Simple sequence repeats: genetic modulators of brain function and behavior

When repetita no-longer iuvant: somatic instability of the CAG triplet in Huntington's disease

Trinucleotide repeats in DNA exhibit a dual nature due to their inherent instability. While their rapid expansion can diversify gene expression during evolution, exceeding a certain threshold can lead to diseases such as Huntington's disease (HD), a neurodegenerative condition, triggered by >36 C-A-G repeats in exon 1 of the Huntingtin gene. Notably, the discovery of somatic instability (SI) of the tract allows these mutations, inherited from an affected parent, to further expand throughout the patient's lifetime, resulting in a mosaic brain with specific neurons exhibiting variable and often extreme CAG lengths, ultimately leading to their death. Genome-wide association studies have identified genetic variants-both cis and trans, including mismatch repair modifiers-that modulate SI, as shown in blood cells, and influence HD's age of onset. This review will explore the evidence for SI in HD and its role in disease pathogenesis, as well as the therapeutic implications of these findings. We conclude by emphasizing the urgent need for reliable methods to quantify SI for diagnostic and prognostic purposes.

Mutant Huntingtin Drives Development of an Advantageous Brain Early in Life: Evidence in Support of Antagonistic Pleiotropy

OBJECTIVE

Huntington's disease (HD) is a neurodegenerative disease caused by a triplet repeat expansion within the gene huntingtin (HTT). Antagonistic pleiotropy is a theory of aging that posits that some genes, facilitating individual fitness early in life through adaptive evolutionary changes, also augment detrimental aging-related processes. Antagonistic pleiotropy theory may explain a positive evolutionary pressure toward functionally advantageous brain development that is vulnerable to rapid degeneration. The current study investigated antagonistic pleiotropy in HD using a years-to-onset paradigm in a unique sample of children and young adults at risk for HD.

METHODS

Cognitive, behavioral, motor, and brain structural measures from premanifest gene-expanded (n = 79) and gene nonexpanded (n = 112) participants (6-21 years) in the Kids-HD study were examined. All measures in the gene-expanded group were modeled using a mixed-effects regression approach to assess years-to-onset-based changes while controlling for normal growth. Simultaneously, structure-function associations were also examined.

RESULTS

Decades from motor onset, gene-expanded participants showed significantly better cognitive, behavioral, and motor scores versus gene nonexpanded controls, along with larger cerebral volumes and cortical features. After this initial peak, a prolonged deterioration was observed in both functional and structural measures. Far from onset, brain measures were positively correlated with functional measures, supporting the view that functional advantages were mediated by structural differences.

INTERPRETATION

Mutant HTT may drive the development of a larger than normal brain that subserves superior early-life function. These findings support the antagonistic pleiotropy theory of HTT in HD, where this gene drives early advantage followed by accelerated aging processes. ANN NEUROL 2024;96:1006-1019.

Microsatellite density landscapes illustrate short tandem repeats aggregation in the complete reference human genome

BACKGROUND

Microsatellites are increasingly realized to have biological significance in human genome and health in past decades, the assembled complete reference sequence of human genome T2T-CHM13 brought great help for a comprehensive study of short tandem repeats in the human genome.

RESULTS

Microsatellites density landscapes of all 24 chromosomes were built here for the first complete reference sequence of human genome T2T-CHM13. These landscapes showed that short tandem repeats (STRs) are prone to aggregate characteristically to form a large number of STRs density peaks. We classified 8,823 High Microsatellites Density Peaks (HMDPs), 35,257 Middle Microsatellites Density Peaks (MMDPs) and 199, 649 Low Microsatellites Density Peaks (LMDPs) on the 24 chromosomes; and also classified the motif types of every microsatellites density peak. These STRs density aggregation peaks are mainly composing of a single motif, and AT is the most dominant motif, followed by AATGG and CCATT motifs. And 514 genomic regions were characterized by microsatellite density feature in the full T2T-CHM13 genome.

CONCLUSIONS

These landscape maps exhibited that microsatellites aggregate in many genomic positions to form a large number of microsatellite density peaks with composing of mainly single motif type in the complete reference genome, indicating that the local microsatellites density varies enormously along the every chromosome of T2T-CHM13.

Short Tandem Repeats in the era of next-generation sequencing: from historical loci to population databases

In this study, we explore the landscape of short tandem repeats (STRs) within the human genome through the lens of evolving technologies to detect genomic variations. STRs, which encompass approximately 3% of our genomic DNA, are crucial for understanding human genetic diversity, disease mechanisms, and evolutionary biology. The advent of high-throughput sequencing methods has revolutionized our ability to accurately map and analyze STRs, highlighting their significance in genetic disorders, forensic science, and population genetics. We review the current available methodologies for STR analysis, the challenges in interpreting STR variations across different populations, and the implications of STRs in medical genetics. Our findings underscore the urgent need for comprehensive STR databases that reflect the genetic diversity of global populations, facilitating the interpretation of STR data in clinical diagnostics, genetic research, and forensic applications. This work sets the stage for future studies aimed at harnessing STR variations to elucidate complex genetic traits and diseases, reinforcing the importance of integrating STRs into genetic research and clinical practice.

Mutation protocols share with sexual reproduction the physiological role of producing genetic variation within 'constraints that deconstrain'

Because the universe of possible DNA sequences is inconceivably vast, organisms have evolved mechanisms for exploring DNA sequence space while substantially reducing the hazard that would otherwise accrue to any process of random, accidental mutation. One such mechanism is meiotic recombination. Although sexual reproduction imposes a seemingly paradoxical 50% cost to fitness, sex evidently prevails because this cost is outweighed by the advantage of equipping offspring with genetic variation to accommodate environmental vicissitudes. The potential adaptive utility of additional mechanisms for producing genetic variation has long been obscured by a presumption that the vast majority of mutations are deleterious. Perhaps surprisingly, the probability for adaptive variation can be increased by several mechanisms that generate mutations abundantly. Such mechanisms, here called 'mutation protocols', implement implicit 'constraints that deconstrain'. Like meiotic recombination, they produce genetic variation in forms that minimize potential for harm while providing a reasonably high probability for benefit. One example is replication slippage of simple sequence repeats (SSRs); this process yields abundant, reversible mutations, typically with small quantitative effect on phenotype. This enables SSRs to function as adjustable 'tuning knobs'. There exists a clear pathway for SSRs to be shaped through indirect selection favouring their implicit tuning-knob protocol. Several other molecular mechanisms comprise probable components of additional mutation protocols. Biologists might plausibly regard such mechanisms of mutation not primarily as sources of deleterious genetic mistakes but also as potentially adaptive processes for 'exploring' DNA sequence space.

Dimeric structures of DNA ATTTC repeats promoted by divalent cations

Structural studies of repetitive DNA sequences may provide insights why and how certain repeat instabilities in their number and nucleotide sequence are managed or even required for normal cell physiology, while genomic variability associated with repeat expansions may also be disease-causing. The pentanucleotide ATTTC repeats occur in hundreds of genes important for various cellular processes, while their insertion and expansion in noncoding regions are associated with neurodegeneration, particularly with subtypes of spinocerebellar ataxia and familial adult myoclonic epilepsy. We describe a new striking domain-swapped DNA-DNA interaction triggered by the addition of divalent cations, including Mg2+ and Ca2+. The results of NMR characterization of d(ATTTC)3 in solution show that the oligonucleotide folds into a novel 3D architecture with two central C:C+ base pairs sandwiched between a couple of T:T base pairs. This structural element, referred to here as the TCCTzip, is characterized by intercalative hydrogen-bonding, while the nucleobase moieties are poorly stacked. The 5'- and 3'-ends of TCCTzip motif are connected by stem-loop segments characterized by A:T base pairs and stacking interactions. Insights embodied in the non-canonical DNA structure are expected to advance our understanding of why only certain pyrimidine-rich DNA repeats appear to be pathogenic, while others can occur in the human genome without any harmful consequences.

