Single-cell sequencing shows mosaic aneuploidy in most human embryos

Mammalian preimplantation embryos often contain chromosomal defects that arose in the first divisions after fertilization and affect a subpopulation of cells - an event known as mosaic aneuploidy. In this issue of the JCI, Chavli et al. report single-cell genomic sequencing data for rigorous evaluation of the incidence and degree of mosaic aneuploidy in healthy human in vitro fertilization (IVF) embryos. Remarkably, mosaic aneuploidy occurred in at least 80% of human blastocyst-stage embryos, with often less than 20% of cells showing defects. These findings confirm that mosaic aneuploidy is prevalent in human embryos, indicating that the process is a widespread event that rarely has clinical consequences. There are major implications for preimplantation genetic testing of aneuploidy (PGT-A), a test commonly used to screen and select IVF embryos for transfer. The application and benefit of this technology is controversial, and the findings provide more cause for caution on its use.

Zebrafish Chromosome 14 Gene Differential Expression in the fmr1 h u2787 Model of Fragile X Syndrome

Zebrafish represent a valuable model for investigating the molecular and cellular basis of Fragile X syndrome (FXS). Reduced expression of the zebrafish FMR1 orthologous gene, fmr1, causes developmental and behavioural phenotypes related to FXS. Zebrafish homozygous for the hu2787 non-sense mutation allele of fmr1 are widely used to model FXS, although FXS-relevant phenotypes seen from morpholino antisense oligonucleotide (morpholino) suppression of fmr1 transcript translation were not observed when hu2787 was first described. The subsequent discovery of transcriptional adaptation (a form of genetic compensation), whereby mutations causing non-sense-mediated decay of transcripts can drive compensatory upregulation of homologous transcripts independent of protein feedback loops, suggested an explanation for the differences reported. We examined the whole-embryo transcriptome effects of homozygosity for fmr1 h u2787 at 2 days post fertilisation. We observed statistically significant changes in expression of a number of gene transcripts, but none from genes showing sequence homology to fmr1. Enrichment testing of differentially expressed genes implied effects on lysosome function and glycosphingolipid biosynthesis. The majority of the differentially expressed genes are located, like fmr1, on Chromosome 14. Quantitative PCR tests did not support that this was artefactual due to changes in relative chromosome abundance. Enrichment testing of the "leading edge" differentially expressed genes from Chromosome 14 revealed that their co-location on this chromosome may be associated with roles in brain development and function. The differential expression of functionally related genes due to mutation of fmr1, and located on the same chromosome as fmr1, is consistent with R.A. Fisher's assertion that the selective advantage of co-segregation of particular combinations of alleles of genes will favour, during evolution, chromosomal rearrangements that place them in linkage disequilibrium on the same chromosome. However, we cannot exclude that the apparent differential expression of genes on Chromosome 14 genes was, (if only in part), caused by differences between the expression of alleles of genes unrelated to the effects of the fmr1 h u2787 mutation and made manifest due to the limited, but non-zero, allelic diversity between the genotypes compared.

Non-self mutation: double-stranded RNA elicits antiviral pathogenic response in a Drosophila model of expanded CAG repeat neurodegenerative diseases

Inflammation is activated prior to symptoms in neurodegenerative diseases, providing a plausible pathogenic mechanism. Indeed, genetic and pharmacological ablation studies in animal models of several neurodegenerative diseases demonstrate that inflammation is required for pathology. However, while there is growing evidence that inflammation-mediated pathology may be the common mechanism underlying neurodegenerative diseases, including those due to dominantly inherited expanded repeats, the proximal causal agent is unknown. Expanded CAG.CUG repeat double-stranded RNA causes inflammation-mediated pathology when expressed in Drosophila. Repeat dsRNA is recognized by Dicer-2 as a foreign or ‘non-self’ molecule triggering both antiviral RNA and RNAi pathways. Neither of the RNAi pathway cofactors R2D2 nor loquacious are necessary, indicating antiviral RNA activation. RNA modification enables avoidance of recognition as ‘non-self’ by the innate inflammatory surveillance system. Human ADAR1 edits RNA conferring ‘self’ status and when co-expressed with expanded CAG.CUG dsRNA in Drosophila the pathology is lost. Cricket Paralysis Virus protein CrPV-1A is a known antagonist of Argonaute-2 in Drosophila antiviral defense. CrPV-1A co-expression also rescues pathogenesis, confirming anti-viral-RNA response. Repeat expansion mutation therefore confers ‘non-self’ recognition of endogenous RNA, thereby providing a proximal, autoinflammatory trigger for expanded repeat neurodegenerative diseases.

Neurodegenerative diseases have genetic hallmarks of autoinflammatory disease

The notion that one common pathogenic pathway could account for the various clinically distinguishable, typically late-onset neurodegenerative diseases might appear unlikely given the plethora of diverse primary causes of neurodegeneration. On the contrary, an autoinflammatory pathogenic mechanism allows diverse genetic and environmental factors to converge into a common chain of causality. Inflammation has long been known to correlate with neurodegeneration. Until recently this relationship was seen as one of consequence rather than cause-with inflammatory cells and events acting to 'clean up the mess' after neurological injury. This explanation is demonstrably inadequate and it is now clear that inflammation is at the very least, rate-limiting for neurodegeneration (and more likely, a principal underlying cause in most if not all neurodegenerative diseases), protective in its initial acute phase, but pernicious in its latter chronic phase.

The Enemy within: Innate Surveillance-Mediated Cell Death, the Common Mechanism of Neurodegenerative Disease

Neurodegenerative diseases comprise an array of progressive neurological disorders all characterized by the selective death of neurons in the central nervous system. Although, rare (familial) and common (sporadic) forms can occur for the same disease, it is unclear whether this reflects several distinct pathogenic pathways or the convergence of different causes into a common form of nerve cell death. Remarkably, neurodegenerative diseases are increasingly found to be accompanied by activation of the innate immune surveillance system normally associated with pathogen recognition and response. Innate surveillance is the cell's quality control system for the purpose of detecting such danger signals and responding in an appropriate manner. Innate surveillance is an "intelligent system," in that the manner of response is relevant to the magnitude and duration of the threat. If possible, the threat is dealt with within the cell in which it is detected, by degrading the danger signal(s) and restoring homeostasis. If this is not successful then an inflammatory response is instigated that is aimed at restricting the spread of the threat by elevating degradative pathways, sensitizing neighboring cells, and recruiting specialized cell types to the site. If the danger signal persists, then the ultimate response can include not only the programmed cell death of the original cell, but the contents of this dead cell can also bring about the death of adjacent sensitized cells. These responses are clearly aimed at destroying the ability of the detected pathogen to propagate and spread. Innate surveillance comprises intracellular, extracellular, non-cell autonomous and systemic processes. Recent studies have revealed how multiple steps in these processes involve proteins that, through their mutation, have been linked to many familial forms of neurodegenerative disease. This suggests that individuals harboring these mutations may have an amplified response to innate-mediated damage in neural tissues, and renders innate surveillance mediated cell death a plausible common pathogenic pathway responsible for neurodegenerative diseases, in both familial and sporadic forms. Here we have assembled evidence in favor of the hypothesis that neurodegenerative disease is the cumulative result of chronic activation of the innate surveillance pathway, triggered by endogenous or environmental danger or damage associated molecular patterns in a progressively expanding cascade of inflammation, tissue damage and cell death.

