Danish type gelsolin related amyloidosis: 654G-T mutation is associated with a disease pathogenetically and clinically similar to that caused by the 654G-A mutation (familial amyloidosis of the Finnish type)

BACKGROUND

Familial amyloidosis of the Finnish type (FAF, Finnish hereditary amyloidosis) is caused by a 654G-A mutation in the gelsolin gene on chromosome 9 resulting in the expression of mutant Asn-187 gelsolin which is abnormally proteolytically processed generating amyloidogenic fragments that polymerize into amyloid fibrils. We have recently shown that in a Danish and a Czech family with a clinical syndrome similar to FAF, including corneal lattice dystrophy, cranial neuropathy and skin changes, the disease is caused by another mutation at the same position, namely 654G-T predicting a Try-for-Asp substitution at 187 in secreted gelsolin.

AIM

To undertake a closer examination of the Danish subtype of FAF and report immunohistochemical and biochemical findings.

RESULTS

Immunostaining of plasma gelsolin isolated from heterozygous FAF of the Danish subtype revealed a pattern similar to that found in FAF-Asn 187. The > 60 kDa gelsolin species contain an epitope characteristic of the amyloid forming region as revealed by an amyloid specific antibody, whereas the approximately 50 kDa fragments are devoid of it. Compared with the wild-type gelsolin peptide (Asp-187), the corresponding mutant peptide (Tyr-187) showed dramatically increased fibrillogenicity as revealed by quantitative thioflavine-T based fluorimetry; ultrastructurally, amyloid-like fibrils were formed by the mutant peptide. Immunohistochemistry showed that antibodies directed against residues 231-242 of secreted gelsolin, representing the carboxy terminus of the sequence forming the amyloid protein (residues 173-243) laid down in the tissues in a fibrillar form in FAF, specifically labelled the amyloid deposited in rectum and skin in the Danish (654G-T) subtype.

CONCLUSIONS

The 654G-T mutation in the gelsolin gene gives rise to an amyloid disease clinically and pathogenetically similar to that caused by the 654G-A mutation.

Predictive genetic testing for hereditary non-polyposis colorectal cancer: uptake and long-term satisfaction

The aim of this prospective study was to assess the uptake of predictive genetic testing for hereditary non-polyposis colorectal cancer (HNPCC) and its associations with sociodemographic and other factors, and long-term satisfaction with taking the test. The test was offered to all high-risk members (n = 446) of 36 Finnish HNPCC families in which the mutation was known. The procedure comprised an educational counselling session, a period for reflection, and a test disclosure session. Data were collected by questionnaires sent before the educational counselling and 1 month and 1 year after the test disclosure. Of those eligible, 85% (n = 381) completed the first questionnaire study. Non-participation was more common among men living alone who had not participated in the clinical cancer surveillance programme. Of the 347 subjects who attended counselling, 334 (75% of all subjects) were actually tested. After logistic-regression analysis, the only significant factor predicting test acceptance proved to be employment status: those employed were more likely than others to accept the test (odds ratio = 2.25; 95% confidence intervals, 1.09 to 4.6 1). At follow-up, over 90% of the subjects were fully satisfied with the decision to take the test. In conclusion, acceptance of the test was considerably higher than in previously reported studies. We attribute this to our careful face-to-face individualized counselling, our health care system, and to attitudes of the Finnish population, which are generally favourable towards health care and disease prevention.

Genetic susceptibility to non-polyposis colorectal cancer

Familial colorectal cancer (CRC) is a major public health problem by virtue of its relatively high frequency. Some 15-20% of all CRCs are familial. Among these, familial adenomatous polyposis (FAP), caused by germline mutations in the APC gene, accounts for less than 1%. Hereditary non-polyposis colorectal cancer (HNPCC), also called Lynch syndrome, accounts for approximately 5-8% of all CRC patients. Among these, some 3% are mutation positive, that is, caused by germline mutations in the DNA mismatch repair genes that have so far been implicated (MLH1, MSH2, MSH6, PMS1, and PMS2). Most of the remaining patients belonging to HNPCC or HNPCC-like families are still molecularly unexplained. Among the remaining familial CRCs, a large proportion is probably caused by gene mutations and polymorphisms of low penetrance, of which the I1307K polymorphism in the APC gene is a prime example. Molecular genetic findings have enabled hereditary CRC to be divided into two groups: (1) tumours that show microsatellite instability (MSI), occur more frequently in the right colon, have diploid DNA, harbour characteristic mutations such as transforming growth factor beta type II receptor and BAX, and behave indolently, of which HNPCC is an example; and (2) tumours with chromosomal instability (CIN), which tend to be left sided, show aneuploid DNA, harbour characteristic mutations such as K-ras, APC, and p53, and behave aggressively, of which FAP is an example. This review focuses most heavily on the clinical features, pathology, molecular genetics, surveillance, and management including prophylactic surgery in HNPCC. Because of the difficulty in diagnosing HNPCC, a detailed differential diagnosis of the several hereditary CRC variants is provided. The extant genetic and phenotypic heterogeneity in CRC leads to the conclusion that it is no longer appropriate to discuss the genetics of CRC without defining the specific hereditary CRC syndrome of concern. Therefore, it is important to ascertain cancer of all anatomical sites, as well as non-cancer phenotypic stigmata (such as the perioral and mucosal pigmentations in Peutz-Jeghers syndrome), when taking a family cancer history.

Missense and nonsense mutations in codon 659 of MLH1 cause aberrant splicing of messenger RNA in HNPCC kindreds

Germline mutations that give rise to premature termination codons in mRNAs have frequently been associated with aberrant processing of the nascent transcripts. This can take the form either of nonsense-mediated mRNA decay or of aberrant splicing of the pre-mRNA. In a family affected by hereditary nonpolyposis colorectal cancer, a two-nucleotide deletion in codon 659, which introduces a frameshift and a new stop codon in exon 17 of the DNA mismatch repair gene MLH1, has been reported to lead to skipping of the exon. We now report that this phenomenon occurs also when there are missense or nonsense mutations in this codon. Our results thus suggest that in aberrant splicing the nature of the mutation may be less important than its position within the exon. These findings are of importance to mutation interpretation, as they imply that aberrant splicing could be associated even with silent mutations that do not lead to amino acid substitutions. Genes Chromosomes Cancer 26:372-375, 1999.

Clinical and genetic heterogeneity in autosomal recessive nemaline myopathy

Autosomal recessive nemaline (rod) myopathy is clinically and genetically heterogeneous. A clinically distinct, typical form, with onset in infancy and a non-progressive or slowly progressive course, has been assigned to a region on chromosome 2q22 harbouring the nebulin gene Mutations have now been found in this gene, confirming its causative role. The gene for slow tropomyosin TPM3 on chromosome 1q21, previously found to cause a dominantly inherited form, has recently been found to be homozygously mutated in one severe consanguineous case. Here we wished to determine the degree of genetic homogeneity or heterogeneity of autosomal recessive nemaline myopathy by linkage analysis of 45 families from 10 countries. Forty-one of the families showed linkage results compatible with linkage to markers in the nebulin region, the highest combined lod scores at zero recombination being 14.13 for the marker D2S2236. We found no indication of genetic heterogeneity for the typical form of nemaline myopathy. In four families with more severe forms of nemaline myopathy, however, linkage to both the nebulin and the TPM3 locus was excluded. Our results indicate that at least three genetic loci exist for autosomal recessive nemaline myopathy. Studies of additional families are needed to localise the as yet unknown causative genes, and to fully elucidate genotype-phenotype correlations.

