Children with fragile X syndrome can have a number of characteristic facial features, including an elongated, thin face and prominent ears and forehead. Image source: Reprinted by permission from Macmillan Publishers Ltd., Garber et al., European Journal of Human Genetics16, 666-672, copyright 2008.
Fragile X Syndrome and Autism
Fragile X syndrome in humans is characterized by a broad collection of physical and mental impairments and is the most common form of inherited mental retardation. The syndrome is accompanied by morphological changes in the facial features as well.
The gene for this disorder was discovered in 1991 and is called fragile X mental retardation 1 gene (FMR1). Scientists recently have shown that two different mutations in the FMR1 gene may lead to autism. This is one of the first links between autism and inheritance.
Autism spectrum disorders affect on average 1 in 100 children in the United States and occurs five times more often in boys than in girls. Autism is characterized by problems with social development, limited eye contact, unusual repetitive movement, and difficulty reading facial cues. Understanding the FMR1 gene’s normal function and the effect of mutations in this gene will lead the way for scientists to develop targeted treatments for autism as well as for fragile X syndrome.
Human X chromosome. The yellow arrow indicates the location of the FMR1 gene. Image source: National Institutes of Health.
What Do We Know About FMR1 in Humans?
We know that the FMR1 gene is found on the X chromosome (image at right). The X chromosome is one of the two sex-determining chromosomes in humans and many other organisms; females have two X chromosomes; males, one X and one Y.
The traits that develop in an individual with a mutant FMR1 gene do so because of a “loss of function” of the gene. Loss of expression of the FMR1 gene occurs as the result of expanded repeats of a three-base DNA sequence (CGG). The number of repeats determines the severity of the disease. The image below shows how repeats of CGG can expand to create the disease.
Premutation and mutation (CGG expansion) associated with fragile X syndrome. Image source: Reprinted by permission from Macmillan Publishers Ltd., Garber et al., European Journal of Human Genetics16, 666-672, copyright 2008.
In normal individuals the number of repeats varies between 29 and 31 copies of CGG, but in those affected by the disease we see greater than 200 copies of the repeat. In autism, we see individuals who have inherited the full mutation but also some with an intermediate number of repeats (55-200). The FMR protein (FMRP) is an RNA binding protein involved in translational regulation of several other genes at the synapse. Two of these genes, neurolgins 3 and 4, have also been shown to be involved in autism. Loss FMRP leads to the up-regulation of many of these genes and consequently the expression of the disease.
Y. Q. Zhang et al., Molecular and Cellular Proteomics4, 278-290 (2005).
What Do We Know About FMR1 in Drosophila?
Drosophila have a single FMR1 gene, dFMR, which is involved in many pathways related to neuronal development. The image at right shows the dFMR1 mutant phenotype in the Drosophila brain.
Drosophila with mutations in and loss of function of the dFMR1 gene show decreased locomotor activity and a change in the timing of the emergence of the adult from its pupal case. The neurons in dfmr mutants also have structural changes that can be monitored, including over-growth, over-branching, and abnormal synapses. You can see the effects of a dFMR1 mutation in the figure at right, in which the mutant brain (bottom panel) has regions with high concentrations of neurotransmitters (signalled with arrows) when compared to the normal fly brain (top panel). These phenotypic changes in the fly allow researches to study the effect of loss of dFMR function on other proteins.
What Other Things Can Drosophila Tell us About Autism?
Using Fruit Flies to Discover New Treatments for Fragile X Syndrome.
The following news announcement talks about exciting new studies being done by Dr. Stephen Warren in the Department of Human Genetics at Emory University. Dr. Warren is using Drosophila to study new drugs that have had promising success in reversing the features of Fragile X Syndrome. This is another great example of how Drosophila can be a tremendously informative animal model of human disease.
R. J. Hagerman, Lessons from fragile X regarding neurobiology, autism, and neurodegeneration, Dev. and Behavior. Ped.27, 63-73 (2006).
L. W. Wang et al., Fragile X: leading the way for targeted treatments in autism, Neurotherapeutics7, 264-274 (2010).
Stephane Jamain, et al., Mutations of the X linked genes encoding neuroligins NLGN3 and NLGN4 are associated with autism, Nat. Genet.34, 27-29 (2003).
T. Sekine T et al., Circadian phenotypes of Drosophila fragile x mutants in alternative genetic backgrounds, Zoolog Sci.25, 561-571 (2008).
