A maternal influence on Reading the mind in the Eyes mediated by executive function: differential parental influences on full and half-siblings

Background

Parent-of-origin effects have been found to influence the mammalian brain and cognition and have been specifically implicated in the development of human social cognition and theory of mind. The experimental design in this study was developed to detect parent-of-origin effects on theory of mind, as measured by the ‘Reading the mind in the eyes’ (Eyes) task. Eyes scores were also entered into a principal components analysis with measures of empathy, social skills and executive function, in order to determine what aspect of theory of mind Eyes is measuring.

Methodology/Principal Findings

Maternal and paternal influences on Eyes scores were compared using correlations between pairs of full (70 pairs), maternal (25 pairs) and paternal siblings (15 pairs). Structural equation modelling supported a maternal influence on Eyes scores over the normal range but not low-scoring outliers, and also a sex-specific influence on males acting to decrease male Eyes scores. It was not possible to differentiate between genetic and environmental influences in this particular sample because maternal siblings tended to be raised together while paternal siblings were raised apart. The principal components analysis found Eyes was associated with measures of executive function, principally behavioural inhibition and attention, rather than empathy or social skills.

Conclusions/Significance

In conclusion, the results suggest a maternal influence on Eye scores in the normal range and a sex-specific influence acting to reduce scores in males. This influence may act via aspects of executive function such as behavioural inhibition and attention. There may be different influences acting to produce the lowest Eyes scores which implies that the heratibility and/or maternal influence on poor theory of mind skills may be qualitatively different to the influence on the normal range.     

The imprinted mouse Igf2r/Air cluster--a model maternal imprinting system

Every diploid organism inherits a complete chromosome set from its father and mother in addition to the sex chromosomes, so that all autosomal genes are available in two copies. For most genes, both copies are expressed without preference. Imprinted genes, however, are expressed depending on their parental origin, being active on the paternal or maternal allele only. To date 73 imprinted genes are known in mouse (www.mgu.har.mrc.ac.uk/research/imprinting), 37 show paternal expression while 36 show maternal expression, indicating no bias for imprinting to occur in one sex or the other. Therefore, two different parental-specific imprinting systems may have evolved in mammals, acting specifically in the paternal or maternal gamete. Similarities and differences between the two imprinting systems will be reviewed, with specific reference to the role of non-coding RNAs and chromatin modifications. The mouse Igf2r/Air cluster is presented as a model of the maternal imprinting system.     

Polyandry, life-history trade-offs and the evolution of imprinting at Mendelian loci

Genomic imprinting causes parental origin-dependent differential expression of a small number of genes in mammalian and angiosperm plant embryos, resulting in non-Mendelian inheritance of phenotypic traits. The "conflict" theory of the evolution of imprinting proposes that reduced genetic relatedness of paternally, relative to maternally, derived alleles in offspring of polygamous females supports parental sex-specific selection at gene loci that influence maternal investment. While the theory's physiological predictions are well supported by observation, the requirement of polyandry in the evolution of imprinting from an ancestral Mendelian state has not been comprehensively analyzed. Here, we use diallelic models to examine the influence of various degrees of polyandry on the evolution of both Mendelian and imprinted autosomal gene loci that influence trade-offs between maternal fecundity and offspring viability. We show that, given a plausible assumption on the physiological relationship between maternal fecundity and offspring viability, low levels of polyandry are sufficient to reinforce exclusively the fixation of "greedy" paternally imprinted alleles that increase offspring viability at the expense of maternal fecundity and "thrifty" maternally imprinted alleles of opposite effect. We also show that, for all levels of polyandry, Mendelian alleles at genetic loci that influence the trade-off between maternal fecundity and offspring viability reach an evolutionary stable state, whereas pairs of reciprocally imprinted alleles do not.     