Strategies for screening for hereditary non-polyposis colorectal cancer

Germline mutations in DNA mismatch repair genes (MLH1, MSH2, PMS1, PMS2, and MSH6) predispose to hereditary non-polyposis colorectal cancer (HNPCC). In the absence of pathognomonic clinical features, diagnosis of HNPCC is often based on family history. Microsatellite instability (MSI) analysis has successfully been used for screening colorectal cancer patients for HNPCC. The aim of this study was to evaluate the feasibility of a recently introduced logistical model based on family history data in detecting HNPCC patients with germline mutations. A series of 509 kindreds with a proband with colorectal cancer was studied. MSI analysis and subsequent germline mutation analysis (MLH1 and MSH2) in MSI positive patients had been performed previously. Of the 509 patients, 63 (12%) were MSI positive and 10 (2%) had a germline mutation in MLH1 or MSH2. The power of the logistical model was tested to determine its value in predicting the probability of a germline mutation. The model proposed a high probability in three out of 10 mutation positive cases when data on cancer in first degree relatives were considered (typically three generation pedigrees, consisting, on average, of eight people). Using extended pedigrees and family cancer data in the 10 mutation positive kindreds (an average of 38 family members available), the model suggested high probabilities in seven out of 10 mutation positive cases. We conclude that for the model to predict germline mutation cases, extensive pedigrees and family history data are required. When screening colorectal cancer patients for HNPCC, a model using a combination of family information and MSI has optimal specificity and sensitivity.

The neuronal ceroid lipofuscinoses in human EPMR and mnd mutant mice are associated with mutations in CLN8

The neuronal ceroid lipofuscinoses (NCLs) are a genetically heterogeneous group of progressive neurodegenerative disorders characterized by the accumulation of autofluorescent lipopigment in various tissues. Progressive epilepsy with mental retardation (EPMR, MIM 600143) was recently recognized as a new NCL subtype (CLN8). It is an autosomal recessive disorder characterized by onset of generalized seizures between 5 and 10 years, and subsequent progressive mental retardation. Here we report the positional cloning of a novel gene, CLN8, which is mutated in EPMR. It encodes a putative transmembrane protein. EPMR patients were homozygous for a missense mutation (70C-->G, R24G) that was not found in homozygosity in 433 controls. We also cloned the mouse Cln8 sequence. It displays 82% nucleotide identity with CLN8, conservation of the codon harbouring the human mutation and is localized to the same region as the motor neuron degeneration mouse, mnd, a naturally occurring mouse NCL (ref. 4). In mnd/mnd mice, we identified a homozygous 1-bp insertion (267-268insC, codon 90) predicting a frameshift and a truncated protein. Our data demonstrate that mutations in these orthologous genes underlie NCL phenotypes in human and mouse, and represent the first description of the molecular basis of a naturally occurring animal model for NCL.

Erythropoietin receptor mutations associated with familial erythrocytosis cause hypersensitivity to erythropoietin in the heterozygous state

Inherited mutations in the erythropoietin receptor (EPOR) causing premature termination of the receptor cytoplasmic region are associated with dominant familial erythrocytosis (FE), a benign clinical condition characterized by hypersensitivity of erythroid progenitor cells to EPO and low serum EPO (S-EPO) levels. We describe a Swedish family with dominant FE in which erythrocytosis segregates with a new truncation in the negative control domain of the EPOR. We show that cells engineered to concomitantly express the wild-type (WT) EPOR and mutant EPORs associated with FE (FE EPORs) are hypersensitive to EPO-stimulated proliferation and activation of Jak2 and Stat5. These results demonstrate that FE is caused by hyperresponsiveness of receptor-mediated signaling pathways and that this is dominant with respect to WT EPOR signaling.

Identification of the Finnish founder mutation for diastrophic dysplasia (DTD)

Diastrophic dysplasia (DTD) is especially prevalent in Finland and the existence of a founder mutation has been previously inferred from the fact that 95% of Finnish DTD chromosomes have a rare ancestral haplotype found in only 4% of Finnish control chromosomes. Here we report the identification of the Finnish founder mutation as a GT-> GC transition (c.-26 + 2T > C) in the splice donor site of a previously undescribed 5'-untranslated exon of the diastrophic dysplasia sulfate transporter gene (DTDST); the mutation acts by severely reducing mRNA levels. Among 84 DTD families in Finland, patients carried two copies of the mutation in 69 families, one copy in 14 families, and no copies in one family. Roughly 90% of Finnish DTD chromosomes thus carry the splice-site mutation, which we have designated DTDST(Fin). Unexpectedly, we found that nine of the DTD chromosomes having the apparently ancestral haplotype did not carry DTDST(Fin), but rather two other mutations. Eight such chromosomes had an R279W mutation and one had a V340del deletion. We consider the possible implications of presence of multiple DTD mutations on this rare haplotype.

Polymorphic variation at the BAT-25 and BAT-26 loci in individuals of African origin. Implications for microsatellite instability testing

Instability in the repeat size of microsatellite sequences has been described in both hereditary nonpolyposis and sporadic colorectal cancers. Tumors expressing microsatellite instability are identified through the comparison of the repeat sizes at multiple microsatellite loci between tumor and matched normal tissue DNA. The use of a five-marker panel including two mononucleotide repeat microsatellites, BAT-25 and BAT-26, has recently been suggested for the clinical determination of tumor microsatellite instability. The BAT-25 and BAT-26 loci included in this panel have both demonstrated sensitivity to microsatellite instability and normal quasimonomorphic allelic patterns, which has simplified the distinction between normal and unstable alleles. However, in this study, we identified allelic variations in the size of the poly(A) tract at BAT-26 in 12.6% of 103 healthy African-Americans screened. In addition, 18.4% exhibited allelic size variations in the poly(T) tract at BAT-25. Finally, 2.9% showed variant alleles at both BAT-25 and BAT-26 loci. Screening a small population of Nigerians confirmed the polymorphic nature of both loci and the ethnic origin of alleles not identified in other populations studied thus far. Our results dispute the quasimonomorphic nature of both BAT-25 and BAT-26 in all populations and support the need for thorough population studies to define the different allelic profiles and frequencies at microsatellite loci.

Loss-of-function mutations in PPAR gamma associated with human colon cancer

The gamma isoform of the peroxisome proliferator-activated receptor, PPAR gamma, regulates adipocyte differentiation and has recently been shown to be expressed in neoplasia of the colon and other tissues. We have found four somatic PPAR gamma mutations among 55 sporadic colon cancers: one nonsense, one frameshift, and two missense mutations. Each greatly impaired the function of the protein. c.472delA results in deletion of the entire ligand binding domain. Q286P and K319X retain a total or partial ligand binding domain but lose the ability to activate transcription through a failure to bind to ligands. R288H showed a normal response to synthetic ligands but greatly decreased transcription and binding when exposed to natural ligands. These data indicate that colon cancer in humans is associated with loss-of-function mutations in PPAR gamma.

The intrinsic factor-vitamin B12 receptor, cubilin, is a high-affinity apolipoprotein A-I receptor facilitating endocytosis of high-density lipoprotein

Cubilin is the intestinal receptor for the endocytosis of intrinsic factor-vitamin B12. However, several lines of evidence, including a high expression in kidney and yolk sac, indicate it may have additional functions. We isolated apolipoprotein A-I (apoA-I), the main protein of high-density lipoprotein (HDL), using cubilin affinity chromatography. Surface plasmon resonance analysis demonstrated a high-affinity binding of apoA-I and HDL to cubilin, and cubilin-expressing yolk sac cells showed efficient 125I-HDL endocytosis that could be inhibited by IgG antibodies against apoA-I and cubilin. The physiological relevance of the cubilin-apoA-I interaction was further emphasized by urinary apoA-I loss in some known cases of functional cubilin deficiency. Therefore, cubilin is a receptor in epithelial apoA-I/HDL metabolism.