L. Pan et al., The Drosophila fragile X gene negatively regulates neuronal elaboration and synaptic differentiation, Curr. Biol.14, 1863-1870 (2004).
Lili Wan et al., Characterization of dFMR1, a Drosophila melanogaster Homolog of the Fragile X Mental Retardation Protein, Mol. Cell Biol.20, 8536-8547 (2000).
Y. Q. Zhang et al., Protein expression profiling of the Drosophila fragile X mutant brain reveals up-regulation of monoamine synthesis, Mol. Cell. Proteom.4, 278-290 (2005).
S. Chang et al., Identification of small molecules rescuing fragile X syndrome phenotypes in Drosophila, Nat. Chem. Biol.4, 256-263 (2008).
Glossary Terms
TEST Monocytes
A cell of the immune system, which circulates through the blood, bone marrow, and spleen, and plays an important role in antimicrobial defense.
A protein in the immune system that recognizes a specific target, or antigen, on a foreign molecule, allowing these two structures to bind together with precision.
A chromosome for which there is an equal number of copies in males and females -- that is, a chromosome that is not a sex chromosome.
Activator
A protein which, by binding to an operator sequence, promotes transcription of a gene.
Base Pairing
The "genetic code" in a DNA molecule is written in four bases -- adenine (A), cytosine (C), guanine (G) and thymine (T) -- that are arrayed along each strand of the twisted, two-stranded molecule (the famous "double helix"). Each base is chemically tuned to pair, via hydrogen bonds, with a corresponding base on the opposite strand -- A with T, G with C. The size of an organism's genome is usually given by the number of base pairs.
A technique for analyzing interactions of proteins with DNA, consisting of chromosomal immunoprecipitation, followed by massively parallel DNA sequencing.
The combination of DNA and proteins found in the nucleus of a cell, which makes up chromosomes. Chromatin helps fold DNA so it will fit into the cell and is involved in both gene expression and DNA replication.
DNA encodes the genetic blueprint of an organsim. This genetic material is composed of deoxyribonucleotides -- individual units combining a sugar, a base, and a phosphate group -- that each have different chemical properties, and are referred to by the different base names adenine (A), cytosine (C), guanine (G) and thymine (T). Combinations of these bases "spell out" the code of a given gene.
DNA-Binding Protein
Proteins with a specific or general affinity for DNA.
The class of organisms, composed of one or more cells, containing a membrane-enclosed nucleus and packaging its DNA with histones in a nucleosome array. Eukaryotic cells typically have complex organelles, such as mitochondria.
An exon is a contiguous segment of a gene found both in the initial transcript and in the final product; the introns are those segments found in the initial transcript which are removed during processing, and so are not found in the finished product.
Gene
In molecular biology, a gene is the molecular unit of inheritance for a single function or phenotype -- or, more precisely, the full sequence of bases within a section of the genome that is necessary and sufficient for the synthesis of a functional product. Usually that product is a polypeptide (a section of a protein), but in some cases it is an RNA molecule.
Genome
The full complement of genetic information recorded in the chromosomal DNA (or, for some organisms, RNA).
Within a genome sequencing project, annotation is the process of identifying biologically relevant elements within the genome sequence (e.g., genes), and adding information to the sequence on how those elements function.
A protein first isolated from the jellyfish Aequorea victoria that exhibits bright green fluorescence when exposed to ultraviolet blue light. The GFP gene can be introduced into organisms and used by scientists to "see" gene expression.
The small, basic proteins used to package the DNA in chromatin. The core histones (H2A, H2B, H3, and H4) are highly conserved over evolution, while histone H1 is more variable.
The hypothesis that combinations of chemical modifications to histone proteins in the chromatin form a complex, separate mechanism for regulating transcription and, thus, gene expression.
A gene found in an organism that shares an ancestral sequence with that of another organism. Homologs are often identified based on the retention of shared genetic or protein-level identities between two different species that share a common evolutionary history.
High-Occupancy Target (HOT) Region
Genomic regions where 15 or more independent transcription factors bind.
Immunoprecipitation
The process of isolating and concentrating a specific protein of interest by trapping an antibody that binds to that protein, using any of a number of lab techniques.
The intermediate developmental stages that an insect (such as Drosophila) undergoes between molts until it reaches sexual maturity.
Metabolome
The full complement of metabolites -- small molecules produced by cellular or organismal metabolism -- that characterize a cell, cell population, tissue, or organism.
