Dicky Halley

Identification and characterization of the tuberous sclerosis gene on chromosome 16

Tuberous sclerosis (TSC) is an autosomal dominant multisystem disorder with loci assigned to chromosomes 9 and 16. Using pulsed-field gel electrophoresis (PFGE), we identified five TSC-associated deletions at 16p13.3. These were mapped to a 120 kb region that was cloned in cosmids and from which four genes were isolated. One gene, designated TSC2, was interrupted by all five PFGE deletions, and closer examination revealed several intragenic mutations, including one de novo deletion. In this case, Northern blot analysis identified a shortened transcript, while reduced expression was observed in another TSC family, confirming TSC2 as the chromosome 16 TSC gene. The 5.5 kb TSC2 transcript is widely expressed, and its protein product, tuberin, has a region of homology to the GTPase-activating protein GAP3.Tuberous sclerosis (TSC) is an autosomal dominant multisystem disorder with loci assigned to chromosomes 9 and 16. Using pulsed-field gel electrophoresis (PFGE), we identified five TSC-associated deletions at 16p 13.3. These were mapped to a 120 kb region that was cloned in cosmids and from which four genes were isolated. One gene, designated TSC2, was interrupted by all five PFGE deletions, and closer examination revealed several intragenic mutations, including one de novo deletion. In this case, Northern blot analysis identified a shortened transcript, while reduced expression was observed in another TSC family, confirming TSC2 as the chromosome 16 TSC gene. The 5.5 kb TSC2 transcript is widely expressed, and its protein product, tuberin, has a region of homology to the GTPaseactivating protein GAP3.

Identification of the familial cylindromatosis tumour-suppressor gene

Familial cylindromatosis is an autosomal dominant genetic predisposition to multiple tumours of the skin appendages. The susceptibility gene (CYLD) has previously been localized to chromosome 16q and has the genetic attributes of a tumour-suppressor gene (recessive oncogene). Here we have identified CYLD by detecting germline mutations in 21 cylindromatosis families and somatic mutations in 1 sporadic and 5 familial cylindromas. All mutations predict truncation or absence of the encoded protein. CYLD encodes three cytoskeletal-associated-protein–glycine-conserved (CAP–GLY) domains, which are found in proteins that coordinate the attachment of organelles to microtubules. CYLD also has sequence homology to the catalytic domain of ubiquitin carboxy-terminal hydrolases (UCH).

Mutational analysis of the TSC1 and TSC2 genes in a diagnostic setting: genotype--phenotype correlations and comparison of diagnostic DNA techniques in Tuberous Sclerosis Complex.

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder characterised by the development of hamartomas in multiple organs and tissues. TSC is caused by mutations in either the TSC1 or TSC2 gene. We searched for mutations in both genes in a cohort of 490 patients diagnosed with or suspected of having TSC using a combination of denaturing gradient gel electrophoresis, single-strand conformational polymorphism, direct sequencing, fluorescent in situ hybridisation and Southern blotting. We identified pathogenic mutations in 362 patients, a mutation detection rate of 74%. Of these 362 patients, 276 had a definite clinical diagnosis of TSC and in these patients 235 mutations were identified, a mutation detection rate of 85%. The ratio of TSC2:TSC1 mutations was 3.4:1. In our cohort, both TSC1 mutations and mutations in familial TSC2 cases were associated with phenotypes less severe than de novo TSC2 mutations. Interestingly, consistent with other studies, the phenotypes of the patients in which no mutation was identified were, overall, less severe than those of patients with either a known TSC1 or TSC2 mutation.

The tuberous sclerosis-1 (TSC1) gene product hamartin suppresses cell growth and augments the expression of the TSC2 product tuberin by inhibiting its ubiquitination

We report here that overexpression of the tuberous sclerosis-1 (TSC1) gene product hamartin results in the inhibition of growth, as well as changes in cell morphology. Growth inhibition was associated with an increase in the endogenous level of the product of the tuberous sclerosis-2 (TSC2) gene, tuberin. As overexpression of tuberin inhibits cell growth, and hamartin is known to bind tuberin, these results suggested that hamartin stabilizes tuberin and this contributes to the inhibition of cell growth. Indeed, transient transfection of TSC1 increased the endogenous level of tuberin, and transient co-transfection of TSC1 with TSC2 resulted in higher tuberin levels. The stabilization was explained by the finding that tuberin is highly ubiquitinated in cells, while the fraction of tuberin that is bound to hamartin is not ubiquitinated. Co-expression of tuberin stabilized hamartin, which is weakly ubiquitinated, in transiently transfected cells. The amino-terminal two-thirds of tuberin was responsible for its ubiquitination and for stabilization of hamartin. A mutant of tuberin from a patient missense mutation of TSC2 was also highly ubiquitinated, and was unable to stabilize hamartin. We conclude that hamartin is a growth inhibitory protein whose biological effect is likely dependent on its interaction with tuberin.

