A role for MLH3 in hereditary nonpolyposis colorectal cancer.

We investigated a possible role of the mismatch-repair gene MLH3 in hereditary nonpolyposis colorectal cancer by scanning for mutations in 39 HNPCC families and in 288 patients suspected of having HNPCC. We identified ten different germline MLH3 variants, one frameshift and nine missense mutations, in 12 patients suspected of HNPCC. Three of the 12 also carried a mutation in MSH6.

SMA carrier testing--validation of hemizygous SMN exon 7 deletion test for the identification of proximal spinal muscular atrophy carriers and patients with a single allele deletion.

To facilitate the detection of carriers of a hemizygous survival motor neuron (SMN) exon 7 deletion we have modified the quantitative SMN exon 7 assay described by McAndrew et al (1997). The major changes include quantitative analysis of the amount of SMN exon 7-specific fluorescently-labelled PCR product on an automated sequencer, and the monitoring of the completeness of a DraI digestion necessary to distinguish the PCR products of exons 7 of SMN and its copy gene. In our method the amount of SMN exon 7 PCR product is compared with the amount of a co-amplified PCR product of the retinoblastoma (RB1) exon containing a DraI restriction site. By co-amplification using the same primers of plasmids included in the reaction as internal standards containing SMN exon 7 with a 36-nucleotide deletion and RB1 exon 13 with a 19-nucleotide deletion, respectively, the relative amplification efficacy can be monitored. The assay has been validated in 63 ascertained carriers and 28 ascertained non-carriers. The sensitivity of the test is approximately 97%, the specificity approaches 100%. In four out of six SMA patients without a homozygous deletion we detected a hemizygous deletion. The implications of the use of this assay for carrier testing and for confirmation of the clinical diagnosis of SMA are discussed.

Prenatal prediction of spinal muscular atrophy. Experience with linkage studies and consequences of present SMN deletion analysis.

With the localisation of the gene for the autosomal recessive forms of proximal spinal muscular atrophies (SMA) to the chromosomal region 5q13 and the later detection of homozygous deletions of the SMN gene located in this region, prenatal prediction of SMA has become feasible and is widely applied now. In our experience with 77 prenatal predictions of SMA, follow-up of the 39 liveborn children from these pregnancies never led to a false-negative result. Application of SMN deletion analysis has consequences for prenatal prediction of SMA. When the index patient has a homozygously deleted exon 7 of the SMN gene, prenatal prediction and interpretation of results are straightforward. In families in which no DNA from the index patient is available, prenatal detection of a homozygous SMN deletion may be considered almost proof of SMA in the fetus. Absence of a deletion, however, will not guarantee an unaffected child. A real problem exists if the index patient does not show a homozygous deletion of SMN exon 7. In such cases with non-homozygous SMN deletions, one cannot be certain of 5q linkage and autosomal recessive inheritance until other SMN mutations are detected. This is an argument to abstain from prenatal diagnosis by linkage analysis in these families.

Adrenocortical adenocarcinoma in an MSH2 carrier: coincidence or causal relation?

A woman is described who developed an ovarian adenocarcinoma, 3 metachronous colorectal adenocarcinomas, and a primary adrenocortical adenocarcinoma. Genetic investigation of the mismatch repair genes MLH1 and MSH2 showed a germline mutation in MSH2. Colorectal and ovarian carcinoma belong to the tumor spectrum of hereditary nonpolyposis colorectal cancer (HNPCC). Adrenocortical adenocarcinoma, however, has never been described as 1 of the HNPCC-associated tumors. To investigate whether the adrenocortical adenocarcinoma in this patient was caused by the MSH2 germline mutation, determination of microsatellite instability (MSI) and immunohistochemical analysis were performed on 1 of the colorectal tumors and the adrenocortical adenocarcinoma. MSI and general loss of MSH2 protein expression could be seen in the colorectal tumor but not in the adrenocortical adenocarcinoma. Therefore, it is highly unlikely that the adrenocortical adenocarcinoma found in this patient was due to her genetic predisposition for HNPCC. HUM PATHOL 31:1522-1527.

Differential methylation at the 5' and the 3' CCGG sites flanking the X chromosomal hypervariable DXS255 locus.