Testing tumors for microsatellite instability

The methods for determining microsatellite instability in tumors are highly heterogeneous. Recently a 5-marker panel of microsatellites was suggested for this purpose. In this review attention is drawn to the fact that microsatellite instability can be assessed by analyzing tumor DNA with a single marker, BAT-26, and that normal tissue DNA from the same individual needs to be analyzed only when an aberrant allele is seen in the tumor. Whilst this simple procedure does not distinguish between different types and degrees of instability, it should be sufficient for many purposes, such as screening colorectal cancers for mismatch repair deficiency.

Genomic characterization of human DSPG3

DSPG3, the human homolog to chick PG-Lb, is a mejrkp6of the small leucine-rich repeat proteoglycan (SLRP) family, including decorin, biglycan, fibromodulin, and lumican. In contrast to the tissue distribution of the other SLRPs, DSPG3 is predominantly expressed in cartilage. In this study, we have determined that the human DSPG3 gene is composed of seven exons: Exon 2 of DSPG3 includes the start codon, exons 4-7 code for the leucine-rich repeats, exons 3 and 7 contain the potential glycosaminoglycan attachment sites, and exon 7 contains the potential N-glycosylation sites and the stop codon. We have identified two polymorphic variations, an insertion/deletion composed of 19 nucleotides in intron 1 and a tetranucleotide (TATT)n repeat in intron 5. Analysis of 1.6 kb of upstream promoter sequence of DSPG3 reveals three TATA boxes, one of which is 20 nucleotides before the transcription start site. The transcription start site precedes the translation start site by 98 nucleotides. There are 14 potential binding sites for SOX9, a transcription factor present in cartilage, in the promoter, and in the first intron of DSPG3. We have examined the evolution of the SLRP gene family and found that gene products clustered together in the evolutionary tree are encoded by genes with similarities in genomic structure. Hence, it appears that the majority of the introns in the SLRP genes were inserted after the differentiation of the SLRP genes from an ancestral gene that was most likely composed of 2-3 exons.

Cancer risk in mutation carriers of DNA-mismatch-repair genes

Excessive incidence of various cancers is a challenging feature of the hereditary-non-polyposis-colorectal-cancer (HNPCC) syndrome. This study estimated the cancer incidences in HNPCC compared with the general population. Individuals in a cohort of 1763 members of 50 genetically diagnosed families were categorized according to their genetic status as mutation carriers, non-carriers, or individuals at 50 or 25% risk of being a carrier. Incidences of cancers in these groups were compared with those in the Finnish population overall. In 360 mutation carriers, standardized incidence ratios (SIR) were significantly increased for colorectal [68; 95% confidence intervals (CI), 56 to 81], endometrial (62; 95% CI, 44 to 86), ovarian (13; 95% CI, 5.3 to 25), gastric (6.9; 95% CI, 3.6 to 12), biliary tract (9.1; 95% CI, 1.1 to 33), uro-epithelial (7.6; 95% CI, 2.5 to 18) and kidney (4.7; 95% CI, 1 to 14) cancers and for central-nervous-system tumours (4.5; 95% CI, 1.2 to 12). The SIR increased with increasing likelihood of being a mutation carrier. The cumulative cancer incidences were 82, 60, 13 and 12% for colorectal, endometrial, gastric and ovarian cancers respectively. For other tumours associated with increased risk, corresponding incidences were below 4%. Interestingly, the incidence of endometrial cancer (60%) exceeded that for colorectal cancer in women (54%). The tumour spectrum associated with germline mutations of DNA-mismatch-repair genes involves 8 or more organ sites, suggesting a need to develop methods to screen for extra-colonic cancer also.

Cancer risk in mutation carriers of DNA-mismatch-repair genes

Excessive incidence of various cancers is a challenging feature of the hereditary-non-polyposis-colorectal-cancer (HNPCC) syndrome. This study estimated the cancer incidences in HNPCC compared with the general population. Individuals in a cohort of 1763 members of 50 genetically diagnosed families were categorized according to their genetic status as mutation carriers, non-carriers, or individuals at 50 or 25% risk of being a carrier. Incidences of cancers in these groups were compared with those in the Finnish population overall. In 360 mutation carriers, standardized incidence ratios (SIR) were significantly increased for colorectal [68; 95% confidence intervals (CI), 56 to 81], endometrial (62; 95% CI, 44 to 86), ovarian (13; 95% CI, 5.3 to 25), gastric (6.9; 95% CI, 3.6 to 12), biliary tract (9.1; 95% CI, 1.1 to 33), uro-epithelial (7.6; 95% CI, 2.5 to 18) and kidney (4.7; 95% CI, 1 to 14) cancers and for central-nervous-system tumours (4.5; 95% CI, 1.2 to 12). The SIR increased with increasing likelihood of being a mutation carrier. The cumulative cancer incidences were 82, 60, 13 and 12% for colorectal, endometrial, gastric and ovarian cancers respectively. For other tumours associated with increased risk, corresponding incidences were below 4%. Interestingly, the incidence of endometrial cancer (60%) exceeded that for colorectal cancer in women (54%). The tumour spectrum associated with germline mutations of DNA-mismatch-repair genes involves 8 or more organ sites, suggesting a need to develop methods to screen for extra-colonic cancer also.

Three widespread founder mutations contribute to high incidence of X-linked juvenile retinoschisis in Finland

X-linked juvenile retinoschisis (RS) is a recessively inherited disorder causing progressive vitreoretinal degeneration in males. The gene defective in retinoschisis, XLRS1, has recently been identified and characterised. This gene consists of six exons encoding a protein with a putative role in cell-cell adhesion and phospholipid binding. Juvenile retinoschisis has been actively studied in Finland over the past 30 years, with over 300 diagnosed RS patients. Based on genealogical studies, approximately 70% of the Finnish RS patients originate from Western Finland and 20% from Northern Finland. In this study, one third of the known Finnish RS patients were screened for mutations of the XLRS1 gene. Haplotype analysis, using nine microsatellite markers spanning 1 cM in Xp22.2, suggested the segregation of eight different mutations in these families. To identify mutations, the six exons were amplified by PCR and analysed by single strand conformation analysis, followed by direct sequencing of the PCR products. We identified seven distinct missense mutations, all in exons 4 and 6. The mutations in exon 4, 214G > A and 221G > T, are accountable for RS in Western Finland. A third mutation in exon 4, 325G > C, gives rise to RS in Northern Finland. These three founder mutations are the predominant cause of RS in Finland and their existence explains the high incidence of the disease. The identification of mutations common in genetically isolated populations, such as Finland, allows the diagnosis of patients with an atypical RS phenotype and enables nationwide carrier testing and improved genetic counselling.

Mutations in the nebulin gene associated with autosomal recessive nemaline myopathy

The congenital nemaline myopathies are rare hereditary muscle disorders characterized by the presence in the muscle fibers of nemaline bodies consisting of proteins derived from the Z disc and thin filament. In a single large Australian family with an autosomal dominant form of nemaline myopathy, the disease is caused by a mutation in the alpha-tropomyosin gene TPM3. The typical form of nemaline myopathy is inherited as an autosomal recessive trait, the locus of which we previously assigned to chromosome 2q21.2-q22. We show here that mutations in the nebulin gene located within this region are associated with the disease. The nebulin protein is a giant protein found in the thin filaments of striated muscle. A variety of nebulin isoforms are thought to contribute to the molecular diversity of Z discs. We have studied the 3' end of the 20. 8-kb cDNA encoding the Z disc part of the 800-kDa protein and describe six disease-associated mutations in patients from five families of different ethnic origins. In two families with consanguineous parents, the patients were homozygous for point mutations. In one family with nonconsanguineous parents, the affected siblings were compound heterozygotes for two different mutations, and in two further families with one detected mutation each, haplotypes are compatible with compound heterozygosity. Immunofluorescence studies with antibodies specific to the C-terminal region of nebulin indicate that the mutations may cause protein truncation possibly associated with loss of fiber-type diversity, which may be relevant to disease pathogenesis.

