Drosophila BubR1 is essential for meiotic sister-chromatid cohesion and maintenance of synaptonemal complex

The partially conserved Mad3/BubR1 protein is required during mitosis for the spindle assembly checkpoint (SAC). In meiosis, depletion causes an accelerated transit through prophase I and missegregation of achiasmate chromosomes in yeast [1], whereas in mice, reduced dosage leads to severe chromosome missegregation [2]. These observations indicate a meiotic requirement for BubR1, but its mechanism of action remains unknown. We identified a viable bubR1 allele in Drosophila resulting from a point mutation in the kinase domain that retains mitotic SAC activity. In males, we demonstrate a dose-sensitive requirement for BubR1 in maintaining sister-chromatid cohesion at anaphase I, whereas the mutant BubR1 protein localizes correctly. In bubR1 mutant females, we find that both achiasmate and chiasmate chromosomes nondisjoin mostly equationally consistent with a defect in sister-chromatid cohesion at late anaphase I or meiosis II. Moreover, mutations in bubR1 cause a consistent increase in pericentric heterochromatin exchange frequency, and although the synaptonemal complex is set up properly during transit through the germarium, it is disassembled prematurely in prophase by stage 1. Our results demonstrate that BubR1 is essential to maintain sister-chromatid cohesion during meiotic progression in both sexes and for normal maintenance of SC in females.

Molecular characterization of teflon, a gene required for meiotic autosome segregation in male Drosophila melanogaster

Drosophila melanogaster males lack recombination and have evolved a mechanism of meiotic chromosome segregation that is independent of both the chiasmatic and achiasmatic segregation systems of females. The teflon (tef) gene is specifically required in males for proper segregation of autosomes and provides a genetic tool for understanding recombination-independent mechanisms of pairing and segregation as well as differences in sex chromosome vs. autosome segregation. Here we report on the cloning of the tef gene and the molecular characterization of tef mutations. Rescue experiments using a GAL4-driven pUAS transgene demonstrate that tef corresponds to predicted Berkeley Drosophila Genome Project (BDGP) gene CG8961 and that tef expression is required in the male germ line prior to spermatocyte stage S4. Consistent with this early prophase requirement, expression of tef was found to be independent of regulators of meiotic M phase initiation or progression. The predicted Tef protein contains three C2H2 zinc-finger motifs, one at the amino terminus and two in tandem at the carboxyl terminus. In addition to the zinc-finger motifs, a 44- to 45-bp repeat is conserved in three related Drosophila species. On the basis of these findings, we propose a role for Tef as a bridging molecule that holds autosome bivalents together via heterochromatic connections.

Spectra of antinuclear antibodies in patients with squamous cell carcinoma of the lung and of the head and neck

Squamous cell carcinoma of the head and neck (HNSCC) and of the lung (LSCC) share some important risk factors, but differ substantially in terms of prognosis and treatment. A pulmonary nodule developing in patients with surgically cured HNSCC may pose a diagnostic dilemma. Markers able to distinguish these two common malignancies would be of major clinical importance. In this work we compared the spectrum of antinuclear antibodies (ANA) from 22 patients with SCCL to that of 40 patients with HNSCC. Patient sera were used to probe immunoblots of nuclear extracts from all four major lung cancer cell types, normal lung fibroblasts, cells cultured from a HNSCC, and keratinocytes cultured from the field cancerization. The ability to classify retrospectively LSCC from HNSCC based on serum ANA reactivities was determined by recursive partitioning analyses. We found that while both malignancies share reactivities to a small group of nuclear antigens, other reactivities are directed against proteins uniquely or preferentially expressed in either SCCL or in SCCHN cells. Our work shows that autoimmunity is a prominent feature of squamous cell carcinoma and suggests that molecular characterization of nuclear antigens recognized by ANAs may lead to the discovery of markers valuable to distinguish LSCC from HNSCC.

