Founder TIGR/myocilin mutations for glaucoma in the Québec population

Glaucoma phenotype in a large Swiss pedigree with the myocilin Gly367Arg mutation

AIMS

To characterize genotype, phenotype, and age-related penetrance in a Swiss pedigree with juvenile open-angle glaucoma (JOAG).

METHODS

In a large Swiss family with history of glaucoma and 82 living members of four generations, we conducted molecular analysis and a detailed phenotype characterization in 52 family members. Mutation analysis was carried out using single-strand conformation polymorphism and DNA sequence analyses of the suspected candidate gene, myocilin (MYOC).

RESULTS

We detected a Gly367Arg mutation in the MYOC gene of 13 family members. Nine of them (69.2%) had glaucoma: mean IOP 35.3 mm Hg, range 24-50 mm Hg; mean age at diagnosis 34.9 years, range 28-51 years. Two mutation carriers were glaucoma suspects, one (age 15) was unaffected, and one (age 16) not available for clinical examinations. Age-related glaucoma penetrance was 50% at 30 and 78% at 40. Untreated IOP resulted in rapid disease progression, whereas good IOP control, usually only by means of filtration surgery, could stabilize the disease. None of the wild-type members had glaucoma.

CONCLUSIONS

This Swiss family is the largest reported Gly367Arg pedigree to date. The exact genotype and phenotype characterization allowed a reliable risk and prognosis assessment and targeted eye-care planning for the family. The study demonstrates the importance of genetic investigations in glaucoma families, carrying the potential of long-term socio-economic benefits.

Complex genetics of complex traits: the case of primary open-angle glaucoma

Glaucoma, which is a complex heterogeneous disease, presents an ideal case for genetic investigation. Primary open-angle glaucoma (POAG) is the commonest subtype and will be the focus of this review. When detected early, POAG is amenable to therapeutic intervention. Unfortunately, current population-based clinical screening lacks efficacy. If individuals with a genetic predisposition for developing POAG can be identified, then efficient and cost-effective population-based screening programs could be designed. Although considerable inroads have been made in understanding the natural history of POAG caused by mutations in the myocilin and optineurin genes, other POAG genes accounting for most cases remain to be identified. This review explores the genetic mechanisms that have been unequivocally linked to the glaucomatous process and then discusses potential avenues for future breakthroughs.

Functional analysis of the glaucoma-causing TIGR/myocilin protein: Integrity of amino-terminal coiled-coil regions and olfactomedin homology domain is essential for extracellular adhesion and secretion

TIGR/MYOC mutations account for 2-4% of the primary open-angle glaucoma (POAG) patients. More than 90% of the known mutations are located within its carboxy-terminus olfactomedin-homology (Olf) domain (amino acids (aa) 245-504). In vitro and in vivo studies showed that several Olf domain mutations prevented myocilin secretion. To investigate if intracellular sequestration was a characteristic feature shared by a majority of the mutations, we analyzed the secretion status of 36 myocilin variants. These encompassed 26 glaucoma-causing mutations and 10 non-disease associated or undefined polymorphisms. As several variants were found to be secreted, we tested for their adhesion to the extracellular matrix (ECM) and/or cell surface. Myocilin variants were generated by site-directed mutagenesis of a vector encoding the human MYOC cDNA. COS-7 or immortalized human trabecular meshwork cells were transfected with wild-type or mutated MYOC constructs. Myocilin levels were estimated by immunoprecipitation and/or immunoblotting. All variants showed identical behaviors in both cell lines; the truncated R46X polypeptide being the only variant which could not be detected in our assays. Of the 35 variants monitored, 20 remained sequestered intracellularly. All of them encoded disease-causing polypeptides carrying Olf domain mutations. Of the 15 variants secreted into the culture medium, six (6) were POAG mutants (of which three (3) located within the Olf domain) while the remaining nine (9) were non-disease causing or undefined polymorphisms. Three (3) of the six (6) secreted mutations caused familial POAG; these were the R126W, T377M and A427T mutants. Both, the T377M and A427T mutants located within the Olf domain. When cells were cultured at 30 degrees C, a process known to facilitate protein folding, 11 of the 20 sequestered mutants were released in the extracellular medium. Out of the 15 secreted variants tested for their adhesion to the ECM and/or cell surface, only the R82C and L95P polypeptides displayed loss of their adhesive properties. Deletion experiments revealed that the coiled-coil (aa 78-105) and leucine zipper (aa 114-183) motifs were essential for adhesion. These experiments demonstrate that intracellular sequestration might be the primary mechanism contributing to myocilin-related POAG as it was associated with more than 80% of the disease-causing mutants tested in our study. A second mechanism may involve abnormal interaction(s) between myocilin and ECM and/or cell surface proteins. Our data further revealed the importance of the olfactomedin-homology domain for myocilin secretion and the significant role of the N-terminal region for its extracellular interactions.

