A new genome scan for primary nonsyndromic vesicoureteric reflux emphasizes high genetic heterogeneity and shows linkage and association with various genes already implicated in urinary tract development

Primary vesicoureteric reflux (VUR), the retrograde flow of urine from the bladder toward the kidneys, results from a developmental anomaly of the vesicoureteric valve mechanism, and is often associated with other urinary tract anomalies. It is the most common urological problem in children, with an estimated prevalence of 1–2%, and is a major cause of hypertension in childhood and of renal failure in childhood or adult life. We present the results of a genetic linkage and association scan using 900,000 markers. Our linkage results show a large number of suggestive linkage peaks, with different results in two groups of families, suggesting that VUR is even more genetically heterogeneous than previously imagined. The only marker achieving P < 0.02 for linkage in both groups of families is 270 kb from EMX2. In three sibships, we found recessive linkage to KHDRBS3, previously reported in a Somali family. In another family we discovered sex-reversal associated with VUR, implicating PRKX, for which there was weak support for dominant linkage in the overall data set. Several other candidate genes are suggested by our linkage or association results, and four of our linkage peaks are within copy-number variants recently found to be associated with renal hypodysplasia. Undoubtedly there are many genes related to VUR. Our study gives support to some loci suggested by earlier studies as well as suggesting new ones, and provides numerous indications for further investigations.

A genome-wide scan for genes involved in primary vesicoureteric reflux

BACKGROUND

Vesicoureteric reflux (VUR) is the retrograde flow of urine from the bladder into the ureters. It is the most common urological anomaly in children, and a major cause of end-stage renal failure and hypertension in both children and adults. VUR is seen in approximately 1-2% of Caucasian newborns and is frequently familial.

OBJECTIVE AND METHODS

In order to search for genetic loci involved in VUR, we performed a genome-wide linkage scan using 4710 single-nucleotide polymorphisms (SNPs) in 609 individuals from 129 Irish families with >1 affected member.

RESULTS

Nonparametric linkage (NPL) analysis of the dataset yielded moderately suggestive linkage at chromosome 2q37 (NPL(max) = 2.67, p<0.001). Analysis of a subset without any additional features, such as duplex kidneys, yielded a maximum NPL score of 4.1 (p = 0.001), reaching levels of genome-wide statistical significance. Suggestive linkage was also seen at 10q26 and 6q27, and there were several smaller peaks.

CONCLUSION

Our results confirm the previous conclusion that VUR is genetically heterogeneous, and support the identification of several disease-associated regions indicated by smaller studies, as well as indicating new regions of interest for investigation.

Predictive testing for BRCA1 and 2 mutations: a male contribution

BACKGROUND

Management strategies for women carrying BRCA1 and 2 mutations are becoming clearer and predictive testing for a known family mutation is commonly undertaken. Implications for men are not as clear and they participate less frequently.

PATIENTS AND METHODS

Twenty-six men from 10 extended families underwent predictive testing. Their motivation, reaction and outcome were studied. Subjects had appropriate pre- and post-test counselling. Informed consent was obtained before predictive testing for known deleterious mutations. DNA analysis followed standard procedures.

RESULTS

Eighteen tested positive and eight negative. Four had adverse psychological reactions and three reneged on their commitments to impart results. The spouse of another man had an adverse psychological reaction to the disclosure of his positive result. Two, already suffering from prostate cancer, were phenocopies and paternal lineage transmission was unexpectedly determined in another. Risk was removed from 33 offspring and confirmed for 56.

CONCLUSIONS

Complex themes associated with genetic testing are confirmed and the spectrum extended. Men appear to understand the importance of participating in this process. Methods of avoiding adverse reactions merit further study along with other aspects of the process.

The detection of large deletions or duplications in genomic DNA

While methods for the detection of point mutations and small insertions or deletions in genomic DNA are well established, the detection of larger (>100 bp) genomic duplications or deletions can be more difficult. Most mutation scanning methods use PCR as a first step, but the subsequent analyses are usually qualitative rather than quantitative. Gene dosage methods based on PCR need to be quantitative (i.e., they should report molar quantities of starting material) or semi-quantitative (i.e., they should report gene dosage relative to an internal standard). Without some sort of quantitation, heterozygous deletions and duplications may be overlooked and therefore be under-ascertained. Gene dosage methods provide the additional benefit of reporting allele drop-out in the PCR. This could impact on SNP surveys, where large-scale genotyping may miss null alleles. Here we review recent developments in techniques for the detection of this type of mutation and compare their relative strengths and weaknesses. We emphasize that comprehensive mutation analysis should include scanning for large insertions and deletions and duplications.

Mutations at the mitochondrial DNA polymerase (POLG) locus associated with male infertility

Human mitochondrial DNA polymerase, encoded by POLG, contains a polyglutamine tract encoded by a CAG microsatellite repeat. Analysis of POLG genotypes in different populations identified an association between absence of the common, ten-repeat allele and male infertility typified by a range of sperm quality defects but excluding azoospermia.

Quality control in molecular genetic testing

DNA-based testing for genetic diseases has developed from nothing into a principal part of laboratory medicine over the past 15 years. In the rush to bring these powerful new technologies into medical use, issues of quality have not always been given sufficient attention. Efforts are now being made to assess the quality of the output of genetic testing laboratories, and the results show that there is room for improvement.

