Family-based association study of ITGB3 in autism spectrum disorder and its endophenotypes

The integrin-β 3 gene (ITGB3), located on human chromosome 17q21.3, was previously identified as a quantitative trait locus (QTL) for 5-HT blood levels and has been implicated as a candidate gene for autism spectrum disorder (ASD). We performed a family-based association study in 281 simplex and 12 multiplex Caucasian families. ITGB3 haplotypes are significantly associated with autism (HBAT, global P=0.038). Haplotype H3 is largely over-transmitted to the affected offspring and doubles the risk of an ASD diagnosis (HBAT P=0.005; odds ratio (OR)=2.000), at the expense of haplotype H1, which is under-transmitted (HBAT P=0.018; OR=0.725). These two common haplotypes differ only at rs12603582 located in intron 11, which reaches a P-value of 0.072 in single-marker FBAT analyses. Interestingly, rs12603582 is strongly associated with pre-term delivery in our ASD patients (P=0.008). On the other hand, it is SNP rs2317385, located at the 5' end of the gene, that significantly affects 5-HT blood levels (Mann-Whitney U-test, P=0.001; multiple regression analysis, P=0.010). No gene-gene interaction between ITGB3 and SLC6A4 has been detected. In conclusion, we identify a significant association between a common ITGB3 haplotype and ASD. Distinct markers, located toward the 5' and 3' ends of the gene, seemingly modulate 5-HT blood levels and autism liability, respectively. Our results also raise interest into ITGB3 influences on feto-maternal immune interactions in autism.

Effect of the interaction between the serotonin transporter gene and maternal environment on developing mouse brain

A number of studies have demonstrated that the common polymorphism in the serotonin transporter gene (5-HTT-LPR) moderates the increased risk for major depression seen in persons exposed to early adverse experiences. Several mouse models of this interaction have been recently established to investigate the increased vulnerability of individuals carrying the 5-HTT-LPR S allele to both early and adult life stressful events. Identifying the immediate effects of an adverse early environment on genetically susceptible individuals is critical to develop effective prevention of its long-term negative consequences of such an interaction. For this purpose we investigated molecular and neurochemical effects promoted by variable amount of maternal care in the brain of developing (postnatal day 10) wild type and heterozygous serotonin transporter knockout mice. Pups experiencing low level of maternal care showed increased levels of brain-derived neurotrophic factor (BDNF) messenger RNA within the hippocampus and primary somato-sensory cortex, and increased α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor binding in hippocampus in comparison with pups experiencing high level of maternal care. Strikingly, only heterozygous serotonin transporter knockout pups experiencing high maternal care showed increased hippocampal levels of serotonin and norepinephrine and decreased serotonin turnover compared to wild-type littermates. These findings support the hypothesis that maternal care affects the development of the hippocampus and primary somato-sensory cortex of individuals characterized by genetic variants of the serotonin transporter.

Increased vulnerability to psychosocial stress in heterozygous serotonin transporter knockout mice

Epidemiological evidence links exposure to stressful life events with increased risk for mental illness. However, there is significant individual variability in vulnerability to environmental risk factors, and genetic variation is thought to play a major role in determining who will become ill. Several studies have shown, for example, that individuals carrying the S (short) allele of the serotonin transporter (5-HTT) gene-linked polymorphic region (5-HTTLPR) have an increased risk for major depression following exposure to stress in adulthood. Identifying the molecular mechanisms underlying this gene-by-environment risk factor could help our understanding of the individual differences in resilience to stress. Here, we present a mouse model of the 5-HTT-by-stress risk factor. Wild-type and heterozygous 5-HTT knockout male mice were subjected to three weeks of chronic psychosocial stress. The 5-HTT genotype did not affect the physiological consequences of stress as measured by changes in body temperature, body weight gain and plasma corticosterone. However, when compared with wild-type littermates, heterozygous 5-HTT knockout mice experiencing high levels of stressful life events showed significantly depressed locomotor activity and increased social avoidance toward an unfamiliar male in a novel environment. Heterozygous 5-HTT knockout mice exposed to high stress also showed significantly lower levels of serotonin turnover than wild-type littermates, selectively in the frontal cortex, which is a structure that is known to control fear and avoidance responses, and that is implicated in susceptibility to depression. These data may serve as a useful animal model for better understanding the increased vulnerability to stress reported in individuals carrying the 5-HTTLPR S allele, and suggest that social avoidance represents a behavioral endophenotype of the interaction between 5-HTT and stress.

