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JOURNAL CLUB
Autism and intellectual disability
E Tallantyre • Neil P. Robertson
Published online: 20 February 2013
� Springer-Verlag Berlin Heidelberg 2013
Intellectual disability and autism spectrum disorders affect
approximately 3 % of the population and are commonly
encountered in general neurological practice as a result of
co-morbidity with a range of other neurological disorders,
and in particular with epilepsy. Adult neurologists might
reasonably expect that the diagnosis and classification of
individuals affected by these conditions will have been
achieved in childhood and or adolescence; however, epi-
demiological data suggests that this is frequently not the
case. Intellectual disability in childhood is usually first
identified within a wider backdrop of developmental delay
and precipitates a series of longitudinal assessments and
standard investigations. However, even after searching for
established aetiological factors such as birth asphyxia,
perinatal or neonatal infection and associated genetic
conditions, the underlying cause of intellectual disability
frequently remains obscure. Improved recognition and
characterisation of these conditions will not only provide
clarity about prognosis, but may also facilitate access to
appropriate support or genetic counselling, avoid the need
for unnecessary investigation and may even allow specific
treatment recommendations.
In order to unravel the complex aetiology and patho-
physiology of these conditions, researchers have chosen to
employ increasingly complex research methodologies. As a
result one of the challenges facing modern clinical neu-
rologists is to develop acquaintance with these methods in
order to understand implications for clinical practice. This
is particularly relevant to the rapidly evolving fields of
genetics and imaging as it seems likely that techniques
similar to those currently used in research studies will
emerge into the diagnostic arena before too long. Array
based chromosomal analysis is already used in the diag-
nostic setting of intellectual disability to identify common
causative duplications or deletions that are below the
detection level of conventional karyotyping. Exome-wide
sequencing, that enables sequencing of the entire coding
portion of the genome, can now detect de novo mutations
at relatively low cost. However, as with any unbiased
genome-wide technique, relevant results must be extracted
from a mass of irrelevant data. In particular a major chal-
lenge is to predict which de novo mutation is likely to be
pathogenic, since any new born child will be expected to
have acquired 50–100 de novo mutations in his or her
genome. Similarly, functional MRI has a well established
role in uncovering abnormal circuitry in neurological and
psychiatric illness but also requires complex statistical
analyses and careful interpretation. This is particularly the
case when resting-state imaging is acquired to investigate
the default mode of the brain.
In this month’s journal club we review three papers that
focus on autism spectrum disorders and intellectual dis-
ability. The first uses a novel functional MRI method to
address the question of whether reduced connectivity in
autism occurs at a regional or a whole-brain level. The
second and third papers both use exome-wide sequencing
to identify de novo, potentially pathogenic mutations in
individuals with severe intellectual disability.
Fractionation of social brain circuits in autism
spectrum disorders
Autism is a lifelong neurodevelopmental condition char-
acterised by difficulties in social interaction and
ETallantyre � N. P. Robertson (&)
Department of Neurology, University Hospital of Wales,
Cardiff, UK
e-mail: [email protected]
123
J Neurol (2013) 260:936–939
DOI 10.1007/s00415-013-6861-y
communication, as well as stereotyped or rigid behaviours.
Some evidence suggests that autism spectrum disorders
result from disconnection within widespread brain circuits.
However, parallel research has identified brain regions that
appear to be crucial for social processing. It is therefore
unclear whether autism spectrum disorders occur due to
disconnection on a whole-brain level, or due to more focal
problems in social processing regions. This question has
previously proved difficult to address using functional MRI
(fMRI) because comparison of synchronous brain activity
in multiple regions results in a statistical challenge of
multiple comparisons while the selection of particular
regions of interest for interrogation introduces bias.
Gotts and colleagues used fMRI with a novel ‘‘data-
driven’’ approach to analysis to address the question: to
what extent is abnormal connectivity in autism limited to
the social brain? Thirty-one adolescents with autism
spectrum disorder (with IQ greater than 85 and no known
epilepsy or neurological/genetic conditions) were recruited
and compared with 29 typically developing adolescents.
Functional MRI was used to measure spontaneous brain
BOLD activity while at rest. Maps of ‘‘functional con-
nectedness’’ were derived for each individual by compar-
ing signal over time in each voxel with every other voxel in
the image. The mean connectedness of each region was
compared between cases and controls.
Several regions were identified that appeared to show
significantly abnormal connectivity in patients with autism
spectrum disorder. Almost all of these regions of interest
appeared to fall within areas deemed important for social
processing. Comparison of signal patterns between the
identified regions of interest suggested common patterns of
circuitry across individuals. Three main clusters of altered
activity were identified: limbic structures thought to be
crucial to emotional or affective aspects of social processing,
brain regions involved in linguistic and communicative
aspects and brain regions involved in visuospatial, somato-
sensory or motor aspects of social processing. Regional
correlations that survived adjustment for multiple compari-
sons were almost all long-range connections (as opposed to
local connections) and related to disconnection of the limbic
system from the rest of the brain. Meanwhile, connectedness
within the limbic system appeared to be maintained.
