Mutational analysis within the 3' region of the PKD1 gene

Mutational screening of PKD1 and PKD2 in Indian ADPKD patients identified 95 genetic variants

BACKGROUND

Mutation screening of autosomal dominant polycystic kidney disease (ADPKD) cases imply the major involvement of PKD1 mutations in 85% of patients while rest of the cases harbor mutation in PKD2, DNAJB11 and GANAB. This essentially indicates that individual's genotype holds the key for disease susceptibility and its severity.

METHODS

For finding genetic variability underlying the disease pathophysiology, 84 Indian ADPKD cases, 31 family members (12 susceptible) and 122 age matched control were screened for PKD1 and PKD2 using Sanger sequencing, PCR-RFLP and ARMS-PCR.

RESULTS

Genetic screening of Indian ADPKD cases revealed total 67 variants in PKD1 and 28 variants in PKD2. Among the identified variants in PKD1 and PKD2 genes, 35.79% were novel variants and 64.2% recurrent. Further, subcategorization of PKD1 variants showed 14 truncation/frameshift, 21 nonsynonymous, 25 synonymous and 7 intronic variants. Moreover, we observed 40 families with PKD1 pathogenic variants, 7 families with PKD2 pathogenic variants, 9 families with PKD1 & PKD2 pathogenic variants, and 26 families with PKD1/PKD2/PKD1-PKD2 non-pathogenic genetic variants.

CONCLUSION

Present study represented genetic background of Indian ADPKD cases which will be helpful in disease management as well as finding the genetically matched donor for kidney transplant.

Regulation of polycystin-1 ciliary trafficking by motifs at its C-terminus and polycystin-2 but not by cleavage at the GPS site

Failure to localize membrane proteins to the primary cilium causes a group of diseases collectively named ciliopathies. Polycystin-1 (PC1, also known as PKD1) is a large ciliary membrane protein defective in autosomal dominant polycystic kidney disease (ADPKD). Here, we developed a large set of PC1 expression constructs and identified multiple sequences, including a coiled-coil motif in the C-terminal tail of PC1, regulating full-length PC1 trafficking to the primary cilium. Ciliary trafficking of wild-type and mutant PC1 depends on the dose of polycystin-2 (PC2, also known as PKD2), and the formation of a PC1-PC2 complex. Modulation of the ciliary trafficking module mediated by the VxP ciliary-targeting sequence and Arf4 and Asap1 does not affect the ciliary localization of full-length PC1. PC1 also promotes PC2 ciliary trafficking. PC2 mutations truncating its C-terminal tail but not those changing the VxP sequence to AxA or impairing the pore of the channel, leading to a dead channel, affect PC1 ciliary trafficking. Cleavage at the GPCR proteolytic site (GPS) of PC1 is not required for PC1 trafficking to cilia. We propose a mutually dependent model for the ciliary trafficking of PC1 and PC2, and that PC1 ciliary trafficking is regulated by multiple cis-acting elements. As all pathogenic PC1 mutations tested here are defective in ciliary trafficking, ciliary trafficking might serve as a functional read-out for ADPKD.

Polycystin-1 regulates actin cytoskeleton organization and directional cell migration through a novel PC1-Pacsin 2-N-Wasp complex

How epithelial cells form a tubule with defined length and lumen diameter remains a fundamental question in cell and developmental biology. Loss of control of tubule lumen size in multiple organs including the kidney, liver and pancreas features polycystic kidney disease (PKD). To gain insights into autosomal dominant polycystic kidney disease, we performed yeast two-hybrid screens using the C-terminus of polycystin-1 (PC1) as bait. Here, we report that PC1 interacts with Pacsin 2, a cytoplasmic phosphoprotein that has been implicated in cytoskeletal organization, vesicle trafficking and more recently in cell intercalation during gastrulation. PC1 binds to a 107-residue fragment containing the α3 helix of the F-BAR domain of Pacsin 2 via a coiled-coil domain in its C-tail. PC1 and Pacsin 2 co-localize on the lamellipodia of migrating kidney epithelial cells. PC1 and Pacsin 2-deficient kidney epithelial cells migrate at a slower speed with reduced directional persistency. We further demonstrate that PC1, Pacsin 2 and N-Wasp are in the same protein complex, and both PC1 and Pacsin 2 are required for N-Wasp/Arp2/3-dependent actin remodeling. We propose that PC1 modulates actin cytoskeleton rearrangements and directional cell migration through the Pacsin 2/N-Wasp/Arp2/3 complex, which consequently contributes to the establishment and maintenance of the sophisticated tubular architecture. Disruption of this complex contributes to cyst formation in PKD.

Protein phosphatase-1α interacts with and dephosphorylates polycystin-1

Polycystin signaling is likely to be regulated by phosphorylation. While a number of potential protein kinases and their target phosphorylation sites on polycystin-1 have been identified, the corresponding phosphatases have not been extensively studied. We have now determined that polycystin-1 is a regulatory subunit for protein phosphatase-1α (PP1α). Sequence analysis has revealed the presence of a highly conserved PP1-interaction motif in the cytosolic, C-terminal tail of polycystin-1; and we have shown that transfected PP1α specifically co-immunoprecipitates with a polycystin-1 C-tail construct. To determine whether PP1α dephosphorylates polycystin-1, a PKA-phosphorylated GST-polycystin-1 fusion protein was shown to be dephosphorylated by PP1α but not by PP2B (calcineurin). Mutations within the PP1-binding motif of polycystin-1, including an autosomal dominant polycystic kidney disease (ADPKD)-associated mutation, significantly reduced PP1α-mediated dephosphorylation of polycystin-1. The results suggest that polycystin-1 forms a holoenzyme complex with PP1α via a conserved PP1-binding motif within the polycystin-1 C-tail, and that PKA-phosphorylated polycystin-1 serves as a substrate for the holoenzyme.

