [Color figure can be viewed in the online issue, which is available at http://
onlinelibrary.wiley.com/journal/10.1002/(ISSN) 1552–4833.]
and SIFT (http://sift.jcvi.org/) does not predict any deleterious effect
on the protein, it is unlikely that this variant contributes to ASD risk.
The other three rare variants identified by Sanger sequencing of
the 135 ASD patients are respectively located in the proximal
promoter region, respectively at -241 bp (rs188889623,
c.1–129A>G), at 191 bp (c.1–79G>A), and -182 bp from ATG
(c.1–70_1–69delG) (Table IIa). One out of the three individual with
the -241 bp variant (proband 3377_3) also carries another more
frequent variant in the 5’UTR region (g.32322712C>T,
rs149637464, -86 bp from ATG) (Table IIb). Interestingly, these
AMERICAN JOURNAL OF MEDICAL GENETICS PART A
CC ¼ 75;
CT ¼ 10 (0.059)
CC ¼ 85;
CT ¼ 13 (0.066)
CC ¼ 151;
CT ¼ 23 (0.066)
CC ¼ 112;
CT ¼ 23 (0.085)
g.32460504 G>A
p.E452K c.1466G>A
(rs199504752)
b: 5’-UTR variant found in the same individual carrying the 241 variant
5’-UTR ( 86)
g.32322712C>T
c.27C>T
(rs149637464)
Exon 10
c.1–70_1–69delG
g.32322616 delG
Promoter ( 182)
c.1–79G>A
AA ¼ 172;
AG ¼ 2 (0.006)
GG ¼ 174;
GA ¼ 0 (0)
GG ¼ 174;
G/delG ¼ 0 (0)
GG ¼ 122;
GA ¼ 3 (0.012)
AA ¼ 132;
AG ¼ 3 (0.011)
GG ¼ 134;
GA ¼ 1 (0.004)
GG ¼ 134;
G/delG ¼ 1 (0.004)
GG ¼ 134;
GA ¼ 1 (0.004)
c.1–129A>G
EUR*, Europeans include Utah Residents (CEPH) with Northern and Western European ancestry (CEU); TSI, Toscani in Italia; FIN, Finnish in Finland; GBR, British in England and Scotland; IBS, Iberian population in Spain.
—
—
CC ¼ 336;
CT ¼ 43 (0.057)
—
—
—
—
—
AA ¼ 85 (0)
AA ¼ 96;
AG ¼ 2 (0.010)
—
AA ¼ 374;
AG ¼ 5 (0.007)
—
85 CEU
genotype
count (MAF)
98 TSI
genotype
count (MAF)
Italian
controls genotype
count (MAF)
135 ASD
cases genotype
count (MAF)
Protein
NP_000737.1
cDNA
Region
hg19 position
(bp distance from ATG)
(SNP ID)
a: Rare putative functional variants
Promoter ( 241)
g.32322557A>G
(rs188889623)
Promoter ( 191)
g.32322607G>A
TABLE II. Rare Variants Identified in CHRNA7
Controls (1000 genomes)
379 EUR*
genotype
count (MAF)
6
two variants have been previously reported to have a functional
effect on CHRNA7 gene transcription, being strongly associated
with a significant decrease of promoter activity (P < 0,0001; Leonard et al., 2002). While these two variants have been individually
found in a sample of 174 Italian unaffected individuals and are
separately reported in the 1000 Genome project control individuals
(Table II), none of the 174 Italian control individuals or the 379
EUR patients carry both variants.
Segregation analysis of the -86/-241 variants in the Italian family,
showed that the two variants are not on the same chromosome, as
the -86 bp variant was inherited from the mother while the -241 bp
variant was inherited from the father. It is thus plausible that
CHRNA7 expression is significantly decreased in proband
3377_3. Unfortunately, we were not able to test this hypothesis,
since CHRNA7 mRNA is not detectable in blood.
DISCUSSION
Microdeletions of chromosome 15q13.3 have been associated with
multiple neurological and neuropsychiatric phenotypes, with the
strongest enrichment observed in cases of idiopathic generalized
epilepsy (IGE), but also among patients with ID, autism and
schizophrenia [Marshall et al., 2008; Sharp et al., 2008; Stefansson
et al., 2008; Cooper et al., 2011; Kaminsky et al., 2011; Sanders et al.,
2011]. The reciprocal microduplications of 15q13.3 have been more
challenging to interpret, being detected across the same spectrum of
neuropsychiatric disorders of the microdeletions, but with high
variability in expressivity and reduced penetrance and more often
inherited than de novo as compared with deletions.
CHRNA7 is thought to be the causative gene for the neurological
phenotypes in patients with 15q13.3 CNVs, but, given the genomic
complexity and the presence of an almost identical partial duplication, sequence analysis of the coding region has been performed
only in a handful of patients carrying the microdeletion, with
different clinical phenotypes [Masurel-Paulet et al., 2010].Therefore, in order to analyze the contribution of CHRNA7 rare variants
in ASD susceptibility, we screened a well-characterized cohort of
135 Italian ASD individuals for the presence of structural and
sequence variants.