Involvement of the PRKCB1 gene in autistic disorder: significant genetic association and reduced neocortical gene expression

Protein kinase C enzymes play an important role in signal transduction, regulation of gene expression and control of cell division and differentiation. The fsI and betaII isoenzymes result from the alternative splicing of the PKCbeta gene (PRKCB1), previously found to be associated with autism. We performed a family-based association study in 229 simplex and 5 multiplex families, and a postmortem study of PRKCB1 gene expression in temporocortical gray matter (BA41/42) of 11 autistic patients and controls. PRKCB1 gene haplotypes are significantly associated with autism (P<0.05) and have the autistic endophenotype of enhanced oligopeptiduria (P<0.05). Temporocortical PRKCB1 gene expression was reduced on average by 35 and 31% for the PRKCB1-1 and PRKCB1-2 isoforms (P<0.01 and <0.05, respectively) according to qPCR. Protein amounts measured for the PKCbetaII isoform were similarly decreased by 35% (P=0.05). Decreased gene expression characterized patients carrying the 'normal' PRKCB1 alleles, whereas patients homozygous for the autism-associated alleles displayed mRNA levels comparable to those of controls. Whole genome expression analysis unveiled a partial disruption in the coordinated expression of PKCbeta-driven genes, including several cytokines. These results confirm the association between autism and PRKCB1 gene variants, point toward PKCbeta roles in altered epithelial permeability, demonstrate a significant downregulation of brain PRKCB1 gene expression in autism and suggest that it could represent a compensatory adjustment aimed at limiting an ongoing dysreactive immune process. Altogether, these data underscore potential PKCbeta roles in autism pathogenesis and spur interest in the identification and functional characterization of PRKCB1 gene variants conferring autism vulnerability.

Identifying molecular substrates in a mouse model of the serotonin transporter x environment risk factor for anxiety and depression

BACKGROUND

A polymorphism in the serotonin transporter (5-HTT) gene modulates the association between adverse early experiences and risk for major depression in adulthood. Although human imaging studies have begun to elucidate the neural circuits involved in the 5-HTT x environment risk factor, a molecular understanding of this phenomenon is lacking. Such an understanding might help to identify novel targets for the diagnosis and therapy of mood disorders. To address this need, we developed a gene-environment screening paradigm in the mouse.

METHODS

We established a mouse model in which a heterozygous null mutation in 5-HTT moderates the effects of poor maternal care on adult anxiety and depression-related behavior. Biochemical analysis of brains from these animals was performed to identify molecular substrates of the gene, environment, and gene x environment effects.

RESULTS

Mice experiencing low maternal care showed deficient gamma-aminobutyric acid-A receptor binding in the amygdala and 5-HTT heterozygous null mice showed decreased serotonin turnover in hippocampus and striatum. Strikingly, levels of brain-derived neurotrophic factor (BDNF) messenger RNA in hippocampus were elevated exclusively in 5-HTT heterozygous null mice experiencing poor maternal care, suggesting that developmental programming of hippocampal circuits might underlie the 5-HTT x environment risk factor.

CONCLUSIONS

These findings demonstrate that serotonin plays a similar role in modifying the long-term behavioral effects of rearing environment in diverse mammalian species and identifies BDNF as a molecular substrate of this risk factor.

Reduced availability of brain amines during critical phases of postnatal development in a genetic mouse model of cognitive delay

Serotonin (5-HT), dopamine (DA) and noradrenaline (NE) play important roles in brain postnatal maturation. Therefore, deficits in brain availability of biogenic amines during critical developmental phases might underlie neurodevelopmental disturbances associated with cognitive impairment. To test this hypothesis we evaluated brain availability of 5-HT, DA and NE, of their immediate precursors 5-hydroxytryptophan and 3,4-dihydroxy-l-phenylalanine, and of large neutral amino acids phenylalanine, tyrosine and tryptophan, in developing PahEnu2 mice, the genetic model of Phenylketonuria (PKU) a cause of severe cognitive delay. We found deficits of brain amine levels in PKU pups between day 14 and 35 of postnatal life, when pups of the healthy background showed developmental peak increases of amines and precursors. 5-HT deficits were most pronounced, were unrelated with brain availability of the amino acid precursor tryptophan, but overlapped with peak brain phenylalanine concentrations and reduced availability of 5-HT direct precursor 5-hydroxytryptophan. These results identify a critical window of brain amine availability susceptible to disturbances in a genetic mouse model of pathological neurodevelopment and suggest a mechanism of interference with brain aminergic synthesis in PKU and non-PKU hyperphenylalaninemia.

