A highly informative SNP linkage panel for human genetic studies

Secure Comparisons of Single Nucleotide Polymorphisms Using Secure Multiparty Computation: Method Development

BACKGROUND

While genomic variations can provide valuable information for health care and ancestry, the privacy of individual genomic data must be protected. Thus, a secure environment is desirable for a human DNA database such that the total data are queryable but not directly accessible to involved parties (eg, data hosts and hospitals) and that the query results are learned only by the user or authorized party.

OBJECTIVE

In this study, we provide efficient and secure computations on panels of single nucleotide polymorphisms (SNPs) from genomic sequences as computed under the following set operations: union, intersection, set difference, and symmetric difference.

METHODS

Using these operations, we can compute similarity metrics, such as the Jaccard similarity, which could allow querying a DNA database to find the same person and genetic relatives securely. We analyzed various security paradigms and show metrics for the protocols under several security assumptions, such as semihonest, malicious with honest majority, and malicious with a malicious majority.

RESULTS

We show that our methods can be used practically on realistically sized data. Specifically, we can compute the Jaccard similarity of two genomes when considering sets of SNPs, each with 400,000 SNPs, in 2.16 seconds with the assumption of a malicious adversary in an honest majority and 0.36 seconds under a semihonest model.

CONCLUSIONS

Our methods may help adopt trusted environments for hosting individual genomic data with end-to-end data security.

Biallelic truncating mutations in FMN2, encoding the actin-regulatory protein Formin 2, cause nonsyndromic autosomal-recessive intellectual disability

Dendritic spines represent the major site of neuronal activity in the brain; they serve as the receiving point for neurotransmitters and undergo rapid activity-dependent morphological changes that correlate with learning and memory. Using a combination of homozygosity mapping and next-generation sequencing in two consanguineous families affected by nonsyndromic autosomal-recessive intellectual disability, we identified truncating mutations in formin 2 (FMN2), encoding a protein that belongs to the formin family of actin cytoskeleton nucleation factors and is highly expressed in the maturing brain. We found that FMN2 localizes to punctae along dendrites and that germline inactivation of mouse Fmn2 resulted in animals with decreased spine density; such mice were previously demonstrated to have a conditioned fear-learning defect. Furthermore, patient neural cells derived from induced pluripotent stem cells showed correlated decreased synaptic density. Thus, FMN2 mutations link intellectual disability either directly or indirectly to the regulation of actin-mediated synaptic spine density.

Mutations in LAMB1 cause cobblestone brain malformation without muscular or ocular abnormalities

Cobblestone brain malformation (COB) is a neuronal migration disorder characterized by protrusions of neurons beyond the first cortical layer at the pial surface of the brain. It is usually seen in association with dystroglycanopathy types of congenital muscular dystrophies (CMDs) and ocular abnormalities termed muscle-eye-brain disease. Here we report homozygous deleterious mutations in LAMB1, encoding laminin subunit beta-1, in two families with autosomal-recessive COB. Affected individuals displayed a constellation of brain malformations including cortical gyral and white-matter signal abnormalities, severe cerebellar dysplasia, brainstem hypoplasia, and occipital encephalocele, but they had less apparent ocular or muscular abnormalities than are typically observed in COB. LAMB1 is localized to the pial basement membrane, suggesting that defective connection between radial glial cells and the pial surface mediated by LAMB1 leads to this malformation.

Whole-exome sequencing identifies mutated c12orf57 in recessive corpus callosum hypoplasia

The corpus callosum is the principal cerebral commissure connecting the right and left hemispheres. The development of the corpus callosum is under tight genetic control, as demonstrated by abnormalities in its development in more than 1,000 genetic syndromes. We recruited more than 25 families in which members affected with corpus callosum hypoplasia (CCH) lacked syndromic features and had consanguineous parents, suggesting recessive causes. Exome sequence analysis identified C12orf57 mutations at the initiator methionine codon in four different families. C12orf57 is ubiquitously expressed and encodes a poorly annotated 126 amino acid protein of unknown function. This protein is without significant paralogs but has been tightly conserved across evolution. Our data suggest that this conserved gene is required for development of the human corpus callosum.

Correcting coalescent analyses for panel-based SNP ascertainment

Single-nucleotide polymorphism (SNP) data are routinely obtained by sequencing a region of interest in a small panel, constructing a chip with probes specific to sites found to vary in the panel, and using the chip to assay subsequent samples. The size of the chip is often reduced by removing low-frequency alleles from the set of SNPs. Using coalescent estimation of the scaled population size parameter, Θ, as a test case, we demonstrate the loss of information inherent in this procedure and develop corrections for coalescent analysis of SNPs obtained via a panel. We show that more accurate Θ-estimates can be recovered if the panel size is known, but at considerable computational cost as the panel individuals must be explicitly modeled in the analysis. We extend this technique to apply to the case where rare alleles have been omitted from the SNP panel. We find that when appropriate corrections for panel ascertainment and rare-allele omission are used, the biases introduced by ascertainment are largely correctable, but recovered estimates are less accurate than would be obtained with fully sequenced data. This method is then applied to recombinant multiple population data to investigate the effects of recombination and migration on the estimate of Θ.

Exome sequencing can improve diagnosis and alter patient management

The translation of "next-generation" sequencing directly to the clinic is still being assessed but has the potential for genetic diseases to reduce costs, advance accuracy, and point to unsuspected yet treatable conditions. To study its capability in the clinic, we performed whole-exome sequencing in 118 probands with a diagnosis of a pediatric-onset neurodevelopmental disease in which most known causes had been excluded. Twenty-two genes not previously identified as disease-causing were identified in this study (19% of cohort), further establishing exome sequencing as a useful tool for gene discovery. New genes identified included EXOC8 in Joubert syndrome and GFM2 in a patient with microcephaly, simplified gyral pattern, and insulin-dependent diabetes. Exome sequencing uncovered 10 probands (8% of cohort) with mutations in genes known to cause a disease different from the initial diagnosis. Upon further medical evaluation, these mutations were found to account for each proband's disease, leading to a change in diagnosis, some of which led to changes in patient management. Our data provide proof of principle that genomic strategies are useful in clarifying diagnosis in a proportion of patients with neurodevelopmental disorders.