Tumor suppressor WWOX moderates the mitochondrial respiratory complex

Fragile site FRA16D exhibits DNA instability in cancer, resulting in diminished levels of protein from the WWOX gene that spans it. WWOX suppresses tumor growth by an undefined mechanism. WWOX participates in pathways involving aerobic metabolism and reactive oxygen species. WWOX comprises two WW domains as well as a short-chain dehydrogenase/reductase enzyme. Herein is described an in vivo genetic analysis in Drosophila melanogaster to identify functional interactions between WWOX and metabolic pathways. Altered WWOX levels modulate variable cellular outgrowths caused by genetic deficiencies of components of the mitochondrial respiratory complexes. This modulation requires the enzyme active site of WWOX, and the defective respiratory complex-induced cellular outgrowths are mediated by reactive oxygen species, dependent upon the Akt pathway and sensitive to levels of autophagy and hypoxia-inducible factor. WWOX is known to contribute to homeostasis by regulating the balance between oxidative phosphorylation and glycolysis. Reduction of WWOX levels results in diminished ability to respond to metabolic perturbation of normal cell growth. Thus, the ability of WWOX to facilitate escape from mitochondrial damage-induced glycolysis (Warburg effect) is, therefore, a plausible mechanism for its tumor suppressor activity.

Tumor Suppressor WWOX Contributes to the Elimination of Tumorigenic Cells in Drosophila melanogaster

WWOX is a >1Mb gene spanning FRA16D Common Chromosomal Fragile Site, a region of DNA instability in cancer. Consequently, altered WWOX levels have been observed in a wide variety of cancers. In vitro studies have identified a large number and variety of potential roles for WWOX. Although its normal role in vivo and functional contribution to cancer have not been fully defined, WWOX does have an integral role in metabolism and can suppress tumor growth. Using Drosophila melanogaster as an in vivo model system, we find that WWOX is a modulator of TNFα/Egr-mediated cell death. We found that altered levels of WWOX can modify phenotypes generated by low level ectopic expression of TNFα/Egr and this corresponds to altered levels of Caspase 3 activity. These results demonstrate an in vivo role for WWOX in promoting cell death. This form of cell death is accompanied by an increase in levels of reactive oxygen species, the regulation of which we have previously shown can also be modified by altered WWOX activity. We now hypothesise that, through regulation of reactive oxygen species, WWOX constitutes a link between alterations in cellular metabolism observed in cancer cells and their ability to evade normal cell death pathways. We have further shown that WWOX activity is required for the efficient removal of tumorigenic cells from a developing epithelial tissue. Together these results provide a molecular basis for the tumor suppressor functions of WWOX and the better prognosis observed in cancer patients with higher levels of WWOX activity. Understanding the conserved cellular pathways to which WWOX contributes provides novel possibilities for the development of therapeutic approaches to restore WWOX function in cancer.

WWOX, the chromosomal fragile site FRA16D spanning gene: its role in metabolism and contribution to cancer

The WWOX gene spans the common chromosomal fragile site FRA16D that is located within a massive (780 kb) intron. The WWOX gene is very long, at 1.1 Mb, which may contribute to the very low abundance of the full-length 1.4 kb mRNA. Alternative splicing also accounts for a variety of aberrant transcripts, most of which are devoid of C-terminal sequences required for WWOX to act as an oxidoreductase. The mouse WWOX gene also spans a chromosomal fragile site implying some sort of functional relationship that confers a selective advantage. The encoded protein domains of WWOX are conserved through evolution (between humans and Drosophila melanogaster) and include WW domains, an NAD -binding site, short-chain dehydrogenase/reductase enzyme and nuclear compartmentalization signals. This homology has enabled functional analyses in D. melanogaster that demonstrate roles for WWOX in reactive oxygen species regulation and metabolism. Indeed the human WWOX gene is also responsive to altered metabolism. Cancer cells typically exhibit altered metabolism (Warburg effect). Many cancers exhibit FRA16D DNA instability that results in aberrant WWOX expression and is associated with poor prognosis for these cancers. It is therefore thought that aberrant WWOX expression contributes to the altered metabolism in cancer. In addition, others have found that a specific (low-expression) allele of WWOX genotype contributes to cancer predisposition.

RNA pathogenesis via Toll-like receptor-activated inflammation in expanded repeat neurodegenerative diseases

Previously, we hypothesized that an RNA-based pathogenic pathway has a causal role in the dominantly inherited unstable expanded repeat neurodegenerative diseases. In support of this hypothesis we, and others, have characterized rCAG.rCUG₁₀₀ repeat double-strand RNA (dsRNA) as a previously unidentified agent capable of causing pathogenesis in a Drosophila model of neurodegenerative disease. Dicer, Toll, and autophagy pathways have distinct roles in this Drosophila dsRNA pathology. Dicer dependence is accompanied by cleavage of rCAG.rCUG₁₀₀ repeat dsRNA down to r(CAG)₇ 21-mers. Among the “molecular hallmarks” of this pathway that have been identified in Drosophila, some [i.e., r(CAG)₇ and elevated tumor necrosis factor] correlate with observations in affected people (e.g., Huntington’s disease and amyotrophic lateral sclerosis) or in related animal models (i.e., autophagy). The Toll pathway is activated in the presence of repeat-containing dsRNA and toxicity is also dependent on this pathway. How might the endogenously expressed dsRNA mediate Toll-dependent toxicity in neuronal cells? Endogenous RNAs are normally shielded from Toll pathway activation as part of the mechanism to distinguish “self” from “non-self” RNAs. This typically involves post-transcriptional modification of the RNA. Therefore, it is likely that rCAG.rCUG₁₀₀ repeat dsRNA has a characteristic property that interferes with or evades this normal mechanism of shielding. We predict that repeat expansion leads to an alteration in RNA structure and/or form that perturbs RNA modification, causing the unshielded repeat RNA (in the form of its Dicer-cleaved products) to be recognized by Toll-like receptors (TLRs), with consequent activation of the Toll pathway leading to loss of cell function and then ultimately cell death. We hypothesize that the proximal cause of expanded repeat neurodegenerative diseases is the TLR recognition (and resultant innate inflammatory response) of repeat RNA as “non-self” due to their paucity of “self” modification.