Spatial deficits in a mouse model of Parkinson disease

RATIONALE

Accumulating evidence in humans demonstrated that visuo-spatial deficits are the most consistently reported cognitive abnormalities in Parkinson disease (PD). These deficits have been generally attributed to cortical dopamine degeneration. However, more recent evidence suggests that dopamine loss in the striatum is responsible for the visuo-spatial abnormalities in PD. Studies based on animal models of PD did not specifically address this question.

OBJECTIVES

Thus, the first goal of this study was to analyze the role of dopamine within the dorsal striatum in spatial memory. We tested bilateral 6-OHDA striatal lesioned CD1 mice in an object-place association spatial task. Furthermore, to see whether the effects were selective for spatial information, we measured how the 6-OHDA-lesioned animals responded to a non-spatial change and learned in the one-trial inhibitory avoidance task.

RESULTS

The results demonstrated that bilateral (approximately 75%) dopamine depletion of the striatum impaired spatial change discrimination. On the contrary, no effect of the lesion was observed on non-spatial novelty detection or on passive avoidance learning.

CONCLUSIONS

These results confirm that dopamine depletion is accompanied by cognitive deficits and demonstrate that striatal dopamine dysfunction is sufficient to induce spatial information processing deficits.

Clinical, morphological, and biochemical correlates of head circumference in autism

BACKGROUND

Head growth rates are often accelerated in autism. This study is aimed at defining the clinical, morphological, and biochemical correlates of head circumference in autistic patients.

METHODS

Fronto-occipital head circumference was measured in 241 nonsyndromic autistic patients, 3 to 16 years old, diagnosed according to DSM-IV criteria. We assessed 1) clinical parameters using the Autism Diagnostic Observation Schedule, Autism Diagnostic Interview-Revised, Vineland Adaptive Behavioral Scales, intelligence quotient measures, and an ad hoc clinical history questionnaire; 2) height and weight; 3) serotonin (5-HT) blood levels and peptiduria.

RESULTS

The distribution of cranial circumference is significantly skewed toward larger head sizes (p < .00001). Macrocephaly (i.e., head circumference >97th percentile) is generally part of a broader macrosomic endophenotype, characterized by highly significant correlations between head circumference, weight, and height (p < .001). A head circumference >75th percentile is associated with more impaired adaptive behaviors and with less impairment in IQ measures and motor and verbal language development. Surprisingly, larger head sizes are significantly associated with a positive history of allergic/immune disorders both in the patient and in his/her first-degree relatives.

CONCLUSIONS

Our study demonstrates the existence of a macrosomic endophenotype in autism and points toward pathogenetic links with immune dysfunctions that we speculate either lead to or are associated with increased cell cycle progression and/or decreased apoptosis.

Case-control and family-based association studies of candidate genes in autistic disorder and its endophenotypes: TPH2 and GLO1

Background

The TPH2 gene encodes the enzyme responsible for serotonin (5-HT) synthesis in the Central Nervous System (CNS). Stereotypic and repetitive behaviors are influenced by 5-HT, and initial studies report an association of TPH2 alleles with childhood-onset obsessive-compulsive disorder (OCD) and with autism. GLO1 encodes glyoxalase I, the enzyme which detoxifies α-oxoaldehydes such as methylglyoxal in all living cells. The A111E GLO1 protein variant, encoded by SNP C419A, was identifed in autopsied autistic brains and proposed to act as an autism susceptibility factor. Hyperserotoninemia, macrocephaly, and peptiduria represent some of the best-characterized endophenotypes in autism research.

Methods

Family-based and case-control association studies were performed on clinical samples drawn from 312 simplex and 29 multiplex families including 371 non-syndromic autistic patients and 156 unaffected siblings, as well as on 171 controls. TPH2 SNPs rs4570625 and rs4565946 were genotyped using the TaqMan assay; GLO1 SNP C419A was genotyped by PCR and allele-specific restriction digest. Family-based association analyses were performed by TDT and FBAT, case-control by χ², endophenotypic analyses for 5-HT blood levels, cranial circumference and urinary peptide excretion rates by ANOVA and FBAT.