Mechanistic and Therapeutic Insights into Ataxic Disorders with Pentanucleotide Expansions

Pentanucleotide expansion diseases constitute a special class of neurodegeneration. The repeat expansions occur in non-coding regions, have likely arisen from Alu elements, and often result in autosomal dominant or recessive phenotypes with underlying cerebellar neuropathology. When transcribed (potentially bidirectionally), the expanded RNA forms complex secondary and tertiary structures that can give rise to RNA-mediated toxicity, including protein sequestration, pentapeptide synthesis, and mRNA dysregulation. Since several of these diseases have recently been discovered, our understanding of their pathological mechanisms is limited, and their therapeutic interventions underexplored. This review aims to highlight new in vitro and in vivo insights into these incurable diseases.

A hidden layer of structural variation in transposable elements reveals potential genetic modifiers in human disease-risk loci

Genome-wide association studies (GWAS) have been highly informative in discovering disease-associated loci but are not designed to capture all structural variations in the human genome. Using long-read sequencing data, we discovered widespread structural variation within SINE-VNTR-Alu (SVA) elements, a class of great ape-specific transposable elements with gene-regulatory roles, which represents a major source of structural variability in the human population. We highlight the presence of structurally variable SVAs (SV-SVAs) in neurological disease-associated loci, and we further associate SV-SVAs to disease-associated SNPs and differential gene expression using luciferase assays and expression quantitative trait loci data. Finally, we genetically deleted SV-SVAs in the BIN1 and CD2AP Alzheimer's disease-associated risk loci and in the BCKDK Parkinson's disease-associated risk locus and assessed multiple aspects of their gene-regulatory influence in a human neuronal context. Together, this study reveals a novel layer of genetic variation in transposable elements that may contribute to identification of the structural variants that are the actual drivers of disease associations of GWAS loci.

The evolutionary history of the polyQ tract in huntingtin sheds light on its functional pro-neural activities

Huntington's disease is caused by a pathologically long (>35) CAG repeat located in the first exon of the Huntingtin gene (HTT). While pathologically expanded CAG repeats are the focus of extensive investigations, non-pathogenic CAG tracts in protein-coding genes are less well characterized. Here, we investigated the function and evolution of the physiological CAG tract in the HTT gene. We show that the poly-glutamine (polyQ) tract encoded by CAGs in the huntingtin protein (HTT) is under purifying selection and subjected to stronger selective pressures than CAG-encoded polyQ tracts in other proteins. For natural selection to operate, the polyQ must perform a function. By combining genome-edited mouse embryonic stem cells and cell assays, we show that small variations in HTT polyQ lengths significantly correlate with cells' neurogenic potential and with changes in the gene transcription network governing neuronal function. We conclude that during evolution natural selection promotes the conservation and purity of the CAG-encoded polyQ tract and that small increases in its physiological length influence neural functions of HTT. We propose that these changes in HTT polyQ length contribute to evolutionary fitness including potentially to the development of a more complex nervous system.

Revisiting tandem repeats in psychiatric disorders from perspectives of genetics, physiology, and brain evolution

Genome-wide association studies (GWASs) have revealed substantial genetic components comprised of single nucleotide polymorphisms (SNPs) in the heritable risk of psychiatric disorders. However, genetic risk factors not covered by GWAS also play pivotal roles in these illnesses. Tandem repeats, which are likely functional but frequently overlooked by GWAS, may account for an important proportion in the "missing heritability" of psychiatric disorders. Despite difficulties in characterizing and quantifying tandem repeats in the genome, studies have been carried out in an attempt to describe impact of tandem repeats on gene regulation and human phenotypes. In this review, we have introduced recent research progress regarding the genomic distribution and regulatory mechanisms of tandem repeats. We have also summarized the current knowledge of the genetic architecture and biological underpinnings of psychiatric disorders brought by studies of tandem repeats. These findings suggest that tandem repeats, in candidate psychiatric risk genes or in different levels of linkage disequilibrium (LD) with psychiatric GWAS SNPs and haplotypes, may modulate biological phenotypes related to psychiatric disorders (e.g., cognitive function and brain physiology) through regulating alternative splicing, promoter activity, enhancer activity and so on. In addition, many tandem repeats undergo tight natural selection in the human lineage, and likely exert crucial roles in human brain evolution. Taken together, the putative roles of tandem repeats in the pathogenesis of psychiatric disorders is strongly implicated, and using examples from previous literatures, we wish to call for further attention to tandem repeats in the post-GWAS era of psychiatric disorders.

The Neurodevelopmental Hypothesis of Huntington's Disease

The current dogma of HD pathoetiology posits it is a degenerative disease affecting primarily the striatum, caused by a gain of function (toxicity) of the mutant mHTT that kills neurons. However, a growing body of evidence supports an alternative theory in which loss of function may also influence the pathology.This theory is predicated on the notion that HTT is known to be a vital gene for brain development. mHTT is expressed throughout life and could conceivably have deleterious effects on brain development. The end event in the disease is, of course, neurodegeneration; however the process by which that occurs may be rooted in the pathophysiology of aberrant development.To date, there have been multiple studies evaluating molecular and cellular mechanisms of abnormal development in HD, as well as studies investigating abnormal brain development in HD animal models. However, direct study of how mHTT could affect neurodevelopment in humans has not been approached until recent years. The current review will focus on the most recent findings of a unique study of children at-risk for HD, the Kids-HD study. This study evaluates brain structure and function in children ages 6-18 years old who are at risk for HD (have a parent or grand-parent with HD).

The Effect of CAG Repeats within the Non-Pathological Range in the HTT Gene on Cognitive Functions in Patients with Subjective Cognitive Decline and Mild Cognitive Impairment

The Huntingtin gene (HTT) is within a class of genes containing a key region of CAG repeats. When expanded beyond 39 repeats, Huntington disease (HD) develops. Individuals with less than 35 repeats are not associated with HD. Increasing evidence has suggested that CAG repeats play a role in modulating brain development and brain function. However, very few studies have investigated the effect of CAG repeats in the non-pathological range on cognitive performances in non-demented individuals. In this study, we aimed to test how CAG repeats' length influences neuropsychological scores in patients with subjective cognitive decline (SCD) and mild cognitive impairment (MCI). We included 75 patients (46 SCD and 29 MCI). All patients underwent an extensive neuropsychological battery and analysis of HTT alleles to quantify the number of CAG repeats. Results: CAG repeat number was positively correlated with scores of tests assessing for executive function, visual-spatial ability, and memory in SCD patients, while in MCI patients, it was inversely correlated with scores of visual-spatial ability and premorbid intelligence. When we performed a multiple regression analysis, we found that these relationships still remained, also when adjusting for possible confounding factors. Interestingly, logarithmic models better described the associations between CAG repeats and neuropsychological scores. CAG repeats in the HTT gene within the non-pathological range influenced neuropsychological performances depending on global cognitive status. The logarithmic model suggested that the positive effect of CAG repeats in SCD patients decreases as the number of repeats grows.

An update on the neurological short tandem repeat expansion disorders and the emergence of long-read sequencing diagnostics

BACKGROUND

Short tandem repeat (STR) expansion disorders are an important cause of human neurological disease. They have an established role in more than 40 different phenotypes including the myotonic dystrophies, Fragile X syndrome, Huntington's disease, the hereditary cerebellar ataxias, amyotrophic lateral sclerosis and frontotemporal dementia.

