By ecancer reporter Clare Sansom

One of the general characteristics of cancer is the dysregulation of the epigenetic pathways that control DNA methylation and thus the expression of genes.

A family of enzymes known as the TET (‘ten-eleven-translocation’) proteins catalyse the oxidation of 5-methyl cytosine (5mc) to form 5-hydroxymethylcytosine (5hmC) and other modified cytosines.

These modifications are thought to contribution to both passive and active demethylation of DNA, and decreased TET protein expression and a loss of 5hmC have been reported in a range of human tumours.

Mutations in and translocations involving members of the TET family have also been found in several T-cell and B-cell leukaemias and lymphomas.

These results suggest that the maintenance of cytosine methylation is an important mechanism in tumour suppression, but the precise role of TET1 in tumour suppression is not completely understood.

A large group of researchers led by Rudolf Jaenisch of the Whitehead Institute for Biomedical Research, Cambridge, Massachusetts, USA and Iannis Aifantis of NYU School of Medicine, New York, USA have investigated the role and function of TET1 in haematopoietic malignancies using mouse models and human and mouse tumour cells.

The researchers first monitored a cohort of mice with the gene encoding TET1 deleted (Tet1-/- mice) and heterozygous Tet1 /- mice over two years, and found that both the homozygotes and the heterozygotes had reduced lifespans compared to similar Tet1 / mice.

The spleens and lymph nodes of the Tet1-/- mice showed many characteristics that are consistent with a B-cell lymphoma, and transplant of these organs into congenic Tet1 / mice caused the rapid development of disease.

This suggested that TET1 could act as a suppressor of haematopoietic malignancy, pre-dominantly in the B-cell lineage.

The researchers then sequenced the exomes of TET1-deficient tumour cells derived from the spleens, lymph nodes and peripheral blood of the Tet1-/- and Tet1 /- mice, and identified somatic mutations in 1,785 genes, 157 of which were mutated in two or more samples.

Many of the genes found to be mutated in three or more of the mouse tumours have previously been identified as mutated in patients with B-cell derived non-Hodgkin lymphomas.

These included a gene encoding a histone methyltransferase and others that encode linker histones and histone-modifying enzymes.

However, somatic mutations of TET1 have not yet been reported in human B-cell non-Hodgkin lymphomas (B-NHL).

The researchers therefore investigated alternative mechanisms for inactivating TET1 in these diseases using methylation analysis and RNA sequencing, and discovered that the gene was hypermethylated and downregulated in follicular lymphoma (FL) and diffuse large B-cell lymphoma (DLBCL), both types of B-NHL.

They then explored the mechanisms through which loss of this protein might lead to the development of B-cell malignancy, first using stem cell transplantation to discover that the activity of hematopoietic stem cells is enhanced in TET1-deficient mice.

Knowing that the gene BCL2 is translocated and over-expressed in B-cell lymphomas, particularly FL, they induced BCL2 over-expression in Tet1-/- mice and found that the combination of BCL2 over-expression with TET1 loss drove a rapid expansion of B lymphocytes.

They also found that TET1 expression was highest in the early stages of B-cell differentiation, reduced in mature B cells and absent from immature and mature myeloid cells.

Microarray analysis showed that TET1 loss is associated with the over-expression of transcription factors and other genes that match the expression patterns of mature B lymphocytes and with the under-expression of histones and other genes involved in chromosome maintenance and DNA repair.

Genome-wide analysis of CpG methylation in Tet1-/- cells showed that the DNA in these cells had a loss of 5hmC and a gain of 5mC over all chromosomes relative to Tet1 / cells.

Furthermore, pro-B cells in the Tet1-/- mice showed a higher capacity for self-renewal and more DNA damage than the equivalent cells in Tet1 / mice, establishing them as pre-malignant cells.

Taken together, these results suggest a function for TET1 in mice and humans as a haematopoietic tumour suppressor particularly of B-cell lymphoma.

Reference

Cimmino, L., Dawlaty, M.M., Ndiaye-Lobry, D. and 16 others (2015). TET1 is a tumor suppressor of hematopoietic malignancy. Nature Immunology, published online ahead of print 13 April 2015.