Distinct roles for Toll and autophagy pathways in double-stranded RNA toxicity in a Drosophila model of expanded repeat neurodegenerative diseases

Dominantly inherited expanded repeat neurodegenerative diseases are caused by the expansion of variable copy number tandem repeat sequences in otherwise unrelated genes. Some repeats encode polyglutamine that is thought to be toxic; however, other repeats do not encode polyglutamine indicating either multiple pathogenic pathways or an alternative common toxic agent. As these diseases share numerous clinical features and expanded repeat RNA is a common intermediary, RNA-based pathogenesis has been proposed, based on its toxicity in animal models. In Drosophila, double-stranded (rCAG.rCUG∼100) RNA toxicity is Dicer dependent and generates single-stranded (rCAG)7, an entity also detected in affected Huntington's Disease (HD) brains. We demonstrate that Drosophila rCAG.rCUG∼100 RNA toxicity perturbs several pathways including innate immunity, consistent with the observation in HD that immune activation precedes neuronal toxicity. Our results show that Drosophila rCAG.rCUG∼100 RNA toxicity is dependent upon Toll signaling and sensitive to autophagy, further implicating innate immune activation. In exhibiting molecular and cellular hallmarks of HD, double-stranded RNA-mediated activation of innate immunity is, therefore, a candidate pathway for this group of human genetic diseases.

Ubiquitous expression of CUG or CAG trinucleotide repeat RNA causes common morphological defects in a Drosophila model of RNA-mediated pathology

Expanded DNA repeat sequences are known to cause over 20 diseases, including Huntington's disease, several types of spinocerebellar ataxia and myotonic dystrophy type 1 and 2. A shared genetic basis, and overlapping clinical features for some of these diseases, indicate that common pathways may contribute to pathology. Multiple mechanisms, mediated by both expanded homopolymeric proteins and expanded repeat RNA, have been identified by the use of model systems, that may account for shared pathology. The use of such animal models enables identification of distinct pathways and their 'molecular hallmarks' that can be used to determine the contribution of each pathway in human pathology. Here we characterise a tergite disruption phenotype in adult flies, caused by ubiquitous expression of either untranslated CUG or CAG expanded repeat RNA. Using the tergite phenotype as a quantitative trait we define a new genetic system in which to examine 'hairpin' repeat RNA-mediated cellular perturbation. Further experiments use this system to examine whether pathways involving Muscleblind sequestration or Dicer processing, which have been shown to mediate repeat RNA-mediated pathology in other model systems, contribute to cellular perturbation in this model.

Dynamic mutations: where are they now?

Dynamic mutations are those caused by the expansion of existing polymorphic DNA repeat sequences beyond a copy number threshold. These genetic mutations can give rise to dominant, recessive or X-linked disorders, dependent upon the location of the repeat sequence with respect to the genes that are affected by the expansion. The distinguishing feature of these mutations is their instability, which is a function of the copy number of repeats and can occur in either meiosis or mitosis. For some of the resultant disorders there is a relationship between repeat copy number and age-at-onset and/or severity ofsymptoms ofthe disease. For this reason much effort is now focused on identifying the pathogenic pathways from the mutation to the disease symptoms in the hope of finding means of delaying onset, slowing progression or even preventing symptoms ofthe disease. The growing list ofneurodegenerative and neuromuscular diseases caused by dynamic mutations includes Huntington's disease (HD), spinobulbar muscular atrophy (SBMA), dentatorubral-pallidoluysian atrophy (DRPLA), a number of spinocerebellar ataxias (SCAs), oculopharyngeal muscular dystrophy (OPMD), myotonic dystrophy Type 1 and 2 (DM1 and 2), Huntington's disease-like 2 (HDL-2), Friedrich's ataxia (FRDA), Fragile X associated tremor ataxia syndrome (FXTAS), Fragile XE (FRAXE) and Fragile XA (FRAXA). This chapter aims to give a brief overview of what is currently known about each disease and the mechanisms underlying pathogenesis.

Comparative toxicity of polyglutamine, polyalanine and polyleucine tracts in Drosophila models of expanded repeat disease

Homopolymeric amino acid repeat sequences in proteins are of particular interest due to the discovery that expanded copy numbers of these repeats are the molecular basis for a growing list of human genetic diseases. Repeat copy numbers above a typical normal range of polyglutamine repeats have been found to be the principal pathogenic agents in a number of these diseases, including Huntington's disease. There is emerging evidence that expansions of amino acids encoded by other reading frames of CAG/CUG repeats, including polyalanine and polyleucine, could contribute to toxicity in the 'polyglutamine' diseases. We have therefore used the Drosophila model system to investigate effects of ectopic expression of polyglutamine, polyleucine and polyalanine repeats in vivo to assess their relative toxicities and the common and distinct characteristics of the pathogenesis that they cause. We find that these homopolymeric sequences all exhibit toxicity and are able to form aggregates in Drosophila, although there are marked differences in the degree of toxicity dependent upon the tissue in which they are expressed.

Double-stranded RNA is pathogenic in Drosophila models of expanded repeat neurodegenerative diseases

The pathogenic agent responsible for the expanded repeat diseases, a group of neurodegenerative diseases that includes Huntington's disease is not yet fully understood. Expanded polyglutamine (polyQ) is thought to be the toxic agent in certain cases, however, not all expanded repeat disease genes can encode a polyQ sequence. Since a repeat-containing RNA intermediary is common to all of these diseases, hairpin-forming single-stranded RNA has been investigated as a potential common pathogenic agent. More recently, it has become apparent that most of the expanded repeat disease loci have transcription occurring from both strands, raising the possibility that the complementary repeat RNAs could form a double-stranded structure. In our investigation using Drosophila models of these diseases, we identified a fortuitous integration event that models bidirectional repeat RNA transcription with the resultant flies exhibiting inducible pathology. We therefore established further lines of Drosophila expressing independent complementary repeat RNAs and found that these are toxic. The Dicer pathway is essential for this toxicity and in neuronal cells accounts for metabolism of the high copy number (CAG.CUG)(100) double-stranded RNAs down to (CAG)(7) single-stranded small RNAs. We also observe significant changes to the microRNA profile in neurons. These data identify a novel pathway through which double-stranded repeat RNA is toxic and capable of eliciting symptoms common to neurodegenerative human diseases resulting from dominantly inherited expanded repeats.