The degree of methylation at the 5' and 3' CCGG sequences flanking the variable number of tandem repeat (VNTR) region of the DXS255 locus at Xp11.22 was analysed separately in several haematopoietic cell lineages. The 5' CCGG site on active chromosomes was found to be completely methylated in B and T lymphocytes and granulocytes. Methylation of the 5' site on inactive X chromosomes differed between females (0%-60%), but was consistent in different cell lineages obtained from individual females. In contrast, methylation at the 3' CCGG site on active chromosomes was found to vary in B lymphocytes (40%-100%), whereas complete methylation was found in T lymphocytes and granulocytes. The extent of methylation on inactive X chromosomes was found to differ significantly between B lymphocytes (17%), T lymphocytes (54%) and granulocytes (82%). Thus, methylation at the 5' CCGG site seems to be primarily related to the status of X chromosome inactivation, whereas methylation at the 3' CCGG site is mainly subject to cell-lineage-specific influences.

Mutation analysis of the Bruton's tyrosine kinase gene in X-linked agammaglobulinemia: identification of a mutation which affects the same codon as is altered in immunodeficient xid mice.

X-linked agammaglobulinemia (XLA) is an inherited immunodeficiency disease in man, reflecting an arrest in differentiation of pre-B cells to mature B cell stages. The gene defective in XLA has been identified as a cytoplasmic protein tyrosine kinase, named btk (Bruton's tyrosine kinase). Here we report the characterization of mutations in the btk gene of five unrelated XLA families. Amplified products were generated from cDNA, cloned and sequenced. Three single point mutations and two small insertions were identified. One of the point mutations and the two insertions created stop codons that would lead to truncated btk proteins. In one XLA patient we found a single basepair substitution that altered the highly conserved Arg288 within the SH2 domain and would therefore abrogate interactions with substrate phosphotyrosines. In another XLA patient a single basepair substitution was observed that altered the conserved Arg28 residue in the N-terminal unique region of unknown function. This residue is also mutated in the xid mouse, which has a different, less severe, B cell deficiency. We conclude that a similar mutation in the btk gene leads in man to an almost complete arrest at an early stage of B cell differentiation, but in the mouse to only limited B cell abnormalities.

Diagnosis of Wiskott-Aldrich syndrome by analysis of the X chromosome inactivation patterns in maternal leucocyte populations using the hypervariable DXS255 locus.

The Wiskott-Aldrich syndrome (WAS) is characterized by severe recurrent infections, petachiae and chronic eczema. The syndrome involves differentiation disorders in several haematopoietic cell lineages usually manifested as T lymphocyte deficiency, dysgammaglobulinaemia and thrombocytopenia. The defect is inherited in an X-linked recessive mode. A 1-year-old boy presented with otitis, upper respiratory infections, eczema, a persistent granulocytopenia and a dysgammaglobulinaemia. In his family five males in two generations had been shown to have WAS, which entailed a significant risk for the patient to have WAS. As the WAS gene or gene product is not delineated, the symptoms of the patient presented a diagnostic dilemma. If the boy had inherited the disease, his mother should be a WAS carrier. Segregation analysis in the family using the closely linked restriction fragment length polymorphisms (RFLP) DXS7, DXS255 and DXS14 did not exclude her carriership, although the probability was low. As a result of the differentiation arrest, obligate female WAS carriers manifest a unilateral X chromosome inactivation pattern in several haematopoietic cell lineages. Methylation analysis of the X chromosomal DXS255 loci exposed random X chromosome inactivation patterns in the peripheral blood granulocytes, T lymphocytes and B lymphocytes of the patient's mother. These findings excluded her WAS carriership and therefore excluded the diagnosis of WAS in the patient. This was further substantiated in a 1-year follow up with recovery from the haematological and immunological symptoms. These results demonstrated that X inactivation analysis in maternal leucocytes is decisive in the exclusion of the diagnosis of WAS.

The Bruton's tyrosine kinase gene is expressed throughout B cell differentiation, from early precursor B cell stages preceding immunoglobulin gene rearrangement up to mature B cell stages.