A comparison of methods for gene dosage analysis in HMSN type 1

A number of different approaches are used in diagnostic laboratories to detect the 1.5 Mb duplication at 17p11.2 seen in approximately 70% of patients with hereditary motor and sensory neuropathy type 1 (HMSN1). Here we compare the methods used in UK diagnostic laboratories to detect the duplication. Samples referred to participating centres for HMSN testing were collected, randomised, and distributed for testing. One hundred samples were examined using five different methods; each method was tested by two independent laboratories. Identical results were obtained from all laboratories for 44 samples. The remaining samples were classified as duplication positive or duplication negative on the basis of the same result by two or more methods. A total of 95 samples were classified by more than one method, two were withdrawn from the study as the same result was not obtained by two methods, and three are thought to have a duplication smaller than 1.5 Mb. Seven of 49 duplications were not detected by methods used to detect the common junction fragment and the use of microsatellites failed to yield a result in four of 95 samples. Sequence tagged site (STS) dosage analysis was found to be the most sensitive of the methods tested, although this method was found to be the most likely to require repeat analysis. Eight samples gave discordant results between the two laboratories testing by the same method. Upon retesting, reasons for the initial incorrect result included processing and typographical errors.

The relationship between Y chromosome DNA haplotypes and Y chromosome deletions leading to male infertility

Microdeletions on the short arm of the Y chromosome have defined three non-overlapping regions (AZFa, b, c) recurrently deleted among infertile males. These regions contain several genes or gene families involved in male germ-cell development and maintenance. Even though a meiotic origin for these microdeletions is assumed, the mechanisms and causes leading to microdeletion formation are largely unknown. In order to assess whether some Y chromosome groups (or haplogroups) are predisposed to, or protected against, deletion formation during male meiosis, we have defined and compared Y chromosome haplogroup distribution in a group of infertile/subfertile males harbouring Yq deletions and in a relevant Northwestern European control population. Our analyses suggest that Y chromosome deletion formation is, at least in the study populations, a stochastic event independent of the Y chromosome background on which they arise and may be caused by other genetic and/or environmental factors.

Rapid-onset dystonia-parkinsonism: a clinical and genetic analysis of a new kindred

BACKGROUND

Rapid-onset dystonia-parkinsonism (RDP) is an autosomal dominant disorder linked to chromosome 19q13 that is characterized by sudden onset of primarily bulbar and upper limb dystonia with parkinsonism.

METHODS

The authors evaluated 12 individuals from three generations of an Irish family and obtained detailed medical records on a deceased member. The authors describe the clinical, psychiatric, and genetic features of the affected individuals.

RESULTS

Five of eight affected members developed sudden-onset (several hours to days) dystonia with postural instability. Four of the five also had bulbar symptoms. Two have stable focal or segmental limb dystonia. One has intermittent hemidystonia with dysarthria that comes on abruptly in times of stress or anxiety. Three had a history of profound difficulty socializing, and at presentation two developed depression. Three patients had a trial of dopamine agonists without benefit. Genetic analysis suggests linkage to chromosome 19 with lod score of 2.1 at zero recombination.

CONCLUSION

This is the third reported family with chromosome 19q13 rapid-onset dystonia-parkinsonism. Psychiatric morbidity appeared common in affected members of this family and may be part of the RDP phenotype.

Typical Friedreich's ataxia without GAA expansions and GAA expansion without typical Friedreich's ataxia

We clinically assessed and performed polymerase chain reaction analysis for the GAA trinucleotide repeat expansion in 103 patients from 73 families in Ireland, with a prior clinical diagnosis of Friedreich's ataxia (FA) or an unclassified progressive ataxic syndrome. The patients were classified as "typical" or "atypical" FA according to Harding's mandatory clinical diagnostic criteria. All patients underwent blood glucose analysis, and electrocardiography and echocardiography was performed in 99 and 101 patients, respectively. Mutation screening for expanded CAG trinucleotide repeats, associated with spinocerebellar ataxia (SCA) 1, 2, 3 and 6 was performed in 86 patients overall, including all GAA negative patients. Forty-nine of 56 typical patients and 13 of 47 atypical patients were either homozygous or heterozygous for the GAA expansion. Seven patients with a typical FA phenotype were negative for the GAA expansion. Although one of these patients had vitamin E deficiency, and two had raised alpha-fetoprotein levels, three other GAA negative patients with a typical FA phenotype had no other identifiable cause for their ataxia, once again raising the possibility of locus heterogeneity in FA. It is also possible that these patients have two point mutations in the X25 gene, or that they have another ataxic syndrome mimicking the FA phenotype. Two families who were homozygous for the GAA expansion exhibited intrafamilial phenotypic variability. Only one GAA negative patient had the SCA 3 mutation, and this was the only patient in the study with a possible autosomal dominant inheritance pattern. In the homozygous GAA population typical patients had significantly more repeats on the smaller allele than atypical patients, and there was an inverse relationship between the number of repeats on the smaller allele and the age at presentation. There was also an inverse relationship between the repeat size on both the larger and the smaller of the two alleles and the age at becoming wheelchair bound. There was no significant relationship between repeat size and the other indices of disease severity, including the presence or absence of diabetes or cardiomyopathy. This is the first large study of an Irish population with progressive ataxia that has shown a similar phenotype/genotype relationship to studies of FA in other European and non-European populations. The relatively low sensitivity and specificity of Harding's clinical diagnostic criteria must be appreciated when clinically assessing patients with a progressive ataxic syndrome. Although molecular genetic analysis now plays an essential role in diagnosis and classification, patients with a typical FA phenotype without any identifiable cause for their ataxia exist.