Mutations in CUBN, encoding the intrinsic factor-vitamin B12 receptor, cubilin, cause hereditary megaloblastic anaemia 1

Megaloblastic anaemia 1 (MGA1, OMIM 261100) is a rare, autosomal recessive disorder characterized by juvenile megaloblastic anaemia, as well as neurological symptoms that may be the only manifestations. At the cellular level, MGA1 is characterized by selective intestinal vitamin B12 (B12, cobalamin) malabsorption. MGA1 occurs worldwide, but its prevalence is higher in several Middle Eastern countries and Norway, and highest in Finland (0.8/100,000). We previously mapped the MGA1 locus by linkage analysis in Finnish and Norwegian families to a 6-cM region on chromosome 10p12.1 (ref. 8). A functional candidate gene encoding the intrinsic factor (IF)-B12 receptor, cubilin, was recently cloned; the human homologue, CUBN, was mapped to the same region. We have now refined the MGA1 region by linkage disequilibrium (LD) mapping, fine-mapped CUBN and identified two independent disease-specific CUBN mutations in 17 Finnish MGA1 families. Our genetic and molecular data indicate that mutations in CUBN cause MGA1.

High-resolution physical and genetic mapping of the critical region for Meckel syndrome and Mulibrey Nanism on chromosome 17q22-q23

Previously, we assigned the genes for two autosomal recessive disorders, Meckel syndrome (MKS; MIM 249000) and Mulibrey Nanism [MUL (muscle-liver-brain-eye Nanism); MIM 253250] that are enriched in the Finnish population, to overlapping genomic regions on chromosome 17q. Now, we report the construction of a bacterial clone contig over the critical region for both disorders. Several novel CA-repeat markers were isolated from these clones, which allowed refined mapping of the MKS and MUL loci using haplotype and linkage disequilibrium analysis. The localization of the MKS locus was narrowed to <1 cM between markers D17S1290 and 132-CA, within an approximately 800-kb region. The MUL locus was refined into an approximately 1400-kb interval between markers D17S1290 and 52-CA. The whole MKS region falls within the MUL region. In the common critical region, the conserved haplotypes were different in MKS and MUL patients. A trancript map was constructed by assigning expressed sequence tags (ESTs) and genes, derived from the human gene map, to the bacterial clone contig. Altogether, four genes and a total of 20 ESTs were precisely localized. These data provide the molecular tools for the final identification of the MKS and the MUL genes.

Downregulated in adenoma gene encodes a chloride transporter defective in congenital chloride diarrhea

Congenital chloride diarrhea (CLD) is a recessively inherited disorder characterized by massive loss of chloride in stool. We previously identified mutations in the downregulated in adenoma (DRA) gene in patients with CLD and demonstrated that DRA encodes an intestine-specific sulfate transporter. To determine whether DRA is an intestinal chloride transporter and how mutations affect transport, Xenopus oocytes were injected with wild-type and mutagenized DRA cRNA and uptake of Cl- and SO2-4 was assayed. Both Cl- and SO2-4 were transported by wild-type DRA and an outwardly directed pH gradient stimulated Cl- uptake, consistent with Cl-/OH- exchange. Among three mutants, C307W transported both anions as effectively as wild-type, whereas transport activity was lost in V317del and the double mutant identified in 32 of 32 Finnish CLD patients. We conclude that DRA is a chloride transporter defective in CLD and that V317del is a functional mutation and C307W a silent polymorphism.

Assignment of the muscle-eye-brain disease gene to 1p32-p34 by linkage analysis and homozygosity mapping

Muscle-eye-brain disease (MEB) is an autosomal recessive disease of unknown etiology characterized by severe mental retardation, ocular abnormalities, congenital muscular dystrophy, and a polymicrogyria-pachygyria-type neuronal migration disorder of the brain. A similar combination of muscle and brain involvement is also seen in Walker-Warburg syndrome (WWS) and Fukuyama congenital muscular dystrophy (FCMD). Whereas the gene underlying FCMD has been mapped and cloned, the genetic location of the WWS gene is still unknown. Here we report the assignment of the MEB gene to chromosome 1p32-p34 by linkage analysis and homozygosity mapping in eight families with 12 affected individuals. After a genomewide search for linkage in four affected sib pairs had pinpointed the assignment to 1p, the MEB locus was more precisely assigned to a 9-cM interval flanked by markers D1S200 proximally and D1S211 distally. Multipoint linkage analysis gave a maximum LOD score of 6.17 at locus D1S2677. These findings provide a starting point for the positional cloning of the disease gene, which may play an important role in muscle function and brain development. It also provides an opportunity to test other congenital muscular dystrophy phenotypes, in particular WWS, for linkage to the same locus.

The frequency of an inactivating point mutation (566C-->T) of the human follicle-stimulating hormone receptor gene in four populations using allele-specific hybridization and time-resolved fluorometry

We have described previously in the Finnish population an inactivating point mutation (566C-->T) in the human FSH receptor (FSHR) gene. In women, this mutation causes hypergonadotropic ovarian failure with arrest of follicular maturation and infertility, whereas in men, there is variable suppression of spermatogenesis, but no absolute infertility. To determine whether the same FSHR mutation occurs in other populations, its frequency was determined in Finland, Switzerland, Denmark, and the Chinese population of Singapore. The mutation was screened for using genomic DNA extracted from whole blood or dried blood spots. Exon 7 of the FSHR gene was first amplified using a pair of biotinylated primers. The PCR products were then immobilized on streptavidin-coated microtitration wells and hybridized using short allele-specific oligonucleotide probes labeled with europium. Time-resolved fluorometry was used for europium signal detection. To test the reliability of this method, 40 isolated DNA samples and 35 dried blood spot samples were blindly tested for the 566C-->T FSHR mutation. The analyses yielded identical results with denaturing gradient gel electrophoresis and allele-specific restriction enzyme digestion of the same samples, thus demonstrating the reliability of the tested method. Automation of this procedure allows the screening of large numbers of samples, which was subsequently carried out to investigate the frequency of the 566C-->T mutation in the study populations. A total of 4981 samples from the above-mentioned 4 countries were analyzed. The frequency of the 566C-->T mutation was 0.96% for all Finnish samples (n=1976), with a strong enrichment of the mutant allele in the northeastern part of the country. Only 1 mutation carrier was identified in the samples from Switzerland (n=1162), whereas none was found in samples from Denmark (n=1094) and the Singapore Chinese (n=540). These results suggest that the 566C-->T mutation of the FSHR gene is enriched in Finland, but is uncommon in other populations.