X-linked agammaglobulinemia (XLA) is an immunodeficiency disease in man, resulting from an arrest in early B cell differentiation. The gene defective in XLA has recently been identified and encodes a cytoplasmic protein tyrosine kinase, named Bruton's tyrosine kinase (btk), essential for cell differentiation and proliferation at the transition from pre-B to later B cell stages. In this study we investigated btk expression by Northern blotting experiments in a series of human (precursor-) B cell lines, acute lymphoblastic leukemias and plasmacytomas. btk was found to be already expressed in very early stages of B cell differentiation, even prior to immunoglobulin (Ig) heavy (H) or light (L) chain gene rearrangements. Transcripts were also detected at the pre-B cell stage and in mature B cells, irrespective of the Ig H chain class expressed. Approximately at the transition from mature B cells to plasma cells, expression of the btk gene is down-regulated. In addition, the btk gene was found to be expressed in myeloid cell lines and acute myeloid leukemias. btk expression in myeloid cells is probably not a prerequisite for myeloid differentiation, since myeloid cells in XLA patients seem not to be affected. No btk expression was found in T-lineage cells. The btk expression profile, i.e. from early precursor-B cell stages preceding Ig rearrangement up to mature B cells, supports the hypothesis that the XLA defect resides in a critical step of B cell development which is independent of the Ig gene recombination machinery.

Expression of Bruton's tyrosine kinase in B lymphoblastoid cell lines from X-linked agammaglobulinaemia patients.

X-linked agammaglobulinaemia (XLA) is an immunodeficiency caused by mutations in Bruton's tyrosine kinase (Btk) and is characterized by an almost complete arrest of B cell development. We analysed expression of Btk in B lymphoblastoid cell lines (BLCL) derived from four unrelated XLA patients. In one patient, with a 3 x 5 kb genomic deletion encompassing the first (untranslated) exon, mRNA levels and in vitro kinase activities were very low. The patient manifested a mild phenotype with a delayed onset of the disease. Another mutation, in which the intron 3 donor splice site is lost, was also associated with very low mRNA levels and an absence of detectable Btk protein. Patients with this mutation showed extensive heterogeneity of the immunological phenotype. In the BLCL of a third patient, with an Arg288 substitution in the SH2 domain, the mutation did not appear to affect the expression level, nor to abrogate in vitro phosphorylation activity. In the BLCL of the fourth patient, with an Arg28 mutation in the PH domain, tyrosine kinase activity in BTK precipitates appeared to be decreased compared with control BLCL.

Refinement by linkage analysis in two large families of the candidate region of the third locus (SCA3) for autosomal dominant cerebellar ataxia type I.

The autosomal dominant cerebellar ataxias (ADCA) are clinically and genetically heterogeneous. To date, several loci (SCAI-V) have been identified for ADCA type I. We have studied two large families from the northern part of The Netherlands with ADCA type I with a broad intra-familial variation of symptoms. In both families significant linkage is shown of the disease to the markers of the SCA3 locus on chromosome 14. Through recombinations, the candidate region for SCA3 could be refined to a 13-cM range between D14S256 and D14S81. No recombinations were detected with the markers D14S291 and D14S280, which suggests that the SCA3 gene lies close to these loci. This finding will benefit the individuals at risk in these two families who are seeking predictive testing or prenatal diagnosis.

Association of hereditary nonpolyposis colorectal cancer-related tumors displaying low microsatellite instability with MSH6 germline mutations.

Hereditary nonpolyposis colorectal cancer (HNPCC) (Amsterdam criteria) is often caused by mutations in mismatch repair (MMR) genes, and tumors of patients with HNPCC show microsatellite instability (MSI-high phenotype). Germline mutations of MMR genes have rarely been found in families that have HNPCC or suspected HNPCC and that do not show microsatellite instability (MSI-low phenotype). Therefore, an MSI-high phenotype is often used as an inclusion criterion for mutation testing of MMR genes. Correction of base-base mismatches is the major function of MSH6. Since mismatches present with an MSI-low phenotype, we assumed that the phenotype in patients with HNPCC-related tumors might be associated with MSH6 germline mutations. We divided 36 patients with suspected HNPCC into an MSI-low group (n=18) and an MSI-high group (n=18), on the basis of the results of MSI testing. Additionally, three unrelated patients from Amsterdam families with MSI-low tumors were investigated. All patients were screened for MSH2, MLH1, and MSH6 mutations. Four presumably causative MSH6 mutations were detected in the patients (22%) who had suspected HNPCC and MSI-low tumors. Furthermore, we detected one frameshift mutation in one of the three patients with HNPCC and MSI-low tumors. In the MSI-high group, one MSH6 missense mutation was found, but the same patient also had an MLH1 mutation, which may explain the MSI-high phenotype. These results suggest that MSH6 may be involved in a substantial proportion of patients with HNPCC or suspected HNPCC and MSI-low tumors. Our data emphasize that an MSI-low phenotype cannot be considered an exclusion criterion for mutation testing of MMR genes in general.

