Myf5 and Myogenin in the development of thymic myoid cells - Implications for a murine in vivo model of myasthenia gravis

Thymus and autoimmunity

The thymus prevents autoimmune diseases through mechanisms that operate in the cortex and medulla, comprising positive and negative selection and the generation of regulatory T-cells (Tregs). Egress from the thymus through the perivascular space (PVS) to the blood is another possible checkpoint, as shown by some autoimmune/immunodeficiency syndromes. In polygenic autoimmune diseases, subtle thymic dysfunctions may compound genetic, hormonal and environmental cues. Here, we cover (a) tolerance-inducing cell types, whether thymic epithelial or tuft cells, or dendritic, B- or thymic myoid cells; (b) tolerance-inducing mechanisms and their failure in relation to thymic anatomic compartments, and with special emphasis on human monogenic and polygenic autoimmune diseases and the related thymic pathologies, if known; (c) polymorphisms and mutations of tolerance-related genes with an impact on positive selection (e.g. the gene encoding the thymoproteasome-specific subunit, PSMB11), promiscuous gene expression (e.g. AIRE, PRKDC, FEZF2, CHD4), Treg development (e.g. SATB1, FOXP3), T-cell migration (e.g. TAGAP) and egress from the thymus (e.g. MTS1, CORO1A); (d) myasthenia gravis as the prototypic outcome of an inflamed or disordered neoplastic ‘sick thymus’.

Enhancer reprogramming in mammalian genomes

Background

Transcription factor binding site (TFBS) loss, gain, and reshuffling within the sequence of a regulatory element could alter the function of that regulatory element. Some of the changes will be detrimental to the fitness of the species and will result in gradual removal from a population, while other changes would be either beneficial or just a part of genetic drift and end up being fixed in a population. This “reprogramming” of regulatory elements results in modification of the gene regulatory landscape during evolution.

Results

We identified reprogrammed enhancers (RPEs) by comparing the distribution of tissue-specific enhancers in the human and mouse genomes. We observed that around 30% of mammalian enhancers have been reprogrammed after the human-mouse speciation. In 79% of cases, the reprogramming of an enhancer resulted in a quantifiably different expression of a flanking gene. In the case of the Thy-1 cell surface antigen gene, for example, enhancer reprogramming is associated with cortex to thymus change in gene expression. To understand the mechanisms of enhancer reprogramming, we profiled the evolutionary changes in the TFBS enhancer content and found that enhancer reprogramming took place through the acquisition of new TFBSs in 72% of reprogramming events.

Conclusions

Our results suggest that enhancer reprogramming takes place within well-established regulatory loci with RPEs contributing additively to fine-tuning of the gene regulatory program in mammals. We also found evidence for acquisition of novel gene function through enhancer reprogramming, which allows expansion of gene regulatory landscapes into new regulatory domains.

Electronic supplementary material

The online version of this article (10.1186/s12859-018-2343-7) contains supplementary material, which is available to authorized users.

Metastatic type A thymoma: morphological and genetic correlation

AIMS

The vast majority of type A thymomas are diagnosed in tumour stages 1 or 2, and metastatic cases are exceedingly rare. The histological and genetic features of such metastatic type A thymomas have not been described in detail.

METHODS AND RESULTS

Five metastatic type A thymomas in tumour stage Masaoka IVb that had been reviewed by a panel of expert pathologists were analysed using comparative genomic hybridization (CGH). Cases 1, 2 and 3 showed the prototypical morphology of type A thymomas with mainly solid growth patterns. These cases displayed only very subtle nuclear irregularities and slight nuclear crowding, but no other atypical features. Mitoses were absent. Cases 3 and 4, in contrast, had a distinctly atypical morphology. CGH revealed partially recurrent alterations in four cases (with and without atypical morphology), including gains on chromosome 1q (one case), 17q (two cases), chromosome 19 (three cases) and 22q (one case) and losses on chromosome 17p (two cases) and 22q (one case).

CONCLUSION

Rare metastatic type A thymomas, both with typical and 'atypical' histological features, show partially recurrent genomic alterations that differ from the much more frequent localized and indolent tumours. The fact that these alterations were recurring points to a link between clinical behaviour and molecular features. Our findings may have implications for the management and treatment of such tumours.