MAIN BODY

STR expansions are difficult to detect and may explain unsolved diseases, as highlighted by recent findings including: the discovery of a biallelic intronic 'AAGGG' repeat in RFC1 as the cause of cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS); and the finding of 'CGG' repeat expansions in NOTCH2NLC as the cause of neuronal intranuclear inclusion disease and a range of clinical phenotypes. However, established laboratory techniques for diagnosis of repeat expansions (repeat-primed PCR and Southern blot) are cumbersome, low-throughput and poorly suited to parallel analysis of multiple gene regions. While next generation sequencing (NGS) has been increasingly used, established short-read NGS platforms (e.g., Illumina) are unable to genotype large and/or complex repeat expansions. Long-read sequencing platforms recently developed by Oxford Nanopore Technology and Pacific Biosciences promise to overcome these limitations to deliver enhanced diagnosis of repeat expansion disorders in a rapid and cost-effective fashion.

CONCLUSION

We anticipate that long-read sequencing will rapidly transform the detection of short tandem repeat expansion disorders for both clinical diagnosis and gene discovery.

Association of CAG Repeat Length in the Huntington Gene With Cognitive Performance in Young Adults

OBJECTIVE

To investigate the relationships between CAG repeat length in the huntingtin gene and cognitive performance in participants above and below the disease threshold for Huntington disease (HD), we performed a cross-sectional analysis of the Enroll-HD database.

METHODS

We analyzed data from young, developing adults (≤30 years of age) without a history of depression, apathy, or cognitive deficits. We included participants with and without the gene expansion (CAG ≥36) for HD. All participants had to have a Total Functional Capacity Score of 13, a diagnostic confidence level of zero, and a total motor score of <10 and had to be >28.6 years from their predicted motor onset. We performed regression analyses to investigate the nonlinear relationship between CAG repeat length and various cognitive measures controlling for age, sex, and education level.

RESULTS

There were significant positive relationships between CAG repeat length and the Symbol Digit Modalities, Stroop Color Naming, and Stroop Interference test scores. There were significant negative relationships between CAG repeat length and scores on Parts A and B of the Trails Making Test (p < 0.05), indicating that longer CAG repeat lengths were associated with better performance.

DISCUSSION

An increasing number of CAG repeats in the huntingtin gene below disease threshold and low pathologic CAG ranges were associated with some improvements in cognitive performance. These findings outline the relationship between CAG repeats within the huntingtin gene and cognitive development.

CLASSIFICATION OF EVIDENCE

This study provides Class IV evidence that CAG repeat length is positively associated with cognitive function across a spectrum of CAG repeat lengths.

Huntingtin gene CAG repeat size affects autism risk: Family-based and case-control association study

The Huntingtin (HTT) gene contains a CAG repeat in exon 1, whose expansion beyond 39 repeats consistently leads to Huntington's disease (HD), whereas normal-to-intermediate alleles seemingly modulate brain structure, function and behavior. The role of the CAG repeat in Autism Spectrum Disorder (ASD) was investigated applying both family-based and case-control association designs, with the SCA3 repeat as a negative control. Significant overtransmission of "long" CAG alleles (≥17 repeats) to autistic children and of "short" alleles (≤16 repeats) to their unaffected siblings (all p < 10^-5 ) was observed in 612 ASD families (548 simplex and 64 multiplex). Surprisingly, both 193 population controls and 1,188 neurological non-HD controls have significantly lower frequencies of "short" CAG alleles compared to 185 unaffected siblings and higher rates of "long" alleles compared to 548 ASD patients from the same families (p < .05-.001). The SCA3 CAG repeat displays no association. "Short" HTT alleles seemingly exert a protective effect from clinically overt autism in families carrying a genetic predisposition for ASD, while "long" alleles may enhance autism risk. Differential penetrance of autism-inducing genetic/epigenetic variants may imply atypical developmental trajectories linked to HTT functions, including excitation/inhibition imbalance, cortical neurogenesis and apoptosis, neuronal migration, synapse formation, connectivity and homeostasis.

The long-term environmental risks from the aging of organochlorine pesticide lindane

Although increased contact time (aging) of pesticides in the soil decreases their bioavailability, this does not mean that the bound residues formed during the aging process pose fewer risk to the soil environment. Here the earthworm Eisenia fetida was exposed to organochlorine pesticide lindane in soil under different durations of lindane aging and exposure. The results of de novo RNA sequencing followed by molecular and biochemical validations demonstrated the aged lindane showed a different tendency to disrupt acetylcholine (ACh) transmission with the effects of fresh lindane to gamma-aminobutyric acid. Using own-developed earthworm activity test, we confirmed aged lindane prompted earthworms to exclusively exhibit a significant hypoactivity in locomotion, which could be explained by the inhibition of Ach system. This study suggested that the toxicity of pesticides would not depend solely on their free state components, and the awareness of long-term environmental risks from the bound states needs to be raised.

Characterization of Simple Sequence Repeats (SSRs) in Ciliated Protists Inferred by Comparative Genomics

Simple sequence repeats (SSRs) are prevalent in the genomes of all organisms. They are widely used as genetic markers, and are insertion/deletion mutation hotspots, which directly influence genome evolution. However, little is known about such important genomic components in ciliated protists, a large group of unicellular eukaryotes with extremely long evolutionary history and genome diversity. With recent publications of multiple ciliate genomes, we start to get a chance to explore perfect SSRs with motif size 1-100 bp and at least three motif repeats in nine species of two ciliate classes, Oligohymenophorea and Spirotrichea. We found that homopolymers are the most prevalent SSRs in these A/T-rich species, with AAA (lysine, charged amino acid; also seen as an SSR with one-adenine motif repeated three times) being the codons repeated at the highest frequencies in coding SSR regions, consistent with the widespread alveolin proteins rich in lysine repeats as found in Tetrahymena. Micronuclear SSRs are universally more abundant than the macronuclear ones of the same motif-size, except for the 8-bp-motif SSRs in extensively fragmented chromosomes. Both the abundance and A/T content of SSRs decrease as motif-size increases, while the abundance is positively correlated with the A/T content of the genome. Also, smaller genomes have lower proportions of coding SSRs out of all SSRs in Paramecium species. This genome-wide and cross-species analysis reveals the high diversity of SSRs and reflects the rapid evolution of these simple repetitive elements in ciliate genomes.

ALS Genetics, Mechanisms, and Therapeutics: Where Are We Now?

The scientific landscape surrounding amyotrophic lateral sclerosis (ALS) continues to shift as the number of genes associated with the disease risk and pathogenesis, and the cellular processes involved, continues to grow. Despite decades of intense research and over 50 potentially causative or disease-modifying genes identified, etiology remains unexplained and treatment options remain limited for the majority of ALS patients. Various factors have contributed to the slow progress in understanding and developing therapeutics for this disease. Here, we review the genetic basis of ALS, highlighting factors that have contributed to the elusiveness of genetic heritability. The most commonly mutated ALS-linked genes are reviewed with an emphasis on disease-causing mechanisms. The cellular processes involved in ALS pathogenesis are discussed, with evidence implicating their involvement in ALS summarized. Past and present therapeutic strategies and the benefits and limitations of the model systems available to ALS researchers are discussed with future directions for research that may lead to effective treatment strategies outlined.

