Characterization of a 500 kb region on 17q25 and the exclusion of candidate genes as the familial Tylosis Oesophageal Cancer (TOC) locus

HN1 interacts with γ-tubulin to regulate centrosomes in advanced prostate cancer cells

Prostate cancer is one of the most common cancer for men worldwide with advanced forms showing supernumerary or clustered centrosomes. Hematological and neurological expressed 1 (HN1) also known as Jupiter Microtubule Associated Homolog 1 (JPT1) belongs to a small poorly understood family of genes that are evolutionarily conserved across vertebrate species. The co-expression network of HN1 from the TCGA PRAD dataset indicates the putative role of HN1 in centrosome-related processes in the context of prostate cancer. HN1 expression is low in normal RWPE-1 cells as compared to cancerous androgen-responsive LNCaP and androgen insensitive PC-3 cells. HN1 overexpression resulted in differential response for cell proliferation and cell cycle changes in RWPE-1, LNCaP, and PC-3 cells. Since HN1 overexpression increased the proliferation rate in PC-3 cells, these cells were used for functional characterization of HN1 in advanced prostate carcinogenesis. Furthermore, alterations in HN expression led to an increase in abnormal to normal nuclei ratio and increased chromosomal aberrations in PC-3 cells. We observed the co-localization of HN1 with γ-tubulin foci in prostate cancer cells, further validated by immunoprecipitation. HN1 was observed as physically associated with γ-tubulin and its depletion led to increased γ-tubulin foci and disruption in microtubule spindle assembly. Higher HN1 expression was correlated with prostate cancer as compared to normal tissues. The restoration of HN1 expression after silencing suggested that it has a role in centrosome clustering, implicating a potential role of HN1 in cell division as well as in prostate carcinogenesis warranting further studies.

Downregulation of ST6GALNAC1 is associated with esophageal squamous cell carcinoma development

Tylosis is an inherited disorder characterized by abnormal palmoplantar skin thickening and a highly elevated risk of esophageal squamous cell carcinoma (ESCC). Analyses of tylosis in families have localized the responsible gene locus to a region of chromosome 17q25.1. Frequent loss of heterozygosity (LOH) in 17q25.1 was also observed in the sporadic form of ESCC. A putative tumor suppressor gene for ESCC may exist at this locus. We investigated the expression patterns of genes on 17q25.1 in tumor and corresponding normal tissues from patients with sporadic ESCC using RNA sequence analysis. For candidate genes, quantitative real-time reverse transcription-PCR (qRT-PCR), direct sequence, LOH and methylation analyses were performed using 93 clinical ESCC samples and 10 cell lines. A significant downregulation of ST6GALNAC1 was demonstrated in ESCC tissues compared to its expression in normal tissues by qRT-PCR (n=93, p<0.0001). Frequent LOH (17/27, 62.9%) and hyper‑methylation in ST6GALNAC1 were also observed in all cell lines. Our results indicated that ST6GALNAC1 was downregulated in sporadic ESCC via hyper-methylation and LOH, and it may be a candidate responsible gene for ESCC. Furthermore, recent studies suggest that multiple genes on chromosome 17q25 are involved in ESCC development.

Chemoradiation in oesophageal cancer

Oesophageal cancer is the 8th most common cancer worldwide, and has significant mortality and morbidity rates. The two most common histological types, squamous cell carcinoma and adenocarcinoma, have different localizations, distinctive risk factors, and molecular mechanisms. Survival for patients with locoregional oesophageal cancer is poor when treated with surgery only, with 5-year survival less than 10-15%. Radiation therapy has limited efficacy when given alone. Concurrent chemoradiation improves local-regional control and facilitates margin-free resection when delivered preoperatively. Chemoradiation prolongs survival when given as definitive treatment or combined with surgery. Neoadjuvant chemoradiation also reduces risk of distant recurrence. To date, there is no data supporting the addition of targeted therapy to concurrent chemoradiation. Understanding molecular pathways regulating both radiosensitivity and tumorigenesis/invasion may lead to the discovery of new targeted agents, improving outcome of chemoradiation in terms of both locoregional and systemic control, ultimately resulting in prolonged survival.

