Hairy cell leukemia: Update on molecular profiling and therapeutic advances

Hairy cell leukemia was initially described as a clinicopathologic entity more than 50 years ago. We have subsequently discovered that HCL is really at least two diseases: classical HCL and the hairy cell leukemia variant. The former is among a small group of cancers exceptional for being (nearly) unified by a single genetic lesion, the BRAF V600E mutation. Over the past three decades, tremendous progress in both diagnostic and prognostic clarification has been accompanied by therapeutic advances in classical HCL. Consequently, this once uniformly fatal disease has been converted in most cases into a chronic illness enabling patients to live long and productive lives. In response to standard therapy, patients have high complete remission rates. Unfortunately, the long-term survival curves have not plateaued, revealing that this disease is controlled but not cured. Though rare and representing only about 10% of an already rare disease, those patients with the variant fare exceptionally poorly with standard therapy: complete response rates to purine nucleoside analogs are reported to be less than 50%, whereas the complete response rates in classical HCL are up to 90%. Novel small molecules targeting BRAF and the B-cell receptor signaling complex, and biologic agents like antibodies and immunotoxin conjugates are being explored for those patients who have relapsed. Substantial opportunities for continued research remain. This complex and multi-faceted disease incorporates challenges from altered immunity associated with the underlying disease and its treatments. Considering the rarity of this malignancy, optimization of patient management requires multi-institutional collaboration. The Hairy Cell Leukemia Foundation (www.hairycellleukemia.org) was formed to coordinate these efforts.