Multiplicity in polyp count and extracolonic manifestations in 40 Dutch patients with MYH associated polyposis coli (MAP)

OBJECTIVE

To investigate the contribution of MYH associated polyposis coli (MAP) among polyposis families in the Netherlands, and the prevalence of colonic and extracolonic manifestations in MAP patients.

METHODS

170 patients with polyposis coli, who previously tested negative for APC mutations, were screened by denaturing gradient gel electrophoresis and direct sequencing to identify MYH germline mutations.

RESULTS

Homozygous and compound heterozygous MYH mutations were identified in 40 patients (24%). No difference was found in the percentage of biallelic mutation carriers between patients with 10-99 polyps or 100-1000 polyps (29% in both groups). Colorectal cancer was found in 26 of the 40 patients with MAP (65%) within the age range 21 to 67 years (median 45). Complete endoscopic reports were available for 16 MAP patients and revealed five cases with gastro-duodenal polyps (31%), one of whom also presented with a duodenal carcinoma. Breast cancer occurred in 18% of female MAP patients, significantly more than expected from national statistics (standardised morbidity ratio = 3.75).

CONCLUSIONS

Polyp numbers in MAP patients were equally associated with the attenuated and classical polyposis coli phenotypes. Two thirds of the MAP patients had colorectal cancer, 95% of whom were older than 35 years, and one third of a subset of patients had upper gastrointestinal lesions. Endoscopic screening of the whole intestine should be carried out every two years for all MAP patients, starting from age 25-30 years. The frequent occurrence of additional extraintestinal manifestations, such as breast cancer among female MAP patients, should be thoroughly investigated.

Transgenic mice expressing tamoxifen-inducible Cre for somatic gene modification in renal epithelial cells

Gene inactivation often leads to an embryonic-lethal phenotype. In focal diseases like renal cell carcinomas and polycystic kidney disease, somatic gene inactivation in subsets of cells is likely to occur at later stages. We generated a transgenic mouse line with an inducible form of Cre recombinase for conditional gene modifications in kidney epithelial cells. To this end a 1.4-kb promoter fragment of the kidney-specific cadherin gene (KspCad) was cloned upstream of a tamoxifen-inducible Cre recombinase (CreER(T2)) encoding sequence. Expression and activity of Cre was evaluated using reverse transcriptase polymerase chain reaction (RT-PCR) analysis and by crossbreeding to Z/EG reporter mice. One KspCad-CreER(T2) line showed kidney-specific Cre expression and mediated recombination upon tamoxifen treatment in Z/EG reporter mice. No reporter gene expression was detected in untreated animals or in extrarenal tissues upon treatment. Within the kidneys, enhanced green fluorescent protein (EGFP) fluorescence was observed in epithelial cells in several nephronic segments. In addition, the system successfully recombined a floxed Pkd1 gene.

Diagnostic approach and management of Lynch syndrome (hereditary nonpolyposis colorectal carcinoma): a guide for clinicians

ABSTRACT diagnostic workup of familial colorectal cancer is an elaborate and time consuming process in which the family and several medical specialists closely collaborate. However, establishing a diagnosis can be very rewarding. If a mutation is detected in the family, a satisfactory explanation can be provided for an accumulation of tumors at young age, and often of untimely death. Appropriate presymptomatic testing can be offered to reduce mortality among at-risk family members, and relatives not at risk can avoid uncertainty and needlessly intensive surveillance. We show the differential diagnostic considerations when an individual with a family history of colorectal carcinoma is encountered, with emphasis on Lynch syndrome (Hereditary Nonpolyposis Colorectal Carcinoma [HNPCC]). Practical recommendations for laboratory workup of suspected Lynch syndrome, including analysis of tumor tissue by microsatellite instability analysis and immunohistochemistry, and germline DNA analysis are given. Furthermore, the clinical management after a molecular diagnosis has been made is described. The diagnostic scheme presented here allows efficient and effective analysis of colorectal carcinoma cases with (suspected) Lynch syndrome, making optimal use of currently available technology.

