Autism is a behaviorally defined syndrome characterized by impairment of social interaction, deficiency or abnormality of speech development, and limited activities and interest (American Psychiatric Association, 1994). The last category includes such abnormal behaviors as fascination with spinning objects, repetitive stereotypic movements, obsessive interests, and abnormal aversion to change in the environment. Symptoms are present by 30 months of age. The prevalence rate in recent Canadian studies using total ascertainment is over 1/1,000 (Bryson, S. E. et al., J. Child Psychol. Psychiat., 29, 433 (1988)).
Attempts to identify the cause of the disease have been difficult, in part, because the symptoms do not suggest a brain region or system where injury would result in the diagnostic set of behaviors. Further, the nature of the behaviors included in the criteria preclude an animal model of the diagnostic symptoms and make it difficult to relate much of the experimental literature on brain injuries to the symptoms of autism.
Several quantitative changes have been observed in autistic brains at autopsy. An elevation of about 100 g in brain weight has been reported (Bauman, M. L. and Kemper, T. L., Neurology 35, 866 (1985)). While attempts to find anatomical changes in the cerebral cortex have been unsuccessful (Williams, R. S. et al., Arch. Neurol., 37, 749 (1980); Coleman P. D., et al., J. Autism Dev. Disord., 15, 245 (1985)), several brains have been found to have elevated neuron packing density in structures of the limbic system (Bauman, M. L. and Kemper, T. L., Neurology 35, 866 (1985)), including the amygdala, hippocampus, septal nuclei and mammillary body. Multiple cases in multiple labs have been found to have abnormalities of the cerebellum. A deficiency of Purkinje cell and granule cell number, as well as reduced cell counts in the deep nuclei of the cerebellum and neuron shrinkage in the inferior olive, have been reported (Bauman, M. L. and Kemper, T. L., Neurology, 35, 866 (1985); Bauman, M. L. and Kemper, T. L., Neurology, 36 (suppl. 1), 190 (1986); Bauman, M. L. and Kemper, T. L., The Neurobiology of Autism, Johns Hopkins University Press, 119 (1994); Ritvo, E. R. et al., Am. J. Psychiat., 143, 862 (1986); Kemper, T. L. and Bauman M. L., Neurobiology of Infantile Autism, Elsevier Science Publishers, 43 (1992)).
Imaging studies have allowed examination of some anatomical characteristics in living autistic patients, providing larger samples than those available for histologic evaluation. In general, these confirm that the size of the brain in autistic individuals is not reduced and that most regions are also normal in size (Piven, J. et al., Biol. Psychiat., 31, 491 (1992)). Reports of size reductions in the brainstem have been inconsistent (Gaffney, G. R. et al., Biol. Psychiat., 24, 578 (1988); Hsu, M. et al., Arch. Neurol. 48, 1160 (1991)), but a new, larger study suggests that the midbrain, pons, and medulla are smaller in autistic cases than in controls (Hashimoto, T. et al., J. Aut. Dev. Disord., 25, 1 (1995)). In light of the histological effects reported for the cerebellum, it is interesting that the one region repeatedly identified as abnormal in imaging studies is the neocerebellar vermis (lobules VI and VII; Gaffney, G. R. et al., Am. J. Dis. Child., 141, 1330 (1987); Courchesne E., et al., N. Engl. J. Med., 318, 1349 (1988); Hashimoto, T. et al., J. Aut. Dev. Disord., 25, 1 (1995)). Not all comparisons have found a difference in neocerebellar size (Piven, J. et al., Biol. Psychiat., 31, 491 (1992); Kleiman, M. D. et al., Neurology, 42, 753 (1992)), but a recent reevaluation of positive and negative studies (Courchesne, E. et al, Neurology, 44, 214 (1994)) indicates that a few autistic cases have hyperplasia of the neocerebellar vermis, while many have hypoplasia. Small samples of this heterogeneous population could explain disparate results regarding the size of the neocerebellum in autism. The proposal that the cerebellum in autistic cases can be either large or small is reasonable from an embryological standpoint, because injuries to the developing brain are sometimes followed by rebounds of neurogenesis (e.g., Andreoli, J. et al., Am. J. Anat. 137, 87 (1973); Bohn, M. C. and Lauder, J. M., Dev. Neurosci., 1, 250 (1978); Bohn, M. C., Neuroscience, 5, 2003 (1980)), and it is possible that such rebounds could overshoot the normal cell number. Further, because increased cell density has been observed in the limbic system, the cerebellum is not the only brain region in which some form of overgrowth might account for the neuro-anatomy of autistic cases. It may well be that some autism-inducing injuries occur just prior to a period of rapid growth for the cerebellar lobules in question or the limbic system, leading to excess growth, while other injuries continue to be damaging during the period of rapid growth, leading to hypoplasia. However, the hypothesis that autism occurs with both hypoplastic and hyperplastic cerebella calls into question whether cerebellar anomalies play a major role in autistic symptoms.