Results

TPH2 alleles and haplotypes are not significantly associated in our sample with autism (rs4570625: TDT P = 0.27, and FBAT P = 0.35; rs4565946: TDT P = 0.45, and FBAT P = 0.55; haplotype P = 0.84), with any endophenotype, or with the presence/absence of prominent repetitive and stereotyped behaviors (motor stereotypies: P = 0.81 and 0.84, verbal stereotypies: P = 0.38 and 0.73 for rs4570625 and rs4565946, respectively). Also GLO1 alleles display no association with autism (191 patients vs 171 controls, P = 0.36; TDT P = 0.79, and FBAT P = 0.37), but unaffected siblings seemingly carry a protective gene variant marked by the A419 allele (TDT P < 0.05; patients vs unaffected siblings TDT and FBAT P < 0.00001).

Conclusion

TPH2 gene variants are unlikely to contribute to autism or to the presence/absence of prominent repetitive behaviors in our sample, although an influence on the intensity of these behaviors in autism cannot be excluded. GLO1 gene variants do not confer autism vulnerability in this sample, but allele A419 apparently carries a protective effect, spurring interest into functional correlates of the C419A SNP.

The Medial Prefrontal Cortex Determines the Accumbens Dopamine Response to Stress through the Opposing Influences of Norepinephrine and Dopamine

Although the medial prefrontal cortex (mpFC) appears to constrain stress responses, indirect evidences suggest that it might determine the stress response of the mesoaccumbens dopamine (DA) system. To test this hypothesis, we first evaluated the dynamics of norepinephrine (NE) and DA release in the mpFC and of DA release in the nucleus accumbens (NAc) of acutely stressed rats. Then, we tested the effects of selective depletion of NE or DA in the mpFC (by local 6-hydroxydopamine infusion following desipramine or 1-[2[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine(GBR 12909) on stress-induced changes in mesoaccumbens DA release. Rats experiencing restraint stress for 240 min showed an initial, short-lived increase of NE outflow in the mpFC and of DA in the NAc. These responses were followed by a sustained increase of DA in the mpFC and by a decrease to below resting levels of DA in the NAc. Moreover, selective prefrontal NE depletion eliminated the increase of NE in the mpFC and of DA in the NAc, and selective depletion of mesocortical DA eliminated the enhancement of mpFC DA as well as the inhibition of mesoaccumbens DA, without affecting basal catecholamines outflow. These results demonstrate that the opposing influences of mpFC NE and DA determine mesoaccumbens DA response to stress and suggest that alterations of this mechanism may be responsible for some major psychopathological outcomes of stress.

TLQP-21, a VGF-derived peptide, increases energy expenditure and prevents the early phase of diet-induced obesity

The vgf gene has been identified as an energy homeostasis regulator. Vgf encodes a 617-aa precursor protein that is processed to yield an incompletely characterized panel of neuropeptides. Until now, it was an unproved assumption that VGF-derived peptides could regulate metabolism. Here, a VGF peptide designated TLQP-21 was identified in rat brain extracts by means of immunoprecipitation, microcapillary liquid chromatography-tandem MS, and database searching algorithms. Chronic intracerebroventricular (i.c.v.) injection of TLQP-21 (15 mug/day for 14 days) increased resting energy expenditure (EE) and rectal temperature in mice. These effects were paralleled by increased epinephrine and up-regulation of brown adipose tissue beta2-AR (beta2 adrenergic receptor) and white adipose tissue (WAT) PPAR-delta (peroxisome proliferator-activated receptor delta), beta3-AR, and UCP1 (uncoupling protein 1) mRNAs and were independent of locomotor activity and thyroid hormones. Hypothalamic gene expression of orexigenic and anorexigenic neuropeptides was unchanged. Furthermore, in mice that were fed a high-fat diet for 14 days, TLQP-21 prevented the increase in body and WAT weight as well as hormonal changes that are associated with a high-fat regimen. Biochemical and molecular analyses suggest that TLQP-21 exerts its effects by stimulating autonomic activation of adrenal medulla and adipose tissues. In conclusion, we present here the identification in the CNS of a previously uncharacterized VGF-derived peptide and prove that its chronic i.c.v. infusion effected an increase in EE and limited the early phase of diet-induced obesity.