Genome-wide mining of microsatellites in king cobra (Ophiophagus hannah) and cross-species development of tetranucleotide SSR markers in Chinese cobra (Naja atra)

The complete genome sequence provides the opportunity for genome-wide and coding region analysis of SSRs in the king cobra and for cross-species identification of microsatellite markers in the Chinese cobra. In the Ophiophagus hannah genome, tetranucleotide repeats (38.03%) were the most abundant category, followed by dinucleotides (23.03%), pentanucleotides (13.07%), mononucleotides (11.78%), trinucleotides (11.49%) and hexanucleotides (2.6%). Twenty predominant motifs in the O. hannah genome were (A)n (C)n, (AC)n, (AG)n, (AT)n, (AGG)n, (AAT)n, (AAG)n, (AAC)n, (ATG)n, (ATAG)n, (AAGG)n, (ATCT)n, (CCTT)n, (ATTT)n, (AAAT)n, (AATAG)n, (ATTCT)n, (ATATGT)n, (AGATAT)n. In total, 4344 SSRs were found in coding sequences (CDSs). Tetranucleotides (52.79%) were the most abundant microsatellite type in CDS, followed by trinucleotides (28.50%), dinucleotides (11.02%), pentanucleotides (4.42%), mononucleotides (1.77%), and hexanucleotides (1.50%). A total of 984 CDSs containing microsatellites were assigned 11152 Gene Ontology (GO) functional terms. Gene Ontology (GO) analysis demonstrated that cellular process, cell and binding were the most frequent GO terms in biological process, cellular component and molecular function, respectively. Thirty-two novel highly polymorphic (PIC > 0.5) SSR markers for Naja atra were developed from cross-species amplification based on the tetranucleotide microsatellite sequences in the king cobra genome. The number of alleles (N_A) per locus had between 3 and 11 alleles with an average of 6.5, the polymorphism information content (PIC) value ranged from 0.521 to 0.858 (average = 0.707), the observed heterozygosity (Ho) of 32 microsatellite loci ranged from 0.292 to 0.875 (mean = 0.678), the expected heterozygosity (H_E) ranged from 0.561 to 0.889 (average = 0.761), and 3 microsatellite loci exhibited statistically significant departure from Hardy-Weinberg equilibrium (HWE) after Bonferroni correction (p < 0.003).

Prairie Voles as a Model for Understanding the Genetic and Epigenetic Regulation of Attachment Behaviors

Over a lifetime, humans build relationships with family, friends, and partners that are critically important for our mental and physical health. Unlike commonly used laboratory mice and rats, Microtine rodents provide a unique model to study the neurobiology underlying pair bonding and the selective attachments that form between adults. Comparisons between monogamous prairie voles and the closely related but nonmonogamous meadow and montane voles have revealed that brain-region-specific neuropeptide receptor patterning modulates social behavior between and within species. In particular, diversity in vasopressin 1a receptor (V1aR) distribution has been linked to individual and species differences in monogamy-related behaviors such as partner preference, mate guarding, and space use. Given the importance of differential receptor expression for regulating social behavior, a critical question has emerged: What are the genetic and epigenetic mechanisms that underlie brain-region-specific receptor patterns? This review will summarize what is known about how the vasopressin (AVP)-V1aR axis regulates social behaviors via signaling in discrete brain regions. From this work, we propose that brain-region-specific regulatory mechanisms facilitate robust evolvability of V1aR expression to generate diverse sociobehavioral traits. Translationally, we provide a perspective on how these studies have contributed to our understanding of human social behaviors and how brain-region-specific regulatory mechanisms might be harnessed for targeted therapies to treat social deficits in psychiatric disorders such as depression, complicated grief, and autism spectrum disorder.

Massive variation of short tandem repeats with functional consequences across strains of Arabidopsis thaliana

Short tandem repeat (STR) mutations may comprise more than half of the mutations in eukaryotic coding DNA, yet STR variation is rarely examined as a contributor to complex traits. We assessed this contribution across a collection of 96 strains of Arabidopsis thaliana, genotyping 2046 STR loci each, using highly parallel STR sequencing with molecular inversion probes. We found that 95% of examined STRs are polymorphic, with a median of six alleles per STR across these strains. STR expansions (large copy number increases) are found in most strains, several of which have evident functional effects. These include three of six intronic STR expansions we found to be associated with intron retention. Coding STRs were depleted of variation relative to noncoding STRs, and we detected a total of 56 coding STRs (11%) showing low variation consistent with the action of purifying selection. In contrast, some STRs show hypervariable patterns consistent with diversifying selection. Finally, we detected 133 novel STR-phenotype associations under stringent criteria, most of which could not be detected with SNPs alone, and validated some with follow-up experiments. Our results support the conclusion that STRs constitute a large, unascertained reservoir of functionally relevant genomic variation.

Investigation of short tandem repeats in major depression using whole-genome sequencing data

BACKGROUND

Major depressive disorder (MDD) is a leading contributor to global disease burden. Recent studies have shown that genetic factors play significant roles in the susceptibility to this condition; however, the underlying genetic basis currently remains largely unknown. Short tandem repeat (STR) has been proposed as an explanatory factor in the "missing heritability" of complex diseases or traits.

METHODS

We investigated STR variations from 15 MDD patients and 10 ethnically matched healthy controls based on their deep whole-genome sequencing (WGS) data. The lobSTR software was used to computationally determine STRs.

RESULTS

The results of the Mexican-American sample showed that STRs are significantly richer in healthy controls than in MDD cases on each of the 23 chromosomes (all false discovery rates, FDR P-values < 0.0062); while for the Australian of European-ancestry sample, there was no statistically significant STRs difference between MDD cases and controls.

LIMITATIONS

High quality WGS costs limited obtaining larger datasets.

CONCLUSIONS

This preliminary work is the first study that STR variations are applied to investigate MDD based on WGS data. The results on Mexican-American population may imply that within the same ancestry, targeted sequencing on a specific chromosome or region of genome would be sufficient for examining the relationship between STR and MDD. Further studies should examine larger sequencing datasets on other ethnic groups.

Genome-scale portrait and evolutionary significance of human-specific core promoter tri- and tetranucleotide short tandem repeats

BACKGROUND

While there is an ongoing trend to identify single nucleotide substitutions (SNSs) that are linked to inter/intra-species differences and disease phenotypes, short tandem repeats (STRs)/microsatellites may be of equal (if not more) importance in the above processes. Genes that contain STRs in their promoters have higher expression divergence compared to genes with fixed or no STRs in the gene promoters. In line with the above, recent reports indicate a role of repetitive sequences in the rise of young transcription start sites (TSSs) in human evolution.

RESULTS

Following a comparative genomics study of all human protein-coding genes annotated in the GeneCards database, here we provide a genome-scale portrait of human-specific short- and medium-size (≥ 3-repeats) tri- and tetranucleotide STRs and STR motifs in the critical core promoter region between - 120 and + 1 to the TSS and evidence of skewing of this compartment in reference to the STRs that are not human-specific (Levene's test p < 0.001). Twenty-five percent and 26% enrichment of human-specific transcripts was detected in the tri and tetra human-specific compartments (mid-p < 0.00002 and mid-p < 0.002, respectively).

CONCLUSION

Our findings provide the first evidence of genome-scale skewing of STRs at a specific region of the human genome and a link between a number of these STRs and TSS selection/transcript specificity. The STRs and genes listed here may have a role in the evolution and development of characteristics and phenotypes that are unique to the human species.