Autosomal dominant polycystic kidney disease: comprehensive mutation analysis of PKD1 and PKD2 in 700 unrelated patients

Autosomal dominant polycystic kidney disease (ADPKD), the most common inherited kidney disorder, is caused by mutations in PKD1 or PKD2. The molecular diagnosis of ADPKD is complicated by extensive allelic heterogeneity and particularly by the presence of six highly homologous sequences of PKD1 exons 1-33. Here, we screened PKD1 and PKD2 for both conventional mutations and gross genomic rearrangements in up to 700 unrelated ADPKD patients--the largest patient cohort to date--by means of direct sequencing, followed by quantitative fluorescent multiplex polymerase chain reaction or array-comparative genomic hybridization. This resulted in the identification of the largest number of new pathogenic mutations (n = 351) in a single publication, expanded the spectrum of known ADPKD pathogenic mutations by 41.8% for PKD1 and by 23.8% for PKD2, and provided new insights into several issues, such as the population-dependent distribution of recurrent mutations compared with founder mutations and the relative paucity of pathogenic missense mutations in the PKD2 gene. Our study, together with others, highlights the importance of developing novel approaches for both mutation detection and functional validation of nondefinite pathogenic mutations to increase the diagnostic value of molecular testing for ADPKD.

Genetic variations of patients with familial or multiple melanoma in Southern Brazil

BACKGROUND

Patients with familial melanoma or multiple primary melanoma represent a high-risk population to hereditary melanoma. Mutations in susceptibility genes, such as CDKN2A, CDK4 and MC1R, have been associated with the development of melanoma.

OBJECTIVES

The purpose of this study was to determine the genotypic background of patients with familial and/or multiple melanoma in southern Brazil.

METHODS

This study analysed 33 cases (5 patients with multiple primary melanoma and 28 patients from families with at least two well documented cases) and 29 controls. Genomic analysis of CDKN2A and CDK4 genes by PCR-SSCP analysis and sequencing and direct sequencing of MC1R were performed in all individuals.

RESULTS

No functional mutations in CDKN2A or CDK4 were detected in the 62 individuals. Infrequent variants in polymorphic loci of CDKN2A gene were identified in 15 participants (24.2%) and 24/33 (72.8%) cases and 19/27 (70.4%) controls reported at least one infrequent variant in MC1R (P = 0.372). Furthermore, a non-significant tendency towards an association between melanoma risk and MC1R variants G274A and C451T and a non-significant linear tendency to the number of infrequent high-risk variants in MC1R were observed.

CONCLUSIONS

These results suggest that in southern Brazilian population, CDKN2A or CDK4 germinal alterations may have a weaker influence than previously thought and environmental risk factors may play a central role in melanoma susceptibility. However, considering the tendency observed for gene MC1R, low-penetrance genes may be a relevant aetiological factor in southern Brazil with fair skin population and high sunlight exposure.

Biogenesis of mammalian microRNAs by a non-canonical processing pathway

Canonical microRNA biogenesis requires the Microprocessor components, Drosha and DGCR8, to generate precursor-miRNA, and Dicer to form mature miRNA. The Microprocessor is not required for processing of some miRNAs, including mirtrons, in which spliceosome-excised introns are direct Dicer substrates. In this study, we examine the processing of putative human mirtrons and demonstrate that although some are splicing-dependent, as expected, the predicted mirtrons, miR-1225 and miR-1228, are produced in the absence of splicing. Remarkably, knockout cell lines and knockdown experiments demonstrated that biogenesis of these splicing-independent mirtron-like miRNAs, termed 'simtrons', does not require the canonical miRNA biogenesis components, DGCR8, Dicer, Exportin-5 or Argonaute 2. However, simtron biogenesis was reduced by expression of a dominant negative form of Drosha. Simtrons are bound by Drosha and processed in vitro in a Drosha-dependent manner. Both simtrons and mirtrons function in silencing of target transcripts and are found in the RISC complex as demonstrated by their interaction with Argonaute proteins. These findings reveal a non-canonical miRNA biogenesis pathway that can produce functional regulatory RNAs.

High Resolution Melt analysis for mutation screening in PKD1 and PKD2

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary kidney disorder. It is characterized by focal development and progressive enlargement of renal cysts leading to end-stage renal disease. PKD1 and PKD2 have been implicated in ADPKD pathogenesis but genetic features and the size of PKD1 make genetic diagnosis tedious.

METHODS

We aim to prove that high resolution melt analysis (HRM), a recent technique in molecular biology, can facilitate molecular diagnosis of ADPKD. We screened for mutations in PKD1 and PKD2 with HRM in 37 unrelated patients with ADPKD.

RESULTS

We identified 440 sequence variants in the 37 patients. One hundred and thirty eight were different. We found 28 pathogenic mutations (25 in PKD1 and 3 in PKD2 ) within 28 different patients, which is a diagnosis rate of 75% consistent with literature mean direct sequencing diagnosis rate. We describe 52 new sequence variants in PKD1 and two in PKD2.

CONCLUSION

HRM analysis is a sensitive and specific method for molecular diagnosis of ADPKD. HRM analysis is also costless and time sparing. Thus, this method is efficient and might be used for mutation pre-screening in ADPKD genes.