Hyperinsulinism caused by paternal-specific inheritance of a recessive mutation in the sulfonylurea-receptor gene

Neonatal hyperinsulinism (HI) is a genetic disorder of pancreatic beta-cells characterized by failure to suppress insulin secretion in the presence of hypoglycemia, resulting in brain damage or death if not adequately treated. Germline mutations in four genes have been associated with HI. Some patients have focal regions of beta-cell proliferation (focal HI). Seventy HI probands in whom at least one SUR-1 mutation was identified were studied. Clinical data from patients with two SUR-1 mutant alleles were compared with those from patients with single paternally inherited mutations. Thirty-seven probands were homozygous or compound heterozygous for SUR-1 mutations. In 33 probands, only a single mutation was identified, and in 31, the parental origin of the proband could be determined; in 29, the mutation was on the paternal allele (P < 0.0002). For three of these, pancreatic tissue was available and showed focal beta-cell hyperplasia. DNA extracted from the focal lesion and adjacent normal pancreas revealed loss of the maternal chromosome 11p15, resulting in reduction to homozygosity for the SUR-1 mutation within the focal lesion only. Using the Tdt-mediated dUTP nick end labeling (TUNEL) reaction, apoptotic beta-cells were identified exclusively within the focal region. At diagnosis, disease severity was similar in patients with paternally inherited mutations and those with two mutations. For patients who did not undergo surgery, those with only paternal mutations entered clinical remission within 16 +/- 6.2 months, compared with 48 +/- 23 months for those with two SUR-1 mutations (P = 0.001). In conclusion, we identified a novel mechanism to explain the pathophysiology of focal HI and provide evidence to suggest that this entity may be self-limiting, since affected beta-cells undergo apoptosis.

Fine-resolution mapping by haplotype evaluation: the examples of PFIC1 and BRIC

Loci for two inherited liver diseases, benign recurrent intrahepatic cholestasis (BRIC) and progressive familial intrahepatic cholestasis type 1 (PFIC1), have previously been mapped to 18q21 by a search for shared haplotypes in patients in two isolated populations. This paper describes the use of further haplotype evaluation with a larger sample of patients for both disorders, drawn from several different populations. Our assessment places both loci in the same interval of less than 1 cM and has led to the discovery of the PFIC1/BRIC gene, FIC1; this discovery permits retrospective examination of the general utility of haplotype evaluation and highlights possible caveats regarding this method of genetic mapping.

Hyperinsulinism: molecular aetiology of focal disease

Persistent hypoglycaemia in infancy is most commonly caused by hyperinsulinism. A case is reported of the somatic loss of the maternal 11p in an insulin secreting focal adenoma in association with a germline SUR-1 mutation on the paternal allele in a baby boy with hyperinsulinism diagnosed at 49 days old. A reduction to homozygosity of an SUR-1 mutation is proposed as a critical part of the cause of focal hyperinsulinism.

Instability of normal (CTG)n alleles in the DM kinase gene

We report on a myotonic dystrophy (DM) family exhibiting instability of normal sized (CTG)n alleles in the DM kinase gene on the non-DM chromosome. At least two mutational events involving normal DM alleles must have occurred in this family; one was characterised as a 34-35 (CTG)n repeat mutation. These findings represent a dissociation between (CTG)n repeat instability and myotonic dystrophy. Furthermore, this family highlights genetic counselling issues relating to the pathogenicity of alleles at the upper end of the normal size range and the risk of further expansion into the disease range.

A Molecular, Clinical and Immunohistochemical Study of Vestibular Schwannoma

The molecular pathogenesis of vestibular schwannoma has been investigated by determining the extent of chromosome 22 loss of heterozygosity in 77 tumors and relating these findings to clinical and immunohistochemical indexes of tumor behavior. Loss of heterozygosity was looked for at eight chromosome 22q loci. Clinical details were obtained in all 77 cases, and a clinical growth index was calculated for each tumor. The proliferative index was estimated in all tumors by using a monoclonal antibody to the proliferating cell nuclear antigen and by calculating the labeling index. Forty percent (31 of 77) of the tumors showed allele loss, and in each case this loss involved the region of the neurofibromatosis type 2 gene. No evidence was found that the presence of chromosome 22 allele loss was associated with the clinical growth index. On the log scale, however, an association was seen between the clinical growth index and the proliferating cell nuclear antigen labeling index p = 0.001). These results suggest that chromsome 22 allele loss is a frequent event in vestibular schwannoma. Tumor behavior, however, appears to be independent of the chromosome 22 mutation. It is proposed that chromosome 22 allele loss and neurofibromatosis type 2 gene inactivation is an early event, possibly involved in the initiation of tumorigenesis in vestibular schwannoma. Tumor growth appears to be independent of this mutation and is likely to be determined by other as yet undefined factors.

A molecular, clinical, and immunohistochemical study of vestibular schwannoma

The molecular pathogenesis of vestibular schwannoma has been investigated by determining the extent of chromosome 22 loss of heterozygosity in 77 tumors and relating these findings to clinical and immunohistochemical indexes of tumor behavior. Loss of heterozygosity was looked for at eight chromosome 22q loci. Clinical details were obtained in all 77 cases, and a clinical growth index was calculated for each tumor. The proliferative index was estimated in all tumors by using a monoclonal antibody to the proliferating cell nuclear antigen and by calculating the labeling index. Forty percent (31 of 77) of the tumors showed allele loss, and in each case this loss involved the region of the neurofibromatosis type 2 gene. No evidence was found that the presence of chromosome 22 allele loss was associated with the clinical growth index. On the log scale, however, an association was seen between the clinical growth index and the proliferating cell nuclear antigen labeling index p = 0.001). These results suggest that chromsome 22 allele loss is a frequent event in vestibular schwannoma. Tumor behavior, however, appears to be independent of the chromosome 22 mutation. It is proposed that chromosome 22 allele loss and neurofibromatosis type 2 gene inactivation is an early event, possibly involved in the initiation of tumorigenesis in vestibular schwannoma. Tumor growth appears to be independent of this mutation and is likely to be determined by other as yet undefined factors.