Linkage disequilibrium mapping in isolated populations: the example of Finland revisited

Linkage disequilibrium analysis can provide high resolution in the mapping of disease genes because it incorporates information on recombinations that have occurred during the entire period from the mutational event to the present. A circumstance particularly favorable for high-resolution mapping is when a single founding mutation segregates in an isolated population. We review here the population structure of Finland in which a small founder population some 100 generations ago has expanded into 5.1 million people today. Among the 30-odd autosomal recessive disorders that are more prevalent in Finland than elsewhere, several appear to have segregated for this entire period in the "panmictic" southern Finnish population. Linkage disequilibrium analysis has allowed precise mapping and determination of genetic distances at the 0.1-cM level in several of these disorders. Estimates of genetic distance have proven accurate, but previous calculations of the confidence intervals were too small because sampling variation was ignored. In the north and east of Finland the population can be viewed as having been "founded" only after 1500. Disease mutations that have undergone such a founding bottleneck only 20 or so generations ago exhibit linkage disequilibrium and haplotype sharing over long genetic distances (5-15 cM). These features have been successfully exploited in the mapping and cloning of many genes. We review the statistical issues of fine mapping by linkage disequilibrium and suggest that improved methodologies may be necessary to map diseases of complex etiology that may have arisen from multiple founding mutations.

Genetic background of congenital chloride diarrhea in high-incidence populations: Finland, Poland, and Saudi Arabia and Kuwait

Congenital chloride diarrhea (CLD) is an inherited intestinal disorder caused by mutations in the down-regulated in adenoma gene. In Finland, the disease is prevalent because of a founder effect, and all but one of the CLD-associated chromosomes carry the same mutation, V317del. In Poland, another area with a high incidence of CLD, as many as seven different mutations have been detected so far. A third known cluster of CLD, around the Persian Gulf, has not been genetically studied. We studied the allelic diversity of CLD in Poland, in Saudi Arabia and Kuwait, and in three isolated families from North America and Hong Kong. Altogether, eight novel mutations were identified, making a total of 19 known CLD gene mutations. The Polish major mutation I675-676ins was found in 47% of the Polish CLD-associated chromosomes. Haplotype analysis and clustering of the I675-676ins mutation supported a founder effect and common ancestral origin. As in Finland, a major founder effect was observed in Arab patients: 94% of the CLD-associated chromosomes carried a nonsense mutation, G187X, which occurred in either a conserved ancestral haplotype or its derivative. Our data confirm that the same locus is mutated in all cases of CLD studied so far. In Poland, a relatively common founder mutation is likely to highlight a set of rare mutations that would very rarely produce homozygosity alone. This suggests that mutations in the CLD locus are not rare events. Although the disease is thought to be rare, undiagnosed patients may not be uncommon.

Mismatch repair genes and mononucleotide tracts as mutation targets in colorectal tumors with different degrees of microsatellite instability

Microsatellite instability occurs in 15% of colorectal carcinomas and may be due to replication errors (RER). The pattern of instability--'severe' vs 'mild'--and the tumorigenic pathway, as reflected by the involvement of functionally important genes, may vary according to the underlying gene(s). We defined 'mild' RER as mono- or tetranucleotide repeat instability in the absence of widespread instability at dinucleotide repeats and studied 15 colorectal tumors with this phenotype for mutations in the DNA mismatch repair genes MSH2, MLH1, MSH3, and MSH6. No mutations were found, suggesting that these genes were not implicated. We then compared colorectal cancers with 'mild' RER (n = 15), and those with 'severe' RER without (n = 11) or with (n = 22) detectable mutations in MSH2 or MLH1 to assess the involvement of mononucleotide repeats contained in the coding regions of MSH3, MSH6, BAX, and TGFbeta RII. The combined mutation rates of the above mentioned loci varied significantly between the three groups of tumors, being 0%, 25% and 52%, respectively. Furthermore, the individual genes showed specific patterns of involvement; for example, among tumors with 'severe' RER, TGFbeta RII displayed uniformly high mutation rates while MSH3, MSH6, and BAX were more frequently altered in tumors that also showed MSH2 or MLH1 mutations. Our findings suggest that different subcategories exist among unstable tumors, defined by the RER pattern on the one hand and tumorigenic pathway on the other, and structural changes of MSH2 and MLH1 are likely to explain only a proportion of these cases.

Genomic structure of the human congenital chloride diarrhea (CLD) gene

Congenital chloride diarrhea (CLD) is caused by mutations in a gene which encodes an intestinal anion transporter. We report here the complete genomic organization of the human CLD gene which spans approximately 39kb, and comprises 21 exons. All exon/intron boundaries conform to the GT/AG rule. An analysis of the putative promoter region sequence shows a putative TATA box and predicts multiple transcription factor binding sites. The genomic structure was determined using DNA from several sources including multiple large-insert libaries and genomic DNA from Finnish CLD patients and controls. Exon-specific primers developed in this study will facilitate mutation screening studies of patients with the disease. Genomic sequencing of a BAC clone H_RG364P16 revealed the presence of another, highly homologous gene 3' of the CLD gene, with a similar genomic structure, recently identified as the Pendred syndrome gene (PDS).

The genetics of hereditary common cancers

Subsets of patients with common cancers belong to families in which the predisposition is inherited in a regular Mendelian fashion. Genes underlying these cancers are now recognized in colorectal cancer (APC, mismatch repair genes, LKB1) and in breast cancer (BRCA1, BRCA2) whereas, in prostate cancer, a locus in chromosome 1 (HPC1) has been proposed on the basis of linkage analysis. Major challenges are to determine the population incidence of these mutations, their penetrance, phenotypic expression, and the effects of modifier genes and epigenetic factors. Finally, the role of encoded proteins in carcinogenesis is a matter of major interest.

Incidence of hereditary nonpolyposis colorectal cancer and the feasibility of molecular screening for the disease

BACKGROUND

Genetic disorders that predispose people to colorectal cancer include the polyposis syndromes and hereditary nonpolyposis colorectal cancer. In contrast to the polyposis syndromes, hereditary nonpolyposis colorectal cancer lacks distinctive clinical features. However, a germ-line mutation of DNA mismatch-repair genes is a characteristic molecular feature of the disease. Since clinical screening of carriers of such mutations can help prevent cancer, it is important to devise strategies applicable to molecular screening for this disease.

METHODS

We prospectively screened tumor specimens obtained from 509 consecutive patients with colorectal adenocarcinomas for DNA replication errors, which are characteristic of hereditary colorectal cancers. These replication errors were detected through microsatellite-marker analyses of tumor DNA. DNA from normal tissue from the patients with replication errors was screened for germ-line mutations of the mismatch-repair genes MLH1 and MSH2.

RESULTS

Among the 509 patients, 63 (12 percent) had replication errors. Specimens of normal tissue from 10 of these 63 patients had a germ-line mutation of MLH1 or MSH2. Of these 10 patients (2 percent of the 509 patients), 9 had a first-degree relative with endometrial or colorectal cancer, 7 were under 50 years of age, and 4 had had colorectal or endometrial cancer previously.

CONCLUSIONS

In this series of patients with colorectal cancer in Finland, at least 2 percent had hereditary nonpolyposis colorectal cancer. We recommend testing for replication errors in all patients with colorectal cancer who meet one or more of the following criteria: a family history of colorectal or endometrial cancer, an age of less than 50 years, and a history of multiple colorectal or endometrial cancers. Patients found to have replication errors should undergo further analysis for germ-line mutations in DNA mismatch-repair genes.