Improved approach to identify cancer-associated autoantigens

The failure to identify biomarkers of clinical significance for cancer diagnosis and prognosis generated a great deal of skepticism in regard to the usefulness of autoantibody-based methods. SEREX was a major advancement in immunoscreening that resulted in the identification of a large group of autoantigens recognized by cancer sera. However, few SEREX-defined autoantigens have proven to have definitive diagnostic value in clinical practice. Often, the identified antigens are patient-specific rather than tumor-specific and many tumor-associated antigens are rare in expression libraries made from non-autologous cells. Since autoantibodies are part of the normal immune response, it can be difficult to single out tumor-associated antibodies from the scores of irrelevant patient-specific responses. In our view, any practical approach for identifying cancer-related autoantigens must include an integral strategy for demonstrating tumor relevance early in the screening process. Care must also be taken not to exclude potentially important autoantibodies by pre-screening manipulations to patient sera. We have introduced substantial modifications in SEREX, designed to minimize confounding effects of unrelated autoantibodies and to eliminate steps that preclude the identification of cancer-related autoantigens commonly recognized by cancer sera. In addition, we incorporate methodology to identify candidate antigens that have potential diagnostic or prognostic value prior to their molecular cloning and characterization.

Mutational analysis of the central centromere targeting domain of human centromere protein C, (CENP-C)

Human centromere protein C (CENP-C) is an essential component of the inner kinetochore plate. A central region of CENP-C can bind DNA in vitro and is sufficient for targeting the protein to centromeres in vivo, raising the possibility that this domain mediates centromere localization via direct DNA binding. We performed a detailed molecular dissection of this domain to understand the mechanism by which CENP-C assembles at centromeres. By a combination of PCR mutagenesis and transient expression of GFP-tagged proteins in HeLa cells, we identified mutations that disrupt centromere localization of CENP-C in vivo. These cluster in a 12 amino acid region adjacent to the core domain required for in vitro DNA binding. This region is conserved between human and mouse, but is divergent or absent in invertebrate and plant CENP-C homologues. We suggest that these 12 amino acids are essential to confer specificity to DNA binding by CENP-C in vivo, or to mediate interaction with another as yet unidentified centromere component. A differential yeast two-hybrid screen failed to identify interactions specific to this sequence, but nonetheless identified 14 candidate proteins that interact with the central region of CENP-C. This collection of mutations and interacting proteins comprise a useful resource for further elucidating centromere assembly.

Chromosomally-induced meiotic drive in Drosophila males: checkpoint or fallout?

In male Drosophila melanogaster, anomalies in sex chromosome pairing at meiosis often lead to complete or partial sperm dysfunction. This observation has led to the suggestion that defects in either the efficiency or configuration of chromosome pairing at metaphase trigger a checkpoint mechanism that leads to the elimination of meiotic products. Here, we discuss this model in consideration of recent observations on the conservation of metaphase checkpoint components in male meiosis, and on the phenotype of new alleles of the male-specific meiotic mutant teflon. Based on these observations, we propose an alternative hypothesis for the cause of sperm dysfunction in cases of chromosomal sterility and drive. We suggest that disruption of the prophase compartmentalization of sex chromatin, rather than abnormal pairing at metaphase, may be the causative defect. Such disruption may occur as a result of perturbations in sex chromosome pairing, or by translocations involving autosomal and sex chromatin. We discuss how this hypothesis may account for previously described examples chromosomal causes of meiotic drive and sterility in Drosophila.

The teflon gene is required for maintenance of autosomal homolog pairing at meiosis I in male Drosophila melanogaster

In recombination-proficient organisms, chiasmata appear to mediate associations between homologs at metaphase of meiosis I. It is less clear how homolog associations are maintained in organisms that lack recombination, such as male Drosophila. In lieu of chiasmata and synaptonemal complexes, there must be molecules that balance poleward forces exerted across homologous centromeres. Here we describe the genetic and cytological characterization of four EMS-induced mutations in teflon (tef), a gene involved in this process in Drosophila melanogaster. All four alleles are male specific and cause meiosis I-specific nondisjunction of the autosomes. They do not measurably perturb sex chromosome segregation, suggesting that there are differences in the genetic control of autosome and sex chromosome segregation in males. Meiotic transmission of univalent chromosomes is unaffected in tef mutants, implicating the tef product in a pairing-dependent process. The segregation of translocations between sex chromosomes and autosomes is altered in tef mutants in a manner that supports this hypothesis. Consistent with these genetic observations, cytological examination of meiotic chromosomes suggests a role of tef in regulating or mediating pairing of autosomal bivalents at meiosis I. We discuss implications of this finding in regard to the evolution of heteromorphic sex chromosomes and the mechanisms that ensure chromosome disjunction in the absence of recombination.