Analysis of the cytoplasmic interaction between polycystin-1 and polycystin-2

Autosomal dominant polycystic kidney disease (ADPKD) arises following mutations of either Pkd1 or Pkd2. The proteins these genes encode, polycystin-1 (PC1) and polycystin-2 (PC2), form a signaling complex using direct intermolecular interactions. Two distinct domains in the C-terminal tail of PC2 have recently been identified, an EF-hand and a coiled-coil domain. Here, we show that the PC2 coiled-coil domain interacts with the C-terminal tail of PC1, but that the PC2 EF-hand domain does not. We measured the K0.5 of the interaction between the C-terminal tails of PC1 and PC2 and showed that the direct interaction of these proteins is abrogated by a PC1 point mutation that was identified in ADPKD patients. Finally, we showed that overexpression of the PC1 C-terminal tail in MDCK cells alters the Ca2+ response, but that overexpression of the PC1 C-terminal tail containing the disease mutation does not. These results allow a more detailed understanding of the mechanism of pathogenic mutations in the cytoplasmic regions of PC1 and PC2.

New mutations in the PKD1 gene in Czech population with autosomal dominant polycystic kidney disease

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary renal disease. The disease is caused by mutations of the PKD1 (affecting roughly 85% of ADPKD patients) and PKD2 (affecting roughly 14% of ADPKD patients) genes, although in several ADPKD families, the PKD1 and/or PKD2 linkage was not found. Mutation analysis of the PKD1 gene is complicated by the presence of highly homologous genomic duplications of the first two thirds of the gene.

METHODS

The direct detection of mutations in the non-duplicated region of the PKD1 gene was performed in 90 unrelated individuals, consisting of 58 patients with end-stage renal failure (manifesting before their 50th year of life) and 32 individuals from families where the disease was clearly linked to the PKD1 gene. Mutation screening was performed using denaturing gradient gel electrophoresis (DGGE). DNA fragments showing an aberrant electrophoretic banding pattern were sequenced.

RESULTS

In the non-duplicated region of the PKD1 gene, 19 different likely pathogenic germline sequence changes were identified in 19 unrelated families/individuals. Fifteen likely pathogenic sequence changes are unique for the Czech population. The following probable mutations were identified: 9 nonsense mutations, 6 likely pathogenic missense mutations, 2 frameshifting mutations, one in-frame deletion and probable splice site mutation. In the non-duplicated region of the PKD1 gene, 16 different polymorphisms or unclassified variants were detected.

CONCLUSION

Twenty probable mutations of the PKD1 gene in 90 Czech individuals (fifteen new probable mutations) were detected. The establishment of localization and the type of causal mutations and their genotype phenotype correlation in ADPKD families will improve DNA diagnosis and could help in the assessment of the clinical prognosis of ADPKD patients.

CDKN2A mutations in melanoma families from Uruguay

BACKGROUND

Familial melanoma, a cluster of several cases within a single family, accounts for approximately 10% of cases of melanoma. Hereditary melanoma is defined as two or more first-degree relatives having melanoma. A member of a melanoma-prone family has a 35-70-fold increased relative risk of developing a melanoma. Genetic susceptibility is linked to the major susceptibility genes CDKN2A and CDK4, and the minor susceptibility gene MC1R.

OBJECTIVES

To determine the clinical and genetic characteristics of cutaneous melanoma in melanoma-prone families from Uruguay.

METHODS

We studied 13 individuals from six melanoma-prone families living in Uruguay. Phenotype, familial and personal history were recorded. Molecular screening of CDKN2A and CDK4 was done by polymerase chain reaction-single strand conformational polymorphism analysis. The MC1R gene was sequenced.

RESULTS

Mutations in CDKN2A were detected in five of six families: c.-34G>T, p.G101W and p.E88X. A novel germline mutation p.E88X, associated with hereditary melanoma in two unrelated families, is described. We hypothesize that a founder effect occurred probably in the Mediterranean region. No mutations in CDK4 were detected. Six different MC1R variants, all previously reported, were present in Uruguayan families.

CONCLUSIONS

The overall rate of deleterious CDKN2A mutations in our familial melanoma pedigrees, even though the sample size is small, was considerably higher (83%) than the often quoted range.

Dermoscopic features of melanomas associated with MC1R variants in Spanish CDKN2A mutation carriers

BACKGROUND

The presence of at least one MC1R gene variant is associated with a reduction in age at melanoma diagnosis in families with CDKN2A mutations.

OBJECTIVES

To describe dermoscopic features of early melanoma in CDKN2A gene mutation-positive Spanish individuals and to evaluate the possibility of a correlation between particular dermatoscopic pattern and MC1R gene variants.

METHODS

Patients in whom a melanoma was diagnosed during specific follow up of high-risk individuals carrying CDKN2A mutations (with familial or personal history of previous melanoma) were included in this study. The decision to remove such melanomas was taken on the basis of history, clinical and dermoscopic evaluations including total body photography and digital dermoscopy.

RESULTS

Of the nine patients included in this study, three were noncarriers of the red hair MC1R polymorphism, three patients had one red hair MC1R polymorphism and three patients had two red hair MC1R polymorphisms. On dermoscopic analysis of suspect melanocytic lesions we found that the mean +/- SD ABCD total dermoscopy score (TDS) was significantly higher in noncarriers of red hair MC1R polymorphisms than in carriers of two MC1R gene red hair variants (6.8 +/- 0.4 vs. 4.4 +/- 0.9; P = 0.014).