A non-human species used in experimental biology to study biological processes that might illuminate workings of the same processes in other organisms, for which the same experiments might be infeasible or unethical.
Molting, or ecdysis, is the periodic shedding of the outer skeleton, or exoskeleton, that accompanies the growth of most arthropods, including insects.
A family of techniques for DNA sequencing that rely on massively parallel processing of many millions of DNA fragments, followed by analysis and re-assembly of those fragments using computer techniques.
The null hypothesis generally corresponds to what we expect if nothing "interesting" is happening. If you flip a coin many times, and generally get roughly 50% heads and 50% tails, that is consistent with the null hypothesis that the coin is fair. If you flip a coin many times and get 99% heads, the coin may be unfair, and hence you may have cause to reject the null hypothesis that it is fair.
Ortholog
A gene that has similar sequence in each species in which it's found because the species have a common ancestor during evolutionary time. For example, the alcohol dehydrogenase and Malic Enzyme 1 genes are similar in both Drosophila melanogaster and Homo sapiens. Normally, orthologous genes have the same function in each species in which they are found; therefore, studying the function of a gene in a model organism can provide good evidence for the function of the orthologous gene in humans
The observable characteristics or traits of an organism, resulting from the interaction of the expression of the organism's genes with the influence of environmental factors.
A type of nonverbal communication, usually a chemical or hormone secreted by an animal, which often influences the behavior of other members of the same species. Pheromones are used to establish territory and attract mates.
Giant chromosomes formed by some cells that undergo multiple rounds of DNA replication without actual cell division. The salivary glands of Drosophila contain examples of such chromosomes. Their size makes them especially convenient for work in the lab.
Biological compounds made up of one or more polypeptides (a chain of amino acids) typically folded into a 3-D form. The sequence of amino acids in a protein is defined by the sequence of a gene.
Protein Purification
A variety of processes used to isolate a particular protein from a biological tissue or culture, and thereby to allow the further characterization of the protein's structure and function.
A protein which, by binding to an operator sequence, prevents transcription of a gene.
Reference Genome
A genome sequence assembled from the experimentally obtained sequences of a number of individuals in a species, designed to serve as a representative example of the "typical" gene sequence of that species.
An enzyme that uses RNA as a template to transcribe single-stranded DNA -- thereby reversing the more familiar information flow from DNA to RNA. In addition to its use in the lab, RT has been extensively studied in retroviruses (particularly HIV) that have an RNA genome but must produce double-stranded DNA that becomes integrated into the host cell genome as part of their replication cycle.
RNA is composed of nucleotides, just like DNA -- three major differences between the two: (1) RNA contains the sugar ribose, while DNA contains the slightly different sugar deoxyribose (2) RNA has the nucleobase uracil, while DNA contains thymine; (3) unlike DNA, most RNA molecules are single-stranded.
The silencing or reduction of RNA expression (which generally correspondes to protein production) for a given gene in a cell or organism. It occurs as a natural process within living cells, but is also a powerful technique for studies of gene expression in the lab.
The sequence of a small fragment of DNA, obtained as part of a high-throughput sequencing experiment.
Sex Chromosomes
A pair of chromosomes, usually designated X or Y, in the germ cells of most animals and some plants, that combine to determine the sex and sex-linked characteristics of an individual.
A difference in a single base pair in a given gene sequence, between two or more individuals, or between an individual and a reference genome, that is associated with a difference in phenotype or expressed trait.
A consensus sequence, TATA(A/T)A, found about 25 base pairs upstream from the start site of a group of eukaryotic genes encoding messenger RNA -- often those that can be transcribed at a high rate. The TATA box binds the TATA box binding protein (TBP), a subunit of TFIID, initiating the process of RNA polymerase II assembly at the promoter in vitro, and plays a key role as a recognition sequence for RNA polymerase II in eukaryotic organisms.
A protein that binds to a DNA sequence and controls (increases or decreases) the rate of transcription (the flow of genetic information from DNA to RNA).
The process in which RNA, produced during transcription, is decoded to produce an amino acid chain (polypeptide) that will then fold into an active protein.
Slang term for the domain of classic lab experiments handling actual and analyzing actual biological materials, as opposed to experiments and work performed using computer analysis.
Wild-Type
This is the typical or most common form, appearance, or strain of an organism that exists in the wild, as opposed to the lab. It can also refer to the normal, non-mutated form of a gene that's common in nature.
Zygote
The earliest developmental stage of the embryo, occurring when two gamete cells are joined by means of sexual reproduction.
