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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.

Intercellular differences of chromosomal content in the same individual are defined as chromosomal mosaicism (alias intercellular or somatic genomic variations or, in a number of publications, mosaic aneuploidy). It has long been suggested that this phenomenon poorly contributes both to intercellular (interindividual) diversity and to human disease. However, our views have recently become to change due to a series of communications demonstrated a higher incidence of chromosomal mosaicism in diseased individuals (major psychiatric disorders and autoimmune diseases) as well as depicted chromosomal mosaicism contribution to genetic diversity, the central nervous system development, and aging. The later has been produced by significant achievements in the field of molecular cytogenetics. Recently, Molecular Cytogenetics has published an article by Maj Hulten and colleagues that has provided evidences for chromosomal mosaicism to underlie formation of germline aneuploidy in human female gametes using trisomy 21 (Down syndrome) as a model. Since meiotic aneuploidy is suggested to be the leading genetic cause of human prenatal mortality and postnatal morbidity, these data together with previous findings define chromosomal mosaicism not as a casual finding during cytogenetic analyses but as a more significant biological phenomenon than previously recognized. Finally, the significance of chromosomal mosaicism can be drawn from the fact, that this phenomenon is involved in genetic diversity, normal and abnormal prenatal development, human diseases, aging, and meiotic aneuploidy, the intrinsic cause of which remains, as yet, unknown.

Ataxia telangiectasia (AT) is a chromosome instability (CIN) neurological syndrome arising from DNA damage response defects due to ATM gene mutations. The hallmark of AT is progressive cerebellar degeneration. However, the intrinsic cause of the neurodegeneration remains poorly understood. To highlight the relationship between CIN and neurodegeneration in AT, we monitored aneuploidy and interphase chromosome breaks (chromosomal biomarkers of genomic instability) in the normal and diseased brain. We observed a 2-3-fold increase of stochastic aneuploidy affecting different chromosomes in the cerebellum and the cerebrum of the AT brain. The global aneuploidization of the brain is, therefore, a new genetic phenomenon featuring AT. Degenerating cerebellum in AT was remarkably featured by a dramatic 5-20-fold increase of non-random DNA double-strand breaks and aneuploidy affecting chromosomes 14 and, to a lesser extend, chromosomes 7 and X. Novel recurrent chromosome hot spots associated with cerebellar degeneration were mapped within 14q12. In silico analysis has revealed that this genomic region contains two candidate genes (FOXG1B and NOVA1). The existence of non-random breaks disrupting specific chromosomal loci in neural cells with DNA repair deficiency supports the hypothesis that neuronal genome may undergo programmed somatic rearrangements. Investigating chromosome integrity in neural cells, we provide the first evidence that increased CIN can result into neurodegeneration, whereas it is generally assumed to be associated with cancer. Our data suggest that mosaic instability of somatic genome in cells of the central nervous system is more significant genetic factor predisposing to the brain pathology than previously recognized.

Large-scale variations of the human genome can be produced by losses or gains of whole chromosomes (aneuploidy). In contrast to DNA sequences variations at subchromosomal level (single nucleotide polymorphisms, short tandem repeat variations) or interindividual subtle chromosome region changes (deletions, duplications, large-scale copynumber variants, fragile sites), aneuploidy simultaneously involves hundreds or even thousands of genes and, therefore, dramatically affects functional genome activity. Aneuploidy originates from either meiotic or mitotic chromosome instability and, in some instances, manifests as somatic chromosomal mosaicism. Although the real incidence of mosaic aneuploidy in somatic human tissues remains to be determined, one can suppose an overlooked fraction of cells with unshared genomes due to large-scale genomic alterations among 10 14 cells forming the human body. Intercellular differences in chromosome number can be considered an overlooked type of structural and functional genome variations, which produce genetic mosaicism. This review refers to somatic chromosomal mosaicism and aims to describe its mechanisms and consequences. Moreover, the effect of somatic chromosomal mosaicism on both interindividual and intercellular diversity as well as human diseases is discussed. Finally, since the identification of these genomic variations faces numerous difficulties, we found pertinent to describe available approaches towards the detection of chromosomal mosaicism in human somatic tissues.

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...

Human karyotype is usually studied by classical cytogenetic (banding) techniques. To perform it, one has to obtain metaphase chromosomes of mitotic cells. This leads to the impossibility of analyzing all the cell types, to moderate cell scoring, and to the extrapolation of cytogenetic data retrieved from a couple of tens of mitotic cells to the whole organism, suggesting that all the remaining cells possess these genomes. However, this is far from being the case inasmuch as chromosome abnormalities can occur in any cell along ontogeny. Since somatic cells of eukaryotes are more likely to be in interphase, the solution of the problem concerning studying postmitotic cells and larger cell populations is interphase cytogenetics, which has become more or less applicable for specific biomedical tasks due to achievements in molecular cytogenetics (i.e. developments of fluorescence in situ hybridization -- FISH, and multicolor banding -- MCB). Numerous interphase molecular cytogenetic approaches are restricted to studying specific genomic loci (regions) being, however, useful for identification of chromosome abnormalities (aneuploidy, polyploidy, deletions, inversions, duplications, translocations). Moreover, these techniques are the unique possibility to establish biological role and patterns of nuclear genome organization at suprachromosomal level in a given cell. Here, it is to note that this issue is incompletely worked out due to technical limitations. Nonetheless, a number of state-of-the-art molecular cytogenetic techniques (i.e multicolor interphase FISH or interpahase chromosome-specific MCB) allow visualization of interphase chromosomes in their integrity at molecular resolutions. Thus, regardless numerous difficulties encountered during studying human interphase chromosomes, molecular cytogenetics does provide for high-resolution single-cell analysis of genome organization, structure and behavior at all stages of cell cycle.