Within the group with autism spectrum disorders, the
degree of functional connectedness was compared with
disease severity using the social responsiveness scale. A
close relationship was demonstrated between the severity
of autism and the connectedness of the limbic system with
other social processing regions. The brain regions most
responsible for driving correlations with social respon-
siveness scale were the ventromedial prefrontal cortex and
an area extending from the left amygdala into the left
anterior temporal cortex.
Comments and conclusions
Differences in brain connectedness between high func-
tioning adolescents with an autism spectrum disorder and
typically developing adolescents were found to be con-
centrated preferentially amongst social brain regions. In
addition, the severity of social impairments could be pre-
dicted by the connectedness of social brain regions. The
results implied disconnection between the limbic circuit
and other regions of the social brain rather than degener-
ation of the limbic system itself. These findings are con-
sistent with a mechanism of decoupling of brain regions
involved in affective aspects of social processing from
regions that mediate communication, perception and
action. Possible mechanisms suggested by the authors
include disruption of long-range anatomical connections or
alternatively local limbic cellular or structural abnormali-
ties that inhibit synchronisation with distant circuits. Future
work using magnetoencephalography (MEG) is purported
to offer promise in distinguishing between these mecha-
nisms. The high temporal and spatial resolution of MEG
might establish the degree of coherence of neural activity
between connected regions.
Gotts et al. (2012) Brain 135:2711–2725
Range of genetic mutations associated with severe
non-syndromic sporadic intellectual disability:
an exome sequencing study
The majority of cases of severe intellectual disability are
thought to have a genetic aetiology, and it is likely that de
novo genetic mutations account for a significant proportion
of unexplained cases. However, the reduced fecundity of
individuals with severe intellectual disability implies that a
wide variety of low-prevalence causative mutations are
likely. In view of this and presumed aetiological genetic
heterogeneity, an unbiased genome-wide approach seems
appropriate. Techniques of whole genome sequencing are
rapidly evolving and may not remain confined to research
laboratories indefinitely. Whole-exome sequencing has been
available since 2005 and has already been used successfully
by research groups to detect new genes underlying intellec-
tual disability.
Rauch and colleagues recruited 51 patients with severe
intellectual disability (IQ \ 60) along with their healthy
nonconsanguineous parents, to be analysed as a ‘‘trio’’.
Twenty control trios were recruited from a family study of
diabetes. Patients had sporadic, non-syndromic intellectual
disability and had been screened for causative copy number
variants by genome-wide, high-resolution arrays. Exomic
DNA from each trio was sequenced and used to identify de
novo mutations occurring in the individual with intellectual
J Neurol (2013) 260:936–939 937
123
disability. Any detected variants were validated using
Sanger sequencing to ensure that false positives
(sequencing errors) were excluded. In total, 111 de novo
variants were confirmed (1.41 per case and 1.15 per con-
trol). Synonymous variants were more common in controls
while loss of function mutations were more common in
cases of intellectual disability (39 % of cases versus 10 %
controls; p = 0.022). Genes showing de novo mutation in
individuals with intellectual disability showed significantly
more evolutionary conservation and a higher chance of
haploinsufficiency than the rest of the genome.
Sixteen de novo mutations were identified in genes
known to cause intellectual disability; eight were predicted
to cause loss of function while six others were deemed
likely to be damaging. The possible pathogenicity of other
loss-of-function mutations was assessed according to the
extent of brain expression, the haploinsufficiency index,
functional knowledge about gene homology and knowl-
edge from mouse models. Based on those factors, six other
novel mutations were predicted to have caused disease in
six new candidate genes: ARIH2, CDH2, HIVEP2,
SETD5, SLC6A1, SYNCRIP. A further six genes were
thought to be possibly causative of intellectual disability.
The 12 candidate genes were screened in 22,500 control
exomes and 179 low-coverage genomes of the 1,000
Genomes project to look for loss-of-function mutations.
Only one single nonsense mutation in SYNCRIP was
detected in the genetic material from these non-intellectu-
ally disabled individuals. Autosomal recessive and
X-linked genes appeared to make a low contribution
towards pathogenicity in individuals with severe intellec-
tual disability from nonconsanguineous parents.
Comments and conclusions
Individuals with severe intellectual disability demonstrated
a higher rate of de novo mutation than controls, and these
mutations had more potential to be pathogenic as assessed
by several criteria. The identification of 16 de novo muta-
tions in genes known to be associated with intellectual
disability suggests that exome sequencing could be a
valuable tool in investigation of this group of patients. Six
novel genes were considered likely to be causative of
intellectual disability in this cohort according to predictive
modelling. However, we are not told whether the phenotype
of those individuals matches that of other individuals with
mutations in the same gene and further proof of pathoge-
nicity might be to demonstrate consistent phenotype.
Despite this the absence of loss-of-function mutations
within twelve candidate genes in 22,500 healthy exomes is
highly suggestive of a pathogenic role for these mutations.