A particularly instructive result has appeared in an MRI study on the cerebral cortex (Piven, J. et al., Am. J. Psychiat., 14, 734 (1992)). Of a small sample of autistic cases, the majority showed gyral anomalies (e.g., patches of pachygyria). However, the abnormal areas were not located in the same regions from case to case. That is, while the functional symptoms were similar in all the subjects, the brain damage observed was not. The investigators argue convincingly that the cortical anomalies were not responsible for the functional abnormalities. This is a central problem in all attempts to screen for pathology in living patients or in autopsy cases. While abnormalities may be present, it is not necessarily true that they are related to the symptoms of autism.
To teratologists, the physical anomalies of a neonate, child, or adult can serve as a guide to when the embryo was injured. Years of research have amplified the details of that timetable for the nervous system (Rodier, P. M., Dev. Med. Child Neurol., 22, 525 (1980); Bayer, S. A. et al., Neurotoxicology, 14, 83 (1993)). In the case of autism, lack of specific information on the neuroanatomy associated with the disease has made it difficult to estimate the stage of development when the disorder arises. However, in 1993, Miller and Stromland reported a finding that conclusively identified the time of origin for some cases. They observed that the rate of autism was 33% in people exposed to thalidomide between the 20th and 24th days of gestation, and 0% in cases exposed at other times (Stromland, K. et al., Devel. Med. Child. Neurol., 36, 351 (1994)). Their deduction regarding the time of injury was not based on neuroanatomy, which was not known in their living subjects. Instead, it was based on the external stigmata of the cases.
Because thousands of thalidomide-exposed offspring have been evaluated for somatic malformations, the array of injuries associated with the drug is well-known, and the time when each arises has been carefully defined (Miller, M. T., Trans. Am. Ophthalmol. Soc., 89, 623 (1991)). Of five cases of thalidomide-induced autism, four had malformations of the ears, without limb malformation, and the fifth had malformation of the ears, forelimb, and hindlimb. Thalidomide is not teratogenic before the 20th day of gestation. Starting on day 20 exposure causes ear malformation and abnormalities of the thumb. Limb malformations (other than those of the thumb) first appear with exposure on the 25th day, with effects moving from the forelimb to the hindlimb as exposure occurs at later stages. After the 35th day, thalidomide produces no malformations. Thus, the cases with malformations restricted to the ear must have been exposed before day 25, and the one patient with multiple malformations can only be explained as a case of repeated injuries at several stages of development.
In fact, the idea that autism might arise very early in gestation was suggested long ago. Steg and Rapoport (J. Aut. Child. Schiz., 5, 299 (1975)) noted the significant increase in minor physical anomalies among children with autism, and realized that they indicated an injury in the first trimester. Several studies of minor malformations have found ear effects to be the most common anomalies in autism (Walker, H. A., J. Aut. Child. Schiz., 7, 165 (1977); Campbell, M. et al., Am. J. Psychiat., 135, 573 (1978)), and the most recent study shows that they are not only the best discriminator between people with autism and normal controls, but also the only anomaly that discriminates autism from other developmental disabilities (Rodier, P. M. et al., Teratology 55, 319 (1997)). Ear anomalies are among the earliest of all minor physical malformations in their time of origin.