An unexpected function of the Prader-Willi syndrome imprinting center in maternal imprinting in mice

Genomic imprinting is a phenomenon that some genes are expressed differentially according to the parent of origin. Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are neurobehavioral disorders caused by deficiency of imprinted gene expression from paternal and maternal chromosome 15q11-q13, respectively. Imprinted genes at the PWS/AS domain are regulated through a bipartite imprinting center, the PWS-IC and AS-IC. The PWS-IC activates paternal-specific gene expression and is responsible for the paternal imprint, whereas the AS-IC functions in the maternal imprint by allele-specific repression of the PWS-IC to prevent the paternal imprinting program. Although mouse chromosome 7C has a conserved PWS/AS imprinted domain, the mouse equivalent of the human AS-IC element has not yet been identified. Here, we suggest another dimension that the PWS-IC also functions in maternal imprinting by negatively regulating the paternally expressed imprinted genes in mice, in contrast to its known function as a positive regulator for paternal-specific gene expression. Using a mouse model carrying a 4.8-kb deletion at the PWS-IC, we demonstrated that maternal transmission of the PWS-IC deletion resulted in a maternal imprinting defect with activation of the paternally expressed imprinted genes and decreased expression of the maternally expressed imprinted gene on the maternal chromosome, accompanied by alteration of the maternal epigenotype toward a paternal state spread over the PWS/AS domain. The functional significance of this acquired paternal pattern of gene expression was demonstrated by the ability to complement PWS phenotypes by maternal inheritance of the PWS-IC deletion, which is in stark contrast to paternal inheritance of the PWS-IC deletion that resulted in the PWS phenotypes. Importantly, low levels of expression of the paternally expressed imprinted genes are sufficient to rescue postnatal lethality and growth retardation in two PWS mouse models. These findings open the opportunity for a novel approach to the treatment of PWS.     

Methods for detecting interactions between imprinted genes and environmental exposures using birth cohort designs with mother-offspring pairs

Genomic imprinting is a form of epigenetic regulation in mammals in which the same allele of a gene is expressed differently depending on the parental origin of the allele. Traditionally, the detection of imprinted genes that affect complex diseases has been focused on linkage designs with pedigrees or case-parent designs with case-parent trios. In the past two decades, the birth cohort design with mother-offspring pairs has been applied to understand better the effect of environmental influences during pregnancy and beginning of life on the growth and development of children. No work has been done on the detection of imprinted genes using birth cohort designs. Moreover, although the importance of imprinting has been well recognized, no study has looked at how environmental exposures modify the effects of imprinted genes. In this study, we show that the proposed imprinting test using the birth cohort design with mother-offspring pairs is an efficient test for testing the interactions between imprinted genes and environmental exposures. Through extensive simulation studies and a real data application, the proposed imprinting test has demonstrated much improved power in detecting gene-environment interactions than that of a test assuming the Mendelian dominant model when the true underlying genetic model is imprinting.     

Detection of Parent-of-Origin Specific Expression Quantitative Trait Loci by Cis-Association Analysis of Gene Expression in Trios

Parent-of-origin (PofO) effects, such as imprinting are a phenomenon in which homologous chromosomes exhibit differential gene expression and epigenetic modifications according to their parental origin. Such non-Mendelian inheritance patterns are generally ignored by conventional association studies, as these tests consider the maternal and paternal alleles as equivalent. To identify regulatory regions that show PofO effects on gene expression (imprinted expression Quantitative Trait Loci, ieQTLs), here we have developed a novel method in which we associate SNP genotypes of defined parental origin with gene expression levels. We applied this method to study 59 HapMap phase II parent-offspring trios. By analyzing mother/father/child trios, rules of Mendelian inheritance allowed the parental origin to be defined for ～95% of SNPs in each child. We used 680,475 informative SNPs and corresponding expression data for 92,167 probe sets from Affymetrix GeneChip Human Exon 1.0 ST arrays and performed four independent cis-association analyses with the expression level of RefSeq genes within 1 Mb using PLINK. Independent analyses of maternal and paternal genotypes identified two significant cis-ieQTLs (p<10(-7)) at which expression of genes SFT2D2 and SRRT associated exclusively with maternally inherited SNPs rs3753292 and rs6945374, respectively. 28 additional suggestive cis-associations with only maternal or paternal SNPs were found at a lower stringency threshold of p<10(-6), including associations with two known imprinted genes PEG10 and TRAPPC9, demonstrating the efficacy of our method. Furthermore, comparison of our method that utilizes independent analyses of maternal and paternal genotypes with the Likelihood Ratio Test (LRT) showed it to be more effective for detecting imprinting effects than the LRT. Our method represents a novel approach that can identify imprinted regulatory elements that control gene expression, suggesting novel PofO effects in the human genome.     

