Respiratory chain complex III deficiency due to mutated BCS1L : a novel phenotype with encephalomyopathy, partially phenocopied in a Bcs1l mutant mouse model

Background: Mitochondrial diseases due to defective respiratory chain complex III (CIII) are relatively uncommon. The assembly of the eleven-subunit CIII is completed by the insertion of the Rieske iron-sulfur protein, a process for which BCS1L protein is indispensable. Mutations in the BCS1L gene constitute the most common diagnosed cause of CIII deficiency, and the phenotypic spectrum arising from mutations in this gene is wide. Results: A case of CIII deficiency was investigated in depth to assess respiratory chain function and assembly, and brain, skeletal muscle and liver histology. Exome sequencing was performed to search for the causative mutation(s). The patient's platelets and muscle mitochondria showed respiration defects and defective assembly of CIII was detected in fibroblast mitochondria. The patient was compound heterozygous for two novel mutations in BCS1L, c.306A > T and c.399delA. In the cerebral cortex a specific pattern of astrogliosis and widespread loss of microglia was observed. Further analysis showed loss of Kupffer cells in the liver. These changes were not found in infants suffering from GRACILE syndrome, the most severe BCS1L-related disorder causing early postnatal mortality, but were partially corroborated in a knock-in mouse model of BCS1L deficiency. Conclusions: We describe two novel compound heterozygous mutations in BCS1L causing CIII deficiency. The pathogenicity of one of the mutations was unexpected and points to the importance of combining next generation sequencing with a biochemical approach when investigating these patients. We further show novel manifestations in brain, skeletal muscle and liver, including abnormality in specialized resident macrophages (microglia and Kupffer cells). These novel phenotypes forward our understanding of CIII deficiencies caused by BCS1L mutations.Peer reviewe

Intratumor diversity and clonal evolution in cancer-a skeptical standpoint.

Clonal evolution in cancer is intimately linked to the concept of intratumor cellular diversity, as the latter is a prerequisite for Darwinian selection at the micro-level. It has been frequently suggested in the literature that clonal evolution can be promoted by an elevated rate of mutation in tumor cells, so-called genomic instability, the mechanisms of which are now becoming increasingly well characterized. However, several issues need clarification before the presumably complex relationship between mutation rate, intratumor diversity, and clonal evolution can be understood sufficiently well to translate into models that predict the course of tumor disease. In particular, it has to be clarified which of the proposed mechanisms for genomic instability that are able to generate daughter cells with sufficient viability to form novel clones, how clones with different genomic changes differ phenotypically from each other, and what the selective forces are that guide competition among diverse clones in different microenvironments. Furthermore, standardized measurements of mutation rates at the chromosome level, as well as genotypic and phenotypic diversity, are essential to compare data from different studies. Finally, the relationship between clonal variation brought about by genomic instability, on the one hand, and cellular differentiation hierarchies, on the other hand, should be explored to put genomic instability in the context of the tumor stem cell hypothesis

Aneuploidy in cancer: Sudden or sequential?

Comment on: Gisselsson D, et al. Proc Natl Acad Sci USA 2010; 107:20489-93

Mitotic instability in cancer - Is there method in the madness?

It has been known for more than a century that neoplastic cells often exhibit disturbances of the mitotic process, but the causes have only recently been thoroughly explored. In many cancers, a combination of cell cycle checkpoint deficiency and abnormal shortening of telomeres predisposes to unbalanced chromosome segregation at cell division and the development of complex genomic rearrangements. Shortening of telomeric repeats beyond normal limits leads to fusion of chromosome ends and the formation of chromatin bridges at anaphase. In turn, these bridges may trigger at least three types of chromosomes mutation: ( 1) structural rearrangements of chromosomes through extensive chromatin fragmentation beyond the centromeric sequences, typically leading to the formation of isochromosomes and whole-arm translocations, ( 2) loss of whole chromosomes through mechanical detachment from the mitotic spindle machinery, and ( 3) failure of cytokinesis, leading to polyploidisation and supernumerary centrosomes, which may in turn orchestrate multipolar spindle configurations at a subsequent mitosis. Anaphase bridging rarely hinders further survival of tumor daughter cells. In contrast, multipolar mitoses may lead to extensive reshuffling of chromosome copies that compromise further clonal expansion. The telomere-dependent instability can be partly counteracted by expression of telomerase during tumor progression, but genomic stabilisation is rarely, if ever, complete

Mechanisms of Whole Chromosome Gains in Tumors - Many Answers to a Simple Question.