MicroRNA-99a/100 promotes apoptosis by targeting mTOR in human esophageal squamous cell carcinoma

Recently, microRNA-99 family members, such as miR-99a/b and miR-100, have been reported to exhibit abnormal expression in various malignant tumors, but their functions in carcinomas are controversial. In this study, we focused on miR-99a and miR-100, which were determined to be universally downregulated in esophageal squamous cell carcinoma, and investigated their functions and potential mechanisms of action. The downregulation of miR-99a/100 was validated by qRT-PCR in 101 ESCC surgical tissue samples and in 3 ESCC cell lines. The overexpression of miR-99a and miR-100 via the transient transfection of the corresponding precursor molecules inhibited cell proliferation by inducing apoptosis in the ESCC cell lines. To investigate the molecular mechanism of miR-99a/100-induced apoptosis, luciferase reporter assays and Western blots were performed to demonstrate that the overexpression of miR-99a/100 suppressed the expression of mTOR by directly targeting its 3'UTR in a post-transcriptional manner. Clinically, the decreased expression of miR-99a/100 was associated with worse overall survival in ESCC patients. In conclusion, these results indicated that miR-99a and miR-100 inhibited cell proliferation by suppressing mTOR in ESCC cell lines, and therefore, the miR-99a/100-mTOR signaling pathway is a potential therapeutic target for inducing apoptosis to combat ESCC.

Protection from intracellular oxidative stress by cytoglobin in normal and cancerous oesophageal cells

Cytoglobin is an intracellular globin of unknown function that is expressed mostly in cells of a myofibroblast lineage. Possible functions of cytoglobin include buffering of intracellular oxygen and detoxification of reactive oxygen species. Previous work in our laboratory has demonstrated that cytoglobin affords protection from oxidant-induced DNA damage when over expressed in vitro, but the importance of this in more physiologically relevant models of disease is unknown. Cytoglobin is a candidate for the tylosis with oesophageal cancer gene, and its expression is strongly down-regulated in non-cancerous oesophageal biopsies from patients with TOC compared with normal biopsies. Therefore, oesophageal cells provide an ideal experimental model to test our hypothesis that downregulation of cytoglobin expression sensitises cells to the damaging effects of reactive oxygen species, particularly oxidative DNA damage, and that this could potentially contribute to the TOC phenotype. In the current study, we tested this hypothesis by manipulating cytoglobin expression in both normal and oesophageal cancer cell lines, which have normal physiological and no expression of cytoglobin respectively. Our results show that, in agreement with previous findings, over expression of cytoglobin in cancer cell lines afforded protection from chemically-induced oxidative stress but this was only observed at non-physiological concentrations of cytoglobin. In addition, down regulation of cytoglobin in normal oesophageal cells had no effect on their sensitivity to oxidative stress as assessed by a number of end points. We therefore conclude that normal physiological concentrations of cytoglobin do not offer cytoprotection from reactive oxygen species, at least in the current experimental model.

RHBDF2 mutations are associated with tylosis, a familial esophageal cancer syndrome

Tylosis esophageal cancer (TOC) is an autosomal-dominant syndrome characterized by palmoplantar keratoderma, oral precursor lesions, and a high lifetime risk of esophageal cancer. We have previously localized the TOC locus to a small genomic interval within chromosomal region 17q25. Using a targeted capture array and next-generation sequencing, we have now identified missense mutations (c.557T>C [p.Ile186Thr] and c.566C>T [p.Pro189Leu] in RHBDF2, which encodes the inactive rhomboid protease RHBDF2 (also known as iRhom2), as the underlying cause of TOC. We show that the distribution of RHBDF2 in tylotic skin is altered in comparison with that in normal skin, and immortalized tylotic keratinocytes have decreased levels of total epidermal growth factor receptor (EGFR) and display an increased proliferative and migratory potential relative to normal cells, even when normal cells are stimulated with exogenous epidermal growth factor. It would thus appear that EGFR signaling is dysregulated in tylotic cells. Furthermore, we also show an altered localization of RHBDF2 in both tylotic and sporadic squamous esophageal tumors. The elucidation of a role of RHBDF2 in growth-factor signaling in esophageal cancer will help to determine whether targeting this pathway in chemotherapy for this and other squamous cell carcinomas will be effective.