Screening and Diagnosis for the Fragile X Syndrome among the Mentally Retarded: An Epidemiological and Psychological Survey

The fragile X syndrome is an X-linked mental retardation disorder caused by an expanded CGG repeat in the first exon of the fragile X mental retardation (FMR1) gene. Its frequency, X-linked inheritance, and consequences for relatives all prompt for diagnosis of this disorder on a large scale in all affected individuals. A screening for the fragile X syndrome has been conducted in a representative sample of 3,352 individuals in schools and institutes for the mentally retarded in the southwestern Netherlands, by use of a brief physical examination and the DNA test. The attitudes and reactions of (non)consenting parents/guardians were studied by (pre- and posttest) questionnaires. A total of 2,189 individuals (65%) were eligible for testing, since they had no valid diagnosis, cerebral palsy, or a previous test for the FMR1 gene mutation. Seventy percent (1,531/2,189) of the parents/guardians consented to testing. Besides 32 previously diagnosed fragile X patients, 11 new patients (9 males and 2 females) were diagnosed. Scoring of physical features was effective in preselection, especially for males (sensitivity .91 and specificity .92). Major motives to participate in the screening were the wish to obtain a diagnosis (82%), the hereditary implications (80%), and the support of research into mental retardation (81%). Thirty-four percent of the parents/guardians will seek additional diagnostic workup after exclusion of the fragile X syndrome. The prevalence of the fragile X syndrome was estimated at 1/ 6,045 for males (95% confidence interval 1/9,981-1/ 3,851). On the basis of the actual number of diagnosed cases in the Netherlands, it is estimated that >50% of the fragile X cases are undiagnosed at present.

Characterization of the cytosolic tuberin-hamartin complex. Tuberin is a cytosolic chaperone for hamartin

Tuberous sclerosis (TSC) is an autosomal dominant disorder characterized by a broad phenotypic spectrum that includes seizures, mental retardation, renal dysfunction and dermatological abnormalities. Mutations to either the TSC1 orTSC2 gene are responsible for the disease. TheTSC1 gene encodes hamartin, a 130-kDa protein without significant homology to other known mammalian proteins. Analysis of the amino acid sequence of tuberin, the 200-kDa product of theTSC2 gene, identified a region with limited homology to GTPase-activating proteins. Previously, we demonstrated direct binding between tuberin and hamartin. Here we investigate this interaction in more detail. We show that the complex is predominantly cytosolic and may contain additional, as yet uncharacterized components alongside tuberin and hamartin. Furthermore, because oligomerization of the hamartin carboxyl-terminal coiled coil domain was inhibited by the presence of tuberin, we propose that tuberin acts as a chaperone, preventing hamartin self-aggregation.

A Cystic Fibrosis Mutation Associated with Mild Lung Disease

BACKGROUND Cystic fibrosis is the most common lethal autosomal recessive disorder among whites. Among Dutch patients with cystic fibrosis, delta F508 is the most common mutation and A455E the second most common mutation of the cystic fibrosis transmembrane conductance regulator gene on chromosome 7. A455E is associated with preserved pancreatic function and residual secretion of chloride across membranes. We investigated whether it is also associated with less severe pulmonary disease in patients with cystic fibrosis. METHODS A total of 33 patients with compound heterozygosity for the A455E mutation were matched according to age and sex with patients who were homozygous for the delta F508 mutation. The pairs were analyzed with respect to the following outcome variables: age at diagnosis, pulmonary-function values, and the frequency of pseudomonas colonization, pancreatic sufficiency, and diabetes mellitus. RESULTS Cystic fibrosis was diagnosed at a later age in the patients with the A455E mutation than in the delta F508 homozygotes (mean age at diagnosis, 15.0 vs. 3.1 years; P < 0.001). Fewer patients with the A455E mutation had pancreatic insufficiency (21.2 percent vs. 93.9 percent, P < 0.001), and none had diabetes mellitus (0 percent vs. 27.3 percent, P = 0.004). Forced expiratory volume in one second (FEV1) and forced vital capacity (FVC) were significantly higher in the patients with the A455E mutation (mean FEV1, 73.9 percent of the predicted value vs. 54.3 percent of the predicted value; P = 0.002; mean FVC, 88.7 percent of the predicted value vs. 76.3 percent of the predicted value; P = 0.04). Fewer patients with the A455E mutation were colonized with Pseudomonas aeruginosa (33.3 percent vs. 60.6 percent, P = 0.02). CONCLUSIONS A455E is a common mutation causing cystic fibrosis in the Netherlands. Although several mutations are known to be associated with less severe pancreatic disease, our findings demonstrate a correlation between the A455E mutation and mild pulmonary disease. Because mortality in this disease depends primarily on the progression of pulmonary disease, patients with the A455E mutation have a better prognosis than patients who are homozygous for the delta F508 mutation.