A missense founder mutation in VLDLR is associated with Dysequilibrium Syndrome without quadrupedal locomotion

BACKGROUND

Dysequilibrium syndrome is a genetically heterogeneous condition that combines autosomal recessive, nonprogressive cerebellar ataxia with mental retardation. The condition has been classified into cerebellar ataxia, mental retardation and disequilibrium syndrome types 1 (CAMRQ1), 2 (CAMRQ2) and 3 (CAMRQ3) and attributed to mutations in VLDLR, CA8 and WDR81 genes, respectively. Quadrupedal locomotion in this syndrome has been reported in association with mutations in all three genes.

METHODS

SNP mapping and candidate gene sequencing in one consanguineous Omani family from the United Arab Emirates with cerebellar hypoplasia, moderate mental retardation, delayed ambulation and truncal ataxia was used to identify the mutation. In a second unrelated consanguineous Omani family, massively parallel exonic sequencing was used.

RESULTS

We identified a homozygous missense mutation (c.2117 G > T, p.C706F) in the VLDLR gene in both families on a shared affected haplotype block.This is the first reported homozygous missense mutation in VLDLR and it occurs in a highly conserved residue and predicted to be damaging to protein function.

CONCLUSIONS

We have delineated the phenotype associated with dysequilibrium syndrome in two Omani families and identified the first homozygous missense pathogenic mutation in VLDLR gene with likely founder effect in the southeastern part of the Arabian Peninsula.

Microsatellite markers for linkage and association studies

During the 1990s and the first several years of this century, microsatellites or short tandem repeats were the workhorse genetic markers for hypothesis-independent studies in human genetics, facilitating genome-wide linkage studies and allelic imbalance studies. However, the rise of higher throughput and cost-effective single-nucleotide polymorphism (SNP) platforms led to the era of the SNP for genome scans. Nevertheless, it is important to note that microsatellites remain highly informative and useful measures of genomic variation for linkage and association studies. Their continued advantage in complementing SNPs lies in their greater allelic diversity than biallelic SNPs as well as in their population history, in which single-step expansion or contraction of the tandem repeat on the background of ancestral SNP haplotypes can break up common haplotypes, leading to greater haplotype diversity within the linkage disequilibrium block of interest. In fact, microsatellites have starred in association studies leading to widely replicated discoveries of type 2 diabetes (TCF7L2) and prostate cancer genes (the 8q21 region). At the end of the day, it will be important to catalog all variation, including SNPs, microsatellites, copy number variations, and polymorphic inversions in human genetic studies. This article describes the utilities of microsatellites and experimental approaches in their use.

Single-nucleotide polymorphism array karyotyping in clinical practice: where, when, and how?

Single-nucleotide polymorphism array (SNP-A) karyotyping is a new technology that has enabled genome-wide detection of genetic lesions in human cancers, including hematopoietic neoplasms. Taking advantage of very large numbers of allele-specific probes synthesized on microarrays at high density, copy number alterations as well as allelic imbalances can be sensitively detected in a genome-wide manner at unprecedented resolutions. Most importantly, SNP-A karyotyping represents the only platform currently available for genome-scale detection of copy neutral loss of heterozygosity (CN-LOH) or uniparental disomy (UPD), which is widely observed in cancer genomes. Although not applicable to detection of balanced translocations, which are commonly found in hematopoietic malignancies, SNP-A karyotyping technology complements and even outperforms conventional metaphase karyotyping, potentially allowing for more accurate genetic diagnosis of hematopoietic neoplasms in clinical practice. Here, we review the current status of SNP-A karyotyping and its application to hematopoietic neoplasms.

CEP41 is mutated in Joubert syndrome and is required for tubulin glutamylation at the cilium

Tubulin glutamylation is a post-translational modification that occurs predominantly in the ciliary axoneme and has been suggested to be important for ciliary function. However, its relationship to disorders of the primary cilium, termed ciliopathies, has not been explored. Here we mapped a new locus for Joubert syndrome (JBTS), which we have designated as JBTS15, and identified causative mutations in CEP41, which encodes a 41-kDa centrosomal protein. We show that CEP41 is localized to the basal body and primary cilia, and regulates ciliary entry of TTLL6, an evolutionarily conserved polyglutamylase enzyme. Depletion of CEP41 causes ciliopathy-related phenotypes in zebrafish and mice and results in glutamylation defects in the ciliary axoneme. Our data identify CEP41 mutations as a cause of JBTS and implicate tubulin post-translational modification in the pathogenesis of human ciliary dysfunction.

R, Elston R. Some capabilities for model-based and model-free linkage analysis using the program package S.A.G.E. (Statistical Analysis for Genetic Epidemiology)

For both model-free and model-based linkage analysis the S.A.G.E. (Statistical Analysis for Genetic Epidemiology) program package has some unique capabilities in analyzing both continuous traits and binary traits with variable age of onset. Here we highlight model-based linkage analysis of a quantitative trait (plasma dopamine β hydroxylase) that is known to be largely determined by monogenic inheritance, using a prior segregation analysis to produce the best fitting model for the trait. For a binary trait with variable age of onset (schizophrenia), we illustrate how using age of onset information to obtain a quantitative susceptibility trait leads to more statistically significant linkage signals, suggesting better power.

New recessive syndrome of microcephaly, cerebellar hypoplasia, and congenital heart conduction defect

We identified a two-branch consanguineous family in which four affected members (three females and one male) presented with constitutive growth delay, severe psychomotor retardation, microcephaly, cerebellar hypoplasia, and second-degree heart block. They also shared distinct facial features and similar appearance of their hands and feet. Childhood-onset insulin-dependent diabetes mellitus developed in one affected child around the age of 9 years. Molecular analysis excluded mutations in potentially related genes such as PTF1A, EIF2AK3, EOMES, and WDR62. This condition appears to be unique of other known conditions, suggesting a unique clinical entity of autosomal recessive mode of inheritance. © 2011 Wiley Periodicals, Inc.

Co-occurrence of distinct ciliopathy diseases in single families suggests genetic modifiers

Disorders within the “ciliopathy” spectrum include Joubert (JS), Bardet–Biedl syndromes (BBS), and nephronophthisis (NPHP). Although mutations in single ciliopathy genes can lead to these different syndromes between families, there have been no reports of phenotypic discordance within a single family. We report on two consanguineous families with discordant ciliopathies in sibling. In Ciliopathy-672, the older child displayed dialysis-dependent NPHP whereas the younger displayed the pathognomonic molar tooth MRI sign (MTS) of JS. A second branch displayed two additional children with NPHP. In Ciliopathy-1491, the oldest child displayed classical features of BBS whereas the two younger children displayed the MTS. Importantly, the children with BBS and NPHP lacked MTS, whereas children with JS lacked obesity or NPHP, and the child with BBS lacked MTS and NPHP. Features common to all three disorders included intellectual disability, postaxial polydactyly, and visual reduction. The variable phenotypic expressivity in this family suggests that genetic modifiers may determine specific clinical features within the ciliopathy spectrum. © 2011 Wiley Periodicals, Inc.