Molecular studies in Finnish patients with familial juvenile nephronophthisis exclude a founder effect and support a common mutation causing mechanism

Familial juvenile nephronophthisis (NPH) is an autosomal recessive tubulointerstitial kidney disease associated with formation of medullary and corticomedullary cysts. It progresses to end stage renal failure and its biochemical defect is unknown. An NPH locus has been assigned to a 2 cM interval on chromosome 2q13 by linkage studies. Homozygous deletions of approximately 250 kb have been detected in 80% of familial cases and 65% of sporadic cases and a common mutation mechanism has been suggested. We examined 14 Finnish families for the presence or absence of a deletion. After detecting a deletion in 12 patients belonging to nine families, we studied a possible founder effect by haplotype analysis using markers D2S340, D2S1889, and D2S1893. No common ancestral disease associated haplotype was found suggesting no founder effect. Results of pairwise linkage analyses were suggestive of linkage in the nine families with a deletion (lod scores of 1.39-3.89 at a recombination fraction of 0). Negative lod scores were obtained in the five families without a deletion suggesting that the disease locus in these families lies elsewhere. The end stage renal disease occurred at a more advanced age in patients without a deletion compared to patients with a deletion, indicating a phenotypic difference between these two groups.

A serine/threonine kinase gene defective in Peutz-Jeghers syndrome

Studies of hereditary cancer syndromes have contributed greatly to our understanding of molecular events involved in tumorigenesis. Here we investigate the molecular background of the Peutz-Jeghers syndrome (PJS), a rare hereditary disease in which there is predisposition to benign and malignant tumours of many organ systems. A locus for this condition was recently assigned to chromosome 19p. We have identified truncating germline mutations in a gene residing on chromosome 19p in multiple individuals affected by PJS. This previously identified but unmapped gene, LKB1, has strong homology to a cytoplasmic Xenopus serine/threonine protein kinase XEEK1, and weaker similarity to many other protein kinases. Peutz-Jeghers syndrome is therefore the first cancer-susceptibility syndrome to be identified that is due to inactivating mutations in a protein kinase.

Rearrangement of ALL1 (MLL) in acute myeloid leukemia with normal cytogenetics

Approximately 45% of adults with acute myeloid leukemia (AML) have normal cytogenetics and therefore lack structural abnormalities that can assist in the localization and characterization of molecular defects. The partial tandem duplication of the ALL1 (MLL) gene has been found in several such cases of AML, yet its frequency and clinical significance are unclear. We performed Southern analysis of the ALL1 gene in pretreatment samples from 98 AML patients with normal cytogenetics. Eleven of 98 such patients (11%; 95% confidence interval, 6-19%) showed rearrangement of ALL1 at diagnosis. The partial tandem duplication of ALL1 was responsible for ALL1 rearrangement in all such cases examined, making it a frequent molecular defect in adult AML patients with normal cytogenetics. Furthermore, patients with ALL1 rearrangement had a significantly shorter duration of complete remission when compared to patients without ALL1 rearrangement (P = 0.01; median, 7.1 versus 23.2 months). This defect defines for the first time a subset of AML patients with normal cytogenetics who have short durations of complete remission and thus require new therapeutic approaches.

Reconstructing hominid Y evolution: X-homologous block, created by X-Y transposition, was disrupted by Yp inversion through LINE-LINE recombination

The human X and Y chromosomes share many blocks of similar DNA sequence. We conducted mapping and nucleotide sequencing studies of extensive, multi-megabase homologies between Yp and Xq21, which do not recombine during male meiosis. We confirmed and built upon previous evidence that a Yp inversion had occurred during evolution: a single contiguous segment of Xq21 is homologous to two non-contiguous segments of Yp. We precisely defined and sequenced the inversion breakpoints, obtaining evidence that the inversion was mediated by recombination between LINE-1 elements in otherwise non-homologous regions. This inversion appears to have followed a single transposition of an approximately 4 Mb segment from the X to the Y chromosome. These events jointly account for the present arrangement of Yp-Xq21 homologous sequences. Based on Southern blotting studies of primates and of humans drawn from diverse populations, we conclude that both the X-Y transposition and the subsequent, LINE-mediated Yp inversion occurred after the divergence of hominid and chimp lineages but before the radiation of extant human populations. This evolutionary scenario is consistent with our finding of 99.3 +/- 0.2% nucleotide identity between the X and Y chromosomes within the transposed region, which suggests that the transposition occurred approximately 3-4 million years ago, near the time of emergence of Homo . Comparative sequencing of the entire human X and Y chromosomes may reveal a succession of transpositions, inversions and other rearrangements underlying the complex pattern of sequence similarities between the present-day sex chromosomes. With the possible exception of cubitus valgus, phenotypic features of Turner syndrome are absent in individuals monosomic for Yp-Xq21 homologous sequences, suggesting that most of the critical 'Turner genes' are found elsewhere on the X and Y chromosomes.

Peutz-Jeghers disease: most, but not all, families are compatible with linkage to 19p13.3

A locus for Peutz-Jeghers syndrome (PJS) was recently mapped to chromosome 19p13.3. Each of 12 families studied was compatible with linkage to the marker D19S886. We have analysed 20 further families and found that the majority of these are consistent with a PJS gene on 19p13.3. Three families were, however, unlinked to 19p13.3 and none of the available PJS polyps from these families showed allele loss at D19S886. There were no obvious clinicopathological or ethnic differences between the 19p13.3 linked and unlinked families. There appears, therefore, to be a major PJS locus on chromosome 19p13.3 and the possibility exists of a minor locus (or loci) elsewhere.

Genetic control of serum IgE levels and asthma: linkage and linkage disequilibrium studies in an isolated population

Immunoglobulin E (IgE) concentration in serum is elevated in atopic diseases such as asthma. A large genomic region on chromosome 5 has previously been implicated in the control of IgE levels and bronchial hyperreactivity and may, therefore, harbor genes predisposing to asthma. In an effort to confirm this linkage and to delimit the critical region, we took advantage of an isolated founder subpopulation in Finland to study genetic linkage and haplotype associations. Sixteen polymorphic markers, including the Interleukin-4 and -9 genes (IL4, IL9), were physically ordered and genotyped in 157 nuclear families. Genetic linkage studies involving sib- and cousin-pair analyses found no evidence of genetic linkage between markers in 5q and either serum IgE levels or asthma. Haplotype association studies were also performed. Although initial inspection suggested the possibility of linkage disequilibrium in the region of IL9, we developed a rigorous permutation test for assessing association and determined that the association was no greater than would be expected by chance. Sequence analysis of the IL9 gene in three patients sharing a possibly conserved haplotype revealed a T113M coding polymorphism, but this variant showed no association with either serum IgE levels or asthma. We conclude that allelic variation at chromosome 5q31 is not likely to contribute to inheritance of serum IgE levels or the development of asthma in this Finnish subpopulation.

High-resolution mapping and transcript identification at the progressive epilepsy with mental retardation locus on chromosome 8p

Progressive epilepsy with mental retardation (EPMR) is an autosomal recessive central nervous system disorder characterized by childhood onset epilepsy and subsequent mental retardation. The locus for EPMR has been mapped to human chromosome 8p23. We recently reported the construction of a YAC contig across the 4 centimorgan minimum genetic region that harbors the disease locus. We now report further delineation of the critical region to <700 kb. Our mapping strategy relied on the identification of nine novel microsatellite markers and the construction of a complete BAC contig across the critical region. Several partial gene sequences have been identified from the region and are being analyzed as candidate genes for EPMR.