A complex rearrangement on chromosome 22 affecting both homologues; haplo-insufficiency of the Cat eye syndrome region may have no clinical relevance

The presence of highly homologous sequences, known as low copy repeats, predisposes for unequal recombination within the 22q11 region. This can lead to genomic imbalances associated with several known genetic disorders. We report here a developmentally delayed patient carrying different rearrangements on both chromosome 22 homologues, including a previously unreported rearrangement within the 22q11 region. One homologue carries a deletion of the proximal part of chromosome band 22q11. To our knowledge, a 'pure' deletion of this region has not been described previously. Four copies of this 22q11 region, however, are associated with Cat eye syndrome (CES). While the phenotypic impact of this deletion is unclear, familial investigation revealed five normal relatives carrying this deletion, suggesting that haplo-insufficiency of the CES region has little clinical relevance. The other chromosome 22 homologue carries a duplication of the Velocardiofacial/DiGeorge syndrome (VCFS/DGS) region. In addition, a previously undescribed deletion of 22q12.1, located in a relatively gene-poor region, was identified. As the clinical features of patients suffering from a duplication of the VCFS/DGS region have proven to be extremely variable, it is impossible to postulate as to the contribution of the 22q12.1 deletion to the phenotype of the patient. Additional patients with a deletion within this region are needed to establish the consequences of this copy number alteration. This study highlights the value of using different genomic approaches to unravel chromosomal alterations in order to study their phenotypic impact.

Heterozygous mutations in PMS2 cause hereditary nonpolyposis colorectal carcinoma (Lynch syndrome)

BACKGROUND & AIMS

The role of the mismatch repair gene PMS2 in hereditary nonpolyposis colorectal carcinoma (HNPCC) is not fully clarified. To date, only 7 different heterozygous truncating PMS2 mutations have been reported in HNPCC-suspected families. Our aim was to further assess the role of PMS2 in HNPCC.

METHODS

We performed Southern blot analysis in 112 patients from MLH1-, MSH2-, and MSH6-negative HNPCC-like families. A subgroup (n = 38) of these patients was analyzed by denaturing gradient gel electrophoresis (DGGE). In a second study group consisting of 775 index patients with familial colorectal cancer, we performed immunohistochemistry using antibodies against MLH1, MSH2, MSH6, and PMS2 proteins. In 8 of 775 tumors, only loss of PMS2 expression was found. In these cases, we performed Southern blot analysis and DGGE. Segregation analysis was performed in the families with a (possibly) deleterious mutation.

RESULTS

Seven novel mutations were identified: 4 genomic rearrangements and 3 truncating point mutations. Three of these 7 families fulfill the Amsterdam II criteria. The pattern of inheritance is autosomal dominant with a milder phenotype compared with families with pathogenic MLH1 or MSH2 mutations. Microsatellite instability and immunohistochemical analysis performed in HNPCC-related tumors from proven carriers showed a microsatellite instability high phenotype and loss of PMS2 protein expression in all tumors.

CONCLUSIONS

We show that heterozygous truncating mutations in PMS2 do play a role in a small subset of HNPCC-like families. PMS2 mutation analysis is indicated in patients diagnosed with a colorectal tumor with absent staining for the PMS2 protein.

Copy number variation in regions flanked (or unflanked) by duplicons among patients with developmental delay and/or congenital malformations; detection of reciprocal and partial Williams-Beuren duplications