Perturbation of the Akt/Gsk3-β signalling pathway is common to Drosophila expressing expanded untranslated CAG, CUG and AUUCU repeat RNAs

Recent evidence supports a role for RNA as a common pathogenic agent in both the ‘polyglutamine’ and ‘untranslated’ dominant expanded repeat disorders. One feature of all repeat sequences currently associated with disease is their predicted ability to form a hairpin secondary structure at the RNA level. In order to investigate mechanisms by which hairpin-forming repeat RNAs could induce neurodegeneration, we have looked for alterations in gene transcript levels as hallmarks of the cellular response to toxic hairpin repeat RNAs. Three disease-associated repeat sequences—CAG, CUG and AUUCU—were specifically expressed in the neurons of Drosophila and resultant common transcriptional changes assessed by microarray analyses. Transcripts that encode several components of the Akt/Gsk3-β signalling pathway were altered as a consequence of expression of these repeat RNAs, indicating that this pathway is a component of the neuronal response to these pathogenic RNAs and may represent an important common therapeutic target in this class of diseases.

Drosophila orthologue of WWOX, the chromosomal fragile site FRA16D tumour suppressor gene, functions in aerobic metabolism and regulates reactive oxygen species

Common chromosomal fragile sites FRA3B and FRA16D are frequent sites of DNA instability in cancer, but their contribution to cancer cell biology is not yet understood. Genes that span these sites (FHIT and WWOX, respectively) are often perturbed (either increased or decreased) in cancer cells and both are able to suppress tumour growth. While WWOX has some tumour suppressor characteristics, its normal role and functional contribution to cancer has not been fully determined. We find that a significant proportion of Drosophila Wwox interactors identified by proteomics and microarray analyses have roles in aerobic metabolism. Functional relationships between Wwox and either CG6439/isocitrate dehydrogenase (Idh) or Cu–Zn superoxide dismutase (Sod) were confirmed by genetic interactions. In addition, altered levels of Wwox resulted in altered levels of endogenous reactive oxygen species. Wwox (like FHIT) contributes to pathways involving aerobic metabolism and oxidative stress, providing an explanation for the ‘non-classical tumour suppressor’ behaviour of WWOX. Fragile sites, and the genes that span them, are therefore part of a protective response mechanism to oxidative stress and likely contributors to the differences seen in aerobic glycolysis (Warburg effect) in cancer cells.

Selective neuronal requirement for huntingtin in the developing zebrafish

Huntington's disease shares a common molecular basis with eight other neurodegenerative diseases, expansion of an existing polyglutamine tract. In each case, this repeat tract occurs within otherwise unrelated proteins. These proteins show widespread and overlapping patterns of expression in the brain and yet the diseases are distinguished by neurodegeneration in a specific subset of neurons that are most sensitive to the mutation. It has therefore been proposed that expansion of the polyglutamine region in these genes may result in perturbation of the normal function of the respective proteins, and that this perturbation in some way contributes to the neuronal specificity of these diseases. The normal functions of these proteins have therefore become a focus for investigation as potential pathogenic pathways. We have used synthetic antisense morpholinos to inhibit the translation of huntingtin mRNA during early zebrafish development and have previously reported the effects of huntingtin reduction on iron transport and homeostasis. Here we report an analysis of the effects of huntingtin loss-of-function on the developing nervous system, observing distinct defects in morphology of neuromasts, olfactory placode and branchial arches. The potential common origins of these defects were explored, revealing impaired formation of the anterior-most region of the neural plate as indicated by reduced pre-placodal and telencephalic gene expression with no effect on mid- or hindbrain formation. These investigations demonstrate a specific 'rate-limiting' role for huntingtin in formation of the telencephalon and the pre-placodal region, and differing levels of requirement for huntingtin function in specific nerve cell types.

Unusual inheritance patterns due to dynamic mutation in fragile X syndrome

Fragile X syndrome is the most common form of familial mental retardation and is one of the world's most common genetic diseases. The inheritance patterns of the disease have many unusual features. It is an X-linked disorder yet there are asymptomatic carrier males. The disease is expressed only when the gene is inherited from the mother. The risk of a carrier woman having a child with the syndrome depends upon her position in the pedigree (the Sherman paradox) and her own intellectual status. The discovery that the disease is due to dynamic mutation (which is a multistage process) that inactivates FMR1 has provided an explanation for the unusual inheritance patterns. The finding of linkage disequilibrium between the fragile X mutations and closely linked DNA markers (haplotype) has required a reinterpretation of this phenomenon for dynamic mutations. Only a small number of normal alleles at the fragile X locus have long stretches of perfect repeat (2% with more than 24 copies) and these form a reservoir of alleles that can increase in length into the premutation range. Dynamic mutation is, so far, an exclusively human phenomenon, but this is probably because it has yet to be discovered in other species. Unusual inheritance patterns are a hallmark of dynamic mutation diseases.

Huntingtin-deficient zebrafish exhibit defects in iron utilization and development

Huntington's disease (HD) is one of nine neurodegenerative disorders caused by expansion of CAG repeats encoding polyglutamine in their respective, otherwise apparently unrelated proteins. Despite these proteins having widespread and overlapping expression patterns in the brain, a specific and unique subset of neurons exhibits particular vulnerability in each disease. It has been hypothesized that perturbation of normal protein function contributes to the specificity of neuronal vulnerability; however, the normal biological functions of many of these proteins including the HD gene product, Huntingtin (Htt), are unclear. To explore the roles of Htt, we have used antisense morpholino oligonucleotides to observe the effects of Htt deficiency in early zebrafish development. Knockdown of Htt expression resulted in a variety of developmental defects. Most notably, Htt-deficient zebrafish had hypochromic blood due to decreased hemoglobin production, despite the presence of iron within blood cells. Furthermore, transferrin receptor 1 transcripts were increased, suggesting cellular iron starvation. Provision of iron to the cytoplasm in a bio-available form restored hemoglobin production in Htt-deficient embryos. Since erythroid cells acquire iron via receptor-mediated endocytosis of transferrin, these results suggest a role for Htt in making endocytosed iron accessible for cellular utilization. Iron is required for oxidative energy production, and defects in iron homeostasis and energy metabolism are features of HD pathogenesis that are most pronounced in the major region of neurodegeneration. It is therefore plausible that perturbation of Htt's normal role in the iron pathway (by polyglutamine tract expansion) contributes to HD pathology, and particularly to its neuronal specificity.