Tandem repeats mediating genetic plasticity in health and disease

Accumulating evidence suggests that many classes of DNA repeats exhibit attributes that distinguish them from other genetic variants, including the fact that they are more liable to mutation; this enables them to mediate genetic plasticity. The expansion of tandem repeats, particularly of short tandem repeats, can cause a range of disorders (including Huntington disease, various ataxias, motor neuron disease, frontotemporal dementia, fragile X syndrome and other neurological disorders), and emerging data suggest that tandem repeat polymorphisms (TRPs) can also regulate gene expression in healthy individuals. TRPs in human genomes may also contribute to the missing heritability of polygenic disorders. A better understanding of tandem repeats and their associated repeatome, as well as their capacity for genetic plasticity via both germline and somatic mutations, is needed to transform our understanding of the role of TRPs in health and disease.

Sex-specific effects of the Huntington gene on normal neurodevelopment

Huntington disease is a neurodegenerative disorder caused by a gene (HTT) with a unique feature of trinucleotide repeats ranging from 10 to 35 in healthy people; when expanded beyond 39 repeats, Huntington disease develops. Animal models demonstrate that HTT is vital to brain development; however, this has not been studied in humans. Moreover, evidence suggests that triplet repeat genes may have been vital in evolution of the human brain. Here we evaluate brain structure using magnetic resonance imaging and brain function using cognitive tests in a sample of school-aged children ages 6 to 18 years old. DNA samples were processed to quantify the number of CAG repeats within HTT. We find that the number of repeats in HTT, below disease threshold, confers advantageous changes in brain structure and general intelligence (IQ): the higher the number of repeats, the greater the change in brain structure, and the higher the IQ. The pattern of structural brain changes associated with HTT is strikingly different between males and females. HTT may confer an advantage or a disadvantage depending on the repeat length, playing a key role in either the evolution of a superior human brain or development of a uniquely human brain disease. © 2016 Wiley Periodicals, Inc.

Functional Mechanisms of Microsatellite DNA in Eukaryotic Genomes

Microsatellite repeat DNA is best known for its length mutability, which is implicated in several neurological diseases and cancers, and often exploited as a genetic marker. Less well-known is the body of work exploring the widespread and surprisingly diverse functional roles of microsatellites. Recently, emerging evidence includes the finding that normal microsatellite polymorphism contributes substantially to the heritability of human gene expression on a genome-wide scale, calling attention to the task of elucidating the mechanisms involved. At present, these are underexplored, but several themes have emerged. I review evidence demonstrating roles for microsatellites in modulation of transcription factor binding, spacing between promoter elements, enhancers, cytosine methylation, alternative splicing, mRNA stability, selection of transcription start and termination sites, unusual structural conformations, nucleosome positioning and modification, higher order chromatin structure, noncoding RNA, and meiotic recombination hot spots.

The association of insertions/deletions (INDELs) and variable number tandem repeats (VNTRs) with obesity and its related traits and complications

BACKGROUND

Despite the fact that insertions/deletions (INDELs) are the second most common type of genetic variations and variable number tandem repeats (VNTRs) represent a large portion of the human genome, they have received far less attention than single nucleotide polymorphisms (SNPs) and larger forms of structural variation like copy number variations (CNVs), especially in genome-wide association studies (GWAS) of complex diseases like polygenic obesity. This is exemplified by the vast amount of review papers on the role of SNPs and CNVs in obesity, its related traits (like anthropometric measurements, biochemical variables, and eating behavior), and its related complications (like hypertension, hypertriglyceridemia, hypercholesterolemia, and insulin resistance-collectively known as metabolic syndrome). Hence, this paper reviews the types of INDELs and VNTRs that have been studied for association with obesity and its related traits and complications. These INDELs and VNTRs could be found in the obesity loci or genes from the earliest GWAS and candidate gene association studies, like FTO, genes in the leptin-proopiomelanocortin pathway, and UCP2/3. Given the important role of the brain serotonergic and dopaminergic reward system in obesity susceptibility, the association of INDELs and VNTRs in these neurotransmitters' metabolism and transport genes with obesity is also reviewed. Next, the role of INS VNTR in obesity and its related traits is questionable, since recent large-scale studies failed to replicate the earlier positive associations. As obesity results in chronic low-grade inflammation of the adipose tissue, the proinflammatory cytokine gene IL1RA and anti-inflammatory cytokine gene IL4 have VNTRs that are implicated in obesity. A systemic proinflammatory state in combination with activation of the renin-angiotensin system and decreased nitric oxide bioavailability as found in obesity leads to endothelial dysfunction. This explains why VNTR and INDEL in eNOS and ACE, respectively, could be predisposing factors of obesity. Finally, two novel genes, DOCK5 and PER3, which are involved in the regulation of the Akt/MAPK pathway and circadian rhythm, respectively, have VNTRs and INDEL that might be associated with obesity.

SHORT CONCLUSION

In conclusion, INDELs and VNTRs could have important functional consequences in the pathophysiology of obesity, and research on them should be continued to facilitate obesity prediction, prevention, and treatment.

Balancing selection maintains polymorphisms at neurogenetic loci in field experiments

Most variation in behavior has a genetic basis, but the processes determining the level of diversity at behavioral loci are largely unknown for natural populations. Expression of arginine vasopressin receptor 1a (Avpr1a) and oxytocin receptor (Oxtr) in specific regions of the brain regulates diverse social and reproductive behaviors in mammals, including humans. That these genes have important fitness consequences and that natural populations contain extensive diversity at these loci implies the action of balancing selection. In Myodes glareolus, Avpr1a and Oxtr each contain a polymorphic microsatellite locus located in their 5' regulatory region (the regulatory region-associated microsatellite, RRAM) that likely regulates gene expression. To test the hypothesis that balancing selection maintains diversity at behavioral loci, we released artificially bred females and males with different RRAM allele lengths into field enclosures that differed in population density. The length of Avpr1a and Oxtr RRAMs was associated with reproductive success, but population density and the sex interacted to determine the optimal genotype. In general, longer Avpr1a RRAMs were more beneficial for males, and shorter RRAMs were more beneficial for females; the opposite was true for Oxtr RRAMs. Moreover, Avpr1a RRAM allele length is correlated with the reproductive success of the sexes during different phases of reproduction; for males, RRAM length correlated with the numbers of newborn offspring, but for females selection was evident on the number of weaned offspring. This report of density-dependence and sexual antagonism acting on loci within the arginine vasopressin-oxytocin pathway explains how genetic diversity at Avpr1a and Oxtr could be maintained in natural populations.

CRISPR-Cas9-targeted fragmentation and selective sequencing enable massively parallel microsatellite analysis

Microsatellites are multi-allelic and composed of short tandem repeats (STRs) with individual motifs composed of mononucleotides, dinucleotides or higher including hexamers. Next-generation sequencing approaches and other STR assays rely on a limited number of PCR amplicons, typically in the tens. Here, we demonstrate STR-Seq, a next-generation sequencing technology that analyses over 2,000 STRs in parallel, and provides the accurate genotyping of microsatellites. STR-Seq employs in vitro CRISPR-Cas9-targeted fragmentation to produce specific DNA molecules covering the complete microsatellite sequence. Amplification-free library preparation provides single molecule sequences without unique molecular barcodes. STR-selective primers enable massively parallel, targeted sequencing of large STR sets. Overall, STR-Seq has higher throughput, improved accuracy and provides a greater number of informative haplotypes compared with other microsatellite analysis approaches. With these new features, STR-Seq can identify a 0.1% minor genome fraction in a DNA mixture composed of different, unrelated samples.