Duplicons, that is, DNA sequences with minimum length 10 kb and a high sequence similarity, are known to cause unequal homologous recombination, leading to deletions and the reciprocal duplications. In this study, we designed a Multiplex Amplifiable Probe Hybridisation (MAPH) assay containing 63 exon-specific single-copy sequences from within a selection of the 169 regions flanked by duplicons that were identified, at a first pass, in 2001. Subsequently, we determined the frequency of chromosomal rearrangements among patients with developmental delay (DD) and/or congenital malformations (CM). In addition, we tried to identify new regions involved in DD/CM using the same assay. In 105 patients, six imbalances (5.8%) were detected and verified. Three of these were located in microdeletion-related regions, two alterations were polymorphic duplications and the effect of the last alteration is currently unknown. The same study population was tested for rearrangements in regions with no known duplicons nearby, using a set of probes derived from 58 function-selected genes. The latter screening revealed two alterations. As expected, the alteration frequency per unit of DNA is much higher in regions flanked by duplicons (fraction of the genome tested: 5.2%) compared to regions without known duplicons nearby (fraction of the genome tested: 24.5-90.2%). We were able to detect three novel rearrangements, including the previously undescribed reciprocal duplication of the Williams Beuren critical region, a subduplicon alteration within this region and a duplication on chromosome band 16p13.11. Our results support the hypothesis that regions flanked by duplicons are enriched for copy number variations.

Analysis of missense variants in the PKHD1-gene in patients with autosomal recessive polycystic kidney disease (ARPKD)

Autosomal recessive polycystic kidney disease (ARPKD) is a severe form of polycystic kidney disease characterized by enlarged kidneys and congenital hepatic fibrosis. Given the poor prognosis for the majority of children with the severe perinatal ARPKD phenotype, there is a regular request for prenatal testing. ARPKD is caused by mutations in the polycystic kidney and hepatic disease 1 (PKHD1) gene, which consists of 86 exons that are variably assembled into a number of alternatively spliced transcripts. The longest transcript, comprising 67 exons, encodes the protein fibrocystin/polyductin. We have set up mutation analysis by direct sequencing of these 67 exons. In 39 mainly Dutch families we identified: 11 nonsense mutations, 15 deletions/insertions, 5 splice site mutations, and 39 missense mutations. To classify missense variants we combined evolutionary conservation, using the human, chimpanzee, dog, mouse, chicken and frog Pkhd1 sequences, with the Grantham score for chemical differences. Thirty-three missense mutations were considered pathogenic and seven were classified as rare, probably pathogenic variants. In addition to sequence analysis, multiplex ligation-dependent probe amplification (MLPA) was used to identify multiple exon deletions. However, no large deletions in the PKHD1 gene were identified. In 31 index patients two mutations were found, in 6 patients one mutation was found, leading to a mutation detection rate of 87%. The analysis of amino acid conservation as well as applying the Grantham score for chemical differences allowed us to determine the pathogeneity for nearly all new missense mutations and thus proved to be useful tools to classify missense variants.

Genetic analysis of tall stature

Tall stature is less often experienced as an important problem than short stature. However, a correct diagnosis may be of eminent importance, especially when interventions are planned, or to know the natural history. Overgrowth can be caused by endocrine disorders and skeletal dysplasias, but also by several genetic syndromes. Despite a systematic diagnostic approach, there will be patients with tall stature who do not fit a known diagnosis. In this group of patients possibilities of genetic analysis do exist, but are not common practice. The FMR1 gene should be analyzed in patients with tall stature and mental retardation, and in these patients the NSD1 gene can be considered whenever some features of Sotos syndrome do exist. In tall patients without mental retardation and some features of Sotos or Beckwith-Wiedemann syndrome it may still be useful to look for mutations in the NSD1 gene, but also for changes in the 11p15 region. The various possibilities are discussed and placed in a flowchart.

Common regulatory elements in the polycystic kidney disease 1 and 2 promoter regions

