High-throughput conformation-sensitive gel electrophoresis for discovery of SNPs

Conformation-Sensitive Gel Electrophoresis as an Ideal High-Throughput Strategy for Accurate Detection of Sequence Variations in DNA: Screening hTomm and hTimm Genes

Conformation-sensitive gel electrophoresis is a heteroduplex-based method that is particularly well suited to high-throughput analyses. Its simplicity makes it amenable to various adaptations and modifications to enhance its applicability to genome-wide mutation scans. Technical aspects that markedly improve the conformation-sensitive gel electrophoresis performance by combining high throughput and high resolution of the bands facilitating the interpretation of the results are described here. The authors report some of the results they have obtained in the screening of the exon 1 of human Timm8A gene as an example of the suitability of the conformation-sensitive gel electrophoresis–based protocol that has been adapted to optimize its throughput, speed, and simplicity in the recognition of both heterozygous and homozygous DNA mutations. The higher throughput is achieved by using 12 batches per gel. The length of the gel is sufficient for an adequate well-to-read distance for each batch that allows a clear distinction and resolution of the conformation-sensitive gel electrophoresis bands. Standardization of the procedure using multichannel pipettes reduces the preparation time of the 96-well PCRs to 10 min and also accelerates the gel loading. The resulting bands give high-quality images, allowing easy detection of known as well as novel mutations.

The phenotypic distribution of quantitative traits in a wild mouse F1 population

The human complex diseases such as hypertension, precocious puberty, and diabetes have their own diagnostic thresholds, which are usually estimated from the epidemiological data of nature populations. In the mouse models, numerous phenotypic data of complex traits have been accumulated; however, knowledge of the phenotypic distribution of the natural mouse populations remains quite limited. In order to investigate the distribution of quantitative traits of wild mice, 170 F1 progeny aged 8-10 weeks and derived from wild mice collected from eight spots in the suburbs of Shanghai were tested for their values of anatomic, blood chemical, and blood hematological parameters. All the wild mice breeders were of Mus. m. musculus and Mus. m. castaneus maternal origin according to the single nucleotide polymorphism (SNP) markers of the mitochondrial DNA. The results showed that phenotypes in wild mice had a normal distribution with four to six times the standard deviation. For the majority of the traits, the wild outbred mice and laboratory inbred mice have significantly different ranges and mean values, whereas the wild mice did not necessarily show more phenotypic diversity than the inbred ones. Our data also showed that natural populations may have some unique phenotypes related to sugar and protein metabolism, as the mean value of wild mice differ dramatically from the inbred mice in the levels of blood glucose, BUN (blood urea nitrogen), and total blood protein. The epidemiological information of the complex traits in the nature population from our study provided valuable reference for the application of mouse models in those complex disease studies.

Conformation sensitive gel electrophoresis

Conformation sensitive gel electrophoresis (CSGE) is a rapid screening method for the detection of DNA sequence variation, specifically single-base changes or small insertions and deletions. It has been widely used for mutation screening in genetic disorders and for the detection of single nucleotide polymorphisms (SNPs).CSGE is a simple manual method, based on heteroduplex analysis, and compares well in terms of sensitivity with other screening technologies. CSGE also lends itself to automation and such modi-fications have been useful in increasing sample throughput and sensitivity. However, manual CSGE remains a low-cost, accessible, and effective approach for mutation screening, which can be carried out with -minimal specialist equipment. This chapter describes manual CSGE, and outlines some of the uses, -modifications, and limitations of this method.

