Slipped-strand mispairing can function as a phase variation mechanism in Escherichia coli

Slipped-strand mispairing (SSM) has not been identified as a mechanism of phase variation in Escherichia coli. Using a reporter gene, we show that sequences that cause phase variation by SSM in Haemophilus influenzae also lead to phase variation when introduced onto the chromosome of E. coli, and the frequencies of switching are in the biologically relevant range. Thus, the absence of SSM-mediated phase variation in E. coli does not appear to be due to a mechanistic constraint.

Amplification and overexpression of mutant RET in multiple endocrine neoplasia type 2-associated medullary thyroid carcinoma

We have previously identified two second hit mechanisms involved in the development of multiple endocrine neoplasia type 2 (MEN 2)-associated tumors: trisomy 10 with duplication of the mutant RET allele and loss of the wild-type RET allele. However, some of the MEN 2-associated tumors investigated did not demonstrate either mechanism. Here, we studied the TT cell line derived from MEN 2-associated medullary thyroid carcinoma with a RET germline mutation in codon 634, for alternative mechanisms of tumorigenesis. Although we observed a 2:1 ratio between mutant and wild-type RET at the genomic DNA level in this cell line, fluorescence in situ hybridization analysis revealed neither trisomy 10 nor loss of the normal chromosome 10. Instead, a tandem duplication event was responsible for amplification of mutant RET. In further studies we could for the first time demonstrate that the genomic chromosome 10 abnormalities in this cell line cause an increased production of mutant RET mRNA. These findings provide evidence for a third second hit mechanism resulting in overrepresentation and overexpression of mutant RET in MEN 2-associated tumors.

Reassignment of the EPB4.1 gene to 1p36 and assessment of its involvement in neuroblastomas

OBJECTIVES

EPB4.1 has been previously mapped to human chromosome 1p33-p34.2. In contradiction to this chromosomal location, we have mapped EPB4.1-1p36 by using fluorescence in situ hybridization and radiation hybrid mapping. In neuroblastomas, deletions of the telomeric end of chromosome 1 (1p36) are the most common genetic aberration.

METHODS

We investigated whether genetic aberrations of EPB4.1 can be detected in some neuroblastomas by analyzing 72 tumours for EPB4.1 mutation, expression, and alternative splicing pattern. Furthermore, EPB4.1 protein from a neuroblastoma cell line was studied for its subcellular localization.

RESULTS

Sequence changes could be detected in 14 out of 72 neuroblastomas, including missense, silent, and intronic changes. Duplex RT-PCR analysis revealed a subset of 11 tumours expressing significantly low levels of EPB4.1. Significant EPB4.1 sequence changes that were detected included an exon 4 G/A missense mutation (amino acid: V/I) that was shown to be associated with absence of wild-type EPB4.1 expression (3 tumours), an exon 8 G/A missense mutation (V/M) (1 tumour), and an intronic sequence change that was shown to be associated with the presence of an aberrant transcript (1 tumour). Splicing pattern analysis revealed that all EPB4.1 transcripts from tumours exclude exon 3, a splicing pattern for generating the 135 kDa isoform. EPB4.1 cDNA cloned from a neuroblastoma cell line produced a 135-kDa protein with a cytoplasm/membrane localization.

CONCLUSIONS

Out of 72 neuroblastomas we have identified 11 tumours with impaired EPB4.1 expression and 5 tumours with significant sequence changes. We also found that the 135 kDa isoform is the main EPB4.1 product in neuroblastoma. EPB4.1 cDNA from a neuroblastoma cell line produced a 135-kDa protein and displayed a cytoplasm/membrane localization in transfected cells.