Septin9 is involved in septin filament formation and cellular stability

Septin9 (Sept9) is a member of the filament-forming septin family of structural proteins and is associated with a variety of cancers and with hereditary neuralgic amyotrophy. We have generated mice with constitutive and conditional Sept9 knockout alleles. Homozygous deletion of Sept9 results in embryonic lethality around day 10 of gestation whereas mice homozygous for the conditional allele develop normally. Here we report the consequences of homozygous loss of Sept9 in immortalized murine embryonic fibroblasts. Proliferation rate was not changed but cells without Sept9 had an altered morphology compared to normal cells, particularly under low serum stress. Abnormal, fragmented, and multiple nuclei were more frequent in cells without Sept9. Cell migration, as measured by gap-filling and filter-invasion assays, was impaired, but individual cells did not move less than wild-type cells. Sept9 knockout cells showed a reduced resistance to hypo-osmotic stress. Stress fiber and vinculin staining at focal adhesion points was less prominent. Long septin filaments stained for Sept7 disappeared. Instead, staining was found in short, often curved filaments and rings. Furthermore, Sept7 was no longer localized to the mitotic spindle. Together, these data reveal the importance of Sept9 for septin filament formation and general cell stability.

Survivin promoter -31G/C polymorphism in oral cancer cell lines

Survivin (SVV) is a protein that belongs to the inhibitor of apoptosis proteins (IAP) family and is involved in the G2/M phase progression of the cell cycle as a spindle‑associated molecule. The biological features of this protein are well documented and its activity appears to be involved in mitochondria-dependent and -independent antiapoptotic pathways. Overexpression of SVV at the transcriptional and translational level has been associated with cancer, a multifactorial disorder in which the occurrence of a -31G to C polymorphism in the promoter region may significantly contribute to the development of this pathology. To verify this hypothesis, the occurrence of a single nucleotide polymorphism (SNP) in cis-acting cell cycle-dependent elements (CDEs) and in cell cycle homology regions (CHRs) of the survivin TATA-less promoter was investigated. A total of 23 oral squamous cell carcinoma (OSCC) cell lines and normal epithelium-derived normal human epidermal keratinocyte (NHEK) cell lines were analyzed by RFLP and direct DNA sequencing of their promoter region. Furthermore, survivin expression at the transcriptional and translational levels was evaluated in these cells by RT-PCR and Western blotting, respectively. The findings indicate that the presence of a G or C allele is not directly correlated to survivin expression, at the mRNA or at the protein level, at least in the OSCC lines analyzed in this study.

Upper gastrointestinal cancer predisposition syndromes

Malignancies of the upper gastrointestinal tract form a heterogeneous group of cancers characterized by unique epidemiology and biology. Despite these differences, survival for advanced disease remains poor across the panel of diseases, from cancers of the esophagus, stomach, pancreas, and, until recently, even gastrointestinal stromal tumors. Genetic predisposition syndromes associated with these diseases comprise an emerging subset of these diseases that may provide valuable information on cause and etiology. They may provide insight into molecular drivers for the disease, or disease subtypes, and also insights into novel gene/environment interactions. This review summarizes the current understanding of genetic predisposition syndromes of cancers of the upper gastrointestinal tract.

Cytoglobin, the newest member of the globin family, functions as a tumor suppressor gene

Cytoglobin (CYGB) is a recently discovered vertebrate globin distantly related to myoglobin with unknown function. CYGB is assigned to chromosomal region 17q25, which is frequently lost in multiple malignancies. Previous studies failed to detect evidence for mutations in the CYGB gene. Recent studies provided preliminary evidence for increased methylation of the gene in lung cancer. Our study was aimed at investigating the role of CYGB as a tumor suppressor gene. By nested methylation-specific DNA sequencing analysis of lung and breast cancer cell lines and bronchial and mammary epithelial cell lines, we identified that methylation of a 110-bp CpG-rich segment of the CYGB promoter was correlated with gene silencing. We specifically targeted this sequence and developed a quantitative methylation-specific PCR assay, suitable for high-throughput analysis. We showed that the tumor specificity of CYGB methylation in discriminating patients with and without lung cancer, using biopsies and sputum samples. We further showed the tumor specificity of this assay with multiple other epithelial and hematologic malignancies. To show tumor suppressor activity of CYGB, we performed the following: (a) RNA interference-mediated knockdown of CYGB gene on colony formation in a CYGB expression-positive lung cancer cell line, resulting in increased colony formation; (b) enforced gene expression in CYGB expression-negative lung and breast cancer cell lines, reducing colony formation; and (c) identification of potential proximate targets down-stream of the CYGB genes. Our data constitute the first direct functional evidence for CYGB, the newest member of the globin family, as a tumor suppressor gene.

