This study is the first case report of autism associated with an interstitial deletion on chromosome 4q. Deletion of contiguous genes in the 4q32-4q34 region could lead to the specific dysmorphic features as well as the behavioral phenotype seen in the subject. Among the deleted genes, potential candidates for autistic disorder are most likely to be those that are abundantly expressed in the brain.
Glutamate receptors are named for the pharmacological substances that influence them. AMPA glutamate receptors are responsive to alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) [13
]. Abraham and Bear [14
] reported that AMPA receptor mediated neurotransmission plays a key role in several aspects of developmental and adult synaptic plasticity. AMPA glutamate receptors are excitatory ionotropic neurotransmitter receptors. In these the receptor itself forms an ion channel, and binding of the ligand to the receptor is followed by an influx of ions, sodium, potassium or sometimes calcium into the neuronal synapse. AMPA glutamate receptors are composed of four sub-unit types GluR1, GluR2, GluR3 and GluR4. The genes encoding these receptors are named AMPA 1-4
). Each GRIA (AMPA) glutamate receptor gene encodes a protein of 900 amino acids and the different sub-units show 70% amino-acid homology. The C terminal ends of the GluR sub-units determine their interactions with other synapse associated proteins. AMPA receptors are permeable to Na+, K+ and to some degree to Ca+. GluR2 receptor sub-units are particularly involved in the control of Ca+ influx [15
Two different splice forms of each sub-unit occur. Different splice forms predominate in various regions of the brain during adult and fetal life. There are also alterations in the ratio of GluR2 to GluR3 during development of brain. The AMPA glutamate receptors achieve high density in the cerebral cortex, hippocampus, in the basolateral and lateral nuclei of the amygdala, in the caudate, putamen, nucleus accumbens and olfactory bulb. AMPA receptors occur in the pyramidal layer of the hippocampus and in layers II and III of the cerebellar cortex. Levels of GluR2 are particularly high in the cerebellar cortex and in the Bergman glia of the cerebellum [13
Receptors may be composed of identical sub-units or of different sub-units. Mansour et al. [16
] reported that heteromeric receptors are made up of no more than two types of sub-units. GluR2 subunit is expressed abundantly in the hippocampal pyramidal neurons and Purkinje cells in the cerebellum [17
]. Sans et al. [18
] showed that in the pyramidal neurons of the hippocampus the GluR2 sub-unit is the major determinant of AMPA receptor structure. Furthermore at this site the GluR2 sub-unit plays a major role in receptor trafficking. Since the specific sub-units in the receptor determine the functional properties of the receptor, the relative quantities of specific receptor sub-units that are available may be involved in regulating AMPA receptor function. Sans et al. [18
] analyzed glutamate receptors in GluR2 knockout mice. In their studies of the hippocampus in mice with reduced or deleted GluR2 sub-units, they demonstrated that GluR2 sub-units play a critical role in the assembly and synaptic expression of the AMPA receptor complex. When GluR2 is present in sufficient amounts, GluR1-GluR3 receptor sub-types are not found. In GluR2+/- mice, GluR2 protein levels are approximately 51% of normal. In GluR2-/- mice, no GluR2 protein was present. Furthermore, in GluR2+/+ mice, Sans et al. determined that no GluR1/GluR3 receptors occurred. However, in GluR2+/- mice and in GluR2-/- mice, GluR1/Glur3 receptors were present.
Our patient is GluR2+/- and it is therefore likely that there are more GluR1/GluR3 receptors present in his hippocampus than in control subjects. It is possible that this change contributes to the pathogenesis of his autistic symptoms.
Several investigators have attributed the atypical processing of information in autism to abnormalities in hippocampal functioning. Raymond et al. [19
] used the Golgi stain to analyze pyramidal cells in region CA1 and CA2 of the hippocampus. They demonstrated reduced neuronal cell size and decreased dendritic branching in brains from autistic subjects.
The brain regions that have most frequently shown anatomical changes in autism are the hippocampus and the cerebellum [20
]. It is interesting to note that Purcell et al. [21
] reported that the AMPA receptor density was decreased in the cerebellum of individuals with autism.
] hypothesized that autism may be a hypoglutamatergic disorder based on pharmacotherapeutic studies in mice. Administration of glutamate antagonists leads to autism-like characteristics including heightened auditory and tactile perception and decreased pain sensitivity. Jamain et al. [23
] reported linkage of autism to a 11 cM region on 6q21, which includes the gene that encodes the ionotropic kainite glutamate receptor subunit GluR6.