Improved genetic mapping of X linked retinoschisis

X linked retinoschisis (RS) causes poor vision in affected males owing to radial cystic changes at the macula. Genetic linkage analysis was carried out in 16 British families with X linked retinoschisis using markers from the Xp22 region. Linkage was confirmed between the RS locus and the markers DXS207 (lod score, Zmax = 17.9 at recombination fraction theta = 0.03; confidence interval for theta = 0.007-0.09), DXS1053 (Zmax = 18.0 at theta = 0.01, CI = 0.001-0.06), DXS43 (Zmax = 12.9 at theta = 0.03, CI = 0.004-0.09), DXS1195 (Zmax = 6.4 at theta = 0.00), DXS418 (Zmax = 8.2 at theta = 0.00), DXS999 (Zmax = 21.2 at theta = 0.01, CI = 0.001-0.05), DXS443 (Zmax = 14.2 at theta = 0.03, CI = 0.004-0.09), DXS365 (Zmax = 24.5 at theta = 0.008, CI = 0.001-0.04). Key recombinants placed RS between DXS43 distally and DXS999 proximally. Multipoint linkage analysis gave odds of 344:1 in favour of this location for RS and supported the map Xpter-(DXS207, DXS1053)-DXS43-1 cM-RS-1 cM-DXS999-DXS443-DXS365-DXS1052-Xcen.

Molecular genetic investigation of the neurofibromatosis type 2 tumor suppressor gene in sporadic meningioma

The authors investigated the role of somatic mutations of the neurofibromatosis type 2 (NF2) gene in sporadic meningioma. Neurofibromatosis 2 is a dominantly inherited familial tumor syndrome predisposing affected patients to a variety of central nervous system tumors including vestibular schwannoma and meningioma. Neurofibromatosis type 2 is caused by germline mutations in the NF2 tumor suppressor gene. In addition, the authors and others have reported that somatic NF2 gene mutations occur frequently in nonfamilial vestibular schwannoma. In this study, molecular genetic analysis was performed on 23 nonfamilial meningiomas. Paired DNA samples extracted from the blood and tumors of the patients were analyzed for loss of heterozygosity (LOH) in the region of the NF2 gene on chromosome 22 using closely linked DNA markers. The NF2 gene mutations were sought by single-stranded conformation polymorphism analysis and DNA sequencing. Fourteen (61%) of 23 meningiomas showed LOH in the region of the NF2 gene on chromosome 22. Somatic NF2 gene mutations were detected in eight meningiomas (35%) after screening all 17 exons. All tumors with NF2 gene mutations showed simultaneous chromosome 22 LOH. Review of the histopathological findings of the cases studied did not demonstrate any predominance of genetic abnormalities in a particular histological type of meningioma. These results are compatible with the hypothesis that the NF2 gene acts as a tumor suppressor and that its inactivation is important in the pathogenesis of sporadic meningioma.

Molecular genetic analysis of exons 1 to 6 of the APC gene in non-polyposis familial colorectal cancer

Familial adenomatous polyposis coli is caused by constitutional mutations in the APC gene. The hallmark of familial adenomatous polyposis coli is the presence of numerous (> 100) colorectal polyps, but mutations in the 5' end of the APC gene have been associated with familial colorectal cancer without florid polyposis. Although familial adenomatous polyposis coli accounts for only a minority of familial colorectal cancer cases, we hypothesised that APC mutations which were not associated with florid polyposis might make a significant contribution to nonpolyposis familial colorectal cancer. To investigate this possibility, we analysed 40 unrelated patients with familial colorectal cancer without classical familial adenomatous polyposis coli for mutations in exons 1 to 6 (codons 1 to 243) of the APC gene. No mutations were detected, but a C-->T polymorphism at nucleotide 333 (Arg-->Trp at codon 99) was identified. No 5' APC mutations were detected in two patients with desmoid tumours and a family history of colorectal cancer and polyps. We conclude that mutations in exons 1 to 6 of the APC gene are infrequent in patients with familial colorectal cancer who do not have many colorectal polyps.

Genetic linkage analysis in hereditary non-polyposis colon cancer syndrome

Hereditary Non-polyposis Colon Cancer Syndrome (HNPCC) is the most common cause of familial colorectal cancer. Molecular genetic studies of HNPCC have shown evidence of locus heterogeneity, and mutations in four genes (hMSH2, hMLH1, hPMS1, and hPMS2) which encode components of the mismatch enzyme repair system may cause HNPCC. To determine the extent and nature of locus heterogeneity in HNPCC, we performed genetic linkage studies in 14 HNPCC families from eastern and north-western England. Linkage to hMLH1 was excluded in six families, each of which were likely to be linked to hMSH2 (lod score > 1.0 in each family and total lod score for all six families = 7.64). Linkage to hMSH2 was excluded in three families, each of which were likely to be linked to hMLH1 (lod score > 1.0 in each family and total lod score at hMLH1 for all three families = 3.93). In the remaining five families linkage to hMSH2 or hMLH1 could not be excluded. These results confirm locus heterogeneity in HNPCC and suggest that, in the population studied, most large families with HNPCC will have mutations in hMSH2 or hMLH1. We did not detect any correlation between clinical phenotype and the genetic linkage results, but a Muir-Torre syndrome family excluded from linkage to hMLH1 was likely to be linked to hMSH2 and showed microsatellite instability in a tumour from an affected relative.

Normal CAG and CCG repeats in the Huntington's disease genes of Parkinson's disease patients

The clinical features of Parkinson's disease, particularly rigidity and bradykinesia and occasionally tremor, are seen in juvenile-onset Huntington's disease. Therefore, the CAG and CCG repeats in the Huntington's disease gene were investigated in 45 Parkinson's disease patients and compared to 40 control individuals. All of the Parkinson's disease chromosomes fell within the normal size ranges. In addition, the distributions of the two repeats in the Parkinson's disease patients did not differ significantly from those of the control population. Therefore, abnormalities of these trinucleotide repeats in the Huntington's disease gene are not likely to contribute to the pathogenesis of Parkinson's disease.