Strategies for genotyping

The identification of genomic loci linked to or associated with human disease has been greatly facilitated by the evolution of genotyping strategies and techniques. The success of these strategies continues to be based upon clear clinical assessment, accurate sample handling, and careful data management, but also increasingly upon experimental design. Technological advances in the field of genotyping have permitted increasingly complex and large population studies to be performed. An understanding of publicly available genetic variation databases, including an awareness of the limitations of these data, and an appreciation of the strategic approaches that should be used to exploit this information will provide tremendous insight for researchers are aiming to utilize this accessible technology. As genome-wide association studies (GWAS) and Next Generation (NextGen) sequencing become the mainstays of genetic analyses, it is important that their technical strengths and limitations, as well as their impact on study design, be understood before use in a linkage or genetic association study.

Genomewide linkage scan for diabetic renal failure and albuminuria: the FIND study

BACKGROUND

Diabetic nephropathy (DN) is a leading cause of mortality and morbidity in patients with type 1 and type 2 diabetes. The multicenter FIND consortium aims to identify genes for DN and its associated quantitative traits, e.g. the urine albumin:creatinine ratio (ACR). Herein, the results of whole-genome linkage analysis and a sparse association scan for ACR and a dichotomous DN phenotype are reported in diabetic individuals.

METHODS

A genomewide scan comprising more than 5,500 autosomal single nucleotide polymorphism markers (average spacing of 0.6 cM) was performed on 1,235 nuclear and extended pedigrees (3,972 diabetic participants) ascertained for DN from African-American (AA), American-Indian (AI), European-American (EA) and Mexican-American (MA) populations.

RESULTS

Strong evidence for linkage to DN was detected on chromosome 6p (p = 8.0 × 10(-5), LOD = 3.09) in EA families as well as suggestive evidence for linkage to chromosome 7p in AI families. Regions on chromosomes 3p in AA, 7q in EA, 16q in AA and 22q in MA displayed suggestive evidence of linkage for urine ACR. The linkage peak on chromosome 22q overlaps the MYH9/APOL1 gene region, previously implicated in AA diabetic and nondiabetic nephropathies.

CONCLUSION

These results strengthen the evidence for previously identified genomic regions and implicate several novel loci potentially involved in the pathogenesis of DN.

SRD5A3 is required for converting polyprenol to dolichol and is mutated in a congenital glycosylation disorder

N-linked glycosylation is the most frequent modification of secreted and membrane-bound proteins in eukaryotic cells, disruption of which is the basis of the congenital disorders of glycosylation (CDGs). We describe a new type of CDG caused by mutations in the steroid 5alpha-reductase type 3 (SRD5A3) gene. Patients have mental retardation and ophthalmologic and cerebellar defects. We found that SRD5A3 is necessary for the reduction of the alpha-isoprene unit of polyprenols to form dolichols, required for synthesis of dolichol-linked monosaccharides, and the oligosaccharide precursor used for N-glycosylation. The presence of residual dolichol in cells depleted for this enzyme suggests the existence of an unexpected alternative pathway for dolichol de novo biosynthesis. Our results thus suggest that SRD5A3 is likely to be the long-sought polyprenol reductase and reveal the genetic basis of one of the earliest steps in protein N-linked glycosylation.

High-throughput single nucleotide polymorphism genotyping using nanofluidic Dynamic Arrays

BACKGROUND

Single nucleotide polymorphisms (SNPs) have emerged as the genetic marker of choice for mapping disease loci and candidate gene association studies, because of their high density and relatively even distribution in the human genomes. There is a need for systems allowing medium multiplexing (ten to hundreds of SNPs) with high throughput, which can efficiently and cost-effectively generate genotypes for a very large sample set (thousands of individuals). Methods that are flexible, fast, accurate and cost-effective are urgently needed. This is also important for those who work on high throughput genotyping in non-model systems where off-the-shelf assays are not available and a flexible platform is needed.

RESULTS

We demonstrate the use of a nanofluidic Integrated Fluidic Circuit (IFC) - based genotyping system for medium-throughput multiplexing known as the Dynamic Array, by genotyping 994 individual human DNA samples on 47 different SNP assays, using nanoliter volumes of reagents. Call rates of greater than 99.5% and call accuracies of greater than 99.8% were achieved from our study, which demonstrates that this is a formidable genotyping platform. The experimental set up is very simple, with a time-to-result for each sample of about 3 hours.

CONCLUSION

Our results demonstrate that the Dynamic Array is an excellent genotyping system for medium-throughput multiplexing (30-300 SNPs), which is simple to use and combines rapid throughput with excellent call rates, high concordance and low cost. The exceptional call rates and call accuracy obtained may be of particular interest to those working on validation and replication of genome- wide- association (GWA) studies.

High-throughput single nucleotide polymorphism genotyping using nanofluidic Dynamic Arrays

Background

Single nucleotide polymorphisms (SNPs) have emerged as the genetic marker of choice for mapping disease loci and candidate gene association studies, because of their high density and relatively even distribution in the human genomes. There is a need for systems allowing medium multiplexing (ten to hundreds of SNPs) with high throughput, which can efficiently and cost-effectively generate genotypes for a very large sample set (thousands of individuals). Methods that are flexible, fast, accurate and cost-effective are urgently needed. This is also important for those who work on high throughput genotyping in non-model systems where off-the-shelf assays are not available and a flexible platform is needed.

Results

We demonstrate the use of a nanofluidic Integrated Fluidic Circuit (IFC) - based genotyping system for medium-throughput multiplexing known as the Dynamic Array, by genotyping 994 individual human DNA samples on 47 different SNP assays, using nanoliter volumes of reagents. Call rates of greater than 99.5% and call accuracies of greater than 99.8% were achieved from our study, which demonstrates that this is a formidable genotyping platform. The experimental set up is very simple, with a time-to-result for each sample of about 3 hours.