Genetics of gastroesophageal cancer: paradigms, paradoxes, and prognostic utility

Hereditary diffuse gastric cancer and tylosis are autosomal dominant cancer susceptibility syndromes. Accumulating evidence also suggests a genetic contribution to Barrett's esophagus and adenocarcinoma, traditionally considered acquired disorders. In this article we review the current knowledge on the genetic mechanisms underlying hereditary diffuse gastric cancer, tylosis, and Barrett's esophagus. Hereditary diffuse gastric cancer is a paradigm for hereditary cancer susceptibility syndromes with E-cadherin implicated as the dominant oncogene in up to one-third of cases. Tylosis in contrast remains the paradox as whilst the putative abnormality has been localized to the long arm of chromosome 17, sequencing of this region has failed to reveal a disease causing mutation. In the case of Barrett's esophagus and adenocarcinoma, although a validated specific disease-associated gene is yet to be identified, the increasing body of evidence of a possible genetic link is paving the way for subsequent prognostic studies such as AspECT (Aspirin Esomeprazole Chemoprevention Trial). For the clinician these advances in understanding are already having implications for practice in terms of improved screening and the stratification of management strategies. As the mechanisms continue to be defined, there is the real possibility that these mechanisms could be exploited or subverted in the design of new therapies. In the meantime, however, clinicians should undertake rigorous biopsy programs to ensure early invasive lesions are detected.

CpG island methylation phenotype (CIMP) in oral cancer: associated with a marked inflammatory response and less aggressive tumour biology

Studies in several tumour sites highlight the significance of the CpG island methylation phenotype (CIMP), with distinct features of histology, biological aggression and outcome. We utilise pyrosequencing techniques of quantitative methylation analysis to investigate the presence of CIMP in oral squamous cell carcinoma (OSCC) for the first time, and evaluate its correlation with allelic imbalance, pathology and clinical behaviour. Tumour tissue, control tissue and PBLs were obtained from 74 patients with oral squamous cell carcinoma. Pyrosequencing was used to analyse methylation patterns in 75-200 bp regions of the CpG rich gene promoters of 10 genes with a broad range of cellular functions. Allelic imbalance was investigated using a multiplexed panel of 11 microsatellite markers. Corresponding variables, histopathological staging and grading were correlated with these genetic and epigenetic aberrations. A cluster of tumours with a greater degree of promoter methylation than would be predicted by chance alone (P=0.001) were designated CIMP+ve. This group had less aggressive tumour biology in terms of tumour thickness (p=0.015) and nodal metastasis (P=0.012), this being apparently independent of tumour diameter. Further, it seems that these CIMP+ve tumours excited a greater host inflammatory response (P=0.019). The exact mechanisms underlying CIMP remain obscure but the association with a greater inflammatory host response supports existing theories relating these features in other tumour sites. As CIMP has significant associations with other well documented prognostic indicators, it may prove beneficial to include methylation analyses in molecular risk modelling of tumours.

Promoter methylation of P16, RARbeta, E-cadherin, cyclin A1 and cytoglobin in oral cancer: quantitative evaluation using pyrosequencing

Methylation profiling of cancer tissues has identified this mechanism as an important component of carcinogenesis. Epigenetic silencing of tumour suppressor genes through promoter methylation has been investigated by a variety of means, the most recent of which is pyrosequencing. We have investigated quantitative methylation status in oral squamous cell carcinoma patients. Fresh tumour tissue and normal control tissue from resection margin was obtained from 79 consecutive patients undergoing resection of oral squamous cell carcinoma. DNA was extracted and bisulphite treated. PCR primers were designed to amplify 75-200 bp regions of the CpG rich gene promoters of p16, RARbeta, E-cadherin, cytoglobin and cyclinA1. Methylation status of 4-5 CpG sites per gene was determined by pyrosequencing. Significant CpG methylation of gene promoters within tumour specimens was found in 28% for p16, 73% for RARbeta, 42% for E-cadherin, 65% for cytoglobin and 53% for cyclinA1. Promoter methylation was significantly elevated in tumours compared to normal tissue for p16 (P = 0.048), cytoglobin (P = 0.002) and cyclin A1 (P = 0.001) but not in RARbeta (P = 0.088) or E-cadherin (P = 0.347). Concordant methylation was demonstrated in this tumour series (P = 0.03). Significant differences in degree of methylation of individual CpG sites were noted for all genes except RARbeta and these differences were in a characteristic pattern that was reproduced between tumour samples. Cyclin A1 promoter methylation showed an inverse trend with histological grade. Promoter methylation analysis using pyrosequencing reveals valuable quantitative data from several CpG sites. In contrast to qualitative data generated from methylation specific PCR, our data demonstrated p16 promoter methylation in a highly tumour specific pattern. Significant tumour specific methylation of cyclin A1 promoter was also seen. Cytoglobin is a novel candidate tumour suppressor gene highly methylated in upper aero-digestive tract squamous cancer.