Haplotype analysis of the delta 2642 and (CAG)n polymorphisms in the Huntington's disease (HD) gene provides an explanation for an apparent 'founder' HD haplotype

The discovery of the intragenic delta 2642 deletion/insertion polymorphism in the Huntington's disease (HD) gene provides a tool to explore HD evolution, as the deletion is rare in normal chromosomes but overrepresented in HD chromosomes. Thus, delta 2642 deletion alleles were thought to mark normal chromosomes that are particularly prone to becoming HD chromosomes. We have examined this polymorphism in a range of human and non-human primate populations. Our results suggest that the deletion event probably occurred in the human lineage on a chromosome with a CAG allele length at the upper end of the normal size range, as the deletion seemed to be only associated with human chromosomes with 20 or more repeats. These deletion chromosomes are prone to expansion into the HD range because they are at the upper end of the normal range. These data explain the apparent 'founder' HD haplotype defined by the deletion and suggest that chromosomes carrying the deletion are no more mutable than non-deletion chromosomes with similar sized CAG repeats.

Myotonic dystrophy CTG repeats and the associated insertion/deletion polymorphism in human and primate populations

Myotonic dystrophy (DM) is associated with abnormal expansions of the CTG repeats in the 3' untranslated region of its gene. Previous studies in individuals of European origin demonstrated strong linkage disequilibrium between different CTG repeat length alleles and an Alu element insertion/deletion polymorphism in intron 8 of the DM gene: CTG11-13 chromosomes were almost exclusively associated with the deletion allele, while chromosomes with five or 19-30 repeats or disease chromosomes were only found on the insertion allele. One of the models suggested by these results proposed that the triplet repeats on insertion-associated chromosomes were particularly prone to mutation. Studies of other triplet repeat disorders have suggested that arrays of perfect repeats are more prone to instability than those that are interrupted. We have examined the evolution of this locus by typing a variety of primates and samples drawn from several different human populations for both CTG repeat length and the insertion/deletion polymorphism. DM gene sequences from different primates revealed inter- and intraspecies variability in the number of CTG repeats. Human chromosomes with five repeats or 11-13 repeats (the two major modes of the human CTG repeat distributions) showed no evidence of preferential stabilization of these repeat sizes by imperfect sequences. The insertion and deletion allele frequencies showed large interpopulation variation and the degree of association between these alleles and various CTG repeat lengths is not nearly as complete as was previously supposed. We have found deletion alleles carrying long normal (> 19) CTG repeats and insertion alleles associated with CTG11 - 13 in two african populations.(ABSTRACT TRUNCATED AT 250 WORDS)

Stickler syndrome: correlation between vitreoretinal phenotypes and linkage to COL 2A1

Stickler syndrome is an autosomal dominantly inherited condition characterised by ocular, articular, facial, auditory and oral features. There is locus heterogeneity with about two thirds of families showing linkage to the gene encoding type II procollagen (COL 2A1). Clinical overlap with Marshall's, Wagner's and other syndromes has caused considerable confusion but the importance of the congenital vitreous anomaly, as first described by Scott, has not previously been emphasised. This study examines the linkage of two vitreo-retinal phenotype subgroups of Stickler syndrome to COL 2A1. A total of 97 affected patients from 24 pedigrees were examined. This is the largest published series of Stickler syndrome patients to date and all have undergone full clinical and ophthalmological examination by a single investigator. A clinical classification is proposed based on vitreoretinal phenotype. All patients demonstrating the congenital vitreous anomaly have been designated Stickler syndrome type 1 and those without the congenital vitreous anomaly as Stickler syndrome type 2 patients. There were 69 affected patients from 20 unrelated type 1 pedigrees and 28 affected patients from 4 unrelated type 2 pedigrees. Using two markers at the COL 2A1 locus, Stickler syndrome type 1 pedigrees showed complete linkage to COL 2A1 with a maximum lod score of 12.33 at zero recombination. Linkage to COL 2A1 was excluded in the two type 2 pedigrees that were informative. From these data it appears that this clinical classification is a useful first step in resolving the genetic heterogeneity in this condition.

Chimaerism shown by cytogenetics and DNA polymorphism analysis

A child with ambiguous genitalia, brought up phenotypically male, had a 46,XX/46,XY karyotype. At laparotomy, he had a left sided ovary and uterus, and a right sided scrotal testis. The 46,XX line made up 50% of cells in the blood and 90% of cells in a skin biopsy. There were no cytogenetic polymorphisms. Analysis of lymphocyte DNA with seven polymorphic DNA markers showed him to be chimaeric, with four, three, and two parental alleles at different loci. He had one paternal and one maternal X chromosome at the marker DXS1053. Based on our data, we would suggest that chimaerism arose as a result of postzygotic fusion of two embryos. We have shown by DNA polymorphisms the presence of autosomal chimaerism in a case of sex chromosome chimaerism, and indicated the usefulness of DNA polymorphisms in determining the origin of chimaerism.