CNV analysis led to identification of a small 15q13.3 microduplication, which involves the entire CHRNA7 gene and the first
exon of OTUD7A longer isoform, in an ASD proband. Even if it has
not been determined where the duplicated genetic material exactly
resides, the microduplication is most likely generated by NAHR
mediated by LCRs in BP4 and BP5, thus lying in tandem with itself,
as previously described [Szafranski et al., 2010]. Interestingly this
proband, beyond a clinical diagnosis of autism and moderate
cognitive impairment, developed complex partial seizures with
secondary generalization, at age of 13 years.
Whereas epilepsy has been strongly associated with microdeletions of CHRNA7 [Helbig et al., 2009; Shinawi et al., 2009], seizures
are not reported as a common features of patients carrying the
microduplication [Szafranski et al., 2010]. Notably, two peaks of
seizure onset have been reported in ASD, one in early childhood
[Volkmar and Nelson, 1990] and the other in adolescence and
continuing through adulthood [Tuchman and Cuccaro, 2011].
Therefore, it is not possible to exclude that some participants of
7
BACCHELLI ET AL.
pediatric cohorts may develop epilepsy at a later time. To better
understand the complex genotype-phenotype correlations of the
reciprocal microdeletions and microduplication, a detailed clinical
characterization of the individuals carrying the CNV would be very
useful, especially if followed up over time. Nevertheless, the cooccurrence of ASD and epilepsy in the proband with the CHRNA7
duplication, may suggest that the microduplication involving
CHRNA7 could have the same role of the deletion in ASD/epilepsy
susceptibility although with lower penetrance. In accordance with
this hypothesis, CHRNA7 transcript levels were recently found
reduced in neuronal cells [Meguro-Horike et al., 2011] or brain
samples with maternal 15q duplication [Hogart et al., 2009], in
contrast to what is expected according to the gene copy number.
The observation that deletions and duplications at the same locus
may yield similar phenotypes is quite common and it could be
explained by the sensitivity of certain cellular functions to dosage
imbalance, as described for the 1q21.1 region [Harvard et al., 2011].
Further evidence for dosage sensitivity of CHRNA7 has been
provided by a very recent study that shows the co-segregation of
a CHRNA7 triplication with neuropsychiatric and cognitive phenotypes in a three generations family [Soler-Alfonso et al., 2014].
The phenotypic variability at locus 15q13.3 may also be controlled by second-site CNVs, in line with the recently proposed
“two-hit model” for severe developmental delay [Girirajan et al.,
2010]. Support to this model comes from a recent study where five
out of 11 patients with small microduplications involving CHRNA7
and showing a variety of neuropsychiatric disorders, carried at least
one additional different CNV of potential clinical relevance
[Szafranski et al., 2010]. However, no other clearly pathogenic
CNV has been identified in proband 3474_3, and he does not carry
any of the CHRNA7 sequence variants listed in Table II, even if we
cannot exclude the presence of a sequence variant elsewhere in the
genome that could act within the same pathway to increase the risk
of ASD. Interestingly, two ASD individuals carrying a small
CHRNA7 duplication and a de novo SHANK2 deletion on distinct
parental chromosomes, were recently observed, suggesting the
presence of epistasis between these two loci [Leblond et al.,
2012]. Another report described a boy with severe ID, language
impairment, and behavioral anomalies, carrying a de novo balanced
translocation disrupting the SHANK2 gene as well as an inherited
duplication of CHRNA7 [Chilian et al., 2013]. Thus CHRNA7
duplications might act as modifier in presence of other variants of
larger effect.
Mutation screening of the coding and putative regulatory
regions of CHRNA7 led to the identification of only one nonsynonymous rare variant (p.E452K), which is predicted benign by
bioinformatics tools and it is found with a similar frequency in
controls, therefore not supporting its role in ASD susceptibility.
However, three rare variants have been identified in the proximal
promoter region, one of which (-241 bp from ATG) was found in
one ASD individual who also carries a more frequent 5’-UTR
variant (-86 bp from ATG) on the other chromosome. Since the
presence of each of these two variants has been previously associated
with CHRNA7 decreased transcription in vitro [Leonard et al.,
2002], it is reasonable to suppose a marked reduction of expression
in presence of the double variant. Unfortunately it was not possible
to test the functional effect of this double variant in the carrier
proband 3377_3, as CHRNA7 expression is too low in blood to be
assessed by RT-PCR.
Our results do not support the hypothesis that rare sequence
variants in CHRNA7 contribute to ASD susceptibility in this
current Italian cohort. It is notable that our ASD sample has an
overall lesser degree of medical comorbidities, e.g., the frequency of
epilepsy in our sample is only 1.5% while it is commonly reported to
occur in 30% of individuals with ASD, but prevalence estimates
have varied widely, ranging from 5% to 46% [Viscidi et al., 2013].
Analysis of much larger cohorts of individuals is thus warranted to
elucidate the role of rare CHRNA7 sequence variants in ASD risk
and to discriminate if CHRNA7 might be mainly implicated in ASD
cases associated to other clinical features that would be consistent
with the significant enrichment of 15q13.3 CNVs observed in
individuals with comorbid phenotypes.
In conclusion, even if the sample size of this study is limited, our
results are valuable as they represent the first sequence analysis of
the CHRNA7 coding region in a sample of idiopathic autism.
ACKNOWLEDGMENTS
We gratefully thank all the families who have participated in the
study and the professionals who made this study possible. We thank
the Autism Genome Project (AGP) for family 3474 CNV data. This
work was supported by University of Bologna (RFO).
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