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.

Gene mapping for primary open angle glaucoma

Primary open angle glaucoma (POAG) is a leading cause of visual impairment and blindness worldwide. To date, at least 20 genetic loci for POAG have been reported. Only 3 causative genes are identified from these loci: myocilin (MYOC), optineurin (OPTN) and WD repeat domain 36 (WDR36), which together account for less than 10% of POAG. Only a portion of POAG follows Mendelian inheritance, and a considerable fraction results from a large number of variants in several genes, each contributing small effects. Over the past 10 years, there has been vigorous research on mapping the POAG genes. The main technological approaches are functional cloning, family linkage analysis, genome-wide scan, case-control association study, and microarray analysis. Association studies found 16 genes related to POAG, but reports on glaucoma-causing effects of these genes are conflicting. Ten microarray gene expression studies related to POAG have been published. A number of genes potentially related to POAG have been identified, and they provide a good resource to select candidate genes for mutation analysis in association studies. While linkage studies remain a mainstay, the current trend is to use genome-wide association studies to map genes for POAG. This review gives an overview of the efforts in the past decade to identify the POAG genes through linkage studies, genome-wide scans, case-control association studies and microarray studies. In the near future such comprehensive studies are expected to greatly advance our understanding of the genetic basis of POAG and provide information for effective glaucoma therapy.

A common disease haplotype for the Q368STOP mutation of the myocilin gene in Australian and Canadian glaucoma families

PURPOSE

To ascertain whether there is a common disease haplotype for the Q368STOP mutation of the myocilin gene in Australian and Canadian families with primary open-angle glaucoma (POAG).

DESIGN

Family pedigree study.

METHODS

A disease haplotype for the Q368STOP mutation of the myocilin gene has previously been identified in 15 Tasmanian families with POAG. The four microsatellite markers that constitute this 0.14-megabase (Mb) disease haplotype were genotyped in individuals from a large French Canadian family with POAG (family CT) and two unrelated French Canadian individuals with ocular hypertension.

RESULTS

The Tasmanian Q368STOP disease haplotype was identified in affected individuals from family CT, and the same alleles were shared at the four microsatellite markers in the two unrelated French Canadian individuals.

CONCLUSION

The same disease haplotype for the Q368STOP mutation of the myocilin gene was found in both the Tasmanian and French Canadian populations, supporting the view that this mutation arose from a common Caucasian founder.

