Hairy-cell leukaemia is an uncommon B-cell malignancy, normally classified as a sub-type of chronic lymphocytic leukaemia.

It is characterised by invasion of the bone marrow, liver and spleen by a form of malignant B cell with hairy-looking cytoplasmic projections.

Patients diagnosed with this type of leukaemia usually respond well to treatment with purine analogues such as cladribine and pentostatin. Although the unique immunological features of the "hairy" B cells have been well characterised, until now little has been learned about the genetic changes that give rise to this condition.

Now, however, Brunangelo Falini of the University of Perugia, Perugia, Italy and Raul Rabadan of Columbia University, New York, USA, working with a large team of mainly Italian co-workers, have mapped mutations associated with hairy-cell leukaemia.

They sequenced the complete exome (protein-coding genome) of paired leukaemic and normal cells from a single individual diagnosed with this condition using a massively parallel methodology.

By comparing sequences from the normal and leukaemic cells they identified five missense somatic mutations in the cancer genome, and these were confirmed using Sanger sequencing. Only one of these was in a gene that was known to be associated with cancer; this was a mutation in the BRAF gene that gives rise to a V600E variant protein.

This mutation, a substitution of glutamic acid for valine, has been characterised as oncogenic in some solid tumours (particularly melanoma and papillary thyroid cancer) although it has not been definitively associated with any type of leukaemia.

The researchers then sequenced the BRAF gene from hairy-cell leukaemia cells from a further 47 patients using the Sanger method, and discovered, strikingly, that the V600E mutation occurred in all cases.

In comparison, this mutation has rarely been found in much more than 50% of samples of any solid tumour tested.

Furthermore, it was found to be absent in paired normal cells from ten of these patients, confirming it to be somatic in origin.

Next, the researchers sequenced the same gene from a total of 195 patients diagnosed with other B-cell malignancies. None of these samples, which included some from fairly similar splenic lymphomas and leukaemias, carried the BRAF V600E mutation.

The BRAF gene encodes a kinase that becomes constitutively active (is "switched on") by the V600E mutation. This kinase is part of the RAS-RAF-MAPK signalling pathway, which is involved in cell survival and proliferation.

Its immediate target is MEK, another kinase that in turn phosphorylates ERK. The researchers used immunohistochemistry to determine that ERK was phosphorylated in leukaemia cells from patients diagnosed with hairy-cell leukaemia.

Furthermore, this phosphorylation was abolished when these cells were incubated with a specific BRAF kinase inhibitor, PLX-4720. This promising drug, which is now also known as vemurafenib, is in Phase III trials for melanoma.

Falini, Rabadan and their co-workers concluded that the missense mutation in BRAF that results in the V600E variant protein is a key genetic lesion in hairy-cell leukaemia, found in 100% of cases with this diagnosis but in no other tested B-cell disease.

This mutation is likely to be the oncogenic trigger in this condition, and therefore BRAF inhibitors such as vemurafenib may be effective treatments for it.

Reference: Tiacci, E., Trifonov, V., Schiavoni, G. and 33 others (2011). BRAF Mutations in Hairy-Cell Leukemia. N Engl J Med 364, 2305-15. DOI: 10.1056/NEJMoa1014209