W. Ted Brown

Examination of Factors Associated with Instability of the FMR1 CGG Repeat

We examined premutation-female transmissions and premutation-male transmissions of the FMR1 CGG repeat to carrier offspring, to identify factors associated with instability of the repeat. First we investigated associations between parental and offspring repeat size. Premutation-female repeat size was positively correlated with the risk of having full-mutation offspring, confirming previous reports. Similarly, premutation-male repeat size was positively correlated with the daughters repeat size. However, increasing paternal repeat size was associated also with both increased risk of contraction and decreased magnitude of the repeat-size change passed to the daughter. We hypothesized that the difference between the female and male transmissions was due simply to selection against full-mutation sperm. To test this hypothesis, we simulated selection against full-mutation eggs, by only examining premutation-female transmissions to their premutation offspring. Among this subset of premutation-female transmissions, associations between maternal and offspring repeat size were similar to those observed in premutation-male transmissions. This suggests that the difference between female and male transmissions may be due to selection against full-mutation sperm. Increasing maternal age was associated with increasing risk of expansion to the full mutation, possibly because of selection for smaller alleles within the offsprings soma over time; a similar effect of increasing paternal age may be due to the same selection process. Last, we have evidence that the reported association between offspring sex and risk of expansion may be due to ascertainment bias. Thus, female and male offspring are equally likely to inherit the full mutation.

Contribution of olivofloccular circuitry developmental defects to atypical gaze in autism

Individuals with autism demonstrate atypical gaze, impairments in smooth pursuit, altered movement perception and deficits in facial perception. The olivofloccular neuronal circuit is a major contributor to eye movement control. This study of the cerebellum in 12 autistic and 10 control subjects revealed dysplastic changes in the flocculus of eight autistic (67%) and two control (20%) subjects. Defects of the oculomotor system, including avoidance of eye contact and poor or no eye contact, were reported in 88% of autistic subjects with postmortem-detected floccular dysplasia. Focal disorganization of the flocculus cytoarchitecture with deficit, altered morphology, and spatial disorientation of Purkinje cells (PCs); deficit and abnormalities of granule, basket, stellate and unipolar brush cells; and structural defects and abnormal orientation of Bergmann glia are indicators of profound disruption of flocculus circuitry in a dysplastic area. The average volume of PCs was 26% less in the dysplastic region than in the unaffected region of the flocculus (p<0.01) in autistic subjects. Moreover, the average volume of PCs in the entire cerebellum was 25% less in the autistic subjects than in the control subjects (p<0.001). Findings from this study and a parallel study of the inferior olive (IO) suggest that focal floccular dysplasia combined with IO neurons and PC developmental defects may contribute to oculomotor system dysfunction and atypical gaze in autistic subjects.

Genetic Diseases of Premature Aging as Models of Senescence

The aging process and resulting senescence seem likely to have an underlying basis that is in large part encoded in our genes. This is reflected by the wide range of maximal life-span potentials that animal species have attained. They vary from about 1 day in the adult form of the May fly (Ephemera sp, imago form) to more than 150 years in some turtles (Tedudo summeri) (Lints, 1978). Even among mammals, approximately a 100-fold range is seen, from about 1 year in the smokey shrew (Sorex furneus) (Hamilton, 1940) to about 120 years in humans (Russell, 1988). This 100- to 50,000-fold variation is undoubtedly due to underlying differences in the genetic constitution of species. Analyses of the degree of genetic complexity underlying longevity have suggested it may be encoded by a few genes, perhaps 20–50, which have a major gene effect on aging (Cutler, 1980; Martin, 1977; Sacher, 1980). Therefore, to understand the genetic basis of aging, the most direct approach may be to study appropriate genetic mutations that appear to affect longevity. As such, the genetic diseases of premature aging can serve as useful models for the study of senescence.

Bipolar spectrum disorder and fragile X syndrome: A family study

Bipolar illness has long been postulated to have a genetic locus on the X chromosome (Reich et al 1969). Early studies used color blindness and G6PD enzyme phenotypes as X chromosome genetic markers to look for evidence of a bipolar locus (Reich et al 1969; Mendlewicz et al 1972). Baron and colleagues (1987) noted linkage of the disease to the loci for color-blindness and to G6PD on the distal arm of the X chromosome while suggesting that genetic heterogeneity may exist to produce a common phe-

Neuronal nucleus and cytoplasm volume deficit in children with autism and volume increase in adolescents and adults.

IntroductionCharacterization of the type and topography of structural changes and their alterations throughout the lifespan of individuals with autism is essential for understanding the mechanisms contributing to the autistic phenotype. The aim of this stereological study of neurons in 16 brain structures of 14 autistic and 14 control subjects from 4 to 64xa0years of age was to establish the course of neuronal nuclear and cytoplasmic volume changes throughout the lifespan of individuals with autism.ResultsOur data indicate that a deficit of neuronal soma volume in children with autism is associated with deficits in the volume of the neuronal nucleus and cytoplasm. The significant deficits of neuronal nuclear and cytoplasmic volumes in 13 of 16 examined subcortical structures, archicortex, cerebellum, and brainstem in 4- to 8-year-old autistic children suggest a global nature of brain developmental abnormalities, but with region-specific differences in the severity of neuronal pathology. The observed increase in nuclear volumes in 8 of 16 structures in the autistic teenagers/young adults and decrease in nuclear volumes in 14 of 16 regions in the age-matched control subjects reveal opposite trajectories throughout the lifespan. The deficit in neuronal nuclear volumes, ranging from 7% to 42% in the 16 examined regions in children with autism, and in neuronal cytoplasmic volumes from 1% to 31%, as well as the broader range of interindividual differences for the nuclear than the cytoplasmic volume deficits, suggest a partial distinction between nuclear and cytoplasmic pathology.ConclusionsThe most severe deficit of both neuronal nucleus and cytoplasm volume in 4-to 8-year-old autistic children appears to be a reflection of early developmental alterations that may have a major contribution to the autistic phenotype. The broad range of functions of the affected structures implies that their developmental and age-associated abnormalities contribute not only to the diagnostic features of autism but also to the broad spectrum of clinical alterations associated with autism. Lack of clinical improvement in autistic teenagers and adults indicates that the observed increase in neuron nucleus and cytoplasm volume close to control level does not normalize brain function.