Analysis of the DND1 gene in men with sporadic and familial testicular germ cell tumors

A base substitution in the mouse Dnd1 gene resulting in a truncated Dnd protein has been shown to be responsible for germ cell loss and the development of testicular germ cell tumors (TGCT) in the 129 strain of mice. We investigated the human orthologue of this gene in 263 patients (165 with a family history of TGCT and 98 without) and found a rare heterozygous variant, p. Glu86Ala, in a single case. This variant was not present in control chromosomes (0/4,132). Analysis of the variant in an additional 842 index TGCT cases (269 with a family history of TGCT and 573 without) did not reveal any additional instances. The variant, p. Glu86Ala, is within a known functional domain of DND1 and is highly conserved through evolution. Although the variant may be a rare polymorphism, a change at such a highly conserved residue is characteristic of a disease-causing variant. Whether it is disease-causing or not, mutations in DND1 make, at most, a very small contribution to TGCT susceptibility in adults and adolescents.

Molecular diagnostics of genetic eye diseases

Eye diseases can be simple or complex, and mostly of heterogeneous molecular genetics. Some eye diseases are caused by mutations in a single gene, but some diseases, such as primary open angle glaucoma, can be due to sequence variations in multiple genes. In some diseases, both genetic and epigenetic mechanisms are involved, as was recently revealed in the mechanism of retinoblastoma. Disease causative mutations and phenotypes may vary by ethnicity and geography. To date, more than a hundred candidate genes for eye diseases are known, although less than 20 have definite disease-causing mutations. The three common genetic eye diseases, primary open angle glaucoma, age-related macular degeneration, and retinitis pigmentosa, all have known gene mutations, but these account for only a portion of the patients. While the search for eye disease genes and mutations still goes on, known mutations have been utilized for diagnosis. Genetic markers for pre-symptomatic and pre-natal diagnosis are available for specific diseases such as primary open angle glaucoma and retinoblastoma. This paper reviews the molecular basis of common genetic eye diseases and the available genetic markers for clinical diagnosis. Difficulties and challenges in molecular investigation of some eye diseases are discussed. Establishment of ethnic-specific disease databases that contain both clinical and genetic information for identification of genetic markers with diagnostic, prognostic, or pharmacological value is strongly advocated.

Polymorphisms in the Myocilin Promoter Unrelated to the Risk and Severity of Primary Open-Angle Glaucoma

PURPOSE

To investigate the proximal 2.5 kb promoter in the myocilin (MYOC) gene for mutations in Chinese patients with primary open-angle glaucoma (POAG).

PATIENTS AND METHODS

We screened for sequence alterations in the MYOC promoter in 88 unrelated Chinese patients with POAG and 94 unrelated individuals without glaucoma, aged 50 years or above, as control subjects. In addition, the specific MYOC.mt1 polymorphism was determined in a total of 212 POAG patients and 221 control subjects. The relationships between POAG phenotype and the identified polymorphisms were studied by univariate analysis, multivariable logistic regression analysis, and haplotype analysis.

RESULTS

All polymorphisms identified in this study followed Hardy-Weinberg equilibrium (P > 0.12) both in POAG patients and controls. Both univariate and multivariable logistic regression analyses showed no polymorphism that was significantly associated with the risk of POAG, P > 0.08 and P > 0.044 respectively. Haplotype analysis further indicated no association of MYOC promoter polymorphisms with the susceptibility for POAG (P > 0.1). On the other hand, there was no difference of POAG phenotypes among different genotypes of MYOC.mt1 (P > 0.31).

CONCLUSIONS

In this study on the Chinese population, polymorphisms in the MYOC promoter are not related to the risk of POAG. There is no association between the MYOC.mt1 promoter polymorphism with the severity of POAG.

Low-density lipoprotein receptor-related protein 8 (apolipoprotein E receptor 2) gene polymorphisms in Alzheimer's disease

Apolipoprotein E (ApoE) isoforms affect the risk of developing Alzheimer's disease (AD). ApoE-associated risk may be related to its binding to and clearance by cell surface receptors, such as the members of the low-density lipoprotein (LDL) receptor family. Previous studies had shown association of LDL receptor-related protein (LRP) and AD, therefore we speculated that another member of this LDL receptor family, LRP8 (also called apolipoprotein E receptor 2 or ApoER2), which is predominantly expressed in brain, might be associated with Alzheimer's disease. To explore this hypothesis, we screened exons 2-19 of the LRP8 gene in a total of 204 AD and 184 elderly control subjects for polymorphisms using the conformation-sensitive gel electrophoresis method. Our results revealed four sequence alterations: two predicted to result in amino acid changes (E46D and R952Q), one in an intron (IVS9 + 7G > A), and one synonymous polymorphism (2622T > C). The latter was found in four AD patients (2.0%) and 11 controls (6.0%), a significant difference (P = 0.042). Further study is needed to confirm this possible association of LRP8 with AD.