Population history and its impact on medical genetics in Quebec

Knowledge of the genetic demography of Quebec is useful for gene mapping, diagnosis, treatment, community genetics and public health. The French-Canadian population of Quebec, currently about 6 million people, descends from about 8500 French settlers who arrived in Nouvelle-France between 1608 and 1759. The migrations of those settlers and their descendants led to a series of regional founder effects, reflected in the geographical distribution of genetic diseases in Quebec. This review describes elements of population history and clinical genetics pertinent to the treatment of French Canadians and other population groups from Quebec and summarizes the cardinal features of over 30 conditions reported in French Canadians. Some were discovered in French Canadians, such as autosomal recessive ataxia of the Charlevoix-Saguenay (MIM 270550), agenesis of corpus callosum and peripheral neuropathy (MIM 218000) and French-Canadian-type Leigh syndrome (MIM 220111). Other conditions are particularly frequent or have special genetic characteristics in French Canadians, including oculopharyngeal muscular dystrophy, hepatorenal tyrosinaemia, cystic fibrosis, Leber hereditary optic neuropathy and familial hypercholesterolaemia. Three genetic diseases of Quebec First Nations children are also discussed: Cree encephalitis (MIM 608505), Cree leukoencephalopathy (MIM 603896) and North American Indian childhood cirrhosis (MIM 604901).

Mutations including the promoter region of myocilin/TIGR gene

Mutations in the MYOC/TIGR gene are responsible for autosomal dominant primary open angle glaucoma (POAG). Almost all mutations responsible for POAG have been detected in the coding region (in particular at exon 3). By using the techniques of PCR, SSCP, automated sequencing and restriction analysis, we have studied 79 patients suffering from glaucoma. We have found five patients with sequence variants in the consensus region of the promoter. These sequence variants might be involved in the altered association between the consensus region and the corresponding transcription factor. This possibility might be favouring the association of other transcription factors, which would operate as activators or inhibitors of the transcription, altering the MYOC/TIGR expression.

Novel MYOC gene mutation, Phe369Leu, in Japanese patients with primary open-angle glaucoma detected by denaturing high-performance liquid chromatography

PURPOSE

To screen for mutations in the MYOC gene in Japanese patients with primary open-angle glaucoma (POAG) using denaturing high-performance liquid chromatography (DHPLC).

PATIENTS AND METHODS

Blood samples were collected from 171 patients with POAG and 100 controls from seven institutions in Japan. For high-throughput analysis, seven exonic regions were amplified by polymerase chain reaction using DNA pooled from three patients; each DNA pool was then analyzed chromatographically. For analysis of a small number of samples, 7 exonic regions were amplified separately but simultaneously with annealing at 58 degrees C in each patient and then chromatographed, using 7 wells of the same 96-well plate per sample. When chromatographic patterns were abnormal by either method, the PCR products of the individual samples were sequenced.

RESULTS

Four glaucoma-causing mutations were identified in five POAG patients (2.9%). One missense mutation, Phe369Leu, is new; and three others, Ile360Asn, Ala363Thr, and Thr448Pro, have been reported in Japanese patients. Phe369Leu was associated with adult onset POAG.

CONCLUSIONS

Mutations in the MYOC gene were demonstrated chromatographically in 2.9% of our Japanese POAG patients. The use of pooled DNAs with DHPLC analysis is a time- and labor-saving technique. All mutations detected appear to be specific to Japanese patients.

Low prevalence of MYOC mutations in UK primary open-angle glaucoma patients limits the utility of genetic testing

Primary open angle glaucoma (POAG) affects 1% of people over age 40. Early detection and treatment can prevent blindness, but the disease is often asymptomatic until a late stage. Positive family history is an important risk factor and previous studies indicate that approximately 5% of POAG results from mutations in the myocilin ( MYOC) gene, raising the possibility of identifying individuals genetically predisposed to glaucoma. We collected DNA samples from 426 unselected UK POAG patients and analyzed them for MYOC mutations. The Q368X mutation was found in six patients (1.4%). No other mutations were identified, suggesting that amongst patients unselected for family history, the prevalence of MYOC mutations in the UK is lower than in other populations. Genetic and glaucoma screening was offered to first-degree relatives of these six probands (group 1) and of age/sex-matched mutation-negative controls (group 2). Of 11 group-1 relatives, three carried Q368X, one of whom already had glaucoma. Notably, of the 13 relatives in both groups who were mutation negative, one was already being treated for ocular hypertension. We therefore caution against changing glaucoma surveillance regimens in such individuals and suggest that routine untargeted genetic testing for MYOC mutations in patients with POAG would be of limited value until additional significant genetic risk factors are identified.