Know thy fly

The generation and analysis of mutants is central to studies of gene function in model organisms. Methods for random mutagenesis in Drosophila melanogaster have been available for many years, but an alternative approach--targeted mutagenesis using homologous recombination--has only recently been developed. This approach has the advantage of specificity, because genes of interest can be altered. One might expect with a gene-targeting approach that the frequency of background mutations would be minimal. Unfortunately, we have found that this is not the case. Although the possibility of background mutations arising during homologous-recombination-based gene targeting has been raised in the literature, it is not routinely taken into account when using this technique. Our experience suggests that it can be a considerable problem but that it has a relatively simple solution.

Contribution of mGluR and Fmr1 functional pathways to neurite morphogenesis, craniofacial development and fragile X syndrome

Fragile X Syndrome is a leading heritable cause of mental retardation that results from the loss of FMR1 gene function. Studies in mouse and Drosophila model organisms have been critical in understanding many aspects of the loss of function of the FMR1 gene in the human syndrome. Here, we establish that the zebrafish is a useful model organism for the study of the human fragile X syndrome and can be used to examine phenotypes that are difficult or inaccessible to observation in other model organisms. Using morpholino knockdown of the fmr1 gene, we observed abnormal axonal branching of Rohon-Beard and trigeminal ganglion neurons and guidance and defasciculation defects in the lateral longitudinal fasciculus. We demonstrate that this axonal branching defect can be rescued by treatment with MPEP [2-methyl-6-(phenylethynyl) pyridine]. This is consistent with an interaction between mGluR signalling and fmr1 function in neurite morphogenesis. We also describe novel findings of abnormalities in the abundance of trigeminal ganglion neurons and of craniofacial abnormalities apparently due to dysmorphic cartilage formation. These abnormalities may be related to a role for fmr1 in neural crest cell specification and possibly in migration.

Molecular genetics of pseudoxanthoma elasticum: type and frequency of mutations in ABCC6

Pseudoxanthoma elasticum (PXE) is a systemic heritable disorder that affects the elastic tissue in the skin, eye, and cardiovascular system. Mutations in the ABCC6 gene cause PXE. We performed a mutation screen in ABCC6 using haplotype analysis in conjunction with direct sequencing to achieve a mutation detection rate of 97%. This screen consisted of 170 PXE chromosomes in 81 families, and detected 59 distinct mutations (32 missense, eight nonsense, and six likely splice-site point mutations; one small insertion; and seven small and five large deletions). Forty-three of these mutations are novel variants, which increases the total number of PXE mutations to 121. While most mutations are rare, three nonsense mutations, a splice donor site mutation, and the large deletion comprising exons 23-29 (c.2996_4208del) were identified as relatively frequent PXE mutations at 26%, 5%, 3.5%, 3%, and 11%, respectively. Chromosomal haplotyping with two proximal and two distal polymorphic markers flanking ABCC6 demonstrated that most chromosomes that carry these relatively frequent PXE mutations have related haplotypes specific for these mutations, which suggests that these chromosomes originate from single founder mutations. The types of mutations found support loss-of-function as the molecular mechanism for the PXE phenotype. In 76 of the 81 families, the affected individuals were either homozygous for the same mutation or compound heterozygous for two mutations. In the remaining five families with one uncovered mutation, affected showed allelic compound heterozygosity for the cosegregating PXE haplotype. This demonstrates pseudo-dominance as the relevant inheritance mechanism, since disease transmission to the next generation always requires one mutant allelic variant from each parent. In contrast to other previous clinical and molecular claims, our results show evidence only for recessive PXE. This has profound consequences for the genetic counseling of families with PXE.

Common chromosomal fragile sites and cancer: focus on FRA16D

A growing body of experimental evidence supports the view that certain human chromosomal fragile sites have roles to play in cancer. The principle lines of evidence are at the level of mutation mechanism and gene function. Most research in this area has previously focussed on the FRA3B common fragile site and the FHIT gene that spans this site. Here we review recent progress in characterising the second most readily observed common fragile site, FRA16D, and the WWOX gene that spans it. Comparative analyses of FRA3B/FHIT and FRA16D/WWOX reveal some striking similarities suggesting that these sites and their associated genes may play a part in a normal protective response of cells to environmental stress.

Autosomal dominant congenital non-progressive ataxia overlaps with the SCA15 locus

BACKGROUND

Most patients with pure nonprogressive congenital cerebellar ataxia have a sporadic form of unknown heredity and etiology. Several small families have been reported with a dominantly inherited nonprogressive congenital ataxia (NPCA).

METHODS

The authors ascertained and clinically characterized a four-generation pedigree segregating an autosomal dominant type of congenital nonprogressive cerebellar ataxia associated with cognitive impairment. Following the exclusion of several SCA localizations (SCA-1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 17, IOSCA, and DRPLA), a genome-wide linkage study was performed.

RESULTS

Examination of the family showed that all affected members had gait ataxia and cognitive disability with variable features of dysarthria, dysmetria, dysdiadochokinesia, nystagmus, dystonic movements, and cerebellar hypoplasia on imaging. Clinical signs of pyramidal tract dysfunction and sensory changes were absent. A genome-wide search in this family detected linkage to chromosome 3p with a maximum two-point lod score of 4.26 at D3S3630. This localization to the pter is distal to D3S1304, as defined by a recombination event. This overlaps with the SCA15 locus, with the critical overlapping region between the microsatellite markers, D3S1304 and D3S1620 (approximately 8 cM).

CONCLUSION

Autosomal dominant congenital nonprogressive cerebellar ataxia with or without cerebellar hypoplasia overlaps with the SCA15 locus on chromosome 3pter.

FRA16D common chromosomal fragile site oxido-reductase (FOR/WWOX) protects against the effects of ionizing radiation in Drosophila

Fragile sites are chromosomal structures that have been proposed to have a determining role in cancer-associated DNA instability. The human WWOX gene spans the FRA16D chromosomal fragile site, the common minimal region of homozygous deletion found in adenocarcinomas and three out of five translocation breakpoints in multiple myeloma. Transcripts from the alternatively spliced WWOX gene encode proteins with common N-terminal WW domains and variable homology to the oxidoreductase family of proteins. In this study, the Drosophila orthologue of the WWOX gene was identified and subjected to mutagenesis via homologous recombination. The resultant DmWWOX1 mutants were viable but exhibited an increased sensitivity to ionizing radiation. This radiation sensitivity was rescued by reintroduction and expression of either the wild-type Drosophila or human WWOX genes. Thus, the protective function of DmWWOX in response to irradiation in Drosophila is conserved with human WWOX (hWWOX). This is consistent with a protective role for hWWOX where aberrant expression, as a result of breakage at the associated fragile site, could contribute directly to cancer progression.