Conclusion

Our results demonstrate that the Dynamic Array is an excellent genotyping system for medium-throughput multiplexing (30-300 SNPs), which is simple to use and combines rapid throughput with excellent call rates, high concordance and low cost. The exceptional call rates and call accuracy obtained may be of particular interest to those working on validation and replication of genome- wide- association (GWA) studies.

Illumina DNA test panel-based genotyping of whole genome amplified-DNA extracted from hair samples: performance and agreement with genotyping results from genomic DNA from buccal cells

BACKGROUND

Hair is a DNA source that can be collected easily and inexpensively from participants in epidemiological studies. However, there is concern about DNA quality and quantity. Therefore, we assessed genotyping performance of whole genome amplified (WGA)-DNA extracted from hair using the GenomePlex method and evaluated its agreement with genotyping results of buccal cell DNA from the same individuals, using the Illumina GoldenGate platform.

METHODS

The Illumina DNA test panel includes 360 highly validated single nucleotide polymorphisms (SNPs) selected from the Linkage IV Panel that are distributed across the entire genome. DNA was extracted from both archived hair and buccal cell samples obtained from 44 randomly selected subjects participating in a large cohort study in Canada.

RESULTS

The genotyping success rate was 97.7% for 44 paired samples. However, WGA-DNA from hair failed more during genotyping in comparison to buccal cell DNA. Hair samples with a pre-WGA-DNA>or=1 ng/microL quantified using the PicoGreen assay (n=33) showed an average genotyping completion rate of 98.8% and SNP concordance of 91.2% with genotyping performance of buccal cell DNA. In contrast, samples with a pre-WGA-DNA<1 ng/microL had lower genotyping completion rate (94%) and poor SNP concordance (49%).

CONCLUSIONS

Results suggest that WGA-DNA obtained from hair can produce excellent genotyping call rates and show relatively good SNP concordance with results from buccal cell DNA using high-throughput technology. DNA quantity obtained from hair samples is a crucial determinant of genotyping performance. Larger studies are needed to examine the utility of hair DNA with different genotyping platforms.

Mutations in INPP5E, encoding inositol polyphosphate-5-phosphatase E, link phosphatidyl inositol signaling to the ciliopathies

Phosphotidylinositol (PtdIns) signaling is tightly regulated, both spatially and temporally, by subcellularly localized PtdIns kinases and phosphatases that dynamically alter downstream signaling events 1. Joubert Syndrome (JS) characterized by a specific midbrain-hindbrain malformation (“molar tooth sign”) and variably associated retinal dystrophy, nephronophthisis, liver fibrosis and polydactyly 2, and is included in the newly emerging group of “ciliopathies”. In patients linking to JBTS1, we identified mutations in the INPP5E gene, encoding inositol polyphosphate-5-phosphatase E, which hydrolyzes the 5-phosphate of PtdIns(3,4,5)P3 and PtdIns(4,5)P2. Mutations clustered in the phosphatase domain and impaired 5-phosphatase activity, resulting in altered cellular PtdIns ratios. INPP5E localized to cilia in major organs affected in JS, and mutations promoted premature destabilization of cilia in response to stimulation. Thus, these data links PtdIns signaling to the primary cilium, a cellular structure that is becoming increasingly appreciated for its role in mediating cell signals and neuronal function.

Use of SNPs in cancer predisposition analysis, diagnosis and prognosis: tools and prospects

BACKGROUND

The development of cancer is accompanied by several genetic alterations. Single nucleotide polymorphisms (SNPs) are the most common form of genetic variation found in the human population. SNP arrays offer a high-resolution, high-throughput technology for genome-wide analysis, allowing the simultaneous detection of genotype and copy number changes. The power of SNP arrays as a research tool has accelerated our understanding of the genetic alterations in cancer, providing potential clinical applications.

OBJECTIVE

This manuscript reviews the use of SNPs in cancer research and discusses the potential clinical application of analysing SNPs for cancer predisposition analysis, diagnosis and prognosis. We also discuss potential future applications for the analysis of SNPs.

METHODS

In writing this review, we have reflected on our own extensive experience in the field of cancer genomics and have surveyed peer-reviewed articles focussing on the application of SNPs in cancer research. In addition, we have referred to product websites.

CONCLUSION

Since its development, SNP array technology has been extensively applied in cancer research. Information generated from SNP array analysis has been providing valuable information. With the full understanding of the rich resources of SNPs and their effects on influencing cellular function, SNP arrays will revolutionise the clinical practice in cancer risk assessment, diagnosis and prognosis making the concept of personalised medicine a reality.

A nanoliter fluidic platform for large-scale single nucleotide polymorphism genotyping

Discovery, evaluation, and understanding the biological relevance of single nucleotide polymorphisms (SNPs) and their associated phenotypes is relevant to many applications, including human disease diagnostics, pathogen detection, and identification of genetic traits impacting agricultural practices, both in terms of food quality and production efficiency. Validation of putative SNP associations in large-scale cohorts is currently impeded by the technical challenges and high cost inherent in analyzing large numbers of samples using available SNP genotyping platforms. We describe in this report the implementation of the 5′-exonuclease, biallelic PCR assay for SNP genotyping (TaqMan) in a nanofluidic version of a high-density microplate. System performance was assessed using a panel of 32 TaqMan SNP genotyping assays targeted to human polymorphisms. This functional test of the nanoliter fluidic SNP genotyping platform delivered genotyping call rates and accuracies comparable to the same larger volume reactions in microplate systems.

Genome-wide linkage scan reveals three putative breast-cancer-susceptibility loci

Despite all the research efforts made during the last few decades, most of the cases of families with breast cancer remain unexplained. Mutations in BRCA1 and BRCA2, and in other breast-cancer-susceptibility genes, account for about 25% of familial breast cancer. Linkage studies have failed to identify other breast-cancer-susceptibility genes. The selection criteria of the families, differences in the population background, or clinical and genetic heterogeneity, among other factors, might determine the power to detect the linkage signal. We have performed a SNP-based linkage scan with a total of 6000 SNP markers across the genome in 41 breast-cancer Spanish families, with an average of four breast-cancer cases per family not associated with BRCA1 or BRCA2 germline mutations. In addition, we have included three BRCA-positive families to test the power in linkage detection from a low-complexity family in which a high-penetrance mutation segregates. We have identified three regions of interest, located on 3q25, 6q24, and 21q22. The two former regions showed a suggestive linkage signal (HLOD scores 3.01 and 2.26, respectively), and the latter region showed a significant linkage signal (HLOD score 3.55). Moreover, we found that a subset of 13 families with bilateral breast cancer presented a HLOD of 3.13 on the 3q25 region. Our results suggest that several variables must be taken into account before performing a linkage study in familial breast cancer because of the high heterogeneity within non-BRCA1/2 families. Phenotypic and geographic homogeneity could be the most important factors.