CONCLUSIONS

Early melanomas in patients with two MC1R red hair variants may be difficult to diagnose definitively by dermoscopy because, in our limited experience, they show fewer colours and structures and have a lower TDS. In such melanomas, subtle atypical vessels and other changes detected by digital image follow up may be useful to confirm the diagnosis of melanoma. An integrated approach including clinical history and dermoscopic data (also considering additional information, such as the presence of atypical vessels) should be utilized in evaluating these high-risk patients. Further studies are necessary to confirm our suggestion.

Genetic studies in variegate porphyria in Spain. Identification of gene mutations and family study for carrier detection

BACKGROUND

First, to establish the mutations of the protoporphyrinogen-oxidase (PPOX) gene in four Spanish patients with variegate porphyria (VP). Second, study of carrier status detection in the families, including a four-generation Balearic family. Third, evaluation of the results of carrier detection screening methods.

DESIGN

Blood samples of four patients and of 139 members belonging to four families, including four generations of a Balearic family were processed for mutation analysis of the 13 exons of PPOX gene. Biochemical studies were performed together (blood and faecal porphyrin analysis) and plasma fluorescence scanning for 626 nm peak emission detection. A questionnaire regarding clinical manifestations was submitted to all family members studied.

RESULTS

Single strand conformational analysis (SSCP) of DNA allowed the detection of the following mutations: W224R, 746delT: exon 7, 1077-1082insC: exon 10, and IVS6+2T-->A. Mutation was present in 19 of the 139 members of the families studied. Clinical manifestations or biochemical alterations were checked in the carriers detected and found as not relevant or not present. Only 11 members of the 19 mutation-bearing individuals showed plasma fluorescence PV peak positivity.

CONCLUSION

Demonstration of gene mutation is the most reliable means of detecting carriers in studies of variegate porphyria families. DNA analysis is the most sensitive carrier detection method and also allows transmission behaviour of the genetic defect to be established in successive generations of the affected families.

Genetic and biochemical characterization of 16 acute intermittent porphyria cases with a high prevalence of the R173W mutation

Acute intermittent porphyria (AIP) is a metabolic disease with a variable prevalence among different countries. In some areas of southern Europe it remains to be fully evaluated. We undertook a genetic and biochemical study of 16 unrelated Spanish AIP patients and relatives. The genetic analyses showed they harboured the following mutations in the porphobilinogen deaminase gene: R173W, G111R, L278P, L238P, R116W, R26C, 340insT, 730delCT, 691del30bp, and IVS14+1g>a. The mutation R173W was found in 6 patients (37.5%), including the only patients of our series with >3 recurrent porphyria attacks. While in clinical remission, all AIP patients exhibited sustained increased excretion of porphyrins and precursors. PBG excretion showed a high between-subject variation and was not related to erythrocyte PBG deaminase activity. The study of family members allowed the identification of 22 asymptomatic AIP carriers. These included 8 persons harbouring the R173W mutation belonging to four different families. Six of these latent AIP subjects showed increased PBG elimination, and in two the urinary levels were >10-fold the normal limit. These results reinforce the hypothesis that the R173W mutation may have a high biochemical and clinical penetrance among AIP patients.

Biochemical and genetic characterization of four cases of hereditary coproporphyria in Spain

We report a biochemical and genetic characterization of four cases of hereditary coproporphyria (HCP) in Spain. All patients showed a typical HCP porphyrin excretion pattern with a high concentration of coproporphyrins in feces and inverted I:III isomer ratio. The porphyrin precursors in urine were found elevated in two patients who showed acute symptoms. The analysis of the CPO gene showed that three cases harboured novel mutations: V135A (404T>C; exon 1); L214R (641T>G; exon 2); and P249R (746C>G; exon 3) and in the fourth, a previously described R426X mutation in exon 6.

Role of the CDKN2A Locus in Patients With Multiple Primary Melanomas

Purpose

We have studied a consecutive case series of patients with multiple primary melanoma (MPM) for the involvement of the melanoma susceptibility loci CDKN2A and CDK4.

Patients and Methods

One hundred four MPM patients (81 patients with two primary melanomas, 14 with three, five with four, one with five, two with six, and one with seven) were included.

Results

Seven different CDKN2A germline mutations were identified in 17 patients (16.3%). In total, we identified 15 CDKN2A exon 2, one exon 1α missense mutation, and one exon 1β frameshift mutation. The age of onset was significantly lower and the number of primary melanomas higher in patients with mutations. CDKN2A mutations were more frequent in patients with familial history of melanoma (35.5%) compared with patients without (8.2%), with a relative risk (RR) of 4.32 (95% CI, 1.76 to 10.64; P = .001), and in patients with more than two melanomas (39.1%) compared with patients with only two melanomas (10%) with an RR of 3.29 (95% CI, 1.7 to 6.3; P = .002). The A148T polymorphism was more frequent in patients with MPMs than in the control population (P = .05). A variant of uncertain significance, A127S, was also detected in one patient. No CDK4 mutations were identified, suggesting that it has a low impact in susceptibility to MPM.

Conclusion

MPM patients are good candidates for CDKN2A mutational screening. These patients and some of their siblings should be included in a program of specific follow-up with total body photography and digital dermoscopy, which will result in the early detection of melanoma in this subset of high-risk patients and improve phenotypic characterization.