Variability in a Short Tandem Repeat Mediates Complex Epistatic Interactions in Arabidopsis thaliana

Short tandem repeats (STRs) are hypervariable genetic elements that occur frequently in coding regions. Their high mutation rate readily generates genetic variation, contributing to adaptive evolution and human diseases. We previously reported that natural ELF3 polyglutamine variants cause reciprocal genetic incompatibilities in two divergent Arabidopsis thaliana backgrounds. Here, we dissect the genetic architecture of this incompatibility, revealing as many as four loci putatively interacting with ELF3 We were able to specifically identify one such ELF3-interacting gene, LSH9 We further used a yeast two-hybrid strategy to identify proteins whose physical interactions with ELF3 were affected by polyglutamine tract length. We found two proteins for which this was the case, ELF4 and AtGLDP1. Using these two approaches, we identify specific genetic interactions and physical mechanisms by which the ELF3 polyglutamine tract may mediate the observed genetic incompatibilities. Our work elucidates how STR variation, which is generally underascertained in population-scale sequencing, can contribute to phenotypic variation. Furthermore, our results support our proposal that highly variable STR loci can contribute to the epistatic component of heritability.

Selection pressure on human STR loci and its relevance in repeat expansion disease

Short Tandem Repeats (STRs) comprise repeats of one to several base pairs. Because of the high mutability due to strand slippage during DNA synthesis, rapid evolutionary change in the number of repeating units directly shapes the range of repeat-number variation according to selection pressure. However, the remaining questions include: Why are STRs causing repeat expansion diseases maintained in the human population; and why are these limited to neurodegenerative diseases? By evaluating the genome-wide selection pressure on STRs using the database we constructed, we identified two different patterns of relationship in repeat-number polymorphisms between DNA and amino-acid sequences, although both patterns are evolutionary consequences of avoiding the formation of harmful long STRs. First, a mixture of degenerate codons is represented in poly-proline (poly-P) repeats. Second, long poly-glutamine (poly-Q) repeats are favored at the protein level; however, at the DNA level, STRs encoding long poly-Qs are frequently divided by synonymous SNPs. Furthermore, significant enrichments of apoptosis and neurodevelopment were biological processes found specifically in genes encoding poly-Qs with repeat polymorphism. This suggests the existence of a specific molecular function for polymorphic and/or long poly-Q stretches. Given that the poly-Qs causing expansion diseases were longer than other poly-Qs, even in healthy subjects, our results indicate that the evolutionary benefits of long and/or polymorphic poly-Q stretches outweigh the risks of long CAG repeats predisposing to pathological hyper-expansions. Molecular pathways in neurodevelopment requiring long and polymorphic poly-Q stretches may provide a clue to understanding why poly-Q expansion diseases are limited to neurodegenerative diseases.

Adcyap1 polymorphism covaries with breeding latitude in a Nearctic migratory songbird, the Wilson's warbler (Cardellina pusilla)

Understanding the genetic background of complex behavioral traits, showing multigenic control and extensive environmental effects, is a challenging task. Among such traits, migration is known to show a large additive genetic component. Yet, the identification of specific genes or gene regions explaining phenotypic variance in migratory behavior has received less attention. Migration ultimately depends on seasonal cycles, and polymorphism at phenological candidate genes may underlie variation in timing of migration or other aspects of migratory behavior. In this study of a Nearctic–Neotropical migratory songbird, the Wilson's warbler (Cardellina pusilla), we investigated the association between polymorphism at two phenological candidate genes, Clock and Adcyap1, and two aspects of the migratory phenotype, timing of spring migration through a stopover site and inferred latitude of the breeding destination. The breeding destination of migrating individuals was identified using feather deuterium ratio (δ²H), which reliably reflects breeding latitude throughout the species' western breeding range. Ninety‐eight percent of the individuals were homozygous at Clock, and the rare heterozygotes did not deviate from homozygous migration phenology. Adcyap1 was highly polymorphic, and allele size was not significantly associated with migration date. However, Adcyap1 allele size significantly positively predicted the inferred breeding latitude of males but not of females. Moreover, we found a strong positive association between inferred breeding latitude and Adcyap1 allele size in long‐distance migrating birds from the northern sector of the breeding range (western Canada), while this was not the case in short‐distance migrating birds from the southern sector of the breeding range (coastal California). Our findings support previous evidence for a role of Adcyap1 in shaping the avian migratory phenotype, while highlighting that patterns of phenological candidate gene–phenotype associations may be complex, significantly varying between geographically distinct populations and even between the sexes.

Tandem repeat variation in human and great ape populations and its impact on gene expression divergence

Tandem repeats (TRs) are stretches of DNA that are highly variable in length and mutate rapidly. They are thus an important source of genetic variation. This variation is highly informative for population and conservation genetics. It has also been associated with several pathological conditions and with gene expression regulation. However, genome-wide surveys of TR variation in humans and closely related species have been scarce due to technical difficulties derived from short-read technology. Here we explored the genome-wide diversity of TRs in a panel of 83 human and nonhuman great ape genomes, in a total of six different species, and studied their impact on gene expression evolution. We found that population diversity patterns can be efficiently captured with short TRs (repeat unit length, 1–5 bp). We examined the potential evolutionary role of TRs in gene expression differences between humans and primates by using 30,275 larger TRs (repeat unit length, 2–50 bp). Genes that contained TRs in the promoters, in their 3′ untranslated region, in introns, and in exons had higher expression divergence than genes without repeats in the regions. Polymorphic small repeats (1–5 bp) had also higher expression divergence compared with genes with fixed or no TRs in the gene promoters. Our findings highlight the potential contribution of TRs to human evolution through gene regulation.

Candidate gene-environment interactions and their relationships with timing of breeding in a wild bird population

Monitoring and predicting evolutionary changes underlying current environmental modifications are complex challenges. Recent approaches to achieve these objectives include assessing the genetic variation and effects of candidate genes on traits indicating adaptive potential. In birds, for example, short tandem repeat polymorphism at four candidate genes (CLOCK, NPAS2, ADCYAP1, and CREB1) has been linked to variation in phenological traits such as laying date and timing of migration. However, our understanding of their importance as evolutionary predictors is still limited, mainly because the extent of genotype–environment interactions (GxE) related to these genes has yet to be assessed. Here, we studied a population of Tree swallow (Tachycineta bicolor) over 4 years in southern Québec (Canada) to assess the relationships between those four candidate genes and two phenological traits related to reproduction (laying date and incubation duration) and also determine the importance of GxE in this system. Our results showed that NPAS2 female genotypes were nonrandomly distributed across the study system and formed a longitudinal cline with longer genotypes located to the east. We observed relationships between length polymorphism at all candidate genes and laying date and/or incubation duration, and most of these relationships were affected by environmental variables (breeding density, latitude, or temperature). In particular, the positive relationships detected between laying date and both CLOCK and NPAS2 female genotypes were variable depending on breeding density. Our results suggest that all four candidate genes potentially affect timing of breeding in birds and that GxE are more prevalent and important than previously reported in this context.

Motif mismatches in microsatellites: insights from genome-wide investigation among 20 insect species

We present a detailed genome-wide comparative study of motif mismatches of microsatellites among 20 insect species representing five taxonomic orders. The results show that varying proportions (∼15-46%) of microsatellites identified in these species are imperfect in motif structure, and that they also vary in chromosomal distribution within genomes. It was observed that the genomic abundance of imperfect repeats is significantly associated with the length and number of motif mismatches of microsatellites. Furthermore, microsatellites with a higher number of mismatches tend to have lower abundance in the genome, suggesting that sequence heterogeneity of repeat motifs is a key determinant of genomic abundance of microsatellites. This relationship seems to be a general feature of microsatellites even in unrelated species such as yeast, roundworm, mouse and human. We provide a mechanistic explanation of the evolutionary link between motif heterogeneity and genomic abundance of microsatellites by examining the patterns of motif mismatches and allele sequences of single-nucleotide polymorphisms identified within microsatellite loci. Using Drosophila Reference Genetic Panel data, we further show that pattern of allelic variation modulates motif heterogeneity of microsatellites, and provide estimates of allele age of specific imperfect microsatellites found within protein-coding genes.