The PKD1 and PKD2 genes are mutated in patients with autosomal dominant polycystic kidney disease (ADPKD), a systemic disease, with the formation of renal cysts as main clinical feature. The genes are developmentally regulated and aberrant expression of PKD1 or PKD2 leads to cystogenesis. To date, however, the transcription factors regulating expression of these genes have hardly been studied. To identify conserved putative transcription factor-binding sites, we cloned and characterized the 5'-flanking regions of the murine and canine Pkd1 genes and performed a multispecies comparison by including sequences from the human and Fugu rubripes orthologues as well as the Pkd2 promoters from mouse and human. Sequence analysis revealed a variety of conserved putative binding sites for transcription factors and no TATA-box element. Nine elements were conserved in the mammalian Pkd1 promoters: AP2, E2F, E-Box, EGRF, ETS, MINI, MZF1, SP1, and ZBP-89. Interestingly, six of these elements were also found in the mammalian Pkd2 promoters. Deletion studies with the mouse Pkd1 promoter showed that a approximately 280 bp fragment is capable of driving luciferase reporter gene expression, whereas reporter constructs containing larger fragments of the Pkd1 promoter showed a lower activity. Furthermore, mutating a potential E2F-binding site within this 280 bp fragment diminished the reporter construct activity, suggesting a role for E2F in regulating cell cycle-dependent expression of the Pkd1 gene. Our data define a functional promoter region for Pkd1 and imply that E2F, EGRF, Ets, MZF1, Sp1, and ZBP-89 are potential key regulators of PKD1 and PKD2 in mammals.

Increased activity of activator protein-1 transcription factor components ATF2, c-Jun, and c-Fos in human and mouse autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease is a common inherited disorder that predominantly manifests with the formation of fluid-filled cysts in both kidneys. The disease can be accounted for by a mutation in either the PKD1 or the PKD2 gene. It was demonstrated previously that aberrant expression of the PKD1 gene product, polycystin-1, results in modification of activator protein-1 (AP-1) transcription factor activity in cultured renal epithelial cells. Here, it is reported that activity of the AP-1 components c-Jun, ATF2, and c-Fos is altered in renal cystic tissue of patients with autosomal dominant polycystic kidney disease and of hypomorphic Pkd1 mice with polycystic kidney disease. Data were obtained using immunohistochemical and Western blot analysis. Significant upregulation of Thr71- and Thr69/71-phosphorylated ATF2 and Ser73-phosphorylated c-Jun and increased c-Fos were detected in small cysts and (dilated) ducts and tubules surrounded by fibrotic interstitium. The data indicate that various AP-1 components are constitutively activated in polycystic kidney disease and suggest that aberrant AP-1 activity is relevant for cyst formation.

Mutation screening of EXT1 and EXT2 by direct sequence analysis and MLPA in patients with multiple osteochondromas: splice site mutations and exonic deletions account for more than half of the mutations

Multiple osteochondromas (MO) is an autosomal dominant condition, caused by mutations in either the EXT1 or the EXT2 gene. The DNA of a cohort of 35 patients, clinically suspected to be affected with MO, was screened for mutations by a combination of direct sequence analysis and multiplex ligation-dependent probe amplification (MLPA). In this cohort, 26 pathogenic gene alterations were found (74%). With sequence analysis mutations were detected in 22 patients (63%). In total, 10 mutations were detected in the EXT1 and 12 in the EXT2 gene. The number of the splice site mutations detected was larger than expected from the literature. In addition, with the MLPA four deletions of one or more exons were found in this cohort. Two patients, of whom one had a negative family history, showed deletions of exon 1 of the EXT1 gene, which is possibly a deletion hot spot. In patients suspected to be affected by MO, we recommend a quantitative analysis such as MLPA, followed by direct sequence analysis for the screening of the EXT1 and EXT2 genes.

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.

What’s the message? Interpretation of an uninformative BRCA1/2 test result for women at risk of familial breast cancer

PURPOSE

To test the "false-reassurance hypothesis," which suggests that women who receive an uninformative BRCA1/2 test result may incorrectly conclude that they no longer have an elevated risk, with possible harmful consequences for adherence to breast surveillance guidelines.

METHODS

A prospective questionnaire design was used to compare 183 women with an uninformative BRCA test result (94 affected and 89 unaffected) with 41 proven BRCA mutation-carriers and 49 true negatives before and after BRCA1/2 test disclosure.