Paraoxonase gene variants are associated with autism in North America, but not in Italy: possible regional specificity in gene-environment interactions

Organophosphates (OPs) are routinely used as pesticides in agriculture and as insecticides within the household. Our prior work on Reelin and APOE delineated a gene-environment interactive model of autism pathogenesis, whereby genetically vulnerable individuals prenatally exposed to OPs during critical periods in neurodevelopment could undergo altered neuronal migration, resulting in an autistic syndrome. Since household use of OPs is far greater in the USA than in Italy, this model was predicted to hold validity in North America, but not in Europe. Here, we indirectly test this hypothesis by assessing linkage/association between autism and variants of the paraoxonase gene (PON1) encoding paraoxonase, the enzyme responsible for OP detoxification. Three functional single nucleotide polymorphisms, PON1 C-108T, L55M, and Q192R, were assessed in 177 Italian and 107 Caucasian-American complete trios with primary autistic probands. As predicted, Caucasian-American and not Italian families display a significant association between autism and PON1 variants less active in vitro on the OP diazinon (R192), according to case-control contrasts (Q192R: chi2=6.33, 1 df, P<0.025), transmission/disequilibrium tests (Q192R: TDT chi2=5.26, 1 df, P<0.025), family-based association tests (Q192R and L55M: FBAT Z=2.291 and 2.435 respectively, P<0.025), and haplotype-based association tests (L55/R192: HBAT Z=2.430, P<0.025). These results are consistent with our model and provide further support for the hypothesis that concurrent genetic vulnerability and environmental OP exposure may possibly contribute to autism pathogenesis in a sizable subgroup of North American individuals.

Activation of TRPV1 in the VTA excites dopaminergic neurons and increases chemical- and noxious-induced dopamine release in the nucleus accumbens

Dopamine (DA)-containing neurons of the ventral tegmental area (VTA) provide dopaminergic input to the nucleus accumbens and to the prefrontal cortex within the mesolimbic pathway. In the present study, we combined electrophysiological recordings and microdialysis techniques to investigate the function of transient receptor potential vanilloid 1 (TRPV1) channel in the VTA. In brain slices, application of the TRPV1 receptor agonist capsaicin increased the firing rate of rat dopamine neurons and in a proportion of tested cells (44%) it also induced a bursting behavior. The effects of capsaicin were concentration dependent. The increase in neuronal firing was dependent on enhanced glutamatergic transmission since it was blocked by the superfusion of the ionotropic glutamate antagonists, CNQX and AP5. Interestingly, microinjection of capsaicin into the VTA and noxious tail stimulation transiently enhanced dopamine release into the nucleus accumbens. Both the in vitro and in vivo effects were mediated by TRPV1 activation in the VTA since they were reduced by co-perfusion of the selective TRPV1 receptor antagonist iodoresineferatoxin. Our data suggest a novel role for TRPV1 channels in the mesencephalon of rat, namely activation of the DA system following a peripheral noxious stimulation.

Enhanced APOE2 transmission rates in families with autistic probands

We have previously described linkage/association between reelin gene polymorphisms and autistic disorder. APOE also participates in the Reelin signaling pathway, by competitively antagonizing Reelin binding to APOE receptor 2 and to very-low-density lipoprotein receptors. The APOE2 protein variant displays the lowest receptor binding affinity compared with APOE3 and APOE4. In this study, we assess linkage/association between primary autism and APOE alleles in 223 complete trios, from 119 simplex Italian families and 44 simplex and 29 multiplex Caucasian-American families. Statistically significant disequilibrium favors the transmission of epsilon2 alleles to autistic offspring, over epsilon3 and epsilon4 (allele-wise transmission/disequilibrium test [TDT], chi2 = 6.16, 2 degrees of freedom [d.f.], P<0.05; genotype-wise TDT, chi2 = 10.68, 3 d.f., P<0.05). A novel epsilon3r allele was also discovered in an autistic child and his mother. Autistic patients do not differ significantly from unaffected siblings (allele-wise TDT comparing autistic patients versus unaffected sibs, chi2 = 1.83, 2 d.f., P<0.40, not significant). The major limitation of this study consists of our small sample size of trios including one unaffected sibling, currently not possessing the statistical power necessary to conclusively discriminate a specific association of epsilon2 with autism, from a distorted segregation pattern characterized by enhanced epsilon2 transmission rates both to affected and unaffected offspring. Our findings are thus compatible with either (a) pathogenetic contributions by epsilon2 alleles to autism spectrum vulnerability, requiring additional environmental and/or genetic factors to yield an autistic syndrome, and/or (b) a protective effect of epsilon2 alleles against the enhanced risk of miscarriage and infertility previously described among parents of autistic children.