Mutation analysis of autosomal dominant polycystic kidney disease genes in Han Chinese

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in two genes, PKD1 and PKD2. The complexity of these genes, particularly PKD1, has complicated genetic screening, though recent advances have provided new opportunities for amplifying these genes. In the Han Chinese population, no complete mutational analysis has previously been conducted across the entire span of PKD1 and PKD2. Here, we used single-strand conformation polymorphism (SSCP) analysis to screen the entire coding sequence of PKD1 and PKD2 in 85 healthy controls and 72 Han Chinese from 24 ADPKD pedigrees. In addition to 11 normal variants, we identified 17 mutations (12 in PKD1 and 5 in PKD2), 15 of which were novel ones (11 for PKD1 and 4 for PKD2). We did not identify any seeming mutational hot spots in PKD1 and PKD2. Notably, we found several disease-associated C-T or G-A mutations that led to charge or hydrophobicity changes in the corresponding amino acids. This suggests that the mutations cause conformational alterations in the PKD1 and PKD2 protein products that may impact the normal protein functions. Our study is the first report of screenable mutations in the full-length PKD1 and PKD2 genes of the Han Chinese, and also offers a benchmark for comparisons between Caucasian and Han ADPKD pedigrees and patients.

Autosomal recessive Alport's syndrome and benign familial hematuria are collagen type IV diseases

BACKGROUND

Alport's syndrome (AS) is a genetically heterogeneous renal hereditary disease. Mutations in collagen type IV genes have been described to be responsible for X-linked (COL4A5), autosomal recessive, and autosomal dominant AS (COL4A3/COL4A4). Moreover, at least 40% of benign familial hematuria (BFH) cases cosegregate with the COL4A3/COL4A4 loci, following a dominant pattern of inheritance. Therefore, it has been suggested that BFH may represent the carrier state for autosomal recessive AS.

METHODS

We report a mutational study of the COL4A3 and COL4A4 genes in 14 AS and 2 BFH families. When possible, linkage analysis has been performed to confirm the pattern of inheritance. One affected proband from each family underwent mutation screening by single-strand conformation polymorphism/heteroduplex analysis.

RESULTS

We identified 13 mutations within the COL4A3 gene and 2 mutations within the COL4A4 gene, 9 of which are first reported here. We also detected 14 polymorphisms within the COL4A3 gene and 15 polymorphisms within the COL4A4 gene, 7 of them not previously described. In 2 of our AS families, we found mutations previously reported for BFH, and we characterized a novel mutation shared by an AS and a BFH family.

CONCLUSION

Collagen type IV nephropathy is an entity in itself, and phenotypic manifestations of COL4A3/COL4A4 mutations may range from monosymptomatic hematuria (BFH) to severe renal failure (AS), depending on the gene dosage. In 3 of our families, we genetically confirmed that BFH represents the carrier state for autosomal recessive AS.

Autosomal dominant polycystic kidney disease - clinical and genetic aspects

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common inherited disorders in humans. It accounts for 8-10% of the cases of end-stage renal disease worldwide, thus representing a serious medical, economical and social problem. ADPKD is in fact a systemic disorder, characterized with the development of cysts in the ductal organs (mainly the kidneys and the liver), also with gastrointestinal and cardiovascular abnormalities. In the last decade there was significant progress in uncovering the genetic foundations and in understanding of the pathogenic mechanisms leading to the renal impairment. This review will retrace the current knowledge about the epidemiology, pathogenesis, genetics, genetic and clinical heterogeneity, diagnostics and treatment of ADPKD.

Polycystin-1 activates and stabilizes the polycystin-2 channel

Autosomal dominant polycystic kidney disease (ADPKD) is a prevalent genetic disorder largely caused by mutations in the PKD1 and PKD2 genes that encode the transmembrane proteins polycystin-1 and -2, respectively. Both proteins appear to be involved in the regulation of cell growth and maturation, but the precise mechanisms are not yet well defined. Polycystin-2 has recently been shown to function as a Ca(2+)-permeable, non-selective cation channel. Polycystin-2 interacts through its cytoplasmic carboxyl-terminal region with a coiled-coil motif in the cytoplasmic tail of polycystin-1 (P1CC). The functional consequences of this interaction on its channel activity, however, are unknown. In this report, we show that P1CC enhanced the channel activity of polycystin-2. R742X, a disease-causing polycystin-2 mutant lacking the polycystin-1 interacting region, fails to respond to P1CC. Also, P1CC containing a disease-causing mutation in its coiled-coil motif loses its stimulatory effect on wild-type polycystin-2 channel activity. The modulation of polycystin-2 channel activity by polycystin-1 may be important for the various biological processes mediated by this molecular complex.