Down-regulation of the cytoglobin gene, located on 17q25, in tylosis with oesophageal cancer (TOC): evidence for trans-allele repression

Tylosis (focal non-epidermolytic palmoplantar keratoderma) is an autosomal dominant skin disorder that is associated with the early onset of squamous cell oesophageal cancer (SCOC) in three families. Our previous linkage and haplotype analyses have mapped the tylosis with oesophageal cancer (TOC) locus to a 42.5 kb region on chromosome 17q25 that has also been implicated in the aetiology of sporadically occurring SCOC from a number of different geographical populations. Oesophageal cancer is one of the 10 leading causes of cancer mortality worldwide. No inherited disease-causing mutations have been identified in the genes located in the 42.5 kb minimal region. We now show that cytoglobin gene expression in oesophageal biopsies from tylotic patients is dramatically reduced by approximately 70% compared with normal oesophagus. Furthermore, both alleles are equally repressed. Given the autosomal dominant nature of the disease, these results exclude haploinsufficiency as a mechanism of the disease and instead suggest a novel trans-allele interaction. We also show that the promoter is hypermethylated in sporadic oesophageal cancer samples: this may constitute the 'second hit' of a gene previously implicated in this disease by allelic imbalance studies.

A 6-Mb contig-based comparative gene and linkage map of the rat schwannoma tumor suppressor region at 10q32.3

Frequent genetic aberrations of malignant schwannomas induced by the alkylating agent N-ethyl-N-nitrosourea in hybrids from inbred BD rat strains include allelic imbalances of the telomeric 20 Mb of chromosome 5 (Dis-2) and of the telomeric 5 Mb of chromosome 10q32 (Dis-1) in 59 and 94% of the tumors, respectively. The Dis-1 minimal loss of heterozygosity consensus region extends from D10Rat4 to the telomere and harbors a putative tumor suppressor gene(s). We constructed a 6-Mb BAC/PAC contig containing more than 70 known genes, 18 mapped microsatellites, and further ESTs/reference RNAs. A continuous block of strongly conserved synteny with mouse chromosome 11E2 and human chromosome 17q25.3 was found. Combining the sequence information from the rat and closely related syntenic regions of different mammalian species produces nearly complete gene maps as a basis for a positional candidate approach and gives insight into mammalian genomic evolution.

Focal palmoplantar and gingival keratosis: A distinct palmoplantar ectodermal dysplasia with epidermolytic alterations but lack of mutations in known keratins

Focal palmoplantar and gingival keratosis is a rare autosomal dominant disease whose clinical features, and in particular, pathologic alterations and molecular etiology remain to be well defined. Recently we observed a German family affected by the disease in at least 3 consecutive generations. The 4 patients examined showed circumscribed and painful hyperkeratosis at the weight-bearing plantar skin since infancy, rather mild palmar hyperkeratosis, and continuous leukokeratosis confined to the maxillary and mandibulary attached gingiva. There were no nail changes, subungeal keratoses, or follicular hyperkeratosis. Light and electron microscopy of the plantar and gingival lesions revealed alterations of epidermolytic hyperkeratosis. Mutations in the known keratin genes were excluded by linkage analysis using microsatellite markers. We conclude that focal palmoplantar and gingival keratosis is a clinically distinct palmoplantar ectodermal dysplasia that is pathologically characterized by epidermolytic alterations, but is most probably not caused by a mutation in a keratin gene.