Considerations for the processing and analysis of GoldenGate-based two-colour Illumina platforms

Illumina's GoldenGate technology is a two-channel microarray platform that allows for the simultaneous interrogation of about 1,500 locations in the genome. GoldenGate has proved a flexible platform not only in the choice of those 1,500 locations, but also in the choice of the property being measured at them. It retains the desirable properties of Illumina's BeadArrays in that the probes (in this case 'beads') are randomly arranged across the microarray, there are multiple instances of each probe and many samples can be processed simultaneously. As for other Illumina technologies, however, these properties are not exploited as they might be. Here we review the various common adaptations of the GoldenGate platform, review the analysis methods that are associated with each adaptation and then, with the aid of a number of example data sets we illustrate some of the improvements that can be made over the default analysis.

Nanoliter high-throughput PCR for DNA and RNA profiling

The increasing emphasis in life science research on utilization of genetic and genomic information underlies the need for high-throughput technologies capable of analyzing the expression of multiple genes or the presence of informative single nucleotide polymorphisms (SNPs) in large-scale, population-based applications. Human disease research, disease diagnosis, personalized therapeutics, environmental monitoring, blood testing, and identification of genetic traits impacting agricultural practices, both in terms of food quality and production efficiency, are a few areas where such systems are in demand. This has stimulated the need for PCR technologies that preserves the intrinsic analytical benefits of PCR yet enables higher throughputs without increasing the time to answer, labor and reagent expenses and workflow complexity. An example of such a system based on a high-density array of nanoliter PCR assays is described here. Functionally equivalent to a microtiter plate, the nanoplate system makes possible up to 3,072 simultaneous end-point or real-time PCR measurements in a device, the size of a standard microscope slide. Methods for SNP genotyping with end-point TaqMan PCR assays and quantitative measurement of gene expression with SYBR Green I real-time PCR are outlined and illustrative data showing system performance is provided.

Mutations in the cilia gene ARL13B lead to the classical form of Joubert syndrome

Joubert syndrome (JS) and related disorders are a group of autosomal-recessive conditions sharing the "molar tooth sign" on axial brain MRI, together with cerebellar vermis hypoplasia, ataxia, and psychomotor delay. JS is suggested to be a disorder of cilia function and is part of a spectrum of disorders involving retinal, renal, digital, oral, hepatic, and cerebral organs. We identified mutations in ARL13B in two families with the classical form of JS. ARL13B belongs to the Ras GTPase family, and in other species is required for ciliogenesis, body axis formation, and renal function. The encoded Arl13b protein was expressed in developing murine cerebellum and localized to the cilia in primary neurons. Overexpression of human wild-type but not patient mutant ARL13B rescued the Arl13b scorpion zebrafish mutant. Thus, ARL13B has an evolutionarily conserved role mediating cilia function in multiple organs.

Multiple testing in the genomics era: findings from Genetic Analysis Workshop 15, Group 15

Recent advances in molecular technologies have resulted in the ability to screen hundreds of thousands of single nucleotide polymorphisms and tens of thousands of gene expression profiles. While these data have the potential to inform investigations into disease etiologies and advance medicine, the question of how to adequately control both type I and type II error rates remains. Genetic Analysis Workshop 15 datasets provided a unique opportunity for participants to evaluate multiple testing strategies applicable to microarray and single nucleotide polymorphism data. The Genetic Analysis Workshop 15 multiple testing and false discovery rate group (Group 15) investigated three general categories for multiple testing corrections, which are summarized in this review: statistical independence, error rate adjustment, and data reduction. We show that while each approach may have certain advantages, adequate error control is largely dependent upon the question under consideration and often requires the use of multiple analytic strategies.

Robustness of linkage maps in natural populations: a simulation study

In a number of long-term individual-based studies of vertebrate populations, the genealogical relationships between individuals have been established with molecular markers. As a result, it is possible to construct genetic linkage maps of these study populations by examining the co-segregation of markers through the pedigree. There are now four free-living vertebrate study populations for whom linkage maps have been built. In this study, simulation was used to investigate whether these linkage maps are likely to be accurate. In all four populations, the probability of assigning markers to the correct chromosome is high and framework maps are generally inferred correctly. However, genotyping error can result in incorrect maps being built with very strong statistical support over the correct order. Future applications of linkage maps of natural populations are discussed.

Efficient intermediate fine mapping: confidence set inference with likelihood ratio test statistic

In positional cloning of disease causing genes, identification of a linked chromosomal region via linkage studies is often followed by fine mapping via association studies. Efficiency can be gained with an intermediate step where confidence regions for the locations of disease genes are constructed. The confidence set inference [CSI; Papachristou and Lin, 2006b] achieves this goal by replacing the traditional null hypothesis of no linkage with a new set of null hypotheses where the chromosomal position under consideration is in tight linkage with a trait locus. This approach was shown to perform favorably compared with several competing methods. Using the duality of confidence sets and hypothesis testing, CSI was proposed for the Mean test statistics with affected sibling pair data (CSI-Mean). We postulate that more efficient confidence sets will result if more efficient test statistics are used in the CSI framework. One promising candidate, the maximum LOD score (MLS) statistic, makes maximum use of available identity by descent information, in addition to handling markers with incomplete polymorphism naturally. We propose a procedure that tests the CSI null hypotheses using the MLS statistic (CSI-MLS). Compared with CSI-Mean, CSI-MLS provides tighter confidence regions over a range of single and two-locus disease models. The MLS test is also shown to be more powerful than the Mean test in testing the CSI null over a wide range of disease models, the advantage being most pronounced for recessive models. In addition, CSI-MLS is computationally much more efficient than CSI-Mean.