Object recognition impairment in Fmr1 knockout mice is reversed by amphetamine: involvement of dopamine in the medial prefrontal cortex

Fragile X syndrome is an X-linked form of mental retardation including, among others, symptoms such as stereotypic behaviour, hyperactivity, hyperarousal, and cognitive deficits. We hypothesized that hyperactivity and/or compromised attentional, cognitive functions may lead to impaired performance in cognitive tasks in Fmr1 knockout mice, the most widely used animal model of fragile X syndrome, and suggested that psychostimulant treatment may improve performance by acting on one or both components. Since hyperactivity and cognitive functions have been suggested to depend on striatal and prefrontal cortex dopaminergic dysfunction, we assessed whether amphetamine produced beneficial, positive effects by acting on dopaminergic corticostriatal systems. Our results show that Fmr1 knockout mice are not able to discriminate between a familiar object and a novel one in the object recognition test, thus showing a clear-cut cognitive impairment that, to date, has been difficult to demonstrate in other cognitive tasks. Amphetamine improved performance of Fmr1 knockout mice, leading to enhanced ability to discriminate novel versus familiar objects, without significantly affecting locomotor activity. In agreement with behavioural data, amphetamine produced a greater increase in dopamine release in the prefrontal cortex of Fmr1 knockout compared with the wild-type mice, while a weak striatal dopaminergic response was observed in Fmr1 knockout mice. Our data support the view that the psychostimulant ameliorates performance in Fmr1 knockout mice by improving merely cognitive functions through its action on prefrontal cortical dopamine, irrespective of its action on motor hyperactivity. These results indicate that prefrontal cortical dopamine plays a major role in cognitive impairments characterizing Fmr1 knockout mice, thus pointing to an important aetiological factor in the fragile X syndrome.

Altered vulnerability to kainate excitotoxicity of transgenic-Cu/Zn SOD1 neurones

The neurotoxicity of the AMPA/kainate receptor agonist kainate was investigated in motor and cortical neurones from mice over-expressing the wild-type and G93A mutant form of Cu/Zn superoxide dismutase (SOD1) human gene, a mouse model of familial amyotrophic lateral sclerosis. G93A mutant motor neurones were more vulnerable and wild-type SOD1 motor neurones were more resistant to kainate toxicity than were controls. Voltage-gated Na channels blockage prevented G93A mutant SOD1 motor neurone death. Cortical cultures exhibited fewer differences in their vulnerability to kainate toxicity. These results demonstrate that SOD1 over-expression selectively affects the sensitivity to kainate excitotoxicity of motor neurones but not neocortical neurones, and that wild-type SOD1 expression increases the resistance to excitotoxicity of motor neurones.

Association between the HOXA1 A218G polymorphism and increased head circumference in patients with autism

BACKGROUND

The HOXA1 gene plays a major role in brainstem and cranial morphogenesis. The G allele of the HOXA1 A218G polymorphism has been previously found associated with autism.

METHODS

We performed case-control and family-based association analyses, contrasting 127 autistic patients with 174 ethnically matched controls, and assessing for allelic transmission disequilibrium in 189 complete trios.

RESULTS

A, and not G, alleles were associated with autism using both case-control (chi(2) = 8.96 and 5.71, 1 df, p <.005 and <.025 for genotypes and alleles, respectively), and family-based (transmission/disequilibrium test chi(2) = 8.80, 1 df, p <.005) association analyses. The head circumference of 31 patients carrying one or two copies of the G allele displayed significantly larger median values (95.0th vs. 82.5th percentile, p <.05) and dramatically reduced interindividual variability (p <.0001), compared with 166 patients carrying the A/A genotype.

CONCLUSIONS

The HOXA1 A218G polymorphism explains approximately 5% of the variance in the head circumference of autistic patients and represents to our knowledge the first known gene variant providing sizable contributions to cranial morphology. The disease specificity of this finding is currently being investigated. Nonreplications in genetic linkage/association studies could partly stem from the dyshomogeneous distribution of an endophenotype morphologically defined by cranial circumference.

The behavioral profile of severe mental retardation in a genetic mouse model of phenylketonuria

Pah(enu2) mice, created by chemically induced genetic mutation, are characterized by biochemical phenotypes closely resembling untreated human phenylketonuria (PKU). However, studies conducted in adult Pah(enu2) mice have shown no indices of the severe mental retardation that characterizes untreated PKU. The present experiments explored recognition of novel spatial and non-spatial information in Pah(enu2) mice by two nonassociative tests that do not use explicit reinforcement and avoid lengthy training. Moreover, we evaluated emotional reactivity by the Elevated Plus Maze. Finally, the performance of affected mutants was compared with that of their unaffected and heterozygous littermates and also with that of mice of the C57BL/6 (C57) inbred strain, an increasingly used background for genetic targeted organisms, and with DBA/2 (DBA) mice, known for their nonpathological deficits in spatial learning. The results demonstrated that mutant Pah(enu2) mice are characterized by deficits involving both spatial and nonspatial recognition, that are not related to motor impairment or to high emotional reactivity to novelty. These results indicate that Pah(enu2) mice show pathological cognitive deficits and support their use to test hypotheses about neurodevelopmental disturbances involved in mental retardation.