Loss of heterozygosity and transcriptome analyses of a 1.2 Mb candidate ovarian cancer tumor suppressor locus region at 17q25.1-q25.2

Loss of heterozygosity (LOH) analysis was performed in epithelial ovarian cancers (EOC) to further characterize a previously identified candidate tumor suppressor gene (TSG) region encompassing D17S801 at chromosomal region 17q25.1. LOH of at least one informative marker was observed for 100 (71%) of 140 malignant EOC samples in an analysis of 6 polymorphic markers (cen-D17S1839-D17S785-D17S1817-D17S801-D17S751-D17S722-tel). The combined LOH analysis revealed a 453 kilobase (Kb) minimal region of deletion (MRD) bounded by D17S1817 and D17S751. Human and mouse genome assemblies were used to resolve marker inconsistencies in the D17S1839-D17S722 interval and identify candidates. The region contains 32 known and strongly predicted genes, 9 of which overlap the MRD. The reference genomic sequences share nearly identical gene structures and the organization of the region is highly collinear. Although, the region does not show any large internal duplications, a 1.5 Kb inverted duplicated sequence of 87% nucleotide identity was observed in a 13 Kb region surrounding D17S801. Transcriptome analysis by Affymetrix GeneChip and reverse transcription (RT)-polymerase chain reaction (PCR) methods of 3 well characterized EOC cell lines and primary cultures of normal ovarian surface epithelial (NOSE) cells was performed with 32 candidates spanning D17S1839-D17S722 interval. RT-PCR analysis of 8 known or strongly predicted genes residing in the MRD in 10 EOC samples, that exhibited LOH of the MRD, identified FLJ22341 as a strong candidate TSG. The proximal repeat sequence of D17S801 occurs 8 Kb upstream of the putative promoter region of FLJ22341. RT-PCR analysis of the EOC samples and cell lines identified DKFZP434P0316 that maps proximal to the MRD, as a candidate. While Affymetrix technology was useful for initially eliminating less promising candidates, subsequent RT-PCR analysis of well-characterized EOC samples was essential to prioritize TSG candidates for further study.

Cloning, expression and subcellular localization of HN1 and HN1L genes, as well as characterization of their orthologs, defining an evolutionarily conserved gene family

The present work reported the cloning and characterization of two novel human genes--HN1 (hematopoietic- and neurologic-expressed sequence 1) and HN1L (HN1-like gene) which are proposed to be involved in embryo development. HN1 is mapped on chromosome 17q25.2, with two transcripts (1.0 and 1.6 kb in length, respectively) due to alternative splicing. HN1 is expressed abundantly in testis and skeletal muscle among 16 human tissues, and it is localized in the nucleus indicated by GFP fusion expression. Western blot confirmed that HN1 encodes a 16.5-kDa protein. HN1L is on chromosome 16p13.3, with three splicing in the length of 2.0, 4.0 and 4.2 kb, respectively. HN1L is expressed in a variety of tissues such as liver, kidney, prostate, testis and uterus at varying levels. HN1L gene encodes a 20-kDa protein, which is localized in both the nucleus and cytoplasm. Fourteen of HN1 and sixteen of HN1L homologous genes in different species were determined and analyzed by BLAST searches. Silicon analyses of the 14 orthologous proteins of HN1 and 16 orthologous proteins of HN1L revealed that they share great conservation in vertebrate. Additionally, we identified nine pseudogenes of HN1 (six) and HN1L (three) in the genomes of the human, mouse and rat. Based on sequence alignments and phylogenetic analysis, all these homologous genes and pseudogenes were defined as a HN1 gene family.

Novel microsatellite markers and single nucleotide polymorphisms refine the tylosis with oesophageal cancer (TOC) minimal region on 17q25 to 42.5 kb: sequencing does not identify the causative gene

Tylosis (focal non-epidermolytic palmoplantar keratoderma) is associated with the early onset of squamous cell oesophageal cancer in three families. Linkage and haplotype analyses have previously mapped the tylosis with oesophageal cancer ( TOC) locus to a 500-kb region on chromosome 17q25 that has also been implicated in sporadically occurring squamous cell oesophageal cancer. In the current study, 17 additional putative microsatellite markers were identified within this 500-kb region by using sequence data and seven of these were shown to be polymorphic in the UK and US families. In addition, our complete sequence analysis of the non-repetitive parts of the TOC minimal region identified 53 novel and six known single nucleotide polymorphisms (SNPs) in one or both of these families. Further fine mapping of the TOC disease locus by haplotype analysis of the seven polymorphic markers and 21 of the 59 SNPs allowed the reduction of the minimal region to 42.5 kb. One known and two putative genes are located within this region but none of these genes shows tylosis-specific mutations within their protein-coding regions. Alternative mechanisms of disease gene action must therefore be considered.