Childhood brain tumours due to germline bi-allelic mismatch repair gene mutations.

Childhood brain tumours may be due to germline bi-allelic mismatch repair (MMR) gene mutations in MLH1, MSH2, MSH6 or PMS2. These mutations can also lead to colorectal neoplasia and haematological malignancies. Here, we review this syndrome and present siblings with early-onset rectal adenoma and papillary glioneural brain tumour, respectively, due to novel germline bi-allelic PMS2 mutations. Identification of MMR protein defects can lead to early diagnosis of this condition. In addition, assays for these defects may help to classify brain tumours for research protocols aimed at targeted therapies.

Non-allelic heterogeneity of familial adenomatous polyposis.

Linkage studies on familial adenomatous polyposis (FAP) reported so far suggest that FAP is a genetically homogeneous disease. Recently, we found that the putative gene for Turcot syndrome, an apparently autosomal recessive clinical variant of FAP, is not allelic to FAP. Here we describe another family, segregating for an autosomal dominant disease clinically indistinguishable from FAP but genetically not linked to the APC locus, adding further evidence for the occurrence of non-allelic heterogeneity of FAP. These observations have implications to the linkage-based genetic counselling of persons at risk for FAP especially when they are drawn from small families giving insufficient information.

Facioscapulohumeral muscular dystrophy gene in Dutch families is not linked to markers for familial adenomatous polyposis on the long arm of chromosome 5.

Cosegregation of facioscapulohumeral muscular dystrophy (FSHD) and familial adenomatous polyposis (FAP) has been described in two small families. The gene for FAP is located on the long arm of chromosome 5. We studied two large Dutch families with FSHD and found no evidence for linkage with gene markers closely linked to FAP. These results strongly suggest that the FSHD gene segregating in the Dutch families is not localized close to the FAP locus on chromosome 5.

Molecular, cytogenetic, and phenotypic studies of a constitutional reciprocal translocation t(5;10)(q22;q25) responsible for familial adenomatous polyposis in a Dutch pedigree.

Familial adenomatous polyposis (FAP) is an inherited predisposition to colorectal cancer caused by germline mutations in the adenomatous polyposis coli (APC) gene located on chromosome segment 5q21-q22. We detected a germline rearrangement of the APC gene in a Dutch FAP family by screening genomic DNA samples with APC cDNA probes. Subsequent molecular and cytogenetic studies revealed a constitutional reciprocal translocation t(5;10)(q22;q25) that resulted in the disruption of the APC gene. Southern blot and polymorphic marker analysis indicated that part of the APC gene had been deleted. Analysis of the APC protein product indicated that the translocation breakpoint did not lead to the formation of a detectable truncated APC protein but apparently resulted in a null allele. Evaluation of the clinical phenotypes in the patients suggested that they exhibited features of an unusual form of FAP characterized by a slightly delayed age of onset of colorectal cancer and a reduced number of colorectal polyps. The latter were mainly sessile and were located predominantly in the proximal colon. To our knowledge, this is the first description of FAP caused by a reciprocal translocation disrupting the APC gene.

Presymptomatic diagnosis of familial adenomatous polyposis by bridging DNA markers.

Familial adenomatous polyposis (FAP) is a disorder with autosomal dominant inheritance, which predisposes to colorectal adenocarcinoma. The gene causing the disorder has been assigned to chromosome 5 by means of a polymorphic DNA marker called C11p11. An informative Dutch pedigree showed that two other linked polymorphic DNA markers, Pi227 and YN5.48, closely flank the FAP locus, one on either side. This finding will allow prenatal and presymptomatic diagnosis of FAP, with more than 99.9% reliability in the majority of families, by means of already available markers.

Molecular analysis of the APC gene in 105 Dutch kindreds with familial adenomatous polyposis: 67 germline mutations identified by DGGE, PTT, and southern analysis.

Germline mutations of the adenomatous polyposis coli (APC) gene are responsible for familial adenomatous polyposis (FAP), an autosomal dominant predisposition to colorectal cancer. We screened the entire coding region of the APC gene for mutations in an unselected series of 105 Dutch FAP kindreds. For the analysis of exons 1-14, we employed the GC-clamped denaturing gradient gel electrophoresis (DGGE), while the large exon 15 was examined using the protein truncation test. Using this approach, we identified 65 pathogenic mutations in the above 105 apparently unrelated FAP families. The mutations were predominantly either frameshifts (39/65) or single base substitutions (18/65), resulting in premature stop codons. Mutations that would predict abnormal RNA splicing were identified in seven cases. In one of the families, a nonconservative amino acid change was found to segregate with the disease. In spite of the large number of APC mutations reported to date, we identified 27 novel germline mutations in our patients, which reiterates the great heterogeneity of the mutation spectrum in FAP. In addition to the point mutations identified in our patients, structural rearrangements of APC were found in two pedigrees, by Southern blot analysis. The present study indicates that the combined use of DGGE, protein truncation test, and Southern blot analysis offers an efficient strategy for the presymptomatic diagnosis of FAP by direct mutation detection. We found that the combined use of the currently available molecular approaches still fails to identify the underlying genetic defect in a significant subset of the FAP families. The possible causes for this limitation are discussed.

A new deletion polymorphism at D5S71 raises the linkage information on adenomatous polyposis coli: implications for presymptomatic diagnosis.

Two independent study-groups, one in Britain and the other in the United States, were the first to report linkage between APC and a TaqI restriction fragment length polymorphism (RFLP) at D5S71 (probe C11p11) on chromosome 5q. They found no recombinants in about 50 informative meioses. The same TaqI RFLP was found to be uninformative for linkage in 15 Dutch polyposis families. The recently reported four base-pair deletion polymorphism (DEL1) at D5S71 has raised the polymorphism information content of this marker from 0.17 to 0.40 in the Dutch population. Seven of 20 polyposis families screened for the DEL1 as well as the TaqI polymorphism gave a combined peak lod score of 5.68 with no recombinants in 37 informative meioses. These data, together with those so far reported in the literature, raise the peak lod score to 17.09 at a recombination fraction of 0.05, the 95% upper confidence limit being 0.09. In combination with the use of another informative marker, D5S81 (probe YN5.48) closely mapping on the other side of APC, the presymptomatic diagnosis of the disease can be made with more than 99.9% certainty. It has to be stressed, however, that the the possible existence of more than one polyposis locus cannot, as yet, be excluded.