Mosaic uniparental disomy in Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is a congenital overgrowth syndrome with variable expression. The major features are anterior abdominal wall defects, macroglossia, and gigantism and less commonly neonatal hypoglycaemia, organomegaly, congenital renal anomalies, hemihypertrophy and embryonal tumours occur. BWS is a genetically heterogeneous disorder; most cases are sporadic but approximately 15% are familial and a small number of BWS patients have cytogenetic abnormalities involving chromosome 11p15. Genomic imprinting effects have been implicated in familial and non-familial BWS, and uniparental disomy (UPD) for chromosome 11 has been reported in sporadic cases. We investigated the incidence, pathogenesis, and clinical associations of UPD in 49 patients with non-familial BWS and a normal karyotype. UPD for chromosome 11p15 was detected in nine of 32 (28%) informative patients. A further two patients appeared to be disomic at the WT1 locus in chromosome 11p13, but were uninformative at chromosome 11p15.5 loci tested. In all cases with UPD the affected person was mosaic for a paternal isodisomy and a normal cell line indicating that UPD had arisen as a postzygotic event. Compared to cases in which paternal isodisomy for chromosomes 11p15.5 had been excluded (n = 23), BWS patients with UPD was more likely to have hemihypertrophy (6/9 versus 1/23, p < 0.001) and less likely to have exomphalos (0/9 versus 13/23, p < 0.01), but there were no significant differences between disomic and non-disomic cases in the incidence of hypoglycaemia, nephromegaly, neoplasia, and developmental delay. The detection of UPD in BWS patients allows accurate genetic counselling to be provided and provides an insight into the molecular pathogenesis of BWS.

Study of the Huntington's disease (HD) gene CAG repeats in schizophrenic patients shows overlap of the normal and HD affected ranges but absence of correlation with schizophrenia

The CAG repeats in the Huntington's disease gene were investigated in chromosomes from 71 unrelated schizophrenic persons and 18 patients with schizoaffective disorder in order to determine if any of these patients had abnormal expansions. All of the probands had repeat sizes in the normal range (< 35 repeats) and there was no significant difference between the allele distributions of these patients and the normal controls. The families of two patients with 32 repeats and one patient with 34 repeats were investigated further and showed no uniform segregation of the disease with the large repeat alleles. The proband with 34 repeats inherited a chromosome that originally had 36 repeats in her father. The presence of 36 CAG repeats in members of her family and in HD patients suggests that there is an overlap between the normal and Huntington's disease CAG repeat size ranges. The more recently described CCG polymorphism in this gene was also examined in the schizophrenic and schizoaffective persons. All patients had alleles in the normal range.

Mutational bias provides a model for the evolution of Huntington's disease and predicts a general increase in disease prevalence

Huntington's disease (HD) correlates with abnormal expansion in a block of CAG repeats in the Huntington's disease gene. We have investigated HD evolution by typing CAG alleles in several human populations and in a variety of primates. We find that human alleles have expanded from a shorter ancestral state and exhibit unusual asymmetric length distributions. Computer simulations are used to show that the human state can be derived readily from a primate ancestor, without the need to invoke natural selection. The key element is a simple length-dependent mutational bias towards longer alleles. Our model can explain a number of empirical observations, and predicts an ever-increasing incidence of HD.

Carrier detection in X linked ocular albinism using linked DNA polymorphisms

Sixty two females at 50% carrier risk were assessed from 19 families affected by X linked ocular albinism (OA1). Twenty nine (47%) had definite fundus changes of the carrier state with a mud splattered fundus appearance and 23 (37%) had a normal ophthalmic examination. Ten (16%) had mild peripheral retinal pigmentary changes so that it was difficult to exclude the carrier state; six of these females were shown to be at low risk and only one at high risk of being a carrier by DNA analysis using linked DNA polymorphisms, including a highly informative dinucleotide repeat at the Kallmann locus. Mild peripheral retinal pigmentary changes are not a definite indication of carrier status and in 45 age matched female controls five (11%) had similar changes. No female with a clinically normal fundus was found to be at high risk by DNA analysis. Molecular genetic analysis improves the accuracy of carrier detection in OA1 families and should be considered if the clinical findings are equivocal.

Direct, non-radioactive detection of mutations in multiple endocrine neoplasia type 2A families

We have designed PCR primers that permit the rapid non-isotopic detection of mutations in codon 634 of the RET proto-oncogene, the causative mutations in over 80% of MEN 2A and 50% of FMTC families. In this paper we report the investigation of eleven MEN 2A families referred to the East Anglian Regional Genetics Service. Nine of these families carry codon 634 mutations. We were able to confirm the diagnosis of MEN 2A in twenty six affected individuals and to determine the carrier status of forty one individuals thought to be at risk of developing the disease. Of those at risk, thirty one patients lacked the familial mutation and ten were presymptomatic carriers of MEN 2A. In five cases the direct test proved that patients who had been treated by thyroidectomy but who lacked confirmed cancer, did not carry the familial mutation, removing the perceived risk of MEN 2A from their children. This group included one patient who had been diagnosed as having mild C-cell hyperplasia, confirming that in MEN 2A families C-cell hyperplasia can result from causes other than the presence of the MEN 2A mutation.

Issues in Huntington's disease testing

The gene that is mutated in Huntington's disease has a polymorphic (CAG)n tract close to the 5' end of its message that is unstable and abnormally expanded in disease chromosomes. Rapid PCR tests that measure the CAG repeat number in this gene will become an important clinical tool. However, this test should be approached with caution, as its specificity and sensitivity have not been absolutely determined. The possibility that one might be able to predict the age of onset from the size of the CAG expansion in cases that have more than 50 repeats has been considered. We have also made a case for retaining the diagnostic option of prenatal exclusion testing.

Somatic NF2 gene mutations in familial and non-familial vestibular schwannoma

Vestibular schwannoma occurs both as a sporadic tumour and in the dominantly inherited familial cancer syndrome neurofibromatosis type 2 (NF2). The gene for NF2 has recently been isolated on chromosome 22, and the demonstration of inactivating germline mutations in NF2 patients and NF2 associated tumours suggests that it act as a tumour suppressor. We have investigated 85 sporadic and 2 NF2 associated vestibular schwannomas, and one vagal schwannoma for chromosome 22 allele loss and NF2 gene mutations. A further 7 vestibular schwannomas were investigated for NF2 mutations only. Chromosome 22 allele loss was detected in 34 of 87 vestibular schwannomas and in the vagal nerve schwannoma. Six exons of the NF2 gene were investigated by SSCP analysis in all 95 tumours. Somatic NF2 gene mutations were detected in 13 non-familial vestibular schwannomas and in one of the NF2 vestibular schwannomas. Seven non-familial tumours with an NF2 gene mutation also displayed a chromosome 22 allele loss. Thirteen of the mutations were predicted to produce truncation of the NF2 protein. These results suggest that somatic mutations of the NF2 tumour suppressor gene are a critical step in the pathogenesis of both familial and non-familial vestibular schwannoma and that the mechanism of tumourigenesis complies with a 'two-hit' mutation model.