External malformations are not the only evidence which puts the time of injury in autism at the time of neural tube closure. The cranial nerve dysfunctions observed in the patients with autism secondary to thalidomide exposure--facial nerve palsy, Duane syndrome (lack of abducens innervation with reinnervation of the lateral rectus by the oculomotor nerve), abnormal lacrimation, gaze paresis, and hearing deficits (Stromland, K. et al., Devel. Med. Child. Neurol., 36, 351 (1994))--suggest that the earliest-forming structures of the brain stem were damaged, and it is now known that these form during neural tube closure (Bayer, S. A. et al., Neurotoxicology, 14, 83 (1993)). Subsequent studies have shown that a human brain from a patient with autism has the same pattern of brain stem injury predicted by the thalidomide cases (Rodier, P. M. et al., J. Comp. Neurol., 370, 247 (1996)). Perhaps even more importantly, the autopsied brain has a shortening of the brain stem in the region of the fifth rhombomere, and is missing two of the nuclei known to form from that embryological structure. The rhombomeres exist so briefly (Streeter, G. L., Contr. Embryol. Carneg. Instn., 30,213 (1948)) that the evidence that one failed to form is conclusive in pinpointing the time of injury. Like the thalidomide cases, the autopsy case could have been injured only at the time of neural tube closure.
The effect of injury around neural tube closure has been tested experimentally, to see whether it can produce anatomical results like those suspected in the thalidomide cases and observed in human brain. Animals exposed during the critical period to valproic acid, a teratogen with effects similar to thalidomide, which has also been associated with autism (Christianson, A. L. et al., Devel. Med. Child. Neurol., 36, 357 (1994); Williams, P. G. et al., Dev. Med. Child. Neurol., 39, 632 (1997)) exhibit reductions in the number of cranial nerve motor neurons (Rodier, P. M. et al., J. Comp. Neurol., 370, 247 (1996)). They are distinguished from controls by shortening of the hindbrain in the region which forms from the fifth rhombomere, just as the autopsied brain was (Rodier, P. M., et al., Teratology 55, 319 (1997)). Additional data suggests that the animal model has secondary changes in the cerebellum like those reported in some human cases of autism (Ingram, J. L. et al., Teratology, 53, 86 (1996)).
It has long been known that heritable factors play an important role in the etiology of autism. This was demonstrated by the original twin studies of Folstein and Rutter (J. Child Psychol. Psychiat., 18, 297 (1977)) and the subsequent addition of more twin pairs to the sample has only increased the estimate of the proportion of cases suspected to have a genetic basis (e.g. Bailey, A. et al., Psychol. Med., 25, 63 (1995); LeCouteur, A. et al., J. Child Psychol. Psychiat., 37, 785 (1996)). Family studies of siblings (Smalley, S. L. et al., Arch. Gen. Psychiat., 45, 953 (1988)) and parents (Landa, R. et al., J. Speech Hear. Res., 34, 1339 (1991); Landa, R. et al., Psych. Med., 22, 245 (1992)) also support the conclusion that an inherited risk is involved in many, perhaps all, cases of autism spectrum disorders. While the rate of autism is elevated in close relatives of cases, the rate of symptoms short of the diagnosis is increased much more. That is, individuals known to share genetic factors seem to vary in the degree to which symptoms are expressed. This non-Mendelian pattern (Jorde, L. B. et al., Am. J. Hum. Genet., 49, 932 (1991)) suggests a complex disorder with major contributions from predisposing genetic factors, which interact with the overall genetic background and/or environmental insults to determine the phenotype.
The ability to identify the genetic factors that increase the risk for autism would be a breakthrough for genetic counseling for prevention of the disorder. In addition, it would allow the creation of genetically-engineered animals in which to study the environmental factors that interact with the inherited predispositions. Tests for genetic factors would also serve as biomarkers, valuable for diagnosis, and useful in research on all aspects of the autism spectrum. Unfortunately, neither linkage nor association studies have revealed any chromosomal regions strongly related to autism (e.g. Spence, M. A. et al., Behav. Genet., 15, 1 (1985); Smalley, S. L. et al., Arch. Gen. Psychiat., 45, 953 (1988); Cook, E. H. et al., Molec. Psychiat., 2, 247 (1997); Klauck, S. M. et al., Hum. Molec. Genet., 6, 2233 (1997); Cook, E. H. et al., Am. J. Hum. Genet., 62, 1077 (1998)).
Furthermore, while there is no known medical treatment for autism, some success has been reported for early intervention with behavioral therapies. A biomarker would allow identification of the disease, now typically diagnosed between ages three and five, in infancy or prenatal life. Thus, there is an urgient need for a method of reliably identifying subjects with autism. In particular there is need for a blood test for polymorphisms causing autism spectrum disorders. Families with affected members need to know whether they carry a mutation which could affect future pregnancies. Clinicians need a test as an aid in diagnosis, and researchers would use the test to classify subjects according to the etiology of their disease.