Dorien J.M. Peters

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.

PKD2, a Gene for Polycystic Kidney Disease That Encodes an Integral Membrane Protein

A second gene for autosomal dominant polycystic kidney disease was identified by positional cloning. Nonsense mutations in this gene (PKD2) segregated with the disease in three PKD2 families. The predicted 968-amino acid sequence of the PKD2 gene product has six transmembrane spans with intracellular amino- and carboxyl-termini. The PKD2 protein has amino acid similarity with PKD1, the Caenorhabditis elegans homolog of PKD1, and the family of voltage-activated calcium (and sodium) channels, and it contains a potential calcium-binding domain.

Conjunction dysfunction: CBP/p300 in human disease

CBP and its homolog p300 are large nuclear molecules that coordinate a variety of transcriptional pathways with chromatin remodeling. They interact with transcriptional activators as well as repressors, direct chromatin-mediated transcription, function in TP53-mediated apoptosis, and participate in terminal differentiation of certain tissue types. Recent evidence suggests that the demand for CBP/p300 is greater than the supply, and that competition for CBP/p300 might play an important role in cell growth regulation. Alterations of the human CBP gene have been implicated in hematological malignancies as well as in congenital malformation and mental retardation. Likewise, the p300 gene has been recently implicated in leukemia and mutations in both alleles have been observed in gastric and colorectal carcinomas. The role of these proteins in human disease coupled with biochemical evidence suggests that CBP and p300 are tumor suppressor proteins essential in cell-cycle control, cellular differentiation and human development.

Unified Criteria for Ultrasonographic Diagnosis of ADPKD

Individuals who are at risk for autosomal dominant polycystic kidney disease are often screened by ultrasound using diagnostic criteria derived from individuals with mutations in PKD1. Families with mutations in PKD2 typically have less severe disease, suggesting a potential need for different diagnostic criteria. In this study, 577 and 371 at-risk individuals from 58 PKD1 and 39 PKD2 families, respectively, were assessed by renal ultrasound and molecular genotyping. Using sensitivity data derived from genetically affected individuals and specificity data derived from genetically unaffected individuals, various diagnostic criteria were compared. In addition, data sets were created to simulate the PKD1 and PKD2 case mix expected in practice to evaluate the performance of diagnostic criteria for families of unknown genotype. The diagnostic criteria currently in use performed suboptimally for individuals with mutations in PKD2 as a result of reduced test sensitivity. In families of unknown genotype, the presence of three or more (unilateral or bilateral) renal cysts is sufficient for establishing the diagnosis in individuals aged 15 to 39 y, two or more cysts in each kidney is sufficient for individuals aged 40 to 59 y, and four or more cysts in each kidney is required for individuals > or = 60 yr. Conversely, fewer than two renal cysts in at-risk individuals aged > or = 40 yr is sufficient to exclude the disease. These unified diagnostic criteria will be useful for testing individuals who are at risk for autosomal dominant polycystic kidney disease in the usual clinical setting in which molecular genotyping is seldom performed.

Genetic heterogeneity in Rubinstein-Taybi syndrome: mutations in both the CBP and EP300 genes cause disease

CREB-binding protein and p300 function as transcriptional coactivators in the regulation of gene expression through various signal-transduction pathways. Both are potent histone acetyl transferases. A certain level of CREB-binding protein is essential for normal development, since inactivation of one allele causes Rubinstein-Taybi syndrome (RSTS). There is a direct link between loss of acetyl transferase activity and RSTS, which indicates that the disorder is caused by aberrant chromatin regulation. We screened the entire CREB-binding protein gene (CBP) for mutations in patients with RSTS by using methods that find point mutations and larger rearrangements. In 92 patients, we were able to identify a total of 36 mutations in CBP. By using multiple ligation-dependent probe amplification, we found not only several deletions but also the first reported intragenic duplication in a patient with RSTS. We extended the search for mutations to the EP300 gene and showed that mutations in EP300 also cause this disorder. These are the first mutations identified in EP300 for a congenital disorder.

Polycystin-1 and -2 Dosage Regulates Pressure Sensing

Autosomal-dominant polycystic kidney disease, the most frequent monogenic cause of kidney failure, is induced by mutations in the PKD1 or PKD2 genes, encoding polycystins TRPP1 and TRPP2, respectively. Polycystins are proposed to form a flow-sensitive ion channel complex in the primary cilium of both epithelial and endothelial cells. However, how polycystins contribute to cellular mechanosensitivity remains obscure. Here, we show that TRPP2 inhibits stretch-activated ion channels (SACs). This specific effect is reversed by coexpression with TRPP1, indicating that the TRPP1/TRPP2 ratio regulates pressure sensing. Moreover, deletion of TRPP1 in smooth muscle cells reduces SAC activity and the arterial myogenic tone. Inversely, depletion of TRPP2 in TRPP1-deficient arteries rescues both SAC opening and the myogenic response. Finally, we show that TRPP2 interacts with filamin A and demonstrate that this actin crosslinking protein is critical for SAC regulation. This work uncovers a role for polycystins in regulating pressure sensing.