Examining the effect of linkage disequilibrium between markers on the Type I error rate and power of nonparametric multipoint linkage analysis of two-generation and multigenerational pedigrees in the presence of missing genotype data

Because most multipoint linkage analysis programs currently assume linkage equilibrium between markers when inferring parental haplotypes, ignoring linkage disequilibrium (LD) may inflate the Type I error rate. We investigated the effect of LD on the Type I error rate and power of nonparametric multipoint linkage analysis of two-generation and multigenerational multiplex families. Using genome-wide single nucleotide polymorphism (SNP) data from the Collaborative Study of the Genetics of Alcoholism, we modified the original data set into 30 total data sets in order to consider six different patterns of missing data for five different levels of SNP density. To assess power, we designed simulated traits based on existing marker genotypes. For the Type I error rate, we simulated 1,000 qualitative traits from random distributions, unlinked to any of the marker data. Overall, the different levels of SNP density examined here had only small effects on power (except sibpair data). Missing data had a substantial effect on power, with more completely genotyped pedigrees yielding the highest power (except sibpair data). Most of the missing data patterns did not cause large increases in the Type I error rate if the SNP markers were more than 0.3 cM apart. However, in a dense 0.25-cM map, removing genotypes on founders and/or founders and parents in the middle generation caused substantial inflation of the Type I error rate, which corresponded to the increasing proportion of persons with missing data. Results also showed that long high-LD blocks have severe effects on Type I error rates.

Ignoring intermarker linkage disequilibrium induces false-positive evidence of linkage for consanguineous pedigrees when genotype data is missing for any pedigree member

Missing genotype data can increase false-positive evidence for linkage when either parametric or nonparametric analysis is carried out ignoring intermarker linkage disequilibrium (LD). Previously it was demonstrated by Huang et al. [1] that no bias occurs in this situation for affected sib-pairs with unrelated parents when either both parents are genotyped or genotype data is available for two additional unaffected siblings when parental genotypes are missing. However, this is not the case for autosomal recessive consanguineous pedigrees, where missing genotype data for any pedigree member within a consanguinity loop can increase false-positive evidence of linkage. False-positive evidence for linkage is further increased when cryptic consanguinity is present. The amount of false-positive evidence for linkage, and which family members aid in its reduction, is highly dependent on which family members are genotyped. When parental genotype data is available, the false-positive evidence for linkage is usually not as strong as when parental genotype data is unavailable. For a pedigree with an affected proband whose first-cousin parents have been genotyped, further reduction in the false-positive evidence of linkage can be obtained by including genotype data from additional affected siblings of the proband or genotype data from the proband's sibling-grandparents. For the situation, when parental genotypes are unavailable, false-positive evidence for linkage can be reduced by including genotype data from either unaffected siblings of the proband or the proband's married-in-grandparents in the analysis.

A second-generation combined linkage physical map of the human genome

We have completed a second-generation linkage map that incorporates sequence-based positional information. This new map, the Rutgers Map v.2, includes 28,121 polymorphic markers with physical positions corroborated by recombination-based data. Sex-averaged and sex-specific linkage map distances, along with confidence intervals, have been estimated for all map intervals. In addition, a regression-based smoothed map is provided that facilitates interpolation of positions of unmapped markers on this map. With nearly twice as many markers as our first-generation map, the Rutgers Map continues to be a unique and comprehensive resource for obtaining genetic map information for large sets of polymorphic markers.

Genomewide high-density SNP linkage analysis of non-BRCA1/2 breast cancer families identifies various candidate regions and has greater power than microsatellite studies

Background

The recent development of new high-throughput technologies for SNP genotyping has opened the possibility of taking a genome-wide linkage approach to the search for new candidate genes involved in heredity diseases. The two major breast cancer susceptibility genes BRCA1 and BRCA2 are involved in 30% of hereditary breast cancer cases, but the discovery of additional breast cancer predisposition genes for the non-BRCA1/2 breast cancer families has so far been unsuccessful.

Results

In order to evaluate the power improvement provided by using SNP markers in a real situation, we have performed a whole genome screen of 19 non-BRCA1/2 breast cancer families using 4720 genomewide SNPs with Illumina technology (Illumina's Linkage III Panel), with an average distance of 615 Kb/SNP. We identified six regions on chromosomes 2, 3, 4, 7, 11 and 14 as candidates to contain genes involved in breast cancer susceptibility, and additional fine mapping genotyping using microsatellite markers around linkage peaks confirmed five of them, excluding the region on chromosome 3. These results were consistent in analyses that excluded SNPs in high linkage disequilibrium. The results were compared with those obtained previously using a 10 cM microsatellite scan (STR-GWS) and we found lower or not significant linkage signals with STR-GWS data compared to SNP data in all cases.

Conclusion

Our results show the power increase that SNPs can supply in linkage studies.

Efficacy assessment of SNP sets for genome-wide disease association studies

The power of a genome-wide disease association study depends critically upon the properties of the marker set used, particularly the number and physical spacing of markers, and the level of inter-marker association due to linkage disequilibrium. Extending our previously devised theoretical framework for the entropy-based selection of genetic markers, we have developed a local measure of the efficacy of a marker set, relative to including a maximally polymorphic single nucleotide polymorphism (SNP) at the map position of interest. Using this quantitative criterion, we evaluated five currently available SNP sets, namely Affymetrix 100K and 500K, and Illumina 100K, 300K and 550K in the CEU, YRI and JPT + CHB HapMap populations. At 50% relative efficacy, the commercial marker sets cover between 19 and 68% of the human genome, depending upon the population under study. An optimal technology-independent 500K marker set constructed from HapMap for Caucasians, in contrast, would achieve 73% coverage at the same relative efficacy.

Combining evidence of natural selection with association analysis increases power to detect malaria-resistance variants

Statistical power to detect disease variants can be increased by weighting candidates by their evidence of natural selection. To demonstrate that this theoretical idea works in practice, we performed an association study of 10 putative resistance variants in 471 severe malaria cases and 474 controls from the Luo in Kenya. We replicated associations at HBB (P=.0008) and CD36 (P=.03) but also showed that the same variants are unusually differentiated in frequency between the Luo and Yoruba (who historically have been exposed to malaria) and the Masai and Kikuyu (who have not been exposed). This empirically demonstrates that combining association analysis with evidence of natural selection can increase power to detect risk variants by orders of magnitude--up to P=.000018 for HBB and P=.00043 for CD36.