Common chromosomal fragile site FRA16D mutation in cancer cells

Neither the molecular basis for common fragile site DNA instability nor the contribution of this form of chromosomal instability to cancer is clearly understood. Fragile site FRA16D (16q23.2) is within regions of frequent loss-of-heterozygosity (LOH) in breast and prostate cancers, is associated with homozygous deletions in various adenocarcinomas and t(14;16) chromosomal translocations in multiple myeloma. The FOR (WWOX) gene spans FRA16D and encodes a partner of p53 that also has a role in apoptosis. Previously untested 53 cancer cell lines were screened for deletions within the FOR/WWOX gene. Deletions were detected in Co115, KM12C and KM12SM. Homozygous deletions in these and two previously identified tumour cell lines were intragenic on both alleles, indicating a distinct mutation mechanism from that causing LOH. Identical FRA16D deletions in two cell lines (one derived from the primary carcinoma and the other from a secondary metastasis) demonstrate that FRA16D DNA instability can be an early, transient event. Sequence analysis across one deletion locates one endpoint within a polymorphic AT-dinucleotide repeat and the other adjacent to an AT-rich mini-satellite repeat implicating AT-rich repeats in FRA16D DNA instability. Another deletion is associated with de novo repetition of the 9 bp AT-rich sequence at one of the deletion endpoints. FRA16D deleted cells retain cytogenetic fragile site expression indicating that the deletions are susceptible sites for breakage rather than regions that confer fragility. Most cell lines with FRA16D homozygous deletions also have FRA3B deletions, therefore common fragile sites represent highly susceptible genome-wide targets for a distinct form of mutation.

The pathogenic agent in Drosophila models of 'polyglutamine' diseases

A substantial body of evidence supports the identity of polyglutamine as the pathogenic agent in a variety of human neurodegenerative disorders where the mutation is an expanded CAG repeat. However, in apparent contradiction to this, there are several human neurodegenerative diseases (some of which are clinically indistinguishable from the 'polyglutamine' diseases) that are due to expanded repeats that cannot encode polyglutamine. As polyglutamine cannot be the pathogenic agent in these diseases, either the different disorders have distinct pathogenic pathways or some other common agent is toxic in all of the expanded repeat diseases. Recently, evidence has been presented in support of RNA as the pathogenic agent in Fragile X-associated tremor/ataxia syndrome (FXTAS), caused by expanded CGG repeats at the FRAXA locus. A Drosophila model of FXTAS, in which 90 copies of the CGG repeat are expressed in an untranslated region of RNA, exhibits both neurodegeneration and similar molecular pathology to the 'polyglutamine' diseases. We have, therefore, explored the identity of the pathogenic agent, and specifically the role of RNA, in a Drosophila model of the polyglutamine diseases by the expression of various repeat constructs. These include expanded CAA and CAG repeats and an untranslated CAG repeat. Our data support the identity of polyglutamine as the pathogenic agent in the Drosophila models of expanded CAG repeat neurodegenerative diseases.

Dynamic mutations: a decade of unstable expanded repeats in human genetic disease

The term 'dynamic mutation' was introduced to distinguish the unique properties of expanding, unstable DNA repeat sequences from other forms of mutation. The past decade has seen dynamic mutations uncovered as the molecular basis for a growing number of human genetic diseases and for all of the characterized 'rare' chromosomal fragile sites. The common properties of the repeats in different diseases and fragile sites have given insight into this unique form of DNA instability. While the dynamic mutation mechanism explains some unusual genetic characteristics, unexpected findings have raised new questions and challenged some assumptions about the pathways that lead from mutation to disease. This review will address the current understanding of the molecular mechanisms involved in the dynamic mutation process and elaborate on the pathogenic pathways that lead from expanded repeats to the diseases with which they are associated.

A novel Q378X mutation exists in the transmembrane transporter protein ABCC6 and its pseudogene: implications for mutation analysis in pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE) is an inherited disorder of the elastic tissue with characteristic progressive calcification of elastic fibers in skin, eye, and the cardiovascular system. Recently mutations in the ABCC6 gene, encoding a transmembrane transporter protein, were identified as cause of the disease. Surprisingly, sequence and RFLP analysis for exon 9 with primers corresponding to flanking intronic sequence in diseased and haplotype negative members from all of our families and in a control population revealed either a homozygous or heterozygous state for the Q378X (1132C-->T) nonsense mutation in all individuals. With the publication of the genomic structure of the PXE locus we had identified the starting point of a large genomic segmental duplication within the locus in the cytogenetic interval defined by the Cy19 and Cy185 somatic cell hybrid breakpoints on chromosome 16p13.1. By means of somatic cell hybrid mapping we located this starting point telomeric to exon 10 of ABCC6. The duplication, however, does not include exon 10, but exons 1-9. These findings suggest that one or several copies of an ABCC6 pseudogene (psiABCC6) lie within this large segmental duplication. At least one copy contains exons 1-9 and maps to the chromosomal interval defined by the Cy163 and Cy11 breakpoints. Either this copy and/or an additional copy of psiABCC6 within Cy19-Cy183 carries the Q378X mutation that masks the correct identification of this nonsense mutation as being causative in pseudoxanthoma elasticum. Long-range PCR of exon 9 starting from sequence outside the genomic replication circumvents interference from the psiABCC6 DNA sequences and demonstrates that the Q378X mutation in the ABCC6 gene is associated with PXE in some families. These findings lead us to propose that gene conversion mechanisms from psiABCC6 to ABCC6 play a functional role in mutations causing PXE.

Fragile and unstable chromosomes in cancer: causes and consequences

Cancer cells commonly exhibit various forms of genetic instability, such as changes in chromosome copy number, translocations and point mutations in particular genes. Although transmissible change seems to be an essential part of the neoplastic process, the extent to which DNA instability is a cause rather than a consequence of cancer is unclear. Chromosomal fragile sites have been proposed to be not only susceptible to DNA instability in cancer cells, but also associated with genes that contribute to the neoplastic process. Mutation at fragile site loci might therefore have a causative role in cancer. Recent studies on one class of human chromosomal fragile sites show that instability at fragile site loci can functionally contribute to tumor cell biology.