Evolutionary history of the PER3 variable number of tandem repeats (VNTR): idiosyncratic aspect of primate molecular circadian clock

The PER3 gene is one of the clock genes, which function in the core mammalian molecular circadian system. A variable number of tandem repeats (VNTR) locus in the 18th exon of this gene has been strongly associated to circadian rhythm phenotypes and sleep organization in humans, but it has not been identified in other mammals except primates. To better understand the evolution and the placement of the PER3 VNTR in a phylogenetical context, the present study enlarges the investigation about the presence and the structure of this variable region in a large sample of primate species and other mammals. The analysis of the results has revealed that the PER3 VNTR occurs exclusively in simiiforme primates and that the number of copies of the primitive unit ranges from 2 to 11 across different primate species. Two transposable elements surrounding the 18th exon of PER3 were found in primates with published genome sequences, including the tarsiiforme Tarsius syrichta, which lacks the VNTR. These results suggest that this VNTR may have evolved in a common ancestor of the simiiforme branch and that the evolutionary copy number differentiation of this VNTR may be associated with primate simiiformes sleep and circadian phenotype patterns.

The overdue promise of short tandem repeat variation for heritability

Short tandem repeat (STR) variation has been proposed as a major explanatory factor in the heritability of complex traits in humans and model organisms. However, we still struggle to incorporate STR variation into genotype-phenotype maps. We review here the promise of STRs in contributing to complex trait heritability and highlight the challenges that STRs pose due to their repetitive nature. We argue that STR variants are more likely than single-nucleotide variants to have epistatic interactions, reiterate the need for targeted assays to genotype STRs accurately, and call for more appropriate statistical methods in detecting STR-phenotype associations. Lastly, we suggest that somatic STR variation within individuals may serve as a read-out of disease susceptibility, and is thus potentially a valuable covariate for future association studies.

The conserved PFT1 tandem repeat is crucial for proper flowering in Arabidopsis thaliana

It is widely appreciated that short tandem repeat (STR) variation underlies substantial phenotypic variation in organisms. Some propose that the high mutation rates of STRs in functional genomic regions facilitate evolutionary adaptation. Despite their high mutation rate, some STRs show little to no variation in populations. One such STR occurs in the Arabidopsis thaliana gene PFT1 (MED25), where it encodes an interrupted polyglutamine tract. Although the PFT1 STR is large (∼270 bp), and thus expected to be extremely variable, it shows only minuscule variation across A. thaliana strains. We hypothesized that the PFT1 STR is under selective constraint, due to previously undescribed roles in PFT1 function. We investigated this hypothesis using plants expressing transgenic PFT1 constructs with either an endogenous STR or synthetic STRs of varying length. Transgenic plants carrying the endogenous PFT1 STR generally performed best in complementing a pft1 null mutant across adult PFT1-dependent traits. In stark contrast, transgenic plants carrying a PFT1 transgene lacking the STR phenocopied a pft1 loss-of-function mutant for flowering time phenotypes and were generally hypomorphic for other traits, establishing the functional importance of this domain. Transgenic plants carrying various synthetic constructs occupied the phenotypic space between wild-type and pft1 loss-of-function mutants. By varying PFT1 STR length, we discovered that PFT1 can act as either an activator or repressor of flowering in a photoperiod-dependent manner. We conclude that the PFT1 STR is constrained to its approximate wild-type length by its various functional requirements. Our study implies that there is strong selection on STRs not only to generate allelic diversity, but also to maintain certain lengths pursuant to optimal molecular function.

Microsatellite interruptions stabilize primate genomes and exist as population-specific single nucleotide polymorphisms within individual human genomes

Interruptions of microsatellite sequences impact genome evolution and can alter disease manifestation. However, human polymorphism levels at interrupted microsatellites (iMSs) are not known at a genome-wide scale, and the pathways for gaining interruptions are poorly understood. Using the 1000 Genomes Phase-1 variant call set, we interrogated mono-, di-, tri-, and tetranucleotide repeats up to 10 units in length. We detected ∼26,000–40,000 iMSs within each of four human population groups (African, European, East Asian, and American). We identified population-specific iMSs within exonic regions, and discovered that known disease-associated iMSs contain alleles present at differing frequencies among the populations. By analyzing longer microsatellites in primate genomes, we demonstrate that single interruptions result in a genome-wide average two- to six-fold reduction in microsatellite mutability, as compared with perfect microsatellites. Centrally located interruptions lowered mutability dramatically, by two to three orders of magnitude. Using a biochemical approach, we tested directly whether the mutability of a specific iMS is lower because of decreased DNA polymerase strand slippage errors. Modeling the adenomatous polyposis coli tumor suppressor gene sequence, we observed that a single base substitution interruption reduced strand slippage error rates five- to 50-fold, relative to a perfect repeat, during synthesis by DNA polymerases α, β, or η. Computationally, we demonstrate that iMSs arise primarily by base substitution mutations within individual human genomes. Our biochemical survey of human DNA polymerase α, β, δ, κ, and η error rates within certain microsatellites suggests that interruptions are created most frequently by low fidelity polymerases. Our combined computational and biochemical results demonstrate that iMSs are abundant in human genomes and are sources of population-specific genetic variation that may affect genome stability. The genome-wide identification of iMSs in human populations presented here has important implications for current models describing the impact of microsatellite polymorphisms on gene expression.

Microsatellites are short tandem repeat DNA sequences located throughout the human genome that display a high degree of inter-individual variation. This characteristic makes microsatellites an attractive tool for population genetics and forensics research. Some microsatellites affect gene expression, and mutations within such microsatellites can cause disease. Interruption mutations disrupt the perfect repeated array and are frequently associated with altered disease risk, but they have not been thoroughly studied in human genomes. We identified interrupted mono-, di-, tri- and tetranucleotide MSs (iMS) within individual genomes from African, European, Asian and American population groups. We show that many iMSs, including some within disease-associated genes, are unique to a single population group. By measuring the conservation of microsatellites between human and chimpanzee genomes, we demonstrate that interruptions decrease the probability of microsatellite mutations throughout the genome. We demonstrate that iMSs arise in the human genome by single base changes within the DNA, and provide biochemical data suggesting that these stabilizing changes may be created by error-prone DNA polymerases. Our genome-wide study supports the model in which iMSs act to stabilize individual genomes, and suggests that population-specific differences in microsatellite architecture may be an avenue by which genetic ancestry impacts individual disease risk.

Differential distributions of mononucleotide repeat sequences in 256 viral genomes and its potential implications

Mononucleotide repeats (MNRs) have been systematically investigated in the genomes of eukaryotic and prokaryotic organisms. However, detailed information on the distribution of MNRs in viral genomes is limited. In this study, we examined the distributions of MNRs in 256 fully sequenced virus genomes which showed extensive variations across viral genomes, and is significantly influenced by both genome size and CG content. Furthermore, the ratio of the observed to the expected number of MNRs (O/E ratio) appears to be influenced by both the host range and genome type of a particular virus. Additionally, the densities and frequencies of MNRs in genic regions are lower than in non-coding regions, suggesting that selective pressure acts on viral genomes. We also discuss the potential functional roles that these MNR loci could play in virus genomes. To our knowledge, this is the first analysis focusing on MNRs in viruses, and our study could have potential implications for a deeper understanding of virus genome stability and the co-evolution that occurs between a virus and its host.