Assessment of the frequency of allelic imbalance in human tissue using a multiplex polymerase chain reaction system

Genomic instability can generate chromosome breakage and fusion randomly throughout the genome, frequently resulting in allelic imbalance, a deviation from the normal 1:1 ratio of maternal and paternal alleles. Allelic imbalance reflects the karyotypic complexity of the cancer genome. Therefore, it is reasonable to speculate that tissues with more sites of allelic imbalance have a greater likelihood of having disruption of any of the numerous critical genes that cause a cancerous phenotype and thus may have diagnostic or prognostic significance. For this reason, it is desirable to develop a robust method to assess the frequency of allelic imbalance in any tissue. To address this need, we designed an economical and high-throughput method, based on the Applied Biosystems AmpFlSTR Identifiler multiplex polymerase chain reaction system, to evaluate allelic imbalance at 16 unlinked, microsatellite loci located throughout the genome. This method provides a quantitative comparison of the extent of allelic imbalance between samples that can be applied to a variety of frozen and archival tissues. The method does not require matched normal tissue, requires little DNA (the equivalent of approximately 150 cells) and uses commercially available reagents, instrumentation, and analysis software. Greater than 99% of tissue specimens with >or=2 unbalanced loci were cancerous.

Parallel single nucleotide polymorphism genotyping by surface invasive cleavage with universal detection

Large-scale investigations of sequence variation within the human species will provide information about the basis of heritable variation in disease susceptibility and human migration. The surface invader assay (an adaptation of the invasive cleavage reaction to an array format) is capable of exquisitely sensitive and specific detection of genetic variation. It is shown here that this genotyping technology can be multiplexed in a DNA array format, permitting the parallel analysis of a panel of single nucleotide polymorphisms (SNPs) directly from an unamplified genomic DNA target. In addition, a "universal" mode of detection was developed that makes use of a mixture of degenerate templates for DNA ligation to the surface-bound cleaved oligonucleotides and thereby makes this strategy amenable to any desired SNP site or combination of SNP sites, without regard to their particular DNA sequences. This approach was demonstrated on a proof-of-principle scale using small DNA arrays to genotype 6 SNP markers in the PTPN1 gene and 10 mutations in the cystic fibrosis transmembrane conductance regulator gene. This ability to analyze many different genetic variations in parallel, directly from unamplified human genomic DNA samples, lays the groundwork for the development of high-density arrays able to analyze hundreds of thousands or even millions of SNPs.

Accounting for linkage disequilibrium among markers in linkage analysis: impact of haplotype frequency estimation and molecular haplotypes for a gene in a candidate region for Alzheimer's disease

OBJECTIVES

Linkage disequilibrium (LD) between closely spaced SNPs can be accommodated in linkage analysis by specifying the multi-SNP haplotype frequencies, if known. Phased haplotypes in candidate regions can provide gold standard haplotype frequency estimates, and may be of inherent interest as markers. We evaluated the effects of different methods of haplotype frequency estimation, and the use of marker phase information, on linkage analysis of a multi-SNP cluster in a candidate region for Alzheimer's disease (AD).

METHODS

We performed parametric linkage analysis of a five-SNP cluster in extended pedigrees to compare the use of: (1) haplotype frequencies estimated by molecular phase determination, maximum likelihood estimation, or by assuming linkage equilibrium (LE); (2) AD families or controls as the frequency source; and (3) unphased or molecularly phased SNP data.

RESULTS

There was moderate to strong pairwise LD among the five SNPs. Falsely assuming LE substantially inflated the LOD score, but the method of haplotype frequency estimation and particular sample used made little difference provided that LD was accommodated. Use of phased haplotypes produced a modest increase in the LOD score over unphased SNPs.

CONCLUSIONS

Ignoring LD between markers can lead to substantially inflated evidence for linkage in LOD score analysis of extended pedigrees with missing data. Use of marker phase information in linkage analysis may be important in disease studies where the costs of family recruitment and phenotyping greatly exceed the costs of phase determination.

A case of true hermaphroditism reveals an unusual mechanism of twinning

Traditionally twins are classified as dizygous or fraternal and monozygous or identical (Hall Twinning, 362, 2003 and 735-743). We report a rare case of 46,XX/46,XY twins: Twin A presented with ambiguous genitalia and Twin B was a phenotypically normal male. These twins demonstrate a third, previously unreported mechanism for twinning. The twins underwent initial investigation with 17-hydroxyprogesterone and testosterone levels, pelvic ultrasound and diagnostic laparoscopy. Cytogenetic analysis was performed on peripheral blood cells and skin fibroblasts. Histological examination and Fluorescence in situ hybridization studies on touch imprints were performed on gonadal biopsies. DNA analysis using more than 6,000 DNA markers was performed on skin fibroblast samples from the twins and on peripheral blood samples from both parents. Twin A was determined to be a true hermaphrodite and Twin B an apparently normal male. Both twins had a 46,XX/46,XY chromosome complement in peripheral lymphocytes, skin fibroblasts, and gonadal biopsies. The proportion of XX to XY cells varied between the twins and the tissues evaluated. Most significantly the twins shared 100% of maternal alleles and approximately 50% of paternal alleles in DNA analysis of skin fibroblasts. The twins are chimeric and share a single genetic contribution from their mother but have two genetic contributions from their father thus supporting the existence of a third, previously unreported type of twinning.

Multipoint linkage analysis with many multiallelic or dense diallelic markers: Markov chain-Monte Carlo provides practical approaches for genome scans on general pedigrees

Computations for genome scans need to adapt to the increasing use of dense diallelic markers as well as of full-chromosome multipoint linkage analysis with either diallelic or multiallelic markers. Whereas suitable exact-computation tools are available for use with small pedigrees, equivalent exact computation for larger pedigrees remains infeasible. Markov chain-Monte Carlo (MCMC)-based methods currently provide the only computationally practical option. To date, no systematic comparison of the performance of MCMC-based programs is available, nor have these programs been systematically evaluated for use with dense diallelic markers. Using simulated data, we evaluate the performance of two MCMC-based linkage-analysis programs--lm_markers from the MORGAN package and SimWalk2--under a variety of analysis conditions. Pedigrees consisted of 14, 52, or 98 individuals in 3, 5, or 6 generations, respectively, with increasing amounts of missing data in larger pedigrees. One hundred replicates of markers and trait data were simulated on a 100-cM chromosome, with up to 10 multiallelic and up to 200 diallelic markers used simultaneously for computation of multipoint LOD scores. Exact computation was available for comparison in most situations, and comparison with a perfectly informative marker or interprogram comparison was available in the remaining situations. Our results confirm the accuracy of both programs in multipoint analysis with multiallelic markers on pedigrees of varied sizes and missing-data patterns, but there are some computational differences. In contrast, for large numbers of dense diallelic markers, only the lm_markers program was able to provide accurate results within a computationally practical time. Thus, programs in the MORGAN package are the first available to provide a computationally practical option for accurate linkage analyses in genome scans with both large numbers of diallelic markers and large pedigrees.