Evaluation of optineurin sequence variations in 1,048 patients with open-angle glaucoma

PURPOSE

To investigate the association of sequence variations in the optineurin (OPTN) gene in patients with open-angle glaucoma.

DESIGN

Prospective case control study.

METHODS

The OPTN gene was screened for sequence variations using a combination of single-strand conformational polymorphism analysis and automated DNA sequencing. A total of 1,299 subjects (1048 glaucoma patients and 251 controls) were screened for variations in the four portions of the gene that had been previously associated with glaucoma. A subset of these subjects (376 patients and 176 controls) was screened for variations in the entire coding sequence. Twenty-four percent of the patients and 35% of the controls were Japanese, whereas the remainder were predominantly Caucasian. Allele frequencies were compared with the Fisher exact test.

RESULTS

The OPTN sequence variations were not significantly associated with any form of high-tension open-angle glaucoma. One proband with familial normal-tension glaucoma was found to harbor the previously reported Glu50Lys variation. Another previously reported change, Met98Lys, was associated with normal-tension glaucoma in Japanese but not in Caucasian patients.

CONCLUSIONS

This study provides some additional evidence for the association of the Glu50Lys OPTN sequence variation with familial normal tension glaucoma. However, because familial normal-tension glaucoma is so rare, this change seems to be responsible for less than 0.1% of all open-angle glaucoma. The Arg545Gln variation is likely to be a nondisease-causing polymorphism. The Met98Lys change may be associated with a fraction of normal-tension glaucoma in patients of Japanese ethnicity.

Novel functional polymorphisms in the UGT1A7 and UGT1A9 glucuronidating enzymes in Caucasian and African-American subjects and their impact on the metabolism of 7-ethyl-10-hydroxycamptothecin and flavopiridol anticancer drugs

In vitro metabolic studies revealed that along with UDP-glucuronosyltransferase (UGT) 1A1, the hepatic UGT1A9 and the extrahepatic UGT1A7 are involved in the biotransformation of the active and toxic metabolite of irinotecan, 7-ethyl-10-hydroxycamptothecin (SN-38). Variant UGT1A1 and UGT1A7 alleles have been reported but the polymorphic nature of the UGT1A9 gene has not been revealed yet. To further clarify the molecular determinants of irinotecan-induced toxicity, we have identified and characterized the functionality of novel UGT1A9 polymorphisms and determined whether additional missense polymorphisms exist in UGT1A7. Using direct DNA sequencing, four single nucleotide polymorphisms (SNPs) were identified in the first exons of UGT1A7 and UGT1A9. One of the two amino acid substitutions found in the UGT1A9 gene, UGT1A9*3 (M33T), results in a dramatic decrease in SN-38 glucuronide formation, with 3.8% of the activity of the UGT1A9*1 allele. In turn, the glucuronidation of flavopiridol, an anticancer drug biotransformed predominantly by UGT1A9, remains unaffected, indicating a substrate-dependent impact of this variant. UGT1A9*3 is detected only in Caucasians and 4.4% of the population tested was found heterozygous (*1/*3). Two additional UGT1A7 SNPs were found exclusively in African-American subjects and generate five alleles (UGT1A7*5 to *9) when combined to the four known SNPs present in UGT1A7*2, *3, and *4. Upon functional analysis with SN-38, five out of nine UGT1A7 allozymes exhibited much lower SN-38 glucuronidation activities compared with UGT1A7*1, all having in common the mutational changes at codons 115 or 208. Results suggest that these low SN-38 glucuronidating alleles may represent additional molecular determinants of irinotecan-induced toxicity and warrant further investigations.