MSH2 and MLH1 mutations in sporadic replication error-positive colorectal carcinoma as assessed by two-dimensional DNA electrophoresis

Replication errors (RER) are frequently seen in both sporadic and hereditary forms of colorectal cancer. In hereditary nonpolyposis colorectal cancer (HNPCC), RER is associated with defects in DNA mismatch repair genes. Two of these genes, MSH2 and MLH1, account for a major share of this cancer syndrome. In order to assess the role of these genes in sporadic RER+ colorectal carcinoma, we have carried out a mutation analysis of MSH2 and MLH1 by two-dimensional (2-D) DNA electrophoresis, including heteroduplexing and separation in a denaturing gradient. All exons were amplified using multiplex PCR and were separated on the basis of both size and base pair composition under a single set of experimental conditions. Exons showing a spot position different from normal were sequenced. In screening 33 unselected, sporadic RER+ colorectal tumors, a germline mutation accompanied by loss of heterozygosity in tumor tissue was found in two patients. They were among the 4 patients out of the 33 screened that were diagnosed before the age of 50 years. In 8 of the remaining 31 tumors (26%), presence of somatic mutations (9 in total) could be demonstrated. While suggesting involvement of other genes in a substantial part of sporadic RER+ colorectal carcinomas, our results also demonstrate a clear role of MSH2 and MLH1 in these sporadic tumors and show that young sporadic RER+ colorectal carcinoma patients have a high probability of germline mutations. This has important implications for genetic testing and management of young colorectal cancer patients and their families.

Refined localisation of the genes for nebulin and titin on chromosome 2q allows the assignment of nebulin as a candidate gene for autosomal recessive nemaline myopathy

A locus for autosomal recessive nemaline myopathy (NEM2) has been assigned by linkage analysis to a 13-cM region between the markers D2S150 and D2S142 on 2q21.2-q22. The genes for the giant muscle proteins nebulin and titin have previously been assigned by FISH to 2q24.1-q24.2 and 2q31, respectively. By using radiation hybrid mapping, we have reassigned the nebulin gene close to the microsatellite marker D2S2236 on 2q22 and the titin gene to the vicinity of the markers D2S384 and D2S364 on 2q24.3. The genomic orientation of the nebulin gene was determined as 5'-3' and of TTN as 3'-5' from the centromere. We conclude that the nebulin gene resides within the candidate region for NEM2 on the long arm of chromosome 2, while the titin gene is located outside this region.

Refined mapping of the Cohen syndrome gene by linkage disequilibrium

The Cohen syndrome is a rare autosomal recessively inherited disorder. Contrary to many case reports published elsewhere, the phenotype is uniform in Finland including nonprogressive mental and motor retardation, typical dysmorphic features, granulocytopenia and marked ophthalmological changes. By linkage analysis in five Finnish multiplex nuclear families, the COH1 locus for the Cohen syndrome was recently assigned to a 10-cM region between loci D8S270 and D8S521 on the long arm of chromosome 8. Here we present results of linkage disequilibrium and haplotype analysis in an extended panel of 16 Finnish COH1 families using new markers localized in the COH1 region. By inferring historical recombinations in conserved haplotypes the COH1 gene was assigned in the region of marker loci D8S1808, D8S1762 and D8S546. Calculations of genetic distances based on linkage disequilibrium suggest that the most likely localization of COH1 is in the immediate vicinity of marker locus D8S1762. Haplotype analysis suggests the occurrence of one main COH1 mutation and possibly one or two rare ones in Finland. This information will be useful in the positional cloning of the gene.

Linkage disequilibrium and physical mapping of X-linked juvenile retinoschisis

X-linked juvenile retinoschisis (RS) is a recessively inherited disorder resulting in poor visual acuity. Affected males typically show retinal degeneration and intraretinal splitting. The prevalence of RS is 1:15,000-1:30,000. Elsewhere we have mapped the RS gene between the markers DXS43 and DXS274 in Xp22.1-p22.2. To narrow the RS region, we analyzed 31 Finnish RS families with the markers DXS418, DXS999, DXS7161, and DXS365 and a new polymorphic microsatellite marker, HYAT1. Multipoint linkage analysis allowed us to localize the RS gene between the markers DXS418 and DXS7161 (LOD score = 31.3). We have covered this region with nine YAC clones. On the basis of the sizes of the YACs, sequence-tagged site (STS) content mapping, and restriction mapping, the physical distance between DXS418 and DXS7161 is approximately 0.9 Mb. A total of five potential CpG islands could be identified. For haplotype analysis, eight additional Finnish RS families were analyzed with the markers DXS1195, DXS418, HYAT1, DXS999, DXS7161, and DXS365. On the basis of the linkage-disequilibrium data that were derived from the genetically isolated Finnish population, the critical region for RS could be narrowed to 0.2-0.3 cM, between the markers DXS418 and HYAT1.

The partial tandem duplication of ALL1 in acute myeloid leukemia with normal cytogenetics or trisomy 11 is restricted to one chromosome

The molecular defects responsible for tumorigenesis in adult de novo acute myeloid leukemia (AML) with a normal karyotype or an additional copy of one chromosome (i.e., trisomy) remain largely unknown. We recently discovered that approximately 90% of adult patients with de novo AML and trisomy 11 (+11) as a sole abnormality and 11% of adult patients with de novo AML and normal cytogenetics carry a molecular rearrangement of the ALL1 (MLL, HRX, or HTRX) gene. The rearranged ALL1 gene has been shown to result from the direct tandem duplication of a portion of ALL1 itself. To better understand the underlying mechanisms of leukemogenesis, we asked whether in cytogenetically normal cases one or both chromosomes carry the mutated allele and whether in trisomic cases the mutation is present in one, two, or three chromosomes. Herein we show that in cytogenetically normal cases of AML and in cases with +11 as a sole cytogenetic abnormality, only one chromosome contains the mutated ALL1 allele. Thus a single mutated ALL1 allele with the partial tandem duplication is sufficient for ALL1-associated leukemogenesis, irrespective of the number of normal genes present. The frequently occurring specific association of +11 and ALL1 gene mutation in the leukemic clone remains unexplained.

Unstable minisatellite expansion causing recessively inherited myoclonus epilepsy, EPM1

Progressive myoclonus epilepsy of Unverricht-Lundborg type (EPM1; MIM 254800) is an autosomal recessive disorder that occurs with a low frequency in many populations but is more common in Finland and the Mediterranean region. It is characterized by stimulus-sensitive myoclonus and tonic-clonic seizures with onset at age 6-15 years, typical electroencephalographic abnormalities and a variable rate of progression between and within families. Following the initial mapping of the EPM1 gene to chromosome 21 (ref. 6) and the refinement of the critical region to a small interval, positional cloning identified the gene encoding cystatin B (CST6), a cysteine protease inhibitor, as the gene underlying EPM1 (ref. 10). Levels of messenger RNA encoded by CST6 were dramatically decreased in patients. A 3' splice site and a stop codon mutation were identified in three families, leaving most mutations uncharacterized. In this study, we report a novel type of disease-causing mutation, an unstable 15- to 18-mer minisatellite repeat expansion in the putative promoter region of the CST6 gene. The mutation accounts for the majority of EPM1 patients worldwide. Haplotype data are compatible with a single ancestral founder mutation. The length of the repeat array differs between chromosomes and families, but changes in repeat number seem to be comparatively rare events.

Assignment of the mulibrey nanism gene to 17q by linkage and linkage-disequilibrium analysis

Mulibrey nanism (MUL) is an autosomal recessive disorder with unknown basic metabolic defect. It is characterized by growth failure of prenatal onset, characteristic dysmorphic features, constrictive pericardium, hepatomegaly as a consequence of constrictive pericardium, yellowish dots in the ocular fundi, and J-shaped sella turcica. Hypoplasia of various endocrine glands, causing hormone deficiencies, is common. Here we report the assignment of the MUL gene, by linkage analysis in Finnish families, to a 7-cM region flanked by D17S1799 and D17S948 on chromosome 17q. Multipoint linkage analysis gave a maximum LOD score of 5.01 at loci D17S1606-D17S1853 and at D17S1604. The estimate of the critical MUL region was further narrowed to within approximately 250 kb of marker D17S1853 by linkage disequilibrium analysis. Positional candidate genes that belong to the growth hormone and homeobox B gene clusters were excluded. These data confirm the autosomal recessive inheritance of MUL and allow highly focused attempts to clone the gene.