Serotonin transporter gene promoter variants do not explain the hyperserotoninemia in autistic children

Autism is a biologically-heterogeneous disease. Distinct subgroups of autistic patients may be marked by intermediate phenotypes, such as elevated serotonin (5-HT) blood levels, potentially associated with different underlying disease mechanisms. This could lead to inconsistent genetic association results, such as those of prior studies on serotonin transporter (5-HTT) gene promoter variants and autistic disorder. Contributions of 5-HTT gene promoter alleles to 5-HT blood levels were thus investigated in 134 autistic patients and 291 first-degree relatives. Mean 5-HT blood levels are 11% higher in autistic patients carrying the L/L genotype, compared to patients with the S/S or S/L genotype; this trend is not observed in first-degree relatives. The probability of inheriting L or S alleles is significantly enhanced in patients with 5-HT blood levels above or below the mean, respectively (P < 0.05), but quantitative TDT analyses yield a non-significant trend (P = 0.10), as this polymorphism explains only 2.5% of the variance in 5-HT blood levels of autistic patients. In conclusion, 5-HTT gene promoter variants seemingly exert a small effect on 5-HT blood levels in autistic children, which largely does not account for hyperserotoninemia. Nonetheless, the inconsistent outcome of prior association studies could partly stem from a selection bias of hyper- or hypo-serotoninemic probands.

Deficits in brain serotonin synthesis in a genetic mouse model of phenylketonuria

Although hyperphenylalaninemia causes neurological disturbances and mental retardation, the neuropathological effects of phenylalanine excess are still poorly understood. Brain serotonin depletion may play a major role in such disturbances and is a possible target for feasible pharmacotherapies. In the present study, we investigated hyperphenylalaninemia-related brain serotonin depletion using a genetic mouse model of phenylketonuria, the Pah(enu2) mutant. Mutant mice showed severe depletion of whole brain serotonin, a mild reduction in the brain level of tryptophan, its amino acid precursor, and major deficits in the brain level of 5-hydroxytryptophan, the second rate-limiting factor in serotonin synthesis. These results suggest that interference with brain 5-hydroxytryptophan synthesis may be the major cause of serotonin deficits in hyperphenylalaninemia.

Barrel pattern formation requires serotonin uptake by thalamocortical afferents, and not vesicular monoamine release

Thalamocortical neurons innervating the barrel cortex in neonatal rodents transiently store serotonin (5-HT) in synaptic vesicles by expressing the plasma membrane serotonin transporter (5-HTT) and the vesicular monoamine transporter (VMAT2). 5-HTT knock-out (ko) mice reveal a nearly complete absence of 5-HT in the cerebral cortex by immunohistochemistry, and of barrels, both at P7 and adulthood. Quantitative electron microscopy reveals that 5-HTT ko affects neither the density of synapses nor the length of synaptic contacts in layer IV. VMAT2 ko mice, completely lacking activity-dependent vesicular release of monoamines including 5-HT, also show a complete lack of 5-HT in the cortex but display largely normal barrel fields, despite sometimes markedly reduced postnatal growth. Transient 5-HTT expression is thus required for barrel pattern formation, whereas activity-dependent vesicular 5-HT release is not.

No association between the 4g/5G polymorphism of the plasminogen activator inhibitor-1 gene promoter and autistic disorder

Plasmin, a serine protease, is involved in many physiologically relevant processes, including haemostasis, cellular recruitment during immune response, tumour growth, and also neuronal migration and synaptic remodelling. Both tissue-type and urokinase-type plasminogen activators can be efficiently inhibited by plasminogen activator inhibitor-1 (PAI-1), a protease inhibitor of the serpin family. The human PAI-1 gene is located on chromosome 7q, within or close to a region that has been linked to autism in several linkage studies. Autism seems to be characterized by altered neuronal cytoarchitecture, synaptogenesis and possibly also cellular immune responses. We began addressing the potential involvement of the PAI-1 gene in autistic disorder with this linkage/association study, assessing transmission patterns of the 4G/5G polymorphism in the PAI-1 gene promoter that was previously shown to significantly affect PAI-1 plasma levels. No linkage/association was found in 167 trios with autistic probands, recruited in Italy and in the USA. We thus found no evidence that this polymorphism, or putative functionally relevant gene variants in linkage disequilibrium with it, confer vulnerability to autistic disorder.