Analysis of replication timing at the FRA10B and FRA16B fragile site loci

The molecular basis for the cytogenetic appearance of chromosomal fragile sites is not yet understood. Late replication and further delay of replication at fragile sites expressing alleles has been observed for FRAXA, FRAXE and FRA3B fragile site loci. We analysed the timing of replication at the FRA10B and FRA16B loci to determine whether late replication is a feature which is shared by all fragile sites and, therefore, is a necessary condition for chromosomal fragile site expression. The FRA10B locus was located in a transitional region between early and late zones of replication. Fragile and non-fragile alleles exhibit a similar replication pattern proximal to the repeat but fragile alleles are delayed relative to non-fragile ones on the distal side. Although fragility at FRA10B appears to be caused by expansion of an AT-rich repeat in the region, replication time near the repeat was similar in fragile and non-fragile alleles. The FRA16B locus was late replicating and appeared to replicate even later on fragile chromosomes. While these observations are compatible with the hypothesis that delayed replication may play a role in fragile site expression, they suggest that replication delay may not need to occur at the expanded repeat region itself in order to be permissive for fragility.

Common chromosomal fragile site FRA16D sequence: identification of the FOR gene spanning FRA16D and homozygous deletions and translocation breakpoints in cancer cells

Fluorescence in situ hybridization of a tile path of DNA subclones has previously enabled the cyto-genetic definition of the minimal DNA sequence which spans the FRA16D common chromosomal fragile site, located at 16q23.2. Homozygous deletion of the FRA16D locus has been reported in adenocarcinomas of stomach, colon, lung and ovary. We have sequenced the 270 kb containing the FRA16D fragile site and the minimal homozygously deleted region in tumour cells. This sequence enabled localization of some of the tumour cell breakpoints to regions which contain AT-rich secondary structures similar to those associated with the FRA10B and FRA16B rare fragile sites. The FRA16D DNA sequence also led to the identification of an alternatively spliced gene, named FOR (fragile site FRA16D oxidoreductase), exons of which span both the fragile site and the minimal region of homozygous deletion. In addition, the complete DNA sequence of the FRA16D-containing FOR intron reveals no evidence of additional authentic transcripts. Alternatively spliced FOR transcripts (FOR I, FOR II and FOR III) encode proteins which share N-terminal WW domains and differ at their C-terminus, with FOR III having a truncated oxidoreductase domain. FRA16D-associated deletions selectively affect the FOR gene transcripts. Three out of five previously mapped translocation breakpoints in multiple myeloma are also located within the FOR gene. FOR is therefore the principle genetic target for DNA instability at 16q23.2 and perturbation of FOR function is likely to contribute to the biological consequences of DNA instability at FRA16D in cancer cells.

Chromosomal fragile site FRA16D and DNA instability in cancer

It has been proposed that common aphidicolin-inducible fragile sites, in general, predispose to specific chromosomal breakage associated with deletion, amplification, and/or translocation in certain forms of cancer. Although this appears to be the case for the fragile site FRA3B and may be the case for FRA7G, it is not yet clear whether this association is a general property of this class of fragile site. The major aim of the present study was to determine whether the FRA16D chromosomal fragile site locus has a role to play in predisposing DNA sequences within and adjacent to the fragile site to DNA instability (such as deletion or translocation), which could lead to or be associated with neoplasia. We report the localization of FRA16D within a contig of cloned DNA and demonstrate that this fragile site coincides with a region of homozygous deletion in a gastric adenocarcinoma cell line and is bracketed by translocation breakpoints in multiple myeloma, as reported previously (Chesi, M., et al., Blood, 91: 4457-4463, 1998). Therefore, given similar findings at the FRA3B and FRA7G fragile sites, it is likely that common aphidicolin-inducible fragile sites exhibit the general property of localized DNA instability in cancer cells.

Detection of a novel missense mutation and second recurrent mutation in the CACNA1A gene in individuals with EA-2 and FHM

Mutations in the brain specific P/Q type Ca2+ channel alpha1 subunit gene, CACNA1A, have been identified in three clinically distinct disorders, viz. episodic ataxia type 2 (EA-2), familial hemiplegic migraine (FHM) and spinocerebellar ataxia 6 (SCA6). For individuals with EA-2, the mutations described thus far are presumed to result in a truncated protein product. Several different missense mutations have been identified in patients with FHM. At least two of these mutations have been identified on two different chromosome 19p13 haplotypes and thus represent recurrent mutations. In the present study, we have screened several individuals for mutations in all 47 exons in the CACNA1A gene by single-strand conformation analysis. We have characterised a novel missense mutation, G5260A, in exon 32 in a family segregating for EA-2. The consequence of this mutation is an amino acid substitution at a highly conserved position within the CACNA1A gene. This represents the first point mutation not resulting in a proposed truncated protein. Furthermore, this mutation has been detected in a family member with mild clinical signs including only migraine. Additionally, a second previously identified recurrent muta tion, C2272T, in exon 16 has been discovered in a patient with FHM.

Fragile sites still breaking

Rare fragile sites on chromosomes are the archetypal dynamic mutations. They involve large expansions of the microsatellite CCG or AT-rich minisatellites. The mutation process is an increase in repeat-unit number from within a normal range, through a premutation range, up to full mutation where the fragile site is expressed. Full mutations can inactivate genes and are regions of genomic instability. Common fragile sites, in particular, might have a role in oncogensis by facilitating gene inactivation through chromosomal deletion or amplification, but this requires further exploration. The mechanisms behind the changes that give rise to the cytogenetic manifestation of chromosomal fragility are now beginning to be understood.

Construction of an approximately 700-kb transcript map around the familial Mediterranean fever locus on human chromosome 16p13.3

We used a combination of cDNA selection, exon amplification, and computational prediction from genomic sequence to isolate transcribed sequences from genomic DNA surrounding the familial Mediterranean fever (FMF) locus. Eighty-seven kb of genomic DNA around D16S3370, a marker showing a high degree of linkage disequilibrium with FMF, was sequenced to completion, and the sequence annotated. A transcript map reflecting the minimal number of genes encoded within the approximately 700 kb of genomic DNA surrounding the FMF locus was assembled. This map consists of 27 genes with discreet messages detectable on Northerns, in addition to three olfactory-receptor genes, a cluster of 18 tRNA genes, and two putative transcriptional units that have typical intron-exon splice junctions yet do not detect messages on Northerns. Four of the transcripts are identical to genes described previously, seven have been independently identified by the French FMF Consortium, and the others are novel. Six related zinc-finger genes, a cluster of tRNAs, and three olfactory receptors account for the majority of transcribed sequences isolated from a 315-kb FMF central region (between D16S468/D16S3070 and cosmid 377A12). Interspersed among them are several genes that may be important in inflammation. This transcript map not only has permitted the identification of the FMF gene (MEFV), but also has provided us an opportunity to probe the structural and functional features of this region of chromosome 16.

