BackgroundUnderstanding the mechanisms underlying generation of neuronal variability and complexity remains the central challenge for neuroscience. Structural variation in the neuronal genome is likely to be one important mechanism for neuronal diversity and brain diseases. Large-scale genomic variations due to loss or gain of whole chromosomes (aneuploidy) have been described in cells of the normal and diseased human brain, which are generated from neural stem cells during intrauterine period of life. However, the incidence of aneuploidy in the developing human brain and its impact on the brain development and function are obscure.Methodology/Principal FindingsTo address genomic variation during development we surveyed aneuploidy/polyploidy in the human fetal tissues by advanced molecular-cytogenetic techniques at the single-cell level. Here we show that the human developing brain has mosaic nature, being composed of euploid and aneuploid neural cells. Studying over 600,000 neural cells, we have determined the average aneuploidy frequency as 1.25–1.45% per chromosome, with the overall percentage of aneuploidy tending to approach 30–35%. Furthermore, we found that mosaic aneuploidy can be exclusively confined to the brain.Conclusions/SignificanceOur data indicates aneuploidization to be an additional pathological mechanism for neuronal genome diversification. These findings highlight the involvement of aneuploidy in the human brain development and suggest an unexpected link between developmental chromosomal instability, intercellural/intertissular genome diversity and human brain diseases.

and Institute of Pediatrics and Children's Surgery, Russian Ministry of Health, Moscow, Russia (SGV) S U M M A R Y Despite the lack of direct cytogenetic studies, the neuronal cells of the normal human brain have been postulated to contain normal (diploid) chromosomal complement. Direct proof of a chromosomal mutation presence leading to large-scale genomic alterations in neuronal cells has been missing in the human brain. Large-scale genomic variations due to chromosomal complement instability in developing neuronal cells may lead to the variable level of chromosomal mosaicism probably having a substantial effect on brain development. The aim of the present study was the pilot assessment of chromosome complement variations in neuronal cells of developing and adult human brain tissues using interphase multicolor fluorescence in situ hybridization (mFISH). Chromosome-enumerating DNA probes from the original collection (chromosomes 1, 13 and 21, 18, X, and Y) were used for the present pilot FISH study. As a source of fetal brain tissue, the medulla oblongata was used. FISH studies were performed using uncultured fetal brain samples as well as organotypic cultures of medulla oblongata tissue. Cortex tissues of postmortem adult brain samples (Brodmann area 10) were also studied. In cultured in vitro embryonic neuronal brain cells, an increased level of aneuploidy was found (mean rate in the range of 1.3-7.0% per individual chromosome, in contrast to 0.6-3.0% and 0.1-0.8% in uncultured fetal and postmortem adult brain cells, respectively). The data obtained support the hypothesis regarding aneuploidy occurrence in normal developing and adult human brain. T he human brain is the control center that stores, computes, integrates, and transmits information. It contains ‫ف‬ 10 12 neurons, each forming as many as a thousand connections with other neurons (Lodish et al. 2000). There have been no direct studies of large-scale genomic variations and chromosomal complement in the human central nervous system. Without experimental proof, the neuronal cells of the normal brain were postulated to contain normal (diploid) chromosome complement. However, indirect evidence for some forms of somatic, genomic, and chromosomal alterations in the neurons of mouse brain has been obtained. By means of fluorescence in situ hybridization (FISH) and spectral karyotype analysis of mouse embryonic cerebral cortical neuroblasts in the developing and adult nervous system, more than 30% of neuroblasts were found to be aneuploid (Rehen et al. 2001). Visualization of metaphase chromosomes by a nuclear transfer technique in mouse cortical neurons has indicated that the majority are characterized by an abnormal karyotype (Osada et al. 2002). Therefore, there is evidence for genomic variation at the level of whole chromosomes in developing and adult mouse neurons.There are only a limited number of molecular cytogenetic studies of the human brain using interphase FISH. The use of FISH was reported for examination of the interphase nuclei chromosomal co...

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Background: Autism is a common childhood neurodevelopmental disorder with a possible genetic background. About 5-10% of autism cases are associated with chromosomal abnormalities or monogenic disorders. However, the role of subtle genomic imbalances in autism has not been delineated. This study aimed to investigate a hypothesis suggesting autism to be associated with subtle genomic imbalances presenting as low-level chromosomal mosaicism. Methods: We surveyed stochastic (background) aneuploidy in children with/without autism by interphase three-colour fluorescence in situ hybridisation. The rate of chromosome loss and gain involving six arbitrarily selected autosomes and the sex chromosomes was assessed in the peripheral blood cells of 60 unaffected children and 120 children with autism. Results: Of 120 analysed boys with autism, 4 (3.3%) with rare s t r u c t u r a l c h r o m o s o m a l a b n o r m a l i t i e s ( 4 6, X Y , t ( 1; 6) ( q 42 . 1 ; q 2 7 ) ; 4 6 , XY , i n v ( 2 ) ( p 1 1q 13 ) ; 4 6 , X Y , d e r ( 6 ) , i n s ( 6 ; 1 ) ( q 2 1 ; p 1 3 . 3 p 2 2 , 1 ) p a t ; a n d 46,XY,r(22)(p11q13)) were excluded from further molecular cytogenetic analysis. Studying ,420 000 cells in 60 controls and 116 children with idiopathic autism, we determined the mean frequency of stochastic aneuploidy in control and autism: (1) autosome loss 0.58% (95% CI 0.42 to 0.75%) and 0.60% (95% CI 0.37 to 0.83%), respectively, p = 0.83; (2) autosome gain 0.15% (95% CI 0.09 to 0.21%) and 0.22% (95% CI 0.14 to 0.30%), respectively, p = 0.39; and (3) chromosome X gain 1.11% (95% CI 0.90 to 1.31%) and 1.01% (95% CI 0.85 to 1.17%), respectively, p = 0.30. A frequency of mosaic aneuploidy greater the background level was found in 19 (16%) of 116 children with idiopathic autism, whereas outlier values were not found in controls (p = 0.0019). Conclusions: Our findings identify low-level aneuploidy as a new genetic risk factor for autism. Therefore, molecular cytogenetic analysis of somatic mosaicism is warranted in children with unexplained autism.

Numerical chromosomal imbalances are a common feature of spontaneous abortions. However, the incidence of mosaic forms of chromosomal abnormalities has not been evaluated. We have applied interphase multicolor fluorescence in situ hybridization using original DNA probes for chromosomes 1, 9, 13, 14, 15, 16, 18, 21, 22, X, and Y to study chromosomal abnormalities in 148 specimens of spontaneous abortions. We have detected chromosomal abnormalities in 89/148 (60.1%) of specimens. Among them, aneuploidy was detected in 74 samples (83.1%). In the remaining samples, polyploidy was detected. The mosaic forms of chromosome abnormality, including autosomal and sex chromosomal aneuploidies and polyploidy (31 and 12 cases, respectively), were observed in 43/89 (48.3%) of specimens. The most frequent mosaic form of aneuploidy was related to chromosome X (19 cases). The frequency of mosaic forms of chromosomal abnormalities in samples with male chromosomal complement was 50% (16/32 chromosomally abnormal), and in samples with female chromosomal complement, it was 47.4% (27/57 chromosomally abnormal). The present study demonstrates that the postzygotic or mitotic errors leading to chromosomal mosaicism in spontaneous abortions are more frequent than previously suspected. Chromosomal mosaicsm may contribute significantly to both pregnancy complications and spontaneous fetal loss.