Rubinstein–Taybi syndrome: clinical and molecular overview

Rubinstein-Taybi syndrome is characterised by mental retardation, growth retardation and a particular dysmorphology. The syndrome is rare, with a frequency of approximately one affected individual in 100,000 newborns. Mutations in two genes - CREBBP and EP300 - have been identified to cause the syndrome. These two genes show strong homology and encode histone acetyltransferases (HATs), which are transcriptional co-activators involved in many signalling pathways. Loss of HAT activity is sufficient to account for the phenomena seen in Rubinstein-Taybi patients. Although some mutations found in CREBBP are translocations, inversions and large deletions, most are point mutations or small deletions and insertions. Mutations in EP300 are comparatively rare. Extensive screening of patients has revealed mutations in CREBBP and EP300 in around 50% of cases. The cause of the syndrome in the remaining patients remains to be identified, but other genes could also be involved. Here, we describe the clinical presentation of Rubinstein-Taybi syndrome, review the mutation spectrum and discuss the current understanding of causative molecular mechanisms.

Mutations in genes encoding subunits of RNA polymerases I and III cause Treacher Collins syndrome.

We identified a deletion of a gene encoding a subunit of RNA polymerases I and III, POLR1D, in an individual with Treacher Collins syndrome (TCS). Subsequently, we detected 20 additional heterozygous mutations of POLR1D in 252 individuals with TCS. Furthermore, we discovered mutations in both alleles of POLR1C in three individuals with TCS. These findings identify two additional genes involved in TCS, confirm the genetic heterogeneity of TCS and support the hypothesis that TCS is a ribosomopathy.

Autosomal dominant polycystic kidney disease: Evidence for the existence of a third locus in a Portuguese family

Autosomal dominant polycystic kidney disease is characterized by clinical and genetic heterogeneity. Two loci implicated in the disease have previously been mapped (PKD1 on chromosome 16 and PKD2 on chromosome 4). By two point and multipoint linkage analysis, negative lod scores have been found for both chromosome 16 and chromosome 4 markers in a large Portuguese family, indicating that a third PKD locus is involved in the development of the disease.

Genotype-Renal Function Correlation in Type 2 Autosomal Dominant Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) is a common Mendelian disorder that affects approximately 1 in 1000 live births. Mutations of two genes, PKD1 and PKD2, account for the disease in approximately 80 to 85% and 10 to 15% of the cases, respectively. Significant interfamilial and intrafamilial renal disease variability in ADPKD has been well documented. Locus heterogeneity is a major determinant for interfamilial disease variability (i.e., patients from PKD1-linked families have a significantly earlier onset of ESRD compared with patients from PKD2-linked families). More recently, two studies have suggested that allelic heterogeneity might influence renal disease severity. The current study examined the genotype-renal function correlation in 461 affected individuals from 71 ADPKD families with known PKD2 mutations. Fifty different mutations were identified in these families, spanning between exon 1 and 14 of PKD2. Most (94%) of these mutations were predicted to be inactivating. The renal outcomes of these patients, including the age of onset of end-stage renal disease (ESRD) and chronic renal failure (CRF; defined as creatinine clearance < or = 50 ml/min, calculated using the Cockroft and Gault formula), were analyzed. Of all the affected individuals clinically assessed, 117 (25.4%) had ESRD, 47 (10.2%) died without ESRD, 65 (14.0%) had CRF, and 232 (50.3%) had neither CRF nor ESRD at the last follow-up. Female patients, compared with male patients, had a later mean age of onset of ESRD (76.0 [95% CI, 73.8 to 78.1] versus 68.1 [95% CI, 66.0 to 70.2] yr) and CRF (72.5 [95% CI, 70.1 to 74.9] versus 63.7 [95% CI, 61.4 to 66.0] yr). Linear regression and renal survival analyses revealed that the location of PKD2 mutations did not influence the age of onset of ESRD. However, patients with splice site mutations appeared to have milder renal disease compared with patients with other mutation types (P < 0.04 by log rank test; adjusted for the gender effect). Considerable renal disease variability was also found among affected individuals with the same PKD2 mutations. This variability can confound the determination of allelic effects and supports the notion that additional genetic and/or environmental factors may modulate the renal disease severity in ADPKD.