Molecular genetic analysis of the mechanism of tumorigenesis in acoustic neuroma

OBJECTIVE

Acoustic neuroma, both familial and sporadic, is clinically and biologically a heterogeneous condition with a wide variation in age of presentation, length of history, and tumor growth rate. In an attempt to correlate this clinical diversity with the underlying molecular pathology, we have analyzed 43 paired blood-tumor DNA samples from patients with acoustic neuromas.

DESIGN

Molecular genetic analysis.

SETTING

Molecular genetic research laboratory.

PATIENTS

Paired blood-tumor DNA samples were obtained from 43 patients (41 sporadic and two patients with neurofibromatosis type 2).

MAIN OUTCOME MEASURES

Loss of constitutional heterozygosity was looked for in the region of tumor suppressor genes on chromosomes 3p, 5q, 11p, 17p, 17q, and 22.

RESULTS

We found loss of heterozygosity exclusively for markers on chromosome 22. Thirty-nine percent of tumors showed allele loss, and in each case the loss of heterozygosity included the region of the neurofibromatosis type 2 (NF2) gene. No loss of heterozygosity was detected in the region of known or putative suppressor genes in chromosomes 3p, 5q, 11p, 17p, and 17q.

CONCLUSIONS

This study has demonstrated that (1) chromosome 22 allele loss is a frequent event in sporadic acoustic neuroma; (2) the minimal region of loss of heterozygosity in acoustic neuroma includes the NF2 gene; (3) the known tumor suppressor genes investigated (VHL, adenomatous polyposis coli, WT2, p53, and NF1) do not appear to be important in the pathogenesis of acoustic neuroma; and (4) patients with extensive chromosome 22 loss tended to be younger and with a slightly shorter clinical history than those with no detectable allele loss.

Genetic mapping of the Kallmann syndrome and X linked ocular albinism gene loci

The X linked form of Kallmann syndrome (KAL) and X linked ocular albinism (OA1) have both been mapped to Xp22.3. We have used a dinucleotide repeat polymorphism at the Kallmann locus to type 17 X linked ocular albinism families which had previously been typed for the Xg blood group (XG) and the DNA markers DXS237 (GMGX9), DXS143 (dic56), and DXS85 (782). Close linkage was found between KAL and OA1 with a maximum lod score (Zmax) of 30.14 at a recombination fraction (theta max) of 0.06 (confidence interval for theta: 0.03-0.10). KAL was also closely linked to DXS237 (Zmax = 15.32; theta max = 0.05; CI 0.02-0.12) and DXS143 (Zmax = 14.57; theta max = 0.05; CI 0.02-0.13). There was looser linkage to the Xg blood group (XG) and to DXS85 (782). Multipoint linkage analysis gave the map: Xpter-XG-0.13-DXS237-0.025-KAL-0.025-DXS143-0.01 5-OA1-0.09-DXS85-Xcen. Placement of OA1 proximal to DXS143 was supported by odds of 2300:1 compared to other orders. This confirms our previous localisation of OA1 and improves the genetic mapping of both disease loci.

Evaluation of molecular genetic diagnosis in the management of familial adenomatous polyposis coli: a population based study

A population based clinical and molecular genetic study of familial adenomatous polyposis coli (FAPC) was performed to investigate the value of molecular genetic analysis and ophthalmological assessment in the presymptomatic diagnosis of FAPC. The point prevalence of affected patients was 2.62 x 10(-5) (1/38,000) and the minimum heterozygote prevalence was estimated at 3.8 x 10(-5) (1/26,000). Eight of 33 (24%) probands were new mutations. Forty-eight asymptomatic relatives at 50% prior risk aged between 10 and 40 years were assessed for risk modification with linked DNA markers: in nine subjects (18%) the family structure was unsuitable for linkage based analysis, but 32 subjects were informative with a panel of intragenic and closely linked markers (25 had a combined age/DNA related risk of < 1% (low risk group) and seven were at high risk (DNA predicted risk > 99%)). Ophthalmological assessment for CHRPEs showed that 27/43 (63%) affected patients and high risk relatives and 0/18 low risk relatives had more than three CHRPEs. Interfamilial variation in CHRPE expression was apparent. This study has shown that DNA based risk modification with intragenic and closely linked DNA markers is informative in most FAPC families. In addition to the clinical benefits of presymptomatic diagnosis for FAPC, the reduction in screening for low risk relatives (365 person years in the present study) means that molecular genetic diagnosis of FAPC is a cost effective procedure.

Genotype analysis in cystic fibrosis in relation to the occurrence of diabetes mellitus

Carbohydrate intolerance and frank diabetes mellitus (DM) are recognised features of cystic fibrosis (CF), but their cause has not been established. Damage to the islet cells due to pancreatic fibrosis is the most common explanation. The relationship between the genotype and the occurrence of diabetes mellitus has been examined in 21 cystic fibrosis patients attending our out-patient chest clinics. DNA samples were tested for the presence of the most common mutation (delta F508) and the results related to the degree of carbohydrate intolerance. We found that in CF patients over 18 years of age diabetes is more frequent among those homozygous for the delta F508 allele than the heterozygotes (P < 0.05). Those homozygous for delta F508 were diagnosed as CF at an earlier age than the heterozygotes (P < 0.05). Both the mean age and the female/male ratio were similar in the diabetic and non-diabetic groups. The prevalence of the delta F508 allele was similar to that expected in a random sample of East Anglian cystic fibrosis patients. These data suggest that the occurrence of diabetes mellitus in patients with cystic fibrosis is related to the genotype, which could be secondary to the way the genetic factors influence the degree of pancreatic disease and its rate of progression.

