Silencing of endogenous retroviruses: when and why do histone marks predominate?

Retrotransposons, such as endogenous retroviruses (ERVs), have colonized the genomes of all metazoans. As retrotransposition can be deleterious, numerous pathways have evolved to repress the expression of these parasitic elements. For example, methylation of the fifth carbon of the cytosine base in DNA (5-methylcytosine, 5mC) is required for transcriptional silencing of ERVs in differentiated cells. However, this epigenetic mark is generally dispensable for ERV silencing during early stages of mouse embryogenesis and in mouse embryonic stem cells (mESCs). In this Opinion, we evaluate recent findings on the exceptional role of covalent modifications of histones in ERV silencing in these cell types. In addition, we discuss the potential role of TET proteins, which catalyze the oxidation of 5mC to 5-hydroxymethylcytosine (5hmC), in perturbing transcriptional silencing, and propose that histone modification-based pathways may be used to silence ERVs during those developmental stages when DNA methylation-mediated silencing is compromised.

Lysine methyltransferase G9a is required for de novo DNA methylation and the establishment, but not the maintenance, of proviral silencing.

Methylation on lysine 9 of histone H3 (H3K9me) and DNA methylation play important roles in the transcriptional silencing of specific genes and repetitive elements. Both marks are detected on class I and II endogenous retroviruses (ERVs) in murine embryonic stem cells (mESCs). Recently, we reported that the H3K9-specific lysine methyltransferase (KMTase) Eset/Setdb1/KMT1E is required for H3K9me3 and the maintenance of silencing of ERVs in mESCs. In contrast, G9a/Ehmt2/KMT1C is dispensable, despite the fact that this KMTase is required for H3K9 dimethylation (H3K9me2) and efficient DNA methylation of these retroelements. Transcription of the exogenous retrovirus (XRV) Moloney murine leukemia virus is rapidly extinguished after integration in mESCs, concomitant with de novo DNA methylation. However, the role that H3K9 KMTases play in this process has not been addressed. Here, we demonstrate that G9a, but not Suv39h1 or Suv39h2, is required for silencing of newly integrated Moloney murine leukemia virus-based vectors in mESCs. The silencing defect in G9a(-/-) cells is accompanied by a reduction of H3K9me2 at the proviral LTR, indicating that XRVs are direct targets of G9a. Furthermore, de novo DNA methylation of newly integrated proviruses is impaired in the G9a(-/-) line, phenocopying proviral DNA methylation and silencing defects observed in Dnmt3a-deficient mESCs. Once established, however, maintenance of silencing of XRVs, like ERVs, is dependent exclusively on the KMTase Eset. Taken together, these observations reveal that in mESCs, the H3K9 KMTase G9a is required for the establishment, but not for the maintenance, of silencing of newly integrated proviruses.

Perceptions of professional attributes in medicine: a qualitative study in Hong Kong.

OBJECTIVE. Medical professionalism has been widely discussed in western scholarly literature. However, since Hong Kong has a mixed Chinese-western culture, it remains uncertain whether Hong Kong health care professionals, medical students, and patients see medical professionalism in exactly the same way as westerners. The objective of the present study was to explore perceptions of medical professionalism in Hong Kong. DESIGN. Individual semi-structured interviews. SETTING. Medical faculty preceptors, residents, interns, nurses, and students from the Li Ka Shing Faculty of Medicine of the University of Hong Kong. Subjects were recruited at an out-patient clinic of Queen Mary Hospital. PARTICIPANTS. We interviewed 39 subjects, including six medical faculty preceptors, six hospital residents, four medical interns, eight nurses, eight out-patients, and seven medical students. The interviews were transcribed and coded. Grounded theory was employed for framing and analysing the interviews. RESULTS. A total of 30 primary themes were identified and grouped under three secondary themes, ie 'Expectations of a professional doctor', 'Work values', and 'Patient care'. In general, the primary themes were consistent with recognised professional attributes in western bioethics, such as knowledge and skills, holistic care, and communication skills. A closer analysis suggested that traditional Chinese thought also played an important role in shaping the medical professionalism of Hong Kong. Challenges to be faced by Hong Kong doctors due to recent social changes were also identified. CONCLUSIONS. Medical professionalism in Hong Kong is shaped by both western medical ethics and traditional Chinese thought. The values treasured by Hong Kong health care professionals as well as technological advance, and the city's proximity to Mainland China makes Hong Kong health care unique. It is important to maintain the present work attitudes and at the same time adapt to new social changes.

G9a Plays Distinct Roles in Maintaining DNA Methylation, Retrotransposon Silencing, and Chromatin Looping.

G9a is a lysine methyltransferase that regulates epigenetic modifications, transcription, and genome organization. However, whether these properties are dependent on one another or represent distinct functions of G9a remains unclear. In this study, we observe widespread DNA methylation loss in G9a depleted and catalytic mutant embryonic stem cells. Furthermore, we define how G9a regulates chromatin accessibility, epigenetic modifications, and transcriptional silencing in both catalytic-dependent and -independent manners. Reactivated retrotransposons provide alternative promoters and splice sites leading to the upregulation of neighboring genes and the production of chimeric transcripts. Moreover, while topologically associated domains and compartment A/B definitions are largely unaffected, the loss of G9a leads to altered chromatin states, aberrant CTCF and cohesin binding, and differential chromatin looping, especially at retrotransposons. Taken together, our findings reveal how G9a regulates the epigenome, transcriptome, and higher-order chromatin structures in distinct mechanisms.

Thomas Clifford Allbutt and comparative pathology.

This paper reconceptualizes Thomas Clifford Allbutt's contributions to the making of scientific medicine in late nineteenth-century England. Existing literature on Allbutt usually describes his achievements, such as his design of the pocket thermometer and his advocacy of the use of the ophthalmoscope in general medicine, as independent events; and his work on the development of comparative pathology is largely overlooked. In this paper I focus on this latter aspect. I examine Allbutt's books and addresses and claim that Allbutt argued for the centrality of comparative pathology in the advancement of medical knowledge. He held that diseases should be studied as biological phenomena and that medicine should be made a biological science. He also argued that comparative pathology should be based upon the idea of evolution, and its study should embrace other nineteenth-century sciences including neurology, embryology and bacteriology. Allbutt's writings reveal that his endorsement of comparative pathology (1880s to 1920s), his promotion of the use of the ophthalmoscope and the thermometer in clinical medicine (early 1870s), and his support of the hospital unit system (1910s to 1920s) were part of a single programme. All were grounded in his scientific vision of medicine which emphasized a research culture, a stringent nosological attitude and an integration of laboratory sciences and clinical medicine.