Reelin gene alleles and haplotypes as a factor predisposing to autistic disorder

Autistic disorder (MIM 209850) is currently viewed as a neurodevelopmental disease. Reelin plays a pivotal role in the development of laminar structures including the cerebral cortex, hippocampus, cerebellum and of several brainstem nuclei. Neuroanatomical evidence is consistent with Reelin involvement in autistic disorder. In this study, we describe several polymorphisms identified using RNA-SSCP and DNA sequencing. Association and linkage were assessed comparing 95 Italian patients to 186 ethnically-matched controls, and using the transmission/disequilibrium test and haplotype-based haplotype relative risk in 172 complete trios from 165 families collected in Italy and in the USA. Both case-control and family-based analyses yield a significant association between autistic disorder and a polymorphic GGC repeat located immediately 5' of the reelin gene (RELN) ATG initiator codon, as well as with specific haplotypes formed by this polymorphism with two single-base substitutions located in a splice junction in exon 6 and within exon 50. Triplet repeats located in 5' untranslated regions (5'UTRs) are indicative of strong transcriptional regulation. Our findings suggest that longer triplet repeats in the 5'UTR of the RELN gene confer vulnerability to autistic disorder.

Striatal dopamine sensitization to D-amphetamine in periadolescent but not in adult rats

The neurobiological and behavioral facets of adolescence have been poorly investigated in relation to the vulnerability to psychostimulants. Periadolescent (33-43 days) and adult (>70 days) Sprague-Dawley rats underwent a 3-day treatment history with D-amphetamine (AMPH) at 0, 2, or 10 mg/kg (once a day). After a short 5-day-long withdrawal interval, freely moving animals were challenged with a 2-mg/kg AMPH dose and their behavior as well as in vivo intrastriatum dopamine (DA) release in the CNS were assessed. Microdialysis data indicated that AMPH-history periadolescent rats showed a prominent sensitization of AMPH-stimulated DA release, whereas no such change was found in adult subjects. As expected, acute AMPH administration strongly reduced time spent lying still and increased levels of cage exploration in animals of both ages. A treatment history of high AMPH dosage was associated with a marked sensitization of the exploratory behavior in adults, whereas it induced a quite opposite profile in periadolescents. The latter group only was also characterized by a compulsive involvement in the stereotyped head-bobbing response. These results indicate that differently from adults, marked alterations in neurobiological target mechanisms are observed in rats around periadolescence as a consequence of a quite mild regimen of intermittent AMPH exposure. Thus, a neurobiological substrate for an age-related increased vulnerability towards the addictive risks of these drugs is suggested.

Adenosine deaminase alleles and autistic disorder: case-control and family-based association studies

Adenosine deaminase (ADA) plays a relevant role in purine metabolism, immune responses, and peptidase activity, which may be altered in some autistic patients. Codominant ADA1 and ADA2 alleles code for ADA1 and ADA2 allozymes, the most frequent protein isoforms in the general population. Individuals carrying one copy of the ADA2 allele display 15 to 20% lower catalytic activity compared to ADA1 homozygotes. Recent preliminary data suggest that ADA2 alleles may be more frequent among autistic patients than healthy controls. The present study was undertaken to replicate these findings in a new case-control study, to test for linkage/association using a family-based design, and to characterize ADA2-carrying patients by serotonin blood levels, peptiduria, and head circumference. ADA2 alleles were significantly more frequent in 91 Caucasian autistic patients of Italian descent than in 152 unaffected controls (17.6% vs. 7.9%, P = 0.018), as well as among their fathers. Family-based tests involving these 91 singleton families, as well as 44 additional Caucasian-American trios, did not support significant linkage/association. However, the observed preferential maternal transmission of ADA2 alleles, if replicated, may point toward linkage disequilibrium between the ADA2 polymorphism and an imprinted gene variant located in its vicinity. Racial and ethnic differences in ADA allelic distributions, together with the low frequency of the ADA2 allele, may pose methodological problems to future linkage/association studies. Direct assessments of ADA catalytic activity in autistic individuals and unaffected siblings carrying ADA1/ADA1 vs ADA1/ADA2 genotypes may provide stronger evidence of ADA2 contributions to autistic disorder. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:784-790, 2000.