Background
Autism spectrum disorder (ASD) is characterized by phenotypic and genetic heterogeneity, and an association with abnormal brain growth has long been recognized. For example, there is an association between ASD and specific Mendelian disorders, such as Rett’s syndrome (microcephaly), a microdeletion syndrome at 16p11.2 (macrocephaly) [
1], and copy number variation (CNV) in genes associated with brain growth, such as
PTEN (macrocephaly) [
2]. Moreover, increased head circumference (HC) is a consistent and replicated finding among individuals with ASD, with ~20% labeled macrocephalic given norms for sex, age, and body size [
3‐
5]. Cross-sectional studies have identified significantly larger HC among individuals with ASD, and this is true for both children and adults [
3,
5]. In addition, when longitudinal data are examined, accelerated head growth during the early months of development is observed among individuals who subsequently develop ASD [
4] and there is also some evidence that larger head size is associated with greater ASD symptom severity [
6].
Beyond the known rare single gene associations for ASD, little is known about the genetic architecture of HC variance in ASD. For the population more generally, evidence from genome-wide association (GWA) studies indicates loci at 12q15 and 12q24 are associated with infant (6–18 months) HC [
7] while variants at 6q22 and 17q21 are associated with intracranial volume measured by MRI in older adults [
8]. Common variants within these associated regions tag genes of potential significance to brain growth, such as
HMGA2 (12q15) and
CRHR1 (17q21). None of these loci, however, overlap previously identified ASD genes.
Understanding the genetic architecture of abnormal brain growth in ASD may shed light on the pathogenesis of ASD, as well as identifying new ASD genes and those involved in brain development more generally. With this in mind, we examined the genetic underpinning of HC using QTL-based genome-wide linkage combined with targeted association analysis. HC has already been identified as a highly heritable trait [
6], and so we anticipated that performing genome-wide linkage of HC would be a powerful approach to narrow the genomic search space. We hypothesized that HC loci would overlap linkage signals for ASD in the same families and that family-based quantitative trait association targeted to linkage regions would fine map the identified signal(s).
Discussion
The aim of this study was to identify genetic loci for HC in families segregating ASD. In the Utah families, one locus with two signals was identified with significant evidence of linkage to HC residualized for the effects of sex, age, age2 and height. One family accounted for much of the linkage evidence. These signals were neither identified in our non-overlapping replication sample, nor in the combined discovery and replication sample, although nearby ‘suggestive’ loci were identified. Additionally, in our replication sample, several other loci were identified with ‘suggestive’ evidence of linkage.
By also carrying out linkage-signal targeted association, we were also able to identify an allele of one SNP associated with HC in the one family driving the linkage signal. This combination of linkage and targeted association is an attractive strategy for the identification of familial segregating genetic risk for complex disorders. Although we also had the opportunity to examine allele sharing in this family by way of available Illumina HumanCoreExome data, only three SNPs demonstrated minor alleles segregating among individuals with HC > 1.88 SD from the mean, and none were in coding regions.
None of the linkage signals in our study overlapped the population-level GWAS association results for HC at 18 months of age [
7] or for intracranial volume during late adulthood [
8]. Of course, these analyses were on population-level samples without ASD, the underlying genetics of which may be very different from HC in ASD. Moreover, our power to detect these associated regions using linkage analysis is likely very low.
Similarly, none of these linkage peaks overlapped those demonstrated in previous studies of these same samples using autism and related phenotypes [
9,
21,
22]. One signal for the social responsiveness scale (SRS) was recorded at 6p22.1 (LOD = 2.36, using a qualitative defined cut-off score and a recessive model of inheritance), which does not overlap the HC signal from the current analysis. No signal for SRS was observed on chromosome 1. Instead, the largest signals for all traits measured were on chromosomes 15, 13 and 7. At none of these locations were any linkage signals for HC demonstrated. Considering the previously published AGP genome-wide linkage analyses [
10,
12], suggestive evidence for linkage was found for ASD as a discrete trait on chromosome 11 and chromosomes 11 and 15 for subsets defined by phrase speech delay and IQ > 69, respectively.
The fact that our linkage signals did not overlap those for ASD in the same samples needs some explanation, as this does not support an etiological relationship between HC and ASD in these families. On the one hand, much variation in HC was seen from family to family, with some families segregating larger heads. Among such families, therefore, there may be a more intimate relationship between the aetiological factors for ASD and head size. However, even for the most significantly linked family, no overlap was seen for ASD and HC linkage signals. This does not, of course, rule out the possibility that more than one genetic mechanism, acting in tandem, is involved in the expression of the ASD phenotype. For example, a combination of one locus, influencing brain size, and another, influencing some other brain mechanism, could raise vulnerability to ASD. Additionally, power is low in both analysis, and so false negatives are highly likely.
Although 6p21.31 is a gene-rich region, our associated SNP does not overlap any expressed or regulatory elements. The most proximal genes are
NRSN1 and
DCDC2, and both are of potential interest.
NRSN1 codes for a protein involved in nerve growth and has a possible role in neurite extension [
23]. Association has been identified with ADHD [
24]. Similarly,
DCDC2 is a highly brain expressed gene with a role in neuronal migration [
25] and with exonic variants demonstrating association with developmental dyslexia [
26]. The wider linked region is relatively broad and gene rich and includes the MHC, which is associated with ASD [
27], schizophrenia [
28] and specific language delay [
29]. There is also evidence of linkage to juvenile bipolar disorder [
30].
While confounding any simple explanation for the possibility of shared genes underlying HC and ASD in these families, the study does illustrate the potential utility of the family design in targeting genetics of complex phenotypes, as well as the importance of considering a family-by-family as well as pooled approach. The identification of linkage signals for HC also raises the ongoing need to consider HC as a biomarker for brain growth that may inform the search for genes and regulatory elements that harbor susceptibility to ASD and other developmental disorders of brain growth.
Acknowledgements
We are grateful to the Genetics Study families whose participation contributed to this study. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health. Partial support for all datasets within the Utah Population Database (UPDB) was provided by the University of Utah Huntsman Cancer Institute.