Rauch et al. (2012) Lancet 380:1674–1682
Diagnostic exome sequencing in persons with severe
intellectual disability
A second study of whole exome sequencing in individuals
with severe intellectual disability was published in
another core clinical journal within the same month. The
authors of this slightly larger study went to greater
lengths to characterise the phenotypic variation of the
cohort and their results lend further support for the use of
exome sequencing in the diagnosis of intellectual
disability.
Ligt and colleagues recruited 100 patients with severe
intellectual disability (IQ \ 50) and their unaffected par-
ents. All patients had undergone pre-enrolment genetic
profiling (a single nucleotide polymorphism microarray
looking for copy number variants and targeted gene testing
by a clinical geneticist) as well as metabolic testing where
appropriate. In line with the previous study, any identified
de novo variants were resequenced using Sanger
sequencing. All candidate genes identified in the initial
cohort were retested in a second validation group of ten
patients with severe intellectual disability. The strongest
five candidate genes to emerge were then screened in a
separate cohort of 765 patients with unexplained severe
intellectual disability. After validation, mutations were said
to be possible causes of intellectual disability if either (1)
they occurred in a gene known to be associated with
intellectual disability and the mutation was predicted to be
pathogenic and the phenotype of the individual(s) was
similar to phenotypes known to be associated with muta-
tions in that gene, or (2) the mutation occurred in a novel
gene, was predicted to be pathogenic, gene function was
linked to brain or embryological development, and at least
two further criteria were met according to evolutionary
conservation, brain expression patterns, gene-ontology and
animal models. A gene was considered novel and causative
if the above criteria were met and there was a striking
overlap of phenotypes of more than one individual with a
mutation in that gene.
In total, 690 candidate de novo mutations were detected,
of which 79 remained present on Sanger sequencing (1 to 4
per patient); of these 63 were nonsynonymous. Twelve de
novo mutations occurred in known intellectual disability
genes and 25 mutations were identified in 24 candidate
genes. Retesting of five candidate genes in the large vali-
dation cohort uncovered two further mutations and indi-
viduals demonstrated a consistent phenotype, therefore
confirming them as novel causative intellectual disability
genes (GATAD2B and CTNNB1). Overall, in 100 patients
the authors reported 13 de novo mutations in known
intellectual disability genes (10 autosomal dominant and 3
X-linked) and three in novel causative genes (from the 24
candidate genes).
938 J Neurol (2013) 260:936–939
123
Comments and conclusions
Intuitively, whole-exome sequencing should be well placed
to detect de novo mutations given the anticipated genetic
heterogeneity and absence of suggestive family history. In
this study the technique generated an additional 16 %
diagnostic yield in patients who had already undergone
detailed clinical and genetic evaluation. The diagnostic
yield may increase further if candidate genes are later
validated to be causative by the identification of new
individuals with a similar mutation and a matching phe-
notype. Family-based diagnostic exome sequencing
appeared to be amenable to automation and could there-
fore, be applicable in a clinical setting. Its use was further
supported by the finding that genotype-phenotype correla-
tions were imperfect, so that recognition of ‘‘syndromic’’
features cannot always be relied upon. However, feasibility
remains to be proven. The expense of the initial technique
may be steadily falling, but genetic validation of each
identified variant is required. Furthermore, the detection of
de novo mutations in unknown genes may only contribute
to uncertainty for individuals or families.
In line with the previous study, autosomal recessively
inherited mutations did not appear to play a large role in
causing intellectual disability in children of non-consan-
guineous parents. This paper reminded the reader that an
unbiased approach such as exome sequencing can also
reveal other clinically relevant genes and patients need to
be consented for this. In this study one case of a gene
carrying risk for retinoblastoma/osteosarcoma was identi-
fied so the parents were informed of the need for appro-
priate surveillance.
Ligt et al. (2012) N Engl J Med 367:1921–1929
Summary
The pattern of improving survival and longevity in patients
with complex disabilities, along with rapid advances in
imaging and genetics, offers a growing opportunity to
unravel the fascinating science of brain development.
However, it also poses the jobbing neurologist with the
challenge of keeping up to date with new techniques and
new diagnoses. These studies collectively illustrate the
advantages and disadvantages of unbiased whole-brain or
whole-genome methods. Scientists more comfortable with
hypothesis driven research may use the term ‘‘fishing
expedition’’ in relation to these studies, and yet they appear
to demonstrate that by applying rigorous analysis and val-
idation, it is possible to extract meaningful data that could
not have been collected in alternative way. A key theme that
emerges from these and other genome-wide screening
projects is that widespread collaboration with open access
to results will be crucial to maximise their potential. The
publication of 100 individual case vignettes by Ligt et al.,
deserves recognition in itself as a valuable reference tool for
subsequent studies in order to continually consolidate
genotype-phenotype associations. If whole-exome screen-
ing were ever to become a diagnostic reality, its usefulness
would rest on the availability of a central database of pre-
viously identified genotypes and phenotypes.
J Neurol (2013) 260:936–939 939
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