Genetic mapping of X-linked ocular albinism: linkage analysis in a large Newfoundland kindred

Genetic linkage studies in a large Newfoundland family affected by X-linked ocular albinism (OA1) showed linkage to markers from Xp22.3. One recombinant mapped the disease proximal to DXS143 (dic56) and two recombinants mapped the disease distal to DXS85 (782). Combining the data with that from 16 British families previously published confirmed close linkage between OA1 and DXS143 (dic56; Zmax = 21.96 at theta = 0.01, confidence interval (CI) 0.0005-0.05) and linkage to DXS85 (782; Zmax = 17.60 at theta = 0.07, CI = 0.03-0.13) and DXS237 (GMGX9; Zmax = 15.20 at theta = 0.08, CI = 0.03-0.15). Multipoint analysis (LINKMAP) gave the most likely order as Xpter-XG-DXS237-DXS 143-OA1-DXS85, with odds of 48:1 over the order Xpter-XG-DXS237-OA1-DXS143-DXS85, and odds exceeding 10(10):1 over other locations for the disease locus.

Cystic fibrosis identified by neonatal screening: incidence, genotype, and early natural history

The incidence of cystic fibrosis over the last 10 years in East Anglia (a region of the United Kingdom with a population of 2.1 million) has halved. This has happened during the establishment of a neonatal screening programme, which has enabled early diagnosis, genetic counselling, and lately the option of prenatal diagnosis in subsequent pregnancies. One hundred and seven children were born with cystic fibrosis between 1981 and 1990, eight of whom were siblings. The Guthrie blood spots of 82 infants detected by neonatal immunoreactive trypsin screening between 1981 and 1990 were examined for the presence of the most common cystic fibrosis gene mutation (delta F508). It was present in 135 (82%) of the 164 cystic fibrosis genes analysed with 54 (66%) cases being homozygous and 27 (33%) heterozygous. Sixty nine per cent of infants were symptomatic at the time of diagnosis regardless of genotype. No association was found between the early clinical or biochemical features of the disease and homozygosity or heterozygosity for this mutation. Screening for cystic fibrosis using the blood immunoreactive trypsin assay alone remains an effective method of identifying infants with the disease soon after birth, thereby allowing early therapeutic intervention. Genetic counselling and prenatal diagnosis have contributed to a reduction in the number of children born with cystic fibrosis, but may not entirely explain the decreasing incidence of the disease.

Presymptomatic diagnosis of von Hippel-Lindau disease with flanking DNA markers

Von Hippel-Lindau (VHL) disease is a dominantly inherited cancer syndrome characterised by the development of retinal, cerebellar, and spinal haemangioblastomas, renal cell carcinoma, and phaeochromocytoma. The gene for VHL disease has been mapped to chromosome 3p25-p26 and flanking markers identified. We have investigated the usefulness of currently available DNA markers for the presymptomatic diagnosis of VHL disease. In the first part of this investigation, genetic linkage data from two previously published studies were updated and reanalysed to provide accurate estimates of sex specific recombination fractions and to confirm that there is no evidence of locus heterogeneity. In the second part of this study, 14 families containing 23 asymptomatic subjects at 50% prior risk of VHL disease were investigated with closely linked DNA markers (RAF1, D3S18, D3S732). Seventeen subjects were informative with one or more markers, six of whom were informative at markers flanking the VHL disease gene. By combining age related and DNA based risk information the carrier risk for 11 subjects was reduced to < 2%.

Molecular analysis of the Mov 34 mutation: transcript disrupted by proviral integration in mice is conserved in Drosophila

The Mov 34 mutation is a recessive embryonic lethal mutation caused by retroviral integration in the murine germline. This integration disrupts a transcription unit that appears to encode a novel protein. The Mov 34 proviral integration is located on mouse chromosome 8 and the human homolog of this gene has been mapped to chromosome region 16q23-q24. An evolutionarily conserved syntenic relationship exists between this region of human chromosome 16 and a region of mouse chromosome 8 that also contains oligosyndactyly (Os), another recessive lethal mutation. Genetic studies have ruled out Os as residing at the same locus as the Mov 34 integration. The Mov 34 transcript is conserved in evolution, and a Drosophila homolog appears to encode a protein with 62% amino acid identity to the murine protein. In situ hybridization to Drosophila polytene chromosomes localizes the Drosophila homolog to 60B,C on chromosome 2. Several Drosophila lethal mutations also map to this region.

Isolation, chromosomal mapping, and expression of the mouse tyrosinase gene

Using a human tyrosinase cDNA probe, we have isolated mouse tyrosinase genomic clones and used them to map the mouse tyrosinase locus and to analyze the promoter sequence of the tyrosinase gene. Southern blot analyses of DNA from somatic cell hybrids, interspecies backcross mice, and albino deletion mice have revealed that the locus for mouse tyrosinase resides at or near the albino locus on mouse chromosome 7. There were three TATA-elements, but only one CAT-element, and the CAT-element appeared to be paired with the third TATA-element, located at the position farthest upstream. Mouse tyrosinase mRNA is approximately 2.4 Kb in size. The amount of tyrosinase mRNA reflects the levels of tyrosinase activity in normal melanocytes and Cloudman S-91 melanoma cell line.