CAG repeat expansion in autosomal dominant familial spastic paraparesis: novel expansion in a subset of patients

Autosomal dominant familial spastic paraplegia (FSP) is a genetically heterogeneous neurodegenerative disorder displaying anticipation for which three loci have been mapped to the chromosomal positions 14q11.2-q24.3 (SPG3), 2p21-p24 (SPG4) and 15q11.1 (SPG6). The repeat expansion detection (RED) method has been used to demonstrate expanded CAG repeats in some FSP families that map to SPG4. We analyzed 20 FSP families, including four for which there is evidence for linkage to SPG4, and found that in most cases the repeat expansion detected by RED is due to non-pathogenic expansions of the chromosome 18q21.1 SEF2-1 or 17q21.3 ERDA1 locus. Polymorphic expansions at SEF2-1 and ERDA1 appear frequent and may confound RED studies in the search for genes causing disorders demonstrating anticipation. In six FSP families, however, CAG repeat expansion was detected in a subset of affected and at-risk individuals that did not result from expansion of the SEF2-1 and ERDA1 loci. Overall, 11 of 37 (30%) of the FSP patients with a CAG/CTG repeat expansion are unaccounted for by the SEF2-1 and ERDA1 loci, compared with two of 23 (9%) of the unaffected at-risk individuals and none of 19 controls. In the majority of cases these novel expansions were shorter than those previously reported.

FRA10B structure reveals common elements in repeat expansion and chromosomal fragile site genesis

A common mechanism for chromosomal fragile site genesis is not yet apparent. Folate-sensitive fragile sites are expanded p(CCG)n repeats that arise from longer normal alleles. Distamycin A or bromodeoxyuridine-inducible fragile site FRA16B is an expanded AT-rich approximately 33 bp repeat; however, the relationship between normal and fragile site alleles is not known. Here, we report that bromodeoxyuridine-inducible, distamycin A-insensitive fragile site FRA10B is composed of expanded approximately 42 bp repeats. Differences in repeat motif length or composition between different FRA10B families indicate multiple independent expansion events. Some FRA10B alleles comprise a mixture of different expanded repeat motifs. FRA10B fragile site and long normal alleles share flanking polymorphisms. Somatic and intergenerational FRA10B repeat instability analogous to that found in expanded trinucleotide repeats supports dynamic mutation as a common mechanism for repeat expansion.

Dynamic mutation: possible mechanisms and significance in human disease

Increases in repeat-DNA copy number are the molecular basis of a growing list of human genetic diseases, including fragile X syndrome, myotonic dystrophy, Huntington disease and a form of epilepsy. Repeat-DNA sequences undergo a unique process of dynamic mutation, the common properties of which probably reflect common molecular events. This form of mutation is no longer restricted to trinucleotide repeats, because repeats of different length have been found to undergo expansion.

Genetic heterogeneity in familial acute myelogenous leukemia: evidence for a second locus at chromosome 16q21-23.2

The identification of genes responsible for the rare cases of familial leukemia may afford insight into the mechanism underlying the more common sporadic occurrences. Here we test a single family with 11 relevant meioses transmitting autosomal dominant acute myelogenous leukemia (AML) and myelodysplasia for linkage to three potential candidate loci. In a different family with inherited AML, linkage to chromosome 21q22.1-22.2 was recently reported; we exclude linkage to 21q22.1-22.2, demonstrating that familial AML is a heterogeneous disease. After reviewing familial leukemia and observing anticipation in the form of a declining age of onset with each generation, we had proposed 9p21-22 and 16q22 as additional candidate loci. Whereas linkage to 9p21-22 can be excluded, the finding of a maximum two-point LOD score of 2.82 with the microsatellite marker D16S522 at a recombination fraction theta = 0 provides evidence supporting linkage to 16q22. Haplotype analysis reveals a 23.5-cM (17.9-Mb) commonly inherited region among all affected family members extending from D16S451 to D16S289. In order to extract maximum linkage information with missing individuals, incomplete informativeness with individual markers in this interval, and possible deviance from strict autosomal dominant inheritance, we performed nonparametric linkage analysis (NPL) and found a maximum NPL statistic corresponding to a P-value of .00098, close to the maximum conditional probability of linkage expected for a pedigree with this structure. Mutational analysis in this region specifically excludes expansion of the AT-rich minisatellite repeat FRA16B fragile site and the CAG trinucleotide repeat in the E2F-4 transcription factor. The "repeat expansion detection" method, capable of detecting dynamic mutation associated with anticipation, more generally excludes large CAG repeat expansion as a cause of leukemia in this family.

Chromosomal localization of the human P2y6 purinoceptor gene and phylogenetic analysis of the P2y purinoceptor family

The G-protein-coupled P2Y purinoceptors mediate a variety of physiological effects in response to extracellular nucleotides. With the recent discovery of several new members from a variety of species, the P2Y purinoceptor family now encompasses types P2Y1 to P2Y6. By fluorescence in situ hybridization and utilization of the National Center for Biotechnology Information (NCBI) database, the human P2Y6 gene was localized to chromosome 11q13.5, between polymorphic markers D11S1314 and D11S916. NCBI database analysis of the remaining human P2Y purinoceptor genes revealed that P2Y2 and P2Y6 mapped to within less than 4 cM, and thus constitute the first described chromosomal clustering of this gene family. Phylogenetic analysis of the P2Y purinoceptor family demonstrated the presence of five evolutionary branches and suggests the occurrence of an ancient gene duplication event.

Unbalanced t(4;11)(q32;q23) in a 34-year-old man with manifestations of distal monosomy 11q and trisomy 4q syndromes

We present a 34-year-old man with an unbalanced translocation between the long arms of chromosome 4 and chromosome 11. He had manifestations of monosomy 11(q23)--minor facial anomalies, abnormal head shape, cryptorchidism; trisomy 4(q32)--hirsutism, renal disease; and manifestations attributable to both imbalances--heart disease, musculoskeletal anomalies, and mental retardation. FISH studies showed that the chromosome 11q23.3 translocation breakpoint was distal to the rare folate sensitive fragile site (FRA11B). The patient is the oldest reported with both imbalance of 4q+ and 11q-.