Application of genetic approaches to ocular disease

The human eye is a complex organ whose development requires extraordinary coordination of developmental processes. Multiple genes responsible for the proper development and maintenance of the vertebrate eye have been identified and shown to be involved in a variety of debilitating ocular conditions. Genetic diseases involving the eye represent a leading cause of blindness in children and adults. This article summarizes current genetic approaches and their application to studies of ocular disease.

High-density SNP analysis of 642 Caucasian families with rheumatoid arthritis identifies two new linkage regions on 11p12 and 2q33

We have completed a genome wide linkage scan using >5700 informative single-nucleotide polymorphism (SNP) markers (Illumina IV SNP linkage panel) in 642 Caucasian families containing affected sibling pairs with rheumatoid arthritis (RA), ascertained by the North American Rheumatoid Arthritis Consortium. The results show striking new evidence of linkage at chromosomes 2q33 and 11p12 with logarithm of odds (LOD) scores of 3.52 and 3.09, respectively. In addition to a strong and broad linkage interval surrounding the major histocompatibility complex (LOD>16), regions with LOD>2.5 were observed on chromosomes 5 and 10. Additional linkage evidence (LOD scores between 1.46 and 2.35) was also observed on chromosomes 4, 7, 12, 16 and 18. This new evidence for multiple regions of genetic linkage is partly explained by the significantly increased information content of the Illumina IV SNP linkage panel (75.6%) compared with a standard microsatellite linkage panel utilized previously (mean 52.6%). Stratified analyses according to whether or not the sibling pair members showed elevated anticyclic citrullinated peptide titers indicates significant variation in evidence for linkage among strata on chromosomes 4, 5, 6 and 7. Overall, these new linkage data should reinvigorate efforts to utilize positional information to identify susceptibility genes for RA.

Using high-throughput SNP technologies to study cancer

Identifying genes involved in the development of cancer is crucial to fully understanding cancer biology, for developing novel therapeutics for cancer treatment and for providing methods for cancer prevention and early diagnosis. The use of polymorphic markers, in particular single nucleotide polymorphisms (SNPs), promises to provide a comprehensive tool for analysing the human genome and identifying those genes and genomic regions contributing to the cancer phenotype. This review summarizes the various analytical methodologies in which SNPs are used and presents examples of how each of these methodologies have been used to locate genes and genomic regions of interest for various cancer types. Additionally many of the current SNP-analysing technologies will be reviewed with particular attention paid to the advantages and disadvantages of each and how each technology can be applied to the analysis of the genome for identifying cancer-related genes.

High-throughput SNP genotyping by single-tube PCR with Tm-shift primers

Despite many recent advances in high-throughput single nucleotide polymorphism (SNP) genotyping technologies, there is still a great need for inexpensive and flexible methods with a reasonable throughput. Here we report substantial modifications and improvements to an existing homogenous allele-specific PCR-based SNP genotyping method, making it an attractive new option for researchers engaging in candidate gene studies or following up on genome-wide scans. In this advanced version of the melting temperature (Tm)-shift SNP genotyping method, we attach two GC-rich tails of different lengths to allele-specific PCR primers, such that SNP alleles in genomic DNA samples can be discriminated by the Tms of the PCR products. We have validated 306 SNP assays using this method and achieved a success rate in assay development of greater than 83% under uniform PCR conditions. We have developed a standalone software application to automatically assign genotypes directly from melting curve data. To demonstrate the accuracy of this method, we typed 592 individuals for 6 SNPs and showed a high call rate (>98%) and high accuracy (>99.9%). With this method, 6-10,000 samples can be genotyped per day using a single 384-well real-time thermal cycler with 2-4 standard 384-well PCR instruments.

Reliable high-throughput genotyping and loss-of-heterozygosity detection in formalin-fixed, paraffin-embedded tumors using single nucleotide polymorphism arrays

Most human cancers show genetic instabilities leading to allelic imbalances, including loss of heterozygosity (LOH). Single nucleotide polymorphism (SNP) arrays can be used to detect LOH. Currently, these arrays require intact genomic DNA as obtained from frozen tissue; however, for most cancer cases, only low-quality DNA from formalin-fixed, paraffin-embedded (FFPE) tissue is available. In this study, we tested Illumina BeadArrays to genotype FFPE tissue and detect LOH/allelic imbalances in matched colorectal tumor and normal tissue. Genotypes were compared between leukocyte and FFPE normal tissue as well as between frozen and FFPE tumor tissue. Identical genotypes and LOH profiles were obtained from normal and tumor isolates. LOH was mainly observed on chromosomes 4, 5q, 12q, 14q, 15q, 17p, 18, and 20p, which are commonly detected regions in colorectal cancer. LOH profiles of the BeadArrays were compared with profiles obtained by Affymetrix GeneChip 10K arrays, showing identical LOH patterns. These data show that genome-wide genotyping of FFPE tissue with the BeadArray gives reliable results and is a powerful technique for LOH analysis.

Description of the data from the Collaborative Study on the Genetics of Alcoholism (COGA) and single-nucleotide polymorphism genotyping for Genetic Analysis Workshop 14

The data provided to the Genetic Analysis Workshop 14 (GAW 14) was the result of a collaboration among several different groups, catalyzed by Elizabeth Pugh from The Center for Inherited Disease Research (CIDR) and the organizers of GAW 14, Jean MacCluer and Laura Almasy. The DNA, phenotypic characterization, and microsatellite genomic survey were provided by the Collaborative Study on the Genetics of Alcoholism (COGA), a nine-site national collaboration funded by the National Institute of Alcohol and Alcoholism (NIAAA) and the National Institute of Drug Abuse (NIDA) with the overarching goal of identifying and characterizing genes that affect the susceptibility to develop alcohol dependence and related phenotypes. CIDR, Affymetrix, and Illumina provided single-nucleotide polymorphism genotyping of a large subset of the COGA subjects. This article briefly describes the dataset that was provided.