Genome-wide association study of restless legs syndrome identifies common variants in three genomic regions

A HapMap harvest of insights into the genetics of common disease

The International HapMap Project was designed to create a genome-wide database of patterns of human genetic variation, with the expectation that these patterns would be useful for genetic association studies of common diseases. This expectation has been amply fulfilled with just the initial output of genome-wide association studies, identifying nearly 100 loci for nearly 40 common diseases and traits. These associations provided new insights into pathophysiology, suggesting previously unsuspected etiologic pathways for common diseases that will be of use in identifying new therapeutic targets and developing targeted interventions based on genetically defined risk. In addition, HapMap-based discoveries have shed new light on the impact of evolutionary pressures on the human genome, suggesting multiple loci important for adapting to disease-causing pathogens and new environments. In this review we examine the origin, development, and current status of the HapMap; its prospects for continued evolution; and its current and potential future impact on biomedical science.

Neuroimaging insights into the pathophysiology of sleep disorders

Neuroimaging methods can be used to investigate whether sleep disorders are associated with specific changes in brain structure or regional activity. However, it is still unclear how these new data might improve our understanding of the pathophysiology underlying adult sleep disorders. Here we review functional brain imaging findings in major intrinsic sleep disorders (i.e., idiopathic insomnia, narcolepsy, and obstructive sleep apnea) and in abnormal motor behavior during sleep (i.e., periodic limb movement disorder and REM sleep behavior disorder). The studies reviewed include neuroanatomical assessments (voxel-based morphometry, magnetic resonance spectroscopy), metabolic/functional investigations (positron emission tomography, single photon emission computed tomography, functional magnetic resonance imaging), and ligand marker measurements. Based on the current state of the research, we suggest that brain imaging is a useful approach to assess the structural and functional correlates of sleep impairments as well as better understand the cerebral consequences of various therapeutic approaches. Modem neuroimaging techniques therefore provide a valuable tool to gain insight into possible pathophysiological mechanisms of sleep disorders in adult humans.

Microarray technology and applications in the arena of genome-wide association

BACKGROUND

there is a revolution occurring in single nucleotide polymorphism (SNP) genotyping technology, with high-throughput methods now allowing large numbers of SNPs (10(5)-10(6)) to be genotyped in large cohort studies. This has enabled large-scale genome-wide association (GWA) studies in complex diseases, such as diabetes, asthma, and inflammatory bowel disease, to be undertaken for the first time.

CONTENT

the GWA approach serves the critical need for a comprehensive and unbiased strategy to identify causal genes related to complex disease, and is rapidly replacing the more traditional candidate gene studies and microsatellite-based linkage mapping approaches that have dominated gene discovery attempts for common diseases. As a consequence of employing array-based technologies, over the last 3 years dramatic discoveries of key variants involved in multiple complex diseases and related traits have been reported in the top scientific literature and, most importantly, have been largely replicated by independent investigator groups. As a consequence, several novel genes have been identified, most notably in the metabolic, cardiovascular, autoimmune, and oncology disease areas, that are clearly rooted in the biology of these disorders. These discoveries have opened up new avenues for investigators to address novel molecular pathways that were not previously linked to or thought of in relation with these diseases.

SUMMARY

this review provides a synopsis of recent advances and what we may expect to still emerge from this field.

Genetic determinants of phenotypic diversity in humans

New techniques are enabling the identification of genetic variants underlying complex diseases and phenotypic traits.

New technologies for rapidly assaying DNA sequences have revealed that the degree and nature of human genetic variation is far more complex then previously realized. These same technologies have also resulted in the identification of common genetic variants associated with more than 30 human diseases and traits.

Genome-wide screen for asthma in Puerto Ricans: evidence for association with 5q23 region

While the number of success stories for mapping genes associated with complex diseases using genome-wide association approaches is growing, there is still much work to be done in developing methods for such studies when the samples are collected from a population, which may not be homogeneous. Here we report the first genome-wide association study to identify genes associated with asthma in an admixed population. We genotyped 96 Puerto Rican moderate to severe asthma cases and 88 controls as well as 109 samples representing Puerto Rico's founding populations using the Affymetrix GeneChip Human Mapping 100K array sets. The data from samples representing Puerto Rico's founding populations was used to identify ancestry informative markers for admixture mapping analyses. In addition, a genome-wide association analysis using logistic regression was performed on the data. Although neither admixture mapping nor regression analysis gave any significant association with asthma after correction for multiple testing, an overlap analysis using the top scoring SNPs from different methods suggested chromosomal regions 5q23.3 and 13q13.3 as potential regions harboring genes for asthma in Puerto Ricans. The validation analysis of these two regions in 284 Puerto Rican asthma trios gave significant association for the 5q23.3 region. Our results provide strong evidence that the previously linked 5q23 region is associated with asthma in Puerto Ricans. The detection of causative variants in this region will require fine mapping and functional validation.