RESULTS

After DNA-test disclosure, test applicants differed from each other with regard to their perception of the likelihood of carrying a deleterious gene (P < 0.0001). The BRCA mutation carriers reported the highest perceived likelihood and the true negatives reported the lowest. Compared to the predisclosure measures, women who received an uninformative DNA test result reported a lower perceived risk after disclosure (P < 0.0001), suggesting a relatively high level of reassurance because of the test result. However, after DNA-test disclosure, only 12 women concluded that the risk of carrying a mutation was nonexistent, and perceived likelihood was significantly associated with the pedigree-based risk assessment (P = 0.0001). Moreover, despite the significant decrease in perceived likelihood for uninformative women, intention to obtain mammograms did not change (P = 0.71); it remained at the same almost optimal level as for BRCA mutation carriers.

CONCLUSION

No support was found for the suggestion that the nature of uninformative test results is often misunderstood. Moreover, an uninformative test result did not affect the positive mammography intentions of both affected and unaffected women.

Variants of uncertain clinical significance as a result of BRCA1/2 testing: impact of an ambiguous breast cancer risk message

The identification of an increasing number of variants of uncertain clinical significance (VUCS) in genetic testing for hereditary breast cancer poses serious problems for genetic counseling, because no data are available about the psychosocial impact of discussing such an unclear risk message. The current study is the first to present data on how test applicants actually understand and cope with such a result if communicated by a geneticist. We compared 10 women who received a VUCS result with 34 women who carried the deleterious mutation, 37 women who did not carry the deleterious mutation or 'true negatives,' and 160 women who received a so-called inconclusive result before and after test disclosure. Women, with whom a VUCS result was discussed, reported quite a high level of comprehension of the result. In addition, compared with the pretest measures, they did not report a higher level of perceived risk (p = 0.58) and even reported a decrease in breast cancer distress (p = 0.03). They were very comparable to women who received an inconclusive result on all post-disclosure measures. Our results suggest that discussing a VUCS result in genetic counseling does not give rise for concern.

[From gene to disease; PKHD1 and recessive polycystic kidney disease]

Autosomal recessive polycystic kidney disease (ARPKD) is a severe form of polycystic kidney disease characterised by enlarged kidneys and congenital hepatic fibrosis. The disease has an incidence of 1:7000-:20,000 and is caused by mutations in the PKHD1 gene, which under normal conditions produces the protein fibrocystin, also named polyductin. This protein may be a transmembrane receptor or ligand that plays a role in collecting duct and biliary differentiation. The major site of expression is the primary cilium, and in particular the basal body of the cilium, underlining a link between aberrant cilial function and cystogenesis. Prenatal diagnostics is possible using DNA analysis or ultrasonography.

[From gene to disease; from CLN1, CLN2 and CLN3 to neuronal ceroid lipofuscinosis]

The neuronal ceroid lipofuscinoses (NCL) are worldwide the most common lysosomal storage disorders of childhood. Clinical features often include progressive visual impairment, seizures, psychomotor deterioration, dementia, and premature death. Most NCL cases are caused by mutations in the CLN1, CLN2 and CLN3 genes, which play an essential role in lysosomal protein degradation. Laboratory diagnostics for a patient suspected of NCL should start with enzyme analysis in the case of INCL and LINCL and investigation of lymphocyte vacuolisation for JNCL. Diagnosis at the protein level is not available for JNCL, but CLN3 mutation analysis is possible. The carrier status of healthy relatives in families with known mutations in either CLN1, CLN2, CLN3 or CLN6 can be determined with certainty by mutation analysis.

Feeling at risk: how women interpret their familial breast cancer risk

Women's inaccuracy in recalling their breast cancer risk, even immediately after genetic counseling, has received much attention. However, scarce data are available about how women describe their risk in their own words and about what the risk information actually means to them. The present study aims to address interpretations questions and to assess whether these are congruent with the objective risk. Face-to-face interviews were conducted with 123 women immediately after their (initial) counseling session. N-Vivo software was used to describe the data. The level of accuracy of recall depended strongly on the leniency of the criterion applied. For example, the level of verbal accuracy ranged from 25.8% (an exact match with the verbal label) to 98.4% (a more global awareness of having a high versus a low risk). In assessing the significance of personal risk information, we identified a wide variety of risk beliefs, and stress and coping responses. In general, women associated their risk with the medical options, for example, breast screening, that were available for them given their risk status. The results indicate that the accuracy of recall might be a limited outcome measure for the effectiveness of genetic counseling. First, this is because the level of accuracy of recall depends on how rigorously accuracy is defined. Secondly, because the probability of occurrence is just one of the elements comprising perceived risk, accuracy might rather apply to the distress, and to risk management behaviors that are elicited by the risk information. These beliefs that women hold about their risk status, and concomitant levels of stress should play a prominent role in genetic counseling.