SCA8 in the Spanish population including one homozygous patient

Controversial data have been reported about SCA8 since its description in 1999. The most accepted hypothesis is that CTG expansions within the CTA/CTG combined repeat expansion in the SCA8 locus causes SCA8. It is inherited as a dominant trait with reduced penetrance. The present study, reports the first data regarding SCA8 in the Spanish population and the clinical findings in patients carrying expanded alleles, including one homozygous patient. Two hundred and forty-six individuals from the Spanish population, including controls (149) and ataxic patients (97), were studied. DNA was extracted from blood samples using standard methods. Amplification of the CTA/CTG 3'untranslated region was achieved by PCR using primers SCA8-F3 and SCA8-R4 and conditions described previously. Neurological reevaluation was done in individuals carrying the expanded allele. We detected five unrelated expanded alleles corresponding to three affected patients (one of them homozygous) and one healthy individual. SCA8 represents 4% of the total dominant spinocerebellar ataxias studied in our group (Spanish population) (three index patients out of 75 dominant ataxic independent nucleus). The patient that resulted homozygous for the expansion is a 25-year-old man with a clinical picture of progressive ataxia and dysarthria that began at the age of 12. On neurological examination, he showed ataxia, slight dysarthria and nystagmus to the extreme lateral gaze. A cranial MRI showed global atrophy of cerebellum but the brainstem was spared. Family history showed the presence of ataxia in his grandfather and father. His mother is healthy at the age of 52 and a molecular study of SCA8 reveals one allele that could be considered as premutated. She has no ataxia antecedents in her family. Our results provide additional information about the SCA8 expansion, within the Spanish population. These results are in agreement with the hypothesis of the CTG expansion in the SCA8 locus being responsible for the SCA8 ataxia showing reduced penetrance. Besides homozygous status, advancing age at onset (as previously described for other SCAs) supports this idea.

Three novel mutations of the PKD1 gene in Korean patients with autosomal dominant polycystic kidney disease

Mutations at the PKD1 locus account for 85% of cases of the common genetic disorder called autosomal dominant polycystic kidney disease (ADPKD). Screening for mutations of the PKD1 gene is complicated by the genomic structure of the 5'-duplicated region encoding 75% of the gene. To date, more than 90 mutations of the PKD1 gene have been reported in the European and American populations, and relatively little information is available concerning the pattern of mutations present in the Asian populations. We looked for mutations of the PKD1 gene in 51 unrelated Korean ADPKD patients, using polymerase chain reaction (PCR) with primer pairs located in the 3' single-copy region of the PKD1 gene and by single-strand conformation polymorphism (SSCP) analysis. We found three novel mutations, a G to A substitution at nucleotide 11012 (G3601S), a C to A substitution at nucleotide 11312 (Q3701X), and a C to T substitution at nucleotide 12971 (P4254S), and a single polymorphism involving a G to C substitution at nucleotide 11470 (L3753L). These mutations were not found in control individuals, and no other mutations in the 3' single-copy region of the PKD1 gene of patients with these mutations were observed. In particular, P4254S segregated with the disease phenotype. The clinical data of affected individuals from this study, and of previously reported Korean PKD1 mutations, showed that patients with frameshift or nonsense mutations were more prone to develop end-stage renal failure than those with missense mutations. Our findings indicate that many different PKD1 mutations are likely to be responsible for ADPKD in the Korean population, as in the Western population.

A complete mutation screen of the ADPKD genes by DHPLC

BACKGROUND

Genetic analysis is a useful diagnostic tool in autosomal dominant polycystic kidney disease (ADPKD), especially when imaging results are equivocal. However, molecular diagnostics by direct mutation screening has proved difficult in this disorder due to genetic and allelic heterogeneity and complexity of the major locus, PKD1.

METHODS

A protocol was developed to specifically amplify the exons of PKD1 and PKD2 from genomic DNA as 150 to 450 bp amplicons. These fragments were analyzed by the technique of denaturing high-performance liquid chromatography (DHPLC) using a Wave Fragment Analysis System (Transgenomics) to detect base-pair changes throughout both genes. DHPLC-detected changes were characterized by sequencing.

RESULTS

Cost effective and sensitive mutation screening of the entire coding regions of PKD1 and PKD2 by DHPLC was optimized. All base-pair mutations to these genes that we previously characterized were detected as an altered DHPLC profile. To assess this method for routine diagnostic use, samples from a cohort of 45 genetically uncharacterized ADPKD patients were analyzed. Twenty-nine definite mutations were detected, 26 PKD1, 3 PKD2 and a further five possible missense mutations were characterized leading to a maximal detection rate of 76%. A high level of polymorphism of PKD1 also was detected, with 71 different changes defined. The reproducibility of the DHPLC profile enabled the recognition of many common polymorphisms without the necessity for re-sequencing.

CONCLUSIONS

DHPLC has been demonstrated to be an efficient and effective means for gene-based molecular diagnosis of ADPKD. Differentiating missense mutations and polymorphisms remains a challenge, but family-based segregation analysis is helpful.

Mutations in theCOL4A4 and COL4A3 genes cause familial benign hematuria

Familial benign hematuria (FBH) is a common autosomal dominant disorder characterized by the presence of persistent or recurrent hematuria. The clinical and pathologic features of this syndrome resemble those of early Alport syndrome (AS), and for this reason a common molecular defect has been proposed. The COL4A3/4 genes seem to be involved in both autosomal AS and FBH. This study involves a linkage analysis for the COL4A3/4 loci and a search for mutations within these genes in 11 biopsy-proven FBH families. Haplotype analysis showed that linkage to the COL4A3/4 locus could not be excluded in eight of nine families. One family was not linked to this locus; however, it included three affected women who could be X-linked AS carriers. Two families were too small to perform linkage analysis. COL4A3 and COL4A4 mutation screening disclosed six new pathogenic mutations, two in the COL4A3 gene (G985V and G1015E) and four in the COL4A4 gene (3222insA, IVS23-1G>C, 31del11, and G960R). It is the first time that mutations within the COL4A3 gene are described in families with FBH. This study clearly demonstrates the main role of the COL4A4 and COL4A3 genes in the pathogenesis of FBH.