High rate of mosaicism in tuberous sclerosis complex.

Six families with mosaicism are identified in a series of 62 unrelated families with a mutation in one of the two tuberous sclerosis complex (TSC) genes, TSC1 or TSC2. In five families, somatic mosaicism was present in a mildly affected parent of an index patient. In one family with clinically unaffected parents, gonadal mosaicism was detected after TSC was found in three children. The detection of mosaicism has consequences for genetic counseling of the families involved, as changed risks apply to individuals with mosaicism, both siblings and parents. Clinical investigation of parents of patients with seemingly sporadic mutations is essential to determine their residual chance of gonadal and/or somatic mosaicism, unless a mosaic pattern is detected in the index patient, proving a de novo event. In our data set, the exclusion of signs of TSC in the parents of a patient with TSC reduced the chance of one of the parents to be a (mosaic) mutation carrier from 10% to 2%. In the five families with somatic mosaicism, the parent was given the diagnosis after the diagnosis was made in the child.

Broad spectrum of Pompe disease in patients with the same c.-32-13T -> G haplotype

Background: Pompe disease (acid maltase deficiency, glycogen storage disease type II; OMIM 232300) is an autosomal recessive lysosomal storage disorder characterized by acid α-glucosidase deficiency due to mutations in the GAA gene. Progressive skeletal muscle weakness affects motor and respiratory functions and is typical for all forms of Pompe disease. Cardiac hypertrophy is an additional fatal symptom in the classic infantile subtype. c.-32-13T→G is the most common mutation in adults. Objective: To delineate the disease variation among patients with this mutation and to define the c.-32-13T→G haplotypes in search for genotype–phenotype correlations. Methods: We studied 98 compound heterozygotes with a fully deleterious mutation (11 novel mutations are described) and the common c.-32-13T→G mutation. Results: All patients were Caucasian. None had the classic infantile form of Pompe disease. The clinical course varied far more than anticipated (age at diagnosis <1 to 78 years; age at onset: <1 to 52 years). The acid α-glucosidase activities in a subset of patients ranged from 4 to 19.9 nmol/mg/h. Twelve different c.-32-13T→G haplotypes were identified based on 17 single-nucleotide polymorphisms located in the GAA gene. In 76% of the cases, c.-32-13T→G was encountered in the second most common GAA core haplotype (DHRGEVVT). In only one case was c.-32-13T→G encountered in the major GAA core haplotype (DRHGEIVT). Conclusion: Patients with the same c.-32-13T→G haplotype (c.q. GAA genotype) may manifest first symptoms at different ages, indicating that secondary factors may substantially influence the clinical course of patients with this mutation.

Determinants of mild clinical symptoms in cystic fibrosis patients. Residual chloride secretion measured in rectal biopsies in relation to the genotype.

Previous Ussing chamber measurements of secretagogue-provoked changes in short circuit current in rectal suction biopsies of cystic fibrosis (CF) patients showed that in a minority of patients chloride secretion in response to cholinergic agonists is reduced but not completely absent. To assess a possible relationship between this phenomenon and both the genotype and the phenotype, we performed Ussing chamber experiments on rectal suction biopsies of 51 CF patients. The CF mutation was identified in 89 out of 102 CF alleles. No apparent chloride secretion was found in 30 CF patients (group I). Low residual chloride secretion was found in 11 CF patients (group II), while a relatively high residual secretion appeared in 10 CF patients (group III). Pancreatic function was preserved more frequently in CF patients displaying residual secretion: 0% in group I, 27% in group II, and 60% in group III (P < 0.001). The age at diagnosis (mean +/- SEM) in group III (18.4 +/- 6.6) was significantly different from group I (1.2 +/- 0.4, P < 0.01) and group II (3.5 +/- 1.4, P = 0.05). Residual chloride secretion was found in some of the 28 dF508 homozygous patients (three in group II, and one in group III), disclosing that other factors than the CF gene defect itself affect the transepithelial chloride transport. The age at diagnosis correlates significantly with the magnitude of the secretory response, even within the dF508 homozygous patients (r = 0.4, P < 0.05). We conclude that residual chloride secretion in CF is the pathophysiological basis of preserved pancreatic function and delayed presentation of the disease, which is not exclusively determined by the CF genotype.