Whole chromosome gain is the most common type of gross genomic abnormality observed in human tumors. It is particularly frequent in lympho-haematopoietic and embryonic neoplasms, where trisomies and tetrasomies are typically present together with few or no other cytogenetic imbalances, resulting in hyperdiploid chromosome numbers. Despite the high prevalence of whole chromosome gains in neoplastic cells, their mechanism of origin remains disputed. Here, 4 potential models for the generation of whole chromosome gains are reviewed: (1) loss of chromosomes from the tetraploid level, (2) sequential sister chromatid non-disjunction, (3) multipolar mitosis coupled to sister chromatid non-disjunction, and (4) multipolar mitosis coupled to incomplete cytokinesis. Each of these mechanisms may in theory result in the generation of hyperdiploid neoplastic clones, but none of them were single-handedly able to reproduce the scenario of chromosome copy number alterations in tumors when cell populations resulting from these models were simulated in silico and compared to published cytogenetic data. To develop models for the generation of whole chromosome gains further, it is critical to improve our knowledge of the principles of clonal selection in tumors and of the baseline rate of chromosome segregation errors in human cells. To illustrate this, a model combining multipolar mitosis coupled to incomplete cytokinesis with a low rate of baseline sister chromatid non-disjunction was shown readily to reproduce copy number distributions in hyperdiploid karyotypes from human tumors

Chromosomal Instability and Genomic Amplification in Bone and Soft Tissue Tumours

Acquired genetic abnormalities are found in all types of malignant tumours and may contribute to neoplastic processes by altering protein structure or dosage. Many bone and soft tissue tumours (BSTT) are characterised by complex patterns of chromosome changes, including extensive intratumour heterogeneity and amplification of DNA sequences. The results presented in this thesis demonstrate that the cytogenetic heterogeneity in a number of BSTT may, to a considerable extent, be caused by ring and dicentric chromosomes involved in breakage-fusion-bridge (BFB) events, as shown by the strong correlation between sequences present in mitotically unstable chromosomes and anaphase bridges. In borderline/low-malignant tumours with ring chromosomes carrying amplified sequences from chromosome 12, BFB rearrangements are associated with intratumour variability in the organisation of amplified material and abnormalities in centromeric alpha satellite sequences, including neocentromere formation. After the healing of broken ends by telomeric sequences from other chromosomes, rings may transform into giant marker chromosomes, which may occasionally be dicentric and rearrange further. BFB instability is also present in highly malignant tumours exhibiting a complex, unspecific, and heterogeneous pattern of chromosome aberrations. These neoplasms typically show unstable dicentric chromosomes and a general destabilisation of the chromosome complement. Tumour cells with BFB instability exhibit a decreased elimination rate of mitotically unstable chromosomes, which may be partly related to disruption of the normal TP53–dependent DNA damage response. In both BSTT and epithelial tumours, BFB events are associated with specific abnormalities in nuclear shape, indicating that nuclear atypia may serve as an indicator of ongoing genomic reorganisation in neoplastic cell populations

Classification of chromosome segregation errors in cancer.

Abnormal chromosome segregation at mitosis is one way by which neoplastic cells accumulate the many genetic abnormalities required for tumour development. In this paper, a straightforward morphology-based classification of chromosome segregation errors in cancer is suggested. This classification distinguishes between abnormalities in spindle symmetry (spindle multipolarity, size-asymmetry of ana-telophase poles) and abnormalities in sister chromatid segregation (chromosome bridges, chromatid bridges, chromosome lagging, acentric fragment lagging). Often, these categories of errors must be combined to accurately describe the events in a single abnormal mitotic cell. The suggested categories can to some extent be distinguished by standard chromatin staining. However, labelling of abnormal mitotic figures by fluorescence in situ hybridization and immunofluorescence enhances the accuracy of classification and also allows visualisation of the segregation of individual chromosomes, making it possible to detect non-disjunction also in the absence of gross alterations in mitotic morphology. Further characterisation of the molecular alterations leading to abnormal chromosome segregation together with the current developments in nano-level and real-time imaging will undoubtedly lead to an improved understanding of chromosome dynamics in cancer cells. Any morphology-based classification of chromosome segregation errors in cancer must therefore be taken as provisional, anticipating a satisfactory integration of morphology and molecular biology