[Molecular diagnosis on indication of maturity onset diabetes of the young; results from 184 patients]

OBJECTIVE

To describe the results of mutation analysis of the genes involved in maturity onset diabetes of the young (MODY) types 1-3.

DESIGN

Descriptive.

METHOD

In the period July 2000-October 2003 the DNA from 184 possible MODY patients was analysed for the presence of mutations of the genes involved in MODY types 1, 2 and 3. The patients fulfilled at least one of the following criteria: diabetes mellitus had been diagnosed before the age of 25, or at least before the age of 40, there was a family history of diabetes mellitus at an early age, there were no characteristics to indicate diabetes mellitus type 1 or 2.

RESULTS

In the blood of 65 patients (35%) a pathogenic gene mutation was found. A total of 45 patients had a mutation in the HNF-1alpha-gene (which is linked to MODY3), 11 in the glucokinase gene (MODY2) and 9 in the HNF-4alpha-gene (MODY1). Of all the HNF-1alpha-gene mutations, the insertion of a C in codon 291 was the most frequently seen (in 11 families). A mutation in exon 9 of the HNF-1alpha-gene was also shown in 9 apparently non-related families, which probably was a founder mutation.

CONCLUSION

The MODY subtype was found in one third of the selected patients. This diagnosis may have implications in the clinical management of the patient.

Sarcoglycanopathies and the risk of undetected deletion alleles in diagnosis

We have designed Multiplex Amplifiable Probe Hybridization (MAPH) probes for 28 exons of the sarcoglycan genes SGCA, SGCB, SGCG, and SGCD. The set was used to screen DNA from limb-girdle muscular dystrophy (LGMD) patients for the presence of pathogenic deletion or duplication mutations. An unexpected heterozygous deletion of SGCG exon 7 was detected in a patient from a consanguineous family in which a known c.525delT mutation segregates. The exon 7 deletion was inherited from the father, who was part of the consanguineous c.525delT branch of the family but who did not carry the c.525delT mutation. A similar, homozygous deletion had been identified in two unrelated LGMD patients from southern Italy. The deletion breakpoints were mapped, isolated, and sequenced, and were identical in all cases. Haplotype analysis showed the same alleles segregating with the mutation in all three patients, suggesting a common ancestor. Exonic deletions in sarcoglycanopathies appear to be rare events. However, we recommend screening for exonic deletions/duplications in patients where a mutation has not been identified in both alleles, as well as in seemingly homozygous cases where segregation of the mutations can not be confirmed in the parents.

Bifurcation of the femur with tibial agenesis and additional anomalies

Bifurcation of the femur and tibial agenesis are rare anomalies and have been described in both the Gollop-Wolfgang Complex and the tibial agenesis-ectrodactyly syndrome. We report on two patients with bifurcation of the femur and tibial agenesis. Hand ectrodactyly was seen in one of these patients. Both patients had unusual additional anomalies. The first patient had in addition proximal focal femoral deficiency, the other patient had a tracheo-esophageal fistula and pyloric stenosis. Clinical and genetic aspects are discussed.

Comparison between siblings and twins supports a role for modifier genes in ADPKD

BACKGROUND

Autosomal-dominant polycystic kidney disease (ADPKD) is characterized by intrafamilial variability in renal disease progression, which could result from a combination of environmental and genetic factors. Although a role for modifier genes has been evidenced in mouse models, direct evidence in ADPKD patients is lacking. The analysis of variability in affected siblings and monozygotic (MZ) twins would help evaluate the relative contribution of environment and genetic factors on renal disease progression in ADPKD.