MLH1 and MSH2 protein expression as a pre-screening marker in hereditary and non-hereditary endometrial hyperplasia and cancer.

The predictive value of MLH1 or MSH2 protein expression for the presence of truncating germline mutations was examined in benign and (pre)malignant endometrial samples from 3 patient groups: (I) 10 endometrial cancer patients from hereditary non-polyposis colorectal cancer (HNPCC) families with (n = 6) or without (n = 4) a known germline mutation; (II) 15 women from HNPCC families with (n = 7) or without (n = 8) a known germline mutation, who underwent endometrial sampling for non-malignant reasons; (III) 38 endometrial cancer patients <50 years of age, without HNPCC family history. Immunostaining for MLH1 and MSH2 was performed on paraffin-embedded sections. In group III, tumor DNA was examined for microsatellite instability (MSI) and MLH1, MSH2 and MSH6 mutation analysis was carried out. In 6/6 MLH1/MSH2 mutation carriers with endometrial cancer (group I), concordance was found between protein loss in the tumor and the corresponding mutation. In 3 MLH1 mutation carriers, MLH1 protein loss was also observed in concurrent endometrial hyperplasia. In group II, no protein loss was detected in normal endometrial tissue samples; in 3/4 patients with endometrial hyperplasia, MLH1/MSH2 protein loss was observed. In group III, protein loss was detected in 12/38 patients (9 MLH1, 3 MSH2), while in 3/11 patients with concurrent endometrial hyperplasia protein loss was also observed in the hyperplasia. MSI analysis in group III revealed 26 MSI-low and 12 MSI-high tumors. Mutation analysis in 28/38 patients showed only 1 missense MSH6 and no MLH1 or MSH2 germline mutations. In group III, loss of MLH1/MSH2 protein expression was not related to the presence of MSI or MLH1/MSH2 germline mutations. In conclusion, MLH1 or MSH2 protein loss in HNPCC-related endometrial neoplasia is strongly related to corresponding germline mutations. This relation was not clearly present in young sporadic endometrial cancer patients. Immunohistochemical pre-screening of the MLH1 and MSH2 proteins in endometrial hyperplasia or cancer can thus be helpful in HNPCC families. Frequent loss of MLH1 or MSH2 protein in endometrial hyperplasia indicates that this loss is an early event in endometrial carcinogenesis.

Germline mutations of EXO1 gene in patients with hereditary nonpolyposis colorectal cancer (HNPCC) and atypical HNPCC forms.

BACKGROUND & AIMS: Germline mutations in one of four mismatch repair genes have been found in the majority of families with hereditary nonpolyposis colorectal cancer (HNPCC), but only in a small part of families with atypical HNPCC. The recently cloned EXO1 gene might be involved in the pathogenesis of HNPCC because the EXO1 protein strongly interacts with the MSH2 protein. To determine its role in HNPCC, EXO1 was scanned for germline mutations. METHODS: All 14 exons of EXO1 were scanned for mutations in index patients from 33 families with HNPCC fulfilling the Amsterdam criteria and in 225 index patients suspected of HNPCC. RESULTS: Germline variants of EXO1 were detected in 14 patients, including one splice-site mutation in a family with HNPCC and 13 missense mutations in patients with atypical HNPCC. These variants did not occur in more than 200 control individuals. From 13 of these 14 patients, tumors were available for analysis of microsatellite instability and loss of heterozygosity. Six of the tumors showed microsatellite instability. Heterozygosity analysis showed one case without EXO1 allelic loss and 12 tumors with loss of the mutant allele and retention of the normal one. CONCLUSIONS: The results indicate a possible association of germline EXO1 variants with HNPCC and atypical HNPCC.