Cation channel regulation by COOH-terminal cytoplasmic tail of polycystin-1: mutational and functional analysis

Polycystin-1 (PKD1) mutations account for approximately 85% of autosomal dominant polycystic kidney disease (ADPKD). We have shown previously that oocyte surface expression of a transmembrane fusion protein encoding part of the cytoplasmic COOH terminus of PKD1 increases activity of a Ca2+-permeable cation channel. We show here that human ADPKD mutations incorporated into this fusion protein attenuated or abolished encoded cation currents. Point mutations and truncations showed that cation current expression requires integrity of a region encompassing the putative coiled coil domain of the PKD1 cytoplasmic tail. Whereas these loss-of-function mutants did not exhibit dominant negative phenotypes, coexpression of a fusion protein expressing the interacting COOH-terminal cytoplasmic tail of PKD2 did suppress cation current. Liganding of the ectodomain of the PKD1 fusion protein moderately activated cation current. The divalent cation permeability and pharmacological profile of the current has been extended. Inducible expression of the PKD1 fusion in EcR-293 cells was also associated with activation of cation channels and increased Ca2+ entry.

Novel mutations of PKD1 gene in Chinese patients with autosomal dominant polycystic kidney disease

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is a common disease in China. The major gene responsible for ADPKD, PKD1, has been fully characterized and shown to encode an integral membrane protein, polycystin 1, which is thought to be involved in cell-cell and cell-matrix interaction. Until now, 82 mutations of PKD1 gene have been reported in European, American, and Asian populations. However, there has been no report on mutations of the PKD1 gene in a Chinese population.

METHODS

Eighty Chinese patients in 60 families with ADPKD were screened for mutations in the 3' region of the PKD1 gene using polymerase chain reaction-single-strand conformation polymorphism (PCR-SSCP) and DNA-sequencing techniques.

RESULTS

Three mutations were found. The first mutation is a 12593delA frameshift mutation in exon 45, and the polycystin change is 4129WfsX4197, 107 amino acids shorter than the normal polycystin (4302aa). The second mutation is a 12470InsA frameshift mutation in exon 45, producing 4088DfsX4156, and the predicted protein is 148 amino acids shorter than the normal. The third one is a 11151C-->T transition in exon 37 converting Pro3648 to Leu. In addition, nine DNA variants, including IVS44delG, were identified.

CONCLUSIONS

Three mutations in Chinese ADPKD patients are described and all of them are de novo mutations. Data obtained from mutation analysis also suggests that the mutation rate of the 3' single-copy region of PKD1 in Chinese ADPKD patients is very low, and there are no mutation hot spots in the PKD1 gene. Mutations found in Chinese ADPKD patients, including nucleotide substitution and minor frameshift, are similar to the findings reported by other researchers. Many mutations of the PKD1 gene probably exist in the duplicated region, promoter region, and the introns of PKD1.

Mutational analysis within the 3' region of the PKD1 gene in Japanese families

Autosomal dominant polycystic kidney disease (ADPKD) is a widespread genetic disease that causes renal failure. One of the genes that is responsible for this disease, PKD1, has been identified and characterized. Many mutations of the PKD1 gene have been identified in the Caucasian population. We investigated the occurrence of mutations in this gene in the Japanese population. We analyzed each exon in the 3' single copy region of the gene between exons 35 and 46 in genomic DNA obtained from 69 patients, using a PCR-based direct sequencing method. Four missense mutations (T3509M, G3559R, R3718Q, R3752W), one deletion mutation (11307del61bp) and one polymorphism (L3753L) were identified, and their presence confirmed by allele-specific oligonucleotide (ASO) hybridization. These were novel mutations, except for R3752W, and three of them were identified in more than two families. Mutation analysis of the PKD1 gene in the Japanese population is being reported for the first time.

Pkd1 unusual DNA conformations are recognized by nucleotide excision repair

The 2.5-kilobase pair poly(purine.pyrimidine) (poly(R.Y)) tract present in intron 21 of the polycystic kidney disease 1 (PKD1) gene has been proposed to contribute to the high mutation frequency of the gene. To evaluate this hypothesis, we investigated the growth rates of 11 Escherichia coli strains, with mutations in the nucleotide excision repair, SOS, and topoisomerase I and/or gyrase genes, harboring plasmids containing the full-length tract, six 5'-truncations of the tract, and a control plasmid (pSPL3). The full-length poly(R.Y) tract induced dramatic losses of cell viability during the first few hours of growth and lengthened the doubling times of the populations in strains with an inducible SOS response. The extent of cell loss was correlated with the length of the poly(R.Y) tract and the levels of negative supercoiling as modulated by the genotype of the strains or drugs that specifically inhibited DNA gyrase or bound to DNA directly, thereby affecting conformations at specific loci. We conclude that the unusual DNA conformations formed by the PKD1 poly(R.Y) tract under the influence of negative supercoiling induced the SOS response pathway, and they were recognized as lesions by the nucleotide excision repair system and were cleaved, causing delays in cell division and loss of the plasmid. These data support a role for this sequence in the mutation of the PKD1 gene by stimulating repair and/or recombination functions.

Molecular genetics and pathogenesis of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a common and systemic disease characterized by formation of focal cysts. Of the three potential causes of cysts, downstream obstruction, compositional changes in extracellular matrix, and proliferation of partially dedifferentiated cells, evidence strongly supports the latter as the primary abnormality. In the vast majority of cases, the disease is caused by mutations in PKD1 or PKD2, and appears to be recessive at the cellular level. Somatic second hits in the normal allele of cells containing the germ line mutation initiate or accelerate formation of cysts. The intrinsically high frequency of somatic second hits in epithelia appears to be sufficient to explain the frequent occurrence of somatic second hits in the disease-causing genes. PKD1 and PKD2 encode a putative adhesive/ion channel regulatory protein and an ion channel, respectively. The two proteins interact directly in vitro. Their cellular and subcellular localization suggest that they may also function independently in a common signaling pathway that may involve the membrane skeleton and that links cell-cell and cell-matrix adhesion to the development of cell polarity.