METHODS

The difference in the age at end-stage renal disease (ESRD) and the intraclass correlation coefficient (ICC) were quantified in a large series of ADPKD siblings from western Europe and compared with the values obtained in a series of MZ ADPKD twins from the same geographic area.

RESULTS

Fifty-six sibships (including 129 patients) and nine pairs of MZ twins were included. The difference in the age at ESRD was significantly higher in siblings (6.9 +/- 6.0 years, range 2 months to 23 years) than in MZ twins (2.1 +/- 1.9 years, range 1 month to 6 years; P = 0.02). Furthermore, the intraclass correlation coefficient was significantly lower in siblings than in MZ twins (0.49 vs. 0.92, respectively; P = 0.003). The intrafamilial difference in the age at ESRD was not influenced by gender.

CONCLUSION

These data substantiate the existence of a large intrafamilial variability in renal disease progression in ADPKD siblings. The fact that the variability in siblings is in a significant excess of that found in MZ twins strongly suggests that modifier genes account for a significant part of this variability.

Two-color multiplex ligation-dependent probe amplification: detecting genomic rearrangements in hereditary multiple exostoses

Genomic deletions and duplications play an important role in the etiology of human disease. Versatile tests are required to detect these rearrangements, both in research and diagnostic settings. Multiplex ligation-dependent probe amplification (MLPA) is such a technique, allowing the rapid and precise quantification of up to 40 sequences within a nucleic acid sample using a one-tube assay. Current MLPA probe design, however, involves time-consuming and costly steps for probe generation. To bypass these limitations we set out to use chemically synthesized oligonucleotide probes only. The inherent limitations of this approach are related to oligonucleotide length, and thus the number of probes that can be combined in one assay is also limited. This problem was tackled by designing a two-color assay, combining two sets of probes, each amplified by primers labeled with a different fluorophore. In this way we successfully combined 28 probes in a single reaction. The assay designed was used to screen for the presence of deletions and duplications in patients with hereditary multiple exostoses (HME). Screening 18 patients without detectable point mutations in the EXT1 and EXT2 genes revealed five cases with deletions of one or more exons: four in EXT1 and one in EXT2. Our results show that a two-color MLPA assay using only synthetic oligonucleotides provides an attractive alternative for probe design. The approach is especially suited for cases in which the number of patients to be tested is limited, making it financially unattractive to invest in cloning.

Lowering of Pkd1 expression is sufficient to cause polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a major cause of renal failure and is characterized by the formation of many fluid-filled cysts in the kidneys. It is a systemic disorder that is caused by mutations in PKD1 or PKD2. Homozygous inactivation of these genes at the cellular level, by a 'two-hit' mechanism, has been implicated in cyst formation but does not seem to be the sole mechanism for cystogenesis. We have generated a novel mouse model with a hypomorphic Pkd1 allele, Pkd1(nl), harbouring an intronic neomycin-selectable marker. This selection cassette causes aberrant splicing of intron 1, yielding only 13-20% normally spliced Pkd1 transcripts in the majority of homozygous Pkd1(nl) mice. Homozygous Pkd1(nl) mice are viable, showing bilaterally enlarged polycystic kidneys. This is in contrast to homozygous knock-out mice, which are embryonic lethal, and heterozygous knock-out mice that show only a very mild cystic phenotype. In addition, homozygous Pkd1(nl) mice showed dilatations of pancreatic and liver bile ducts, and the mice had cardiovascular abnormalities, pathogenic features similar to the human ADPKD phenotype. Removal of the neomycin selection-cassette restored the phenotype of wild-type mice. These results show that a reduced dosage of Pkd1 is sufficient to initiate cystogenesis and vascular defects and indicate that low Pkd1 gene expression levels can overcome the embryonic lethality seen in Pkd1 knock-out mice. We propose that in patients reduced PKD1 expression of the normal allele below a critical level, due to genetic, environmental or stochastic factors, may lead to cyst formation in the kidneys and other clinical features of ADPKD.