Brain-expressed imprinted genes and adult behaviour: the example of Nesp and Grb10

Imprinted genes are defined by their parent-of-origin-specific monoallelic expression. Although the epigenetic mechanisms regulating imprinted gene expression have been widely studied, their functional importance is still unclear. Imprinted genes are associated with a number of physiologies, including placental function and foetal growth, energy homeostasis, and brain and behaviour. This review focuses on genomic imprinting in the brain and on two imprinted genes in particular, Nesp and paternal Grb10, which, when manipulated in animals, have been shown to influence adult behaviour. These two genes are of particular interest as they are expressed in discrete and overlapping neural regions, recognised as key “imprinting hot spots” in the brain. Furthermore, these two genes do not appear to influence placental function and/or maternal provisioning of offspring. Consequently, by understanding their behavioural function we may begin to shed light on the evolutionary significance of imprinted genes in the adult brain, independent of the recognised role in maternal care. In addition, we discuss the potential future directions of research investigating the function of these two genes and the behavioural role of imprinted genes more generally.     

Genomic imprinting and endosperm development in flowering plants

Genomic imprinting, the parent-of-origin-specific expression of genes, plays an important role in the seed development of flowering plants. As different sets of genes are imprinted and hence silenced in maternal and paternal gametophyte genomes, the contributions of the parental genomes to the offspring are not equal. Imbalance between paternally and maternally imprinted genes, for instance as a result of interploidy crosses, or in seeds in which imprinting has been manipulated, results in aberrant seed development. It is predominantly the endosperm, and not or to a far lesser extent the embryo, that is affected by such imbalance. Deviation from the normal 2m:1p ratio in the endosperm genome has a severe effect on endosperm development, and often leads to seed abortion. Molecular expression data for imprinted genes suggest that genomic imprinting takes place only in the endosperm of the developing seed. Although far from complete, a picture of how imprinting operates in flowering plants has begun to emerge. Imprinted genes on either the maternal or paternal side are marked and silenced in a process involving DNA methylation and chromatin condensation. In addition, on the maternal side, imprinted genes are most probably under control of the polycomb FIS genes.     

Genomic imprinting in the placenta

Genomic imprinting is an epigenetic mechanism that is important for the development and function of the extra-embryonic tissues in the mouse. Remarkably all the autosomal genes which were found to be imprinted in the trophoblast (placenta) only are active on the maternal and repressed on the paternal allele. It was shown for several of these genes that their paternal silencing is not dependent on DNA methylation, at least not in its somatic maintenance. Rather, recent studies in the mouse suggest that placenta-specific imprinting involves repressive histone modifications and non-coding RNAs. This mechanism of autosomal imprinting is similar to imprinted X chromosome inactivation in the placenta. Although the underlying reasons remain to be explored, this suggests that imprinting in the placenta and imprinted X inactivation are evolutionarily related.     

An extension of the transmission disequilibrium test incorporating imprinting

The recombination rates in meioses of females and males are often different. Some genes that affect development and behavior in mammals are known to be imprinted, and >1% of all mammalian genes are believed to be imprinted. When the gene is imprinted and the recombination fractions are sex specific, the conventional transmission disequilibrium test (TDT) is shown to be still valid for testing for linkage. The power function of the TDT is derived, and the effect of the degree of imprinting on the power of the TDT is investigated. It is learned that imprinting has little effect on the power when the female and male recombination rates are equal. On the basis of case-parents trios, the transmissions from the heterozygous fathers/mothers to their affected children are separated as paternal and maternal, and two TDT-like statistics, TDT(p) and TDT(m), are consequently constructed. It is found that the TDT(p) possesses a higher power than the TDT for maternal imprinting genes, and the TDT(m) is more powerful than the TDT for paternal imprinting genes. On the basis of the parent-of-origin effects test statistic (POET), a novel statistic, TDT incorporating imprinting (TDTI) is proposed to test for linkage in the presence of linkage disequilibrium, which is shown to be more powerful than the TDT when parent-of-origin effects are significant but slightly less powerful than the TDT when parent-of-origin effects are negligible. The validity of the TDT and TDTI is assessed by simulation. The power approximation formulas for the TDT and TDTI are derived and the simulation results show that they are accurate. The simulation study on power comparison shows that the TDTI outperforms the TDT for imprinted genes. The improvement can be substantial in the case of complete paternal/maternal imprinting.     