Our patient is hemizygous for the genes that encode the Glycine receptor sub-units α3 and β. Glycine and GABA are the main inhibitory transmitters in the central nervous system. Glycine acts through ionotropic receptors. In addition it modulates NMDA glutamate receptors. The glycine receptor is a glycoprotein composed of 5 sub-units. Together the sub-units form glycine gated chloride receptors. The alpha sub-unit binds the glycine ligand. Each receptor is composed of three α and two β sub-units. Four different genes encode Glycine receptor α sub-units [13
]. Glycine receptors occur in the spinal cord, brainstem and in the hippocampus, amygdala, striatum and cortex. Homomeric glycine receptors composed of a single type of alpha sub-unit are found in embryonic life and in the early post-natal period [24
]. Glycine receptor α3 subunits are abundant in the frontal and temporal lobes and in the putamen. Two transcripts are derived from the GLRA3
gene. One is 2.4 Kb, the other is 9 Kb in size. The precise role of glycine receptors is not understood. Okabe et al. [25
] demonstrated that glycinergic membrane responses occur early in embryonic neocortical development life in the cortical plate neurons and Cajal Retzius cells. Paton and Richter [26
] and Bracci et al. [27
] postulated that glycinergic inhibition governs the rhythmic output of mammalian motor systems including the medullary respiratory network. Mutations in the GLRA1
subunit have been shown to result in hyperekplexia characterized by excessive startle reactions to unexpected, particularly auditory stimuli [28
]. However, in a subset of families hyperekplexia is not associated with mutations in GLRA1
, suggesting that other genes in the glycinergic pathway, including the α3 and β subunits of the glycine receptor, may be involved [28
]. Increased sensitivity to auditory stimuli is a feature associated with autistic disorder.
There is a growing body of data that confirms that neurogenesis takes place in post-natal and in adult life. The hippocampus is one of the key sites for generation of new neurons after birth. Van Praag [29
] and others demonstrated that newly generated post-natal neurons are integrated into hippocampal circuitry. Gould et al. [30
] reported that hippocampal neurogenesis is enhanced by learning new skills. Collectively these studies provide evidence that post-natal neurogenesis in the hippocampus plays a role in learning and memory. Aylward et al. [31
] reported that there is a reduction in the volume of amygdala and hippocampus in autistic individuals, particularly in relation to total brain volume. They concluded that the histopathology of autism suggests that these volume reductions are related to a reduction in dendritic tree and neuropil development and probably indicate the underdevelopment of the neural connections of limbic structures with other parts of the brain, particularly cerebral cortex.
The subject with autism reported here is hemizygous for genes encoding the neuropeptide Y receptors NPY1R
. Dumont et al. [32
] demonstrated that Neuropeptide Y (NPY) affects cognitive function and learning and memory. Thorsell et al. [33
] reported impaired spatial learning in transgenic rats that over expressed NPY and demonstrated decreased NPY Y1 receptor binding. Michel et al. [34
] reported that in the hippocampus NPY is localized to GABA-ergic interneurons and its activity is mediated through its Y1, Y2 or Y5 receptors. These receptors are coupled to G protein signaling pathways.
Howell and coworkers [35
] demonstrated that NPY has a proliferative effect on hippocampal nestin positive neuronal precursor cells and on hippocampal beta tubulin positive neuroblasts. They carried out studies of the hippocampal cells in NPY Y1 receptor knockout mice. Through studies in these mice and through in vitro
studies in the presence of selective NPY Y1 receptor agonists and antagonists, Howell et al. [35
] demonstrated that the neuroproliferative effect of NPY in the hippocampus is mediated through the neuropeptide Y receptor NPY Y1 (NPY1R
). They postulate that the effect of NPY on learning and memory may be mediated through NPY neurogenesis.
The distal breakpoint in our subject interrupts the Glycoprotein M6A gene GPM6A
. The M6A glycoprotein was first identified in mouse brain by antibody binding assays as a factor that affects the growth of neurites in cultured cerebellar neurons [36
]. The human homolog of this gene, GPM6A
maps to chromosome 4q34 in humans and a second family member GPM6B
maps to Xp22.2-p22.4 [37
]. The human homologs of the murine M6A glycoprotein are yet to be studied thoroughly in terms of function and pathology.
Linkage studies on autism families reported by Yonan et al. [38
] and Buxbaum et al. [39
] provide further support for location of an autism-determining gene or genes on chromosome 4q. Yonan et al. reported a linkage peak for autism at 94 centimorgans (cM) on chromosome 4q. Buxbaum et al. reported linkage peaks between 104.9 and 126 cM on 4q. The deletion in our patient lies approximately between 157 to 177 cM.