Microsatellite instability in cervical and endometrial carcinomas

Microsatellite instability has been found preferentially in tumours associated with the hereditary non-polyposis-colorectal-cancer (HNPCC) syndrome. This phenotype, manifested as new alleles at microsatellite loci, and often the result of a defective mismatch-repair gene, is seen as allelic mobility shifts during electrophoretic runs. We examined possible alterations at 8 dinucleotide loci mapping to 6 different chromosomes in endometrial cancers (n = 20) and cervical cancers (n = 82). Overall instability was found in 30% of the endometrial cancers and in 6% of the cervical cancers, including 3 (15%) and 2 (2%) tumours, respectively, unstable at more than one locus. In contrast to the endometrial cancer sub-group, the affected cervical cancers were characterized by one or two new alleles at one or few loci. By DNA ploidy measurements 5 diploid endometrial cancers were microsatellite-unstable vs. one diploid of 6 unaltered cases (p = 0.015; Fisher's exact test). Our data confirm that a sub-set of diploid sporadic endometrial cancers are characterized by a mutator phenotype similar to that found in colorectal cancer. In contrast, among cervical cancers, not characterized by the HNPCC-tumour spectrum, this mutator phenotype is seen infrequently, and positive cases appear to display only minor alterations.

Clinical significance of cytogenetics in acute myeloid leukemia

Cytogenetic studies of acute myeloid leukemia (AML) have revealed that the majority of patients display acquired chromosome aberrations. Numerous recurrent karyotypic aberrations have been and continue to be discovered in AML. Cytogenetic studies have paved the way for molecular analyses that have identified genes involved in the process of leukemogenesis. Moreover, chromosome aberrations, regardless of whether they have been molecularly characterized or not, have been shown to constitute tumor markers of diagnostic and prognostic value. This review presents current information on cytogenetic findings in AML, and correlations between karyotype and clinical features and outcome of de novo AML.

Mutations predisposing to hereditary nonpolyposis colorectal cancer

Since 1993 four genes have been identified that, when mutated, confer predisposition to a form of hereditary colon cancer (hereditary nonpolyposis colorectal cancer [HNPCC]). These genes belong to the Mut-related family of DNA mismatch repair genes whose protein products are responsible for the recognition and correction of errors that arise during DNA replication. Mutational inactivation of both copies of a DNA mismatch repair gene results in a profound repair defect demonstrable by biochemical assays, and in vivo this defect is presumed to lead to progressive accumulation of secondary mutations throughout the genome, some of which affect important growth-regulatory genes and, hence, give rise to cancer. To date, more than 70 different germline mutations have been detected in DNA mismatch repair genes and shown to be associated with HNPCC. Current evidence suggests that two genes, MSH2 and MLH1, account for roughly equal proportions of HNPCC kindreds, together being responsible for a majority of these families, but striking interethnic differences occur. Most mutations lead to truncated protein products. Mutation screening is quite demanding in HNPCC since, with a few exceptions, the predisposing mutations typically vary from kindred to kindred and individual mutations are scattered throughout the genes. Knowledge of the predisposing mutations allows genotype-phenotype correlations and forms the basis for further studies clarifying the pathogenesis of this disorder. In at-risk individuals, it allows predictive testing for cancer susceptibility and, consequently, appropriate clinical management of mutation carriers and noncarriers.

Uniparental disomy in cartilage-hair hypoplasia

Cartilage-hair hypoplasia (CHH) is an autosomal recessive disorder that presents with pleiotropic manifestations including impaired skeletal growth and cellular immunity. It is most prevalent among two founder populations, the Old Order Amish in the USA and the Finns. The gene has been localized to 9p13 by linkage analysis and linkage disequilibrium mapping. A statistically significant deficiency of affected members resulting in a lower than expected segregation ratio has been reported in the Amish, but was not found in a previous study in Finnish CHH families. Reduced penetrance was the mechanism suggested in the Amish, but could not be verified by haplotype analyses performed after the assignment of the CHH gene. Here we have carried out segregation analysis of 101 Finnish CHH families, but again, evidence of a significant deficiency of affected members was not found. Nevertheless, among 54 uniplex families, 2 patients with CHH and uniparental disomy (UPD) for chromosome 9 were discovered. UPD might contribute to low segregation ratios by increasing the number of families with only 1 affected individual. These observations show that UPD may occur in an unexpectedly high number of the patients and should be taken into account in the genetic counselling and prenatal diagnostics of CHH families.

Localization of a susceptibility locus for Peutz-Jeghers syndrome to 19p using comparative genomic hybridization and targeted linkage analysis

Many human cancer susceptibility genes have been successfully mapped by genetic linkage studies. One that has so far eluded researchers is that for Peutz-Jeghers (P-J) syndrome, a condition characterized by intestinal hamartomatous polyposis and melanin spots of the lips, buccal mucosa and digits. A dramatically elevated risk of malignancy has also been documented. Gastrointestinal tumours as well as cancers of the breast, ovary, testis and uterine cervix appear to be overrepresented in families with this syndrome. The nature of hamartomatous polyps is equivicol. Hamartomas are usually considered histologically benign, but in the case of Peutz-Jeghers patients, there are reports of adenomatous and malignant changes in the polyps, and the possibility of a hamartoma-carcinoma sequence has been discussed. A search for a putative tumour suppressor locus was made using comparative genomic hybridization (CGH) of Peutz-Jeghers polyps, combined with loss of heterozygosity (LOH) study. Genetic linkage analysis in 12 families using markers from a deletion site demonstrated the presence of a high-penetrance locus in distal 19p with a multipoint lod score of 7.00 at marker D19S886 without evidence of genetic heterogeneity. The study demonstrates the power of CGH combined with LOH analysis in identifying putative tumour suppressor loci, and provides molecular evidence of malignant potential in hamartomas.

Age and origin of two common MLH1 mutations predisposing to hereditary colon cancer

Two mutations in the DNA mismatch repair gene MLH1, referred to as mutations 1 and 2, are frequent among Finnish kindreds with hereditary nonpolyposis colorectal cancer (HNPCC). In order to assess the ages and origins of these mutations, we constructed a map of 15 microsatellite markers around MLH1 and used this information in haplotype analyses of 19 kindreds with mutation 1 and 6 kindreds with mutation 2. All kindreds with mutation 1 showed a single allele for the intragenic marker D3S1611 that was not observed on any unaffected chromosome. They also shared portions of a haplotype of 4-15 markers encompassing 2.0-19.0 cM around MLH1. All kindreds with mutation 2 shared another allele for D3S1611 and a conserved haplotype of 5-14 markers spanning 2.0-15.0 cM around MLH1. The degree of haplotype conservation was used to estimate the ages of these two mutations. While some recessive disease genes have been estimated to have existed and spread for as long as thousands of generations worldwide and hundreds of generations in the Finnish population, our analyses suggest that the spread of mutation 1 started 16-43 generations (400-1,075 years) ago and that of mutation 2 some 5-21 generations (125-525 years) ago. These datings are compatible with our genealogical results identifying a common ancestor born in the 16th and 18th century, respectively. Overall, our results indicate that all Finnish kindreds studied to date showing either mutation 1 or mutation 2 are due to single ancestral founding mutations relatively recent in origin in the population. Alternatively, the mutations arose elsewhere earlier and were introduced in Finland more recently.