Genome-wide association studies: progress and potential for drug discovery and development

Although genetic studies have been critically important for the identification of therapeutic targets in Mendelian disorders, genetic approaches aiming to identify targets for common, complex diseases have traditionally had much more limited success. However, during the past year, a novel genetic approach - genome-wide association (GWA) - has demonstrated its potential to identify common genetic variants associated with complex diseases such as diabetes, inflammatory bowel disease and cancer. Here, we highlight some of these recent successes, and discuss the potential for GWA studies to identify novel therapeutic targets and genetic biomarkers that will be useful for drug discovery, patient selection and stratification in common diseases.

Ropinirole treatment for restless legs syndrome

BACKGROUND

Restless legs syndrome (RLS) is a chronic neurological disorder associated with sleep disturbance.

OBJECTIVE

Prepare a drug evaluation of the non-ergot dopamine agonist ropinirole in RLS.

METHODS

Review of scientific literature on RLS, particularly focusing on treatment with ropinirole.

CONCLUSION

Ropinirole has been studied for treatment of moderate to severe primary RLS in a comprehensive clinical development program. Ropinirole significantly improved symptoms of RLS, versus placebo, in patients with primary RLS. These improvements are supported by data from individual studies and pooled analyses. Significant improvements in RLS symptoms were observed within two nights of treatment. Ropinirole also produced significant benefits on objective measures of RLS motor symptoms, such as periodic leg movements, and subjective measures of sleep. Ropinirole was generally well tolerated.

The success of the genome-wide association approach: a brief story of a long struggle

The genome-wide association approach has been the most powerful and efficient study design thus far in identifying genetic variants that are associated with complex human diseases. This approach became feasible as the result of several key advancements in genetic knowledge, genotyping technologies, statistical analysis algorithms and the availability of large collections of cases and controls. With all these necessary tools in hand, many genome-wide association studies were recently completed, and many more studies which will explore the genetic basis of various complex diseases and quantitative traits are soon to come. This approach has started to reap the fruits of its labor over the past several months. Publications of genome-wide association studies in several complex diseases such as inflammatory bowel disease, type-2 diabetes, breast cancer and prostate cancer have been abundant in the first half of this year. The aims of this review are firstly, to provide a timely summary for most of the genome-wide association studies that have been published until June/July 2007 and secondly, to evaluate to what extent these results have been validated in subsequent replication studies.

Update of the pathophysiology of the restless-legs-syndrome

The Restless Legs Syndrome (RLS) is a heterogeneous disease. Symptomatic or secondary forms encompass iron deficiency, uremia, pregnancy, polyneuropathy, and other causes. The so-called idiopathic RLS syndrome preferentially affects patients with a younger onset before the age of 30. Here we summarize pathophysiological results along the anatomical route, beginning at the cortex and followed by the basal ganglia, thalamus, A11 neurones, substantia nigra, brainstem nuclei, and spinal cord. Genetic risk variants for RLS have recently been identified in two genes, one of them the homeobox gene MEIS1, known to be involved in embryonic development and variants in a second locus containing the genes encoding mitogen-activated protein kinase MAP2K5, and the transcription factor LBXCOR1. A third one, the BTBD9 gene with unknown function encodes a BTB(POZ) domain. Accordingly, new concepts on pathophysiology have to bridge conventional knowledge with possible consequences deriving from these findings. Furthermore, this may create a framework to help understand why dopamine, opioid, and some anticonvulsant therapies are effective in RLS patients.

Clinical significance of RLS

While the restless legs syndrome (RLS) may have been known in antiquity, it has only recently come to medical attention. Individuals with RLS fall along a spectrum from mild, infrequent symptoms to those with severe daily life-impairing discomforts and sleep disruption. These problems can cause impaired mood, daytime fatigue, cognitive difficulties, and inability to participate in a variety of quiet activities. This leads to a general reduction in quality of life similar to other significant psychiatric and medical disorders. Recent studies suggest that RLS may be a risk factor for developing both psychiatric disorders (such as major depression and anxiety) and somatic diseases (such as hypertension and cardiovascular disease). In dialysis patients, RLS has been found to be a risk factor for mortality. Therefore, those with RLS who have clinically significant symptoms suffer increased morbidity and are at risk for impaired long-term medical outcomes.

Suggestive evidence for linkage for restless legs syndrome on chromosome 19p13

Five loci for restless legs syndrome (RLS) on chromosomes 12q, 14q, 9p, 2q, and 20p (RLS1-RLS5) have been mapped in RLS families, with a recessive in the first and autosomal-dominant mode of inheritance in the latter cases. Investigations of further RLS families showed evidence for genetic locus heterogeneity. We have conducted a genome-wide linkage analysis in a large RLS family of Italian origin with 12 affected members in 3 generations using 5,861 single nucleotide polymorphisms (SNP, 6K Illumina). Linkage analysis was performed under an autosomal-dominant model with a complete penetrance, an allele frequency of 0.003 and a phenocopy rate of 0.005. The genome-wide scan resulted in suggestive evidence for linkage on chromosome 19p with maximum multipoint logarithm of the odds score of 2.61 between markers rs754292 and rs273265. The locus was replicated in a family-based association study in a set of 159 trios of European origin. This study provides evidence for a further RLS locus, thus supporting the picture of RLS as a genetically heterogenous complex trait.