Inheritance of a stable mutation in a family with early-onset disease

Autosomal/dominant polycystic kidney disease (ADPKD) exhibits a high inter- and intrafamilial heterogeneity partly explained by the involvement of at least 3 different genes in the disorder transmission. PKD1, the major locus, is located on chromosome 16p. The occurrence of very early-onset cases of ADPKD (sometimes in utero) in a few PKD1 families or the increased severity of the disease in successive generations raise the question of anticipation. This is a subject of controversial discussion. This report deals with the molecular analysis in families with very early-onset ADPKD. The finding of the same stable mutation with such different phenotypes rules out a dynamic mutation. The molecular basis of severe childhood PKD in typical ADPKD families remains unclear; it may include segregation of modifying genes or unidentified factors and the two-hit mechanism.

Thirteen novel mutations of the replicated region of PKD1 in an Asian population

BACKGROUND

Mutations of PKD1 are thought to account for approximately 85% of all mutations in autosomal dominant polycystic kidney disease (ADPKD). The search for PKD1 mutations has been hindered by both its large size and complicated genomic structure. To date, few mutations that affect the replicated segment of PKD1 have been described, and virtually all have been reported in Caucasian patients.

METHODS

In the present study, we have used a long-range polymerase chain reaction (PCR)-based strategy previously developed by our laboratory to analyze exons in the replicated region of PKD1 in a population of 41 unrelated Thai and 6 unrelated Korean families with ADPKD. We have amplified approximately 3.5 and approximately 5 kb PKD1 gene-specific fragments (5'MR and 5'LR) containing exons 13 to 15 and 15 to 21 and performed single-stand conformation analysis (SSCA) on nested PCR products.

RESULTS

Nine novel pathogenic mutations were detected, including six nonsense and three frameshift mutations. One of the deletions was shown to be a de novo mutation. Four potentially pathogenic variants, including one 3 bp insertion and three missense mutations, were also discovered. Two of the nonconservative amino acid substitutions were predicted to disrupt the three-dimensional structure of the PKD repeats. In addition, six polymorphisms, including two missense and four silent nucleotide substitutions, were identified. Approximately 25% of both the pathogenic and normal variants were found to be present in at least one of the homologous loci.

CONCLUSION

To our knowledge, this is the first report of mutation analysis of the replicated region of PKD1 in a non-Caucasian population. The methods used in this study are widely applicable and can be used to characterize PKD1 in a number of ethnic groups using DNA samples prepared using standard techniques. Our data suggest that gene conversion may play a significant role in producing variability of the PKD1 sequence in this population. The identification of additional mutations will help guide the study of polycystin-1 and better help us to understand the pathophysiology of this common disease.

Screening the 3' region of the polycystic kidney disease 1 (PKD1) gene in 41 Bulgarian and Australian kindreds reveals a prevalence of protein truncating mutations

Screening for disease-causing mutations in the unique region of the polycystic kidney disease 1 (PKD1) gene was performed in 41 unrelated individuals with autosomal dominant polycystic kidney disease. Exons 34-41 and 43-46 were assayed using PCR amplification and SSCP analysis followed by direct sequencing of amplicons presenting variant SSCP patterns. We have identified seven disease-causing mutations of which five are novel [c.10634-10656del; c.11587delG; IVS37-10C>A; c.11669-11674del; c.13069-13070ins39] and two have been reported previously [Q4010X; Q4041X]. Defects in this part of the gene thus account for 17% of our group of patients. Five of the seven sequence alterations detected are protein-truncating which is in agreement with mutation screening data for this part of the gene by other groups. The two other mutations are in-frame deletions or insertions which could destroy important functional properties of polycystin 1. These findings suggest that the first step toward cyst formation in PKD1 patients is the loss of one functional copy of polycystin 1, which indirectly supports the "two-hit" model of cystogenesis where a second somatic mutation inactivating the normal allele is necessary to occur for development of the disease condition.

Molecular genetics and mechanism of autosomal dominant polycystic kidney disease

Considerable progress toward understanding pathogenesis of autosomal dominant polycystic disease (ADPKD) has been made during the past 15 years. ADPKD is a heterogeneous human disease resulting from mutations in either of two genes, PKD1 and PKD2. The similarity in the clinical presentation and evidence of direct interaction between the COOH termini of polycystin-1 and polycystin-2, the respective gene products, suggest that both proteins act in the same molecular pathway. The fact that most mutations from ADPKD patients result in truncated polycystins as well as evidence of a loss of heterozygosity mechanism in individual PKD cysts indicate that the loss of the function of either PKD1 or PKD2 is the most likely pathogenic mechanism for ADPKD. A novel mouse model, WS25, has been generated with a targeted mutation at Pkd2 locus in which a mutant exon 1 created by inserting a neo(r) cassette exists in tandem with the wild-type exon 1. This causes an unstable allele that undergoes secondary recombination to produce a true null allele at Pkd2 locus. Therefore, the model Pkd2(WS25/-), which carries the WS25 unstable allele and a true null allele, produces somatic second hits during mouse development or adult life and establishes an extremely faithful model of human ADPKD.