An update on conformation sensitive gel electrophoresis

Conformation-sensitive gel electrophoresis (CSGE) was developed as a method of heteroduplex analysis to screen large multi-exon genes for sequence variation. The novelty of the method was in the use of a non-proprietary acrylamide gel matrix that used 1,4-bis (acrolyl) piperazine (BAP) as a cross linker with ethylene glycol and formamide as mildly denaturing solvents. The denaturing environment enhances the conformation polymorphism present in DNA heteroduplexes containing variations as small as single nucleotide polymorphisms (SNPs). CSGE has also been adapted for use on a fluorescent platform (F-CSGE) that resulted in higher throughput and sensitivity. Variation in sensitivity of CSGE has been studied extensively. The results demonstrate that the nature of the mismatched base in a defined sequence context has the most profound effect on the conformation of the heteroduplex. Additionally, the size of the PCR product, as well as the location of the mismatch within the PCR product, are two important parameters that determine the resolution of the mismatch-containing heteroduplexes during CSGE. Like any other mutation scanning technique, CSGE can have limited resolution of two closely linked sequence variations. For specific genes, like BRCA1 and BRCA2 where multiple SNPs are present in the coding sequence, each CSGE shift has to be sequenced to define the exact nature of the sequence change. In conclusion, CSGE scanning provides a powerful, cost-efficient way to scan genes with high sensitivity and specificity.

Identification of a region of homozygous deletion on 8p22-23.1 in medulloblastoma

To identify critical tumor suppressor loci that are associated with the development of medulloblastoma, we performed a comprehensive genome-wide allelotype analysis in a series of 12 medulloblastomas. Non-random allelic imbalances were identified on chromosomes 7q (58.3%), 8p (66.7%), 16q (58.3%), 17p (58.3%) and 17q (66.7%). Comparative genomic hybridization analysis confirmed that allelic imbalances on 8p, 16q and 17p were due to loss of genetic materials. Finer deletion mapping in an expanded series of 23 medulloblastomas localized the common deletion region on 8p to an interval of 18.14 cM on 8p22-23.2. We then searched within the region of loss on 8p for loci that might contain homozygous deletion using comparative duplex PCR. An overlapping homozygous deletion region was identified in a 1.8 cM interval on 8p22-23.1, between markers D8S520 and D8S1130, in two medulloblastomas. This region of homozygous deletion also encompasses the 1.4 cM minimal deletion region detected on 8p in ductal carcinoma in situ of breast. In conclusion, we reported for the first time a detailed deletion mapping on 8p in medulloblastoma and have identified a region of homozygous deletion on 8p22-23.1 that is likely to contain a critical tumor suppressor gene involved in the development of medulloblastoma.

Molecular diagnostics for retinitis pigmentosa

BACKGROUND

At least 1 million people worldwide have retinitis pigmentosa (RP), making it relatively common among the inherited forms of blindness. Mutations in many genes may cause RP. The most common known mutation, Pro347Leu in rhodopsin, is found in no more than about 1% of unrelated patients, implying the impracticality of a diagnostic test which would screen only for a few, common mutation sites.

CONCLUSIONS

Ongoing discovery and study of RP genes makes it feasible to consider a molecular diagnostic test which would screen coding regions of all known RP genes by a mutation detection method such as conformation-sensitive gel electrophoresis followed by sequencing. The parallel development of RP genetic knowledge and treatments such as gene therapy will make such tests both possible and necessary.