From human genetics and genomics to pharmacogenetics and pharmacogenomics: past lessons, future directions

A brief history of human genetics and genomics is provided, comparing recent progress in those fields with that in pharmacogenetics and pharmacogenomics, which are subsets of genetics and genomics, respectively. Sequencing of the entire human genome, the mapping of common haplotypes of single-nucleotide polymorphisms (SNPs), and cost-effective genotyping technologies leading to genome-wide association (GWA) studies - have combined convincingly in the past several years to demonstrate the requirements needed to separate true associations from the plethora of false positives. While research in human genetics has moved from monogenic to oligogenic to complex diseases, its pharmacogenetics branch has followed, usually a few years behind. The continuous discoveries, even today, of new surprises about our genome cause us to question reviews declaring that "personalized medicine is almost here" or that "individualized drug therapy will soon be a reality." As summarized herein, numerous reasons exist to show that an "unequivocal genotype" or even an "unequivocal phenotype" is virtually impossible to achieve in current limited-size studies of human populations. This problem (of insufficiently stringent criteria) leads to a decrease in statistical power and, consequently, equivocal interpretation of most genotype-phenotype association studies. It remains unclear whether personalized medicine or individualized drug therapy will ever be achievable by means of DNA testing alone.

Copy number variants and common disorders: filling the gaps and exploring complexity in genome-wide association studies

Genome-wide association scans (GWASs) using single nucleotide polymorphisms (SNPs) have been completed successfully for several common disorders and have detected over 30 new associations. Considering the large sample sizes and genome-wide SNP coverage of the scans, one might have expected many of the common variants underpinning the genetic component of various disorders to have been identified by now. However, these studies have not evaluated the contribution of other forms of genetic variation, such as structural variation, mainly in the form of copy number variants (CNVs). Known CNVs account for over 15% of the assembled human genome sequence. Since CNVs are not easily tagged by SNPs, might have a wide range of copy number variability, and often fall in genomic regions not well covered by whole-genome arrays or not genotyped by the HapMap project, current GWASs have largely missed the contribution of CNVs to complex disorders. In fact, some CNVs have already been reported to show association with several complex disorders using candidate gene/region approaches, underpinning the importance of regions not investigated in current GWASs. This reveals the need for new generation arrays (some already in the market) and the use of tailored approaches to explore the full dimension of genome variability beyond the single nucleotide scale.

Restless legs syndrome: a common disorder in patients with rheumatologic conditions

OBJECTIVES

To review the symptoms, differential diagnosis, and treatment of the restless legs syndrome (RLS), and its relevance within rheumatologic practice.

METHODS

Review of the scientific literature on RLS to summarize symptom presentation, burden, diagnosis, treatment, and association with rheumatologic conditions.

RESULTS

RLS is a sensorimotor neurological disorder characterized by an irresistible urge to move the legs, usually accompanied or caused by unpleasant sensations within the legs. These sensations are sometimes described as achy or painful. They may cause sleep disruption and impair quality of life. RLS may be primary, of unknown etiology, with a likely genetic basis, or secondary, provoked by other conditions. Secondary RLS often improves when the underlying condition is treated or resolves. Since RLS is common in rheumatologic disorders such as rheumatoid arthritis or Sjögren's syndrome, rheumatologists need to be familiar with the condition. Primary care physicians may misattribute RLS symptoms to other conditions and refer patients to specialists for treatment. Since RLS symptoms can be similar to, and mistaken for, symptoms in rheumatologic diseases, patients may be referred to rheumatologists. Therefore, it is important that rheumatologists be able to recognize, differentiate, diagnose, and treat RLS.

CONCLUSIONS

The clinical diagnosis of RLS is based on 4 essential diagnostic criteria related to the urge to move that characterizes this disorder. Beyond good sleep hygiene and behavioral measures, dopaminergic agents are first-line treatments for primary RLS. Anticonvulsants, opioids, and sedative/hypnotics may also have a role in management.

Molecular analysis of sleep

Rest or sleep in all animal species constitutes a period of quiescence necessary for recovery from activity. Whether rest and activity observed in all organisms share a similar fundamental molecular basis with sleep and wakefulness in mammals has not yet been established. In addition and in contrast to the circadian system, strong evidence that sleep is regulated at the transcriptional level is lacking. Nevertheless, several studies indicate that single genesmay regulate some specific aspects of sleep. Efforts to better understand or confirm the role of known neurotransmission pathways in sleep-wake regulation using transgenic approaches resulted so far in only limited new insights. Recent gene expression profiling efforts in rats, mice, and fruit flies are promising and suggest that only a few gene categories are differentially regulated by behavioral state. How molecular analysis can help us to understand sleep is the focus of this chapter.