The Evolution of Genomic Imprinting

In some mammalian genes, the paternally and maternally derived alleles are expressed differently: this phenomenon is called genomic imprinting. Here we study the evolution of imprinting using multivariate quantitative genetic models to examine the feasibility of the genetic conflict hypothesis. This hypothesis explains the observed imprinting patterns as an evolutionary outcome of the conflict between the paternal and maternal alleles. We consider the expression of a zygotic gene, which codes for an embryonic growth factor affecting the amount of maternal resources obtained through the placenta. We assume that the gene produces the growth factor in two different amounts depending on its parental origin. We show that genomic imprinting evolves easily if females have some probability of multiple partners. This is in conflict with the observation that not all genes controlling placental development are imprinted and that imprinting in some genes is not conserved between mice and humans. We show however that deleterious mutations in the coding region of the gene create selection against imprinting.     

Shared role for differentially methylated domains of imprinted genes

For most imprinted genes, a difference in expression between the maternal and paternal alleles is associated with a corresponding difference in DNA methylation that is localized to a differentially methylated domain (DMD). Removal of a gene's DMD leads to a loss of imprinting. These observations suggest that DMDs have a determinative role in genomic imprinting. To examine this possibility, we introduced sequences from the DMDs of the imprinted Igf2r, H19, and Snrpn genes into a nonimprinted derivative of the normally imprinted RSVIgmyc transgene, created by excising its own DMD. Hybrid transgenes with sequences from the Igf2r DMD2 were consistently imprinted, with the maternal allele being more methylated than the paternal allele. Only the repeated sequences within DMD2 were required for imprinting these transgenes. Hybrid transgenes containing H19 and Snrpn DMD sequences and ones containing sequences from the long terminal repeat of a murine intracisternal A particle retrotransposon were not imprinted. The Igf2r hybrid transgenes are comprised entirely of mouse genomic DNA and behave as endogenous imprinted genes in inbred wild-type and mutant mouse strains. These types of hybrid transgenes can be used to elucidate the functions of DMD sequences in genomic imprinting.     

Sex specific X chromosome expression caused by genomic imprinting

The conflict theory of genomic imprinting predicts that imprinted genes are growth enhancing when paternally expressed and growth suppressing when maternally expressed. The expression pattern of autosomal imprinted genes generally fits these predictions. However, the conflict theory cannot easily account for the pattern of X-linked imprinting in humans and mice. This has led us to propose a novel hypothesis that X-linked imprinting has evolved to control sex specific gene expression in early embryos. The hypothesis links paternal X-imprinting (i.e. paternal copy silencing) to random X-inactivation and the retention of Y-linked copies, and links maternal X-imprinting to escape from random X-inactivation and the loss of Y-linked copies.The hypothesis offers a good explanation of the existing data on X-imprinted genes.     

Imprinting of the murine MAS protooncogene is restricted to its antisense RNA.

The Mas protooncogene encodes a G protein-coupled receptor with the common seven transmembrane domains and may be involved in the actions of angiotensins. The gene is located in close proximity to the paternally imprinted Igf2r gene and its maternal imprinting has been reported but remained controversial. We used mice carrying a targeted deletion of the Mas protooncogene on the maternal or paternal chromosome to clarify this issue. In all Mas-expressing organs of adult mice such as heart, kidney, testis or brain, no Mas mRNA was missing in heterozygous animals inheriting the deleted allele from the father excluding mono-allelic paternal expression. However, we show exclusive paternal expression of a Mas antisense RNA, confirming the maternal imprinting of this antisense RNA in all investigated adult tissues and in embryos. Our results strongly suggest that Mas is not imprinted in mice but that an antisense RNA probably starting in the neighboring Igf2r gene is maternally imprinted in both embryos and adult organs.     

A conceptual contribution to battles in the brain

Badcock and Crespi have advanced the hypothesis that autism and schizophrenia are caused by imbalanced imprinting in the brain. They argue that an imbalance between the effects of paternally and maternally expressed genes on brain development results in either an extreme paternal (autism) or maternal brain (schizophrenia). In this paper their conceptual model is discussed and criticized since it presupposes an incoherent distinction between observable physical and hidden mental phenomena. An alternative model is discussed that may be more fruitful for investigating the possible role of imprinted genes in the development of social behaviour. The development of crying and reactive crying and behaviours necessary for collaborative action are discussed as a promising research area for understanding the effects of imprinted genes.     