Divergence of Drosophila melanogaster repeatomes in response to a sharp microclimate contrast in Evolution Canyon, Israel

Repeat sequences, especially mobile elements, make up large portions of most eukaryotic genomes and provide enormous, albeit commonly underappreciated, evolutionary potential. We analyzed repeatomes of Drosophila melanogaster that have been diverging in response to a microclimate contrast in Evolution Canyon (Mount Carmel, Israel), a natural evolutionary laboratory with two abutting slopes at an average distance of only 200 m, which pose a constant ecological challenge to their local biotas. Flies inhabiting the colder and more humid north-facing slope carried about 6% more transposable elements than those from the hot and dry south-facing slope, in parallel to a suite of other genetic and phenotypic differences between the two populations. Nearly 50% of all mobile element insertions were slope unique, with many of them disrupting coding sequences of genes critical for cognition, olfaction, and thermotolerance, consistent with the observed patterns of thermotolerance differences and assortative mating.

Simple sequence repeats in the national longitudinal study of adolescent health: an ethnically diverse resource for genetic analysis of health and behavior

Simple sequence repeats (SSRs) are one of the earliest available forms of genetic variation available for analysis and have been utilized in studies of neurological, behavioral, and health phenotypes. Although findings from these studies have been suggestive, their interpretation has been complicated by a variety of factors including, among others, limited power due to small sample sizes. The current report details the availability, diversity, and allele and genotype frequencies of six commonly examined SSRs in the ethnically diverse, population-based National Longitudinal Study of Adolescent Health. A total of 106,743 genotypes were generated across 15,140 participants that included four microsatellites and two di-nucleotide repeats in three dopamine genes (DAT1, DRD4, DRD5), the serotonin transporter, and monoamine oxidase A. Allele and genotype frequencies showed a complex pattern and differed significantly between populations. For both di-nucleotide repeats we observed a greater allelic diversity than previously reported. The availability of these six SSRs in a large, ethnically diverse sample with extensive environmental measures assessed longitudinally offers a unique resource for researchers interested in health and behavior.

Identification of two secreted ferritin subunits involved in immune defense of Yesso scallop Patinopecten yessoensis

As an important iron storage protein, ferritin plays a crucial role in the iron-withholding defense system. In this study, two secreted ferritin subunits (PyFerS1 and PyFerS2) were identified from the Yesso scallop, Patinopecten yessoensis. The complete DNA sequences of the two ferritins are 7101 and 5359 bp, consisting of seven and five exons, respectively. The full-length cDNAs of PyFerS1 and PyFerS2 are 960 and 956 bp in length, encoding 228 and 220 amino acids, respectively. They have typical ferritin structures, with four long α-helices, one short α-helix and an L-loop. Signal peptides were found at the N-terminus of both ferritins, and phylogenetic analysis showed that they both clustered with secreted mollusc ferritins. PyFerS1 possesses all seven conserved residues of the ferroxidase center, whereas PyFerS2 only has two. Real-time PCR analysis indicated high expression level of PyFerS2 in the D-shaped larvae, and PyFerS1 in both D-shaped larvae and fertilized eggs. In adult scallops, PyFerS1 was only detected in the hepatopancreas, whereas PyFerS2 was detected in both hepatopancreas and mantle. After the scallops were challenged by iron ion or bacteria Vibrio anguillarum, the expression of both PyFerS1 and PyFerS2 was significantly elevated, suggesting they may play a role in scallop innate immune defense. For the first time, secreted ferritins were cloned and comprehensively characterized in bivalve molluscs. It will assist in better understanding of the role of secreted ferritins in bivalve innate immunity.

Tandem repeat modification during double-strand break repair induced by an engineered TAL effector nuclease in zebrafish genome

Tandem repeats (TRs) are abundant and widely distributed in eukaryotic genomes. TRs are thought to have various functions in gene transcription, DNA methylation, nucleosome position and chromatin organization. Variation of repeat units in the genome is observed in association with a number of diseases, such as Fragile X Syndrome, Huntington's disease and Friedreich's ataxia. However, the underlying mechanisms involved are poorly understood, largely owing to the technical limitations in modification of TRs at definite sites in the genome in vivo. Transcription activator-like effector nucleases (TALENs) are widely used in recent years in gene targeting for their specific binding to target sequences when engineered in vitro. Here, we show that the repair of a double-strand break (DSB) induced by TALENs adjacent to a TR can produce serial types of mutations in the TR region. Sequencing analysis revealed that there are three types of mutations induced by the DSB repair, including indels only within the TR region or within the flanking TALEN target region or simutaneously within both regions. Therefore, desired TR mutant types can be conveniently obtained by using engineered TALENs. These results demonstrate that TALENs can serve as a convenient tool for modifying TRs in the genome in studying the functions of TRs.

STaRRRT: a table of short tandem repeats in regulatory regions of the human genome

Background

Tandem repeats (TRs) are unstable regions commonly found within genomes that have consequences for evolution and disease. In humans, polymorphic TRs are known to cause neurodegenerative and neuromuscular disorders as well as being associated with complex diseases such as diabetes and cancer. If present in upstream regulatory regions, TRs can modify chromatin structure and affect transcription; resulting in altered gene expression and protein abundance. The most common TRs are short tandem repeats (STRs), or microsatellites. Promoter located STRs are considerably more polymorphic than coding region STRs. As such, they may be a common driver of phenotypic variation. To study STRs located in regulatory regions, we have performed genome-wide analysis to identify all STRs present in a region that is 2 kilobases upstream and 1 kilobase downstream of the transcription start sites of genes.

Results

The Short Tandem Repeats in Regulatory Regions Table, STaRRRT, contains the results of the genome-wide analysis, outlining the characteristics of 5,264 STRs present in the upstream regulatory region of 4,441 human genes. Gene set enrichment analysis has revealed significant enrichment for STRs in cellular, transcriptional and neurological system gene promoters and genes important in ion and calcium homeostasis. The set of enriched terms has broad similarity to that seen in coding regions, suggesting that regulatory region STRs are subject to similar evolutionary pressures as STRs in coding regions and may, like coding region STRs, have an important role in controlling gene expression.

Conclusions

STaRRRT is a readily-searchable resource for investigating potentially polymorphic STRs that could influence the expression of any gene of interest. The processes and genes enriched for regulatory region STRs provide potential novel targets for diagnosing and treating disease, and support a role for these STRs in the evolution of the human genome.

A conserved extraordinarily long serine homopolymer in Dictyostelid amoebae

Eukaryotic protein sequences often contain amino-acid homopolymers that consist of a single amino acid repeated from several to dozens of times. Some of these are functional but others may persist largely because of high expansion rates due to DNA slippage. However, very long homopolymers with over a hundred repeats are very rare. We report an extraordinarily long homopolymer consisting of 306 tandem serine repeats from the single-celled eukaryote Dictyostelium discoideum, which also has a multicellular stage. The gene has a paralog with 132 repeats and orthologs, also with high serine repeat numbers, in various other Dictyostelid species. The conserved gene structure and protein sequences suggest that the homopolymer is functional. The high codon diversity and very poor alignment of serine codons in this gene between species similarly indicate functionality. This is because the serine homopolymer is conserved despite much DNA sequence change. A survey of other very long amino-acid homopolymers in eukaryotes shows that high codon diversity is the rule, suggesting that these too may be functional.