The Prenatal Diagnosis of the Fragile X Syndrome

That an excess of males are affected with nonspecific mental retardation has been known for some time. Penrose (1938) noted that there were more mentally retarded institutionalized males than females. Although Martin and Bell (1943) and Renpenning et al. (1962) described families with characteristic X-linked pedigrees and Davison (1973) suggested that the cause of nonspecific mental retardation (MR) may be due to an X-linked etiology, it was Lehrke (1972) and Turner and Turner (1974) who underlined the significance of X-linkage by concluding that it may be the reason for nonspecific MR in 20% or more of affected males.

Isolation and Characterization of a Highly Polymorphic Human Locus (DXS455) in Proximal Xq28

Human Xq28 is highly gene dense with over 27 loci. Because most of these genes have been mapped by linkage to polymorphic loci, only one of which (DXS52) is informative in most families, a search was conducted for new, highly polymorphic Xq28 markers. From a cosmid library constructed using a somatic cell hybrid containing human Xq27.3----qter as the sole human DNA, a human-insert cosmid (c346) was identified and found to reveal variation on Southern blot analyses with female DNA digested with any of several different restriction endonucleases. Two subclones of c346, p346.8 and p346.T, that respectively identify a multiallelic VNTR locus and a frequent two-allele TaqI polymorphism were isolated. Examination of 21 unrelated females showed heterozygosity of 76 and 57%, respectively. These two markers appeared to be in linkage equilibrium, and a combined analysis revealed heterozygosity in 91% of unrelated females. Families segregating the fragile X syndrome with key Xq28 crossovers position this locus (designated DXS455) between the proximal Xq28 locus DXS296 (VK21) and the more distal locus DXS374 (1A1), which is proximal to DXS52. DXS455 is therefore the most polymorphic locus identified in Xq28 and will be useful in the genetic analysis of this gene dense region, including the diagnosis of nearby genetic disease loci by linkage.

Neuroendocrine Alterations in the Fragile X Mouse

The expression of GABA(A) receptors in the fragile X mouse brain is significantly downregulated. We additionally found that the expression of somatostatin and voltage-sensitive calcium channels (VSCCs) is also reduced. GABA(A) and the VSCCs, through a synergistic interaction, perform a critical role in mediating activity-dependent developmental processes. In the developing brain, GABA is excitatory and its actions are mediated through GABA(A) receptors. Subsequent to GABA-mediated depolarization, the VSCCs are activated and intracellular calcium is increased, which mediates gene transcription and other cellular events. GABAergic excitation mediated through GABA(A) receptors and the subsequent activation of the VSCCs are critically important for the establishment of neuronal connectivity within immature neuronal networks. Data from our laboratories suggest that there is a dysregulation of axonal pathfinding during development in the fragile X mouse brain and that this is likely due to a dysregulation of the synergistic interactions of GABA and VSCC. Thus, we hypothesize that the altered expression of these critical channels in the early stages of brain development leads to altered activity-dependent gene expression that may potentially lead to the developmental delay characteristic of the fragile X syndrome.

Delayed Development of the Claustrum in Autism

The clinical phenotype of autism, with deficits in social interactions; verbal and nonverbal communication; and restricted repetitive and stereotyped patterns of behavior, interests and activities, suggests that numerous high-order functions are not processed properly. Extensive reciprocal connections between the claustrum and almost all brain regions suggest that developmental alterations of the claustrum may contribute to autism’s clinical profile, as it integrates a number of physiological processes that collectively maintain contact between the individual and the environment. Stereological studies revealed a significant delay of neuronal growth in the claustrum. Examination of older children and adults with autism shows that during the teenage years acceleration of neuronal growth results in a partial correction of neuronal size in the claustrum. n nDesynchronization of neuronal growth in the claustrum and interacting brain modalities, including the social brain, the sensorimotor system and the memory system, appears to be a significant contributor to the clinical symptoms of autism. A smaller claustrum and undersized neurons could be indicative of underconnectivity in the brains of autistic subjects. Underconnectivity of the claustrum may result in defects/deficits of the integration of cortical and subcortical modalities and may contribute to the cognitive impairment, repetitive behaviors, deficits of social interactions, altered processing of sensory signals and other components of the autistic phenotype.

Fragile X Syndrome and Autism Spectrum Disorders

Fragile X syndrome (FXS) is the most common monogenic form of intellectual disability, and is strongly associated with autism spectrum disorders (ASD). There is wide variation in the severity of symptoms, which may be due in part to modifier genes, such as MAOA . The most consistently observed behavioral feature is social avoidance, which may be secondary to problems with arousal modulation. This social avoidance distinguishes ASD associated with fragile X from idiopathic ASD. The fragile X gene ( FMR1 ) encodes an RNA binding protein (FMRP) that modulates, primarily by suppression of translation, the expression of approximately 4% of brain genes. The mGluR theory of fragile X postulates that over-activity of glutamate excitatory pathways play a major role in symptomatology. Animal models, particularly mouse and Drosophila knockouts, are providing insights into the underlying pathophysiology and are leading to the development of targeted treatments. Current drug trials in FXS are being carried out and may lead to approved drugs, which may be beneficial for ASD as well.