Coadaptation and conflict,misconception and muddle,in the evolution of genomic imprinting

Common misconceptions of the ‘parental conflict'' theory of genomic imprinting are addressed. Contrary to widespread belief, the theory defines conditions for cooperation as well as conflict in mother–offspring relations. Moreover, conflict between genes of maternal and paternal origin is not the same as conflict between mothers and fathers. In theory, imprinting can evolve either because genes of maternal and paternal origin have divergent interests or because offspring benefit from a phenotypic match, or mismatch, to one or other parent. The latter class of models usually require maintenance of polymorphism at imprinted loci for the maintenance of imprinted expression. The conflict hypothesis does not require maintenance of polymorphism and is therefore a more plausible explanation of evolutionarily conserved imprinting.     

Heat stress causes aberrant DNA methylation of H19 and Igf-2r in mouse blastocysts

To gain a better understanding of the methylation imprinting changes associated with heat stress in early development, we used bisulfite sequencing and bisulfite restriction analysis to examine the DNA methylation status of imprinted genes in early embryos (blastocysts). The paternal imprinted genes, H19 and Igf-2r, had lower methylation levels in heat-stressed embryos than in control embryos, whereas the maternal imprinted genes, Peg3 and Peg1, had similar methylation pattern in heat-stressed embryos and in control embryos. Our results indicate that heat stress may induce aberrant methylation imprinting, which results in developmental failure of mouse embryos, and that the effects of heat shock on methylation imprinting may be gene-specific.     

Identification of an Imprinted Gene Cluster in the X-Inactivation Center

Mammalian development is strongly influenced by the epigenetic phenomenon called genomic imprinting, in which either the paternal or the maternal allele of imprinted genes is expressed. Paternally expressed Xist, an imprinted gene, has been considered as a single cis-acting factor to inactivate the paternally inherited X chromosome (Xp) in preimplantation mouse embryos. This means that X-chromosome inactivation also entails gene imprinting at a very early developmental stage. However, the precise mechanism of imprinted X-chromosome inactivation remains unknown and there is little information about imprinted genes on X chromosomes. In this study, we examined whether there are other imprinted genes than Xist expressed from the inactive paternal X chromosome and expressed in female embryos at the preimplantation stage. We focused on small RNAs and compared their expression patterns between sexes by tagging the female X chromosome with green fluorescent protein. As a result, we identified two micro (mi)RNAs–miR-374-5p and miR-421-3p–mapped adjacent to Xist that were predominantly expressed in female blastocysts. Allelic expression analysis revealed that these miRNAs were indeed imprinted and expressed from the Xp. Further analysis of the imprinting status of adjacent locus led to the discovery of a large cluster of imprinted genes expressed from the Xp: Jpx, Ftx and Zcchc13. To our knowledge, this is the first identified cluster of imprinted genes in the cis-acting regulatory region termed the X-inactivation center. This finding may help in understanding the molecular mechanisms regulating imprinted X-chromosome inactivation during early mammalian development.     

Mouse imprinting defect mutations that model Angelman syndrome

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are neurobehavioral disorders resulting from deficiency of imprinted gene expression from paternal or maternal chromosome 15q11-15q13, respectively. In humans, expression of the imprinted genes is under control of a bipartite cis-acting imprinting center (IC). Families with deletions causing PWS imprinting defects localize the PWS-IC to 4.3 kb overlapping with SNRPN exon 1. Families with deletions causing AS imprinting defects localize the AS-IC to 880 bp 35 kb upstream of the PWS-IC. We report two mouse mutations resulting in defects similar to that seen in AS patients with deletion of the AS-IC. An insertion/duplication mutation 13 kb upstream of Snrpn exon 1 resulted in lack of methylation at the maternal Snrpn promoter, activation of maternally repressed genes, and decreased expression of paternally repressed genes. The acquisition of a paternal epigenotype on the maternal chromosome in the mutant mice was demonstrated by the ability to rescue the lethality and growth retardation in a mouse model of a PWS imprinting defect. A second mutation, an 80-kb deletion extending upstream of the first mutation, caused a similar imprinting defect with variable penetrance. These results suggest that there is a mouse functional equivalent to the human AS-IC.