Chromosomal position effects reveal different cis-acting requirements for rDNA transcription and sex chromosome pairing in Drosophila melanogaster

In Drosophila melanogaster, the rDNA loci function in ribosome biogenesis and nucleolar formation and also as sex chromosome pairing sites in male meiosis. These activities are not dependent on the heterochromatic location of the rDNA, because euchromatic transgenes are competent to form nucleoli and restore pairing to rDNA-deficient X chromosomes. These transgene studies, however, do not address requirements for the function of the endogenous rDNA loci within the heterochromatin. Here we describe two chromosome rearrangements that disrupt rDNA functions. Both rearrangements are translocations that cause an extreme bobbed visible phenotype and XY nondisjunction and meiotic drive in males. However, neither rearrangement interacts with a specific Y chromosome, Ymal(+), that induces male sterility in combination with rDNA deletions. Molecular studies show that the translocations are not associated with gross rearrangements of the rDNA repeat arrays. Rather, suppression of the bobbed phenotypes by Y heterochromatin suggests that decreased rDNA function is caused by a chromosomal position effect. While both translocations affect rDNA transcription, only one disrupts meiotic XY pairing, indicating that there are different cis-acting requirements for rDNA transcription and rDNA-mediated meiotic pairing.

Antinuclear antibodies as potential markers of lung cancer

There are multiple case reports of antinuclear antibodies (ANAs) in patients with malignancies, yet to date there has not been a systematic survey of ANAs in lung cancer. We have previously reported that autoantibodies to collagen antigens resembling those found in the connective tissue diseases are consistently detected in the sera from lung cancer patients. In this work, we looked for the presence of ANAs in the sera from these same patients. Sera from 64 patients with lung cancer and 64 subjects without a history of cancer were retrospectively tested for reactivity on immunoblots of nuclear extracts of HeLa, small cell carcinoma, squamous cell carcinoma, adenocarcinoma, large cell carcinoma of the lung, and of normal lung cells. Associations were sought between the reactivities on immunoblots and lung cancer cell type, diagnosis, and progression-free survival by the method of classification and regression trees (CARTs). Cross-validated CART analyses indicated that reactivities to certain bands in immunoblots are associated with different types of lung cancer. Some of these autoantibodies were associated with a prolonged survival without disease progression. Our data suggest that autoimmunity is often a prominent feature of lung cancer and that molecular characterization of these antigens may lead to the discovery of proteins with diagnostic and prognostic value.

Double or nothing: a Drosophila mutation affecting meiotic chromosome segregation in both females and males

We describe a Drosophila mutation, Double or nothing (Dub), that causes meiotic nondisjunction in a conditional, dominant manner. Previously isolated mutations in Drosophila specifically affect meiosis either in females or males, with the exception of the mei-S332 and ord genes which are required for proper sister-chromatid cohesion. Dub is unusual in that it causes aberrant chromosome segregation almost exclusively in meiosis I in both sexes. In Dub mutant females both nonexchange and exchange chromosomes undergo nondisjunction, but the effect of Dub on nonexchange chromosomes is more pronounced. Dub reduces recombination levels slightly. Multiple nondisjoined chromosomes frequently cosegregate to the same pole. Dub results in nondisjunction of all chromosomes in meiosis I of males, although the levels are lower than in females. When homozygous, Dub is a conditional lethal allele and exhibits phenotypes consistent with cell death.

Centromere and kinetochore structure

Recent studies have begun to yield some insight into the structural and regulatory components of centromeres, and new assays have been developed that promise to be of use in advancing our understanding of centromere structure and function. In the budding yeast Saccharomyces cerevisiae new proteins that are required for centromere function have been identified and an in vitro microtubule-binding assay that should assist in dissecting the process of centromere microtubule attachment has been developed. The centromere-specific DNA sequences in the fission yeast Schizosaccharomyces pombe have been identified and partially characterized. In addition, several mammalian centromere proteins have been further characterized, and localization and inhibition studies suggest roles for these proteins in the regulation and assembly of a functional kinetochore.