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Preclinical models of mastocytosis
Mastocytosis designates a group of rare disorders char- acterized by a pathological accumulation of MC in one or more organs.6 Clinical presentations of mastocytosis range from skin-limited disease (cutaneous mastocytosis) occurring mainly in childhood and often regressing spon- taneously, to systemic disease categories (systemic mas- tocytosis; SM), mostly seen in adults. SM variants usually involve the bone marrow and sometimes other internal organs, such as the spleen, liver, and/or gastrointestinal tract.
According to the World Health Organization (WHO), mastocytosis can be classified into three major categories: cutaneous mastocytosis, the most common variant, fol- lowed by SM, and MC sarcoma, a rare localized MC tumor (Online Supplementary Table S1).7 SM is subdivided into five distinct categories: indolent SM (ISM), smolder- ing SM (SSM), SM with an associated hematologic neo- plasm (SM-AHN), aggressive SM (ASM) and MC leukemia (MCL) (Online Supplementary Table S1).7 While patients with ISM have a normal or near-normal life expectancy, patients with SM-AHN, ASM or MCL, col- lectively termed advanced SM, share a poor prognosis.8 The diagnosis of SM is based on WHO criteria and is established when one major criterion and one minor cri- terion or at least three minor criteria are present (Online Supplementary Table S2).9 Once the diagnosis of SM has been established, patients are further graded according to the presence of B-findings reflecting a high MC burden, and of C-findings reflecting organ damage related to MC infiltration (Online Supplementary Table S3).10 The patho- physiology of mastocytosis is complex and if acquired activating mutations in KIT (mostly KIT D816V: NM_000222.2(KIT):c.2447A>T, p.Asp816Val) seem to be major drivers of disease in ISM, the same cannot be said for advanced SM in which, in addition to KIT mutants, KIT-independent signaling pathways are activated and additional genetic defects are frequently found.
Given the complex pathophysiology of mastocytosis, in
vitro models mimicking neoplastic MC found in SM
patients could be useful for developing new therapeutic
approaches. To date only a few human MC lines have
been described, namely HMC-111 and its subclones
(HMC-1.1 and HMC-1.2),12 LAD (subclones 1 through
5),13 LUVA,14 ROSAKIT WT and its subclone ROSAKIT D816V,15
Pathophysiology of mastocytosis
The pathophysiology of mastocytosis is governed by the presence of KIT activating mutations in neoplastic MC.18 Indeed, various KIT activating mutations have been described, initially in patients with SM,19 then in children with cutaneous mastocytosis.20 In adult SM patients, KIT mutations affect primarily exon 17 encoding for the phos- photransferase domain, usually D816V (>80% of all
exons 13 and 14 encoding for kinase domain 1.21 By con- trast, in children, KIT mutations are found in nearly 75% of biopsies of skin lesions, but the KIT D816V mutation is found in only 30% of all cases.20 Indeed, a significant percentage of children present with KIT mutants located in the extracellular domain (codons 8 and 9) (Figure 1).20
In KIT D816V+ SM patients, the development of neo- plastic MC is principally governed by the PI3K/AKT and JAK/STAT5 signaling pathways activated downstream of KIT.22,23 Indeed, AKT and STAT5 are constitutively acti-
Figure 1. Normal structure of the KIT receptor and KIT mutations described in human mast cell leukemia-like cell lines and in patients with mastocytosis. In humans, KIT, located on chromosome 4q12, contains 21 exons transcribed/translated into a transmembrane receptor with tyrosine kinase activity (145 kDa and 976 amino acids). KIT is presented in its monomeric form, but dimerizes as a result of stem cell factor (SCF) ligation. The extracellular domain, in yellow, comprises five immunoglobulin (Ig)-like subunits where the lig- and binding site (SCF for KIT) and the dimerization site are located. The cytoplas- mic region contains a transmembrane domain (TMD) made by a single helix, in blue. The intracellular portion of KIT, in gray, contains an auto-inhibitory jux- tamembrane domain (JMD) and a kinase domain (in dark gray) which is split into two parts: an ATP-binding domain (ABD) and a phosphotransferase domain (PTD) linked by a large kinase insert (KI) domain of ~ 60–100 residues. For the sake of clarity and readability, only the most frequent mutations found in patients and/or in human MC lines are represented. In red, mutants found in adult patients and in cell lines (KIT D816V found in >80% of adult patients with sys- temic mastocytosis and 30% of children with cutaneous mastocytosis, as well as in HMC-1.2 and ROSAKIT D816V MCL-like cell lines, and KIT V560G found in the MCL- like cell lines HMC-1.1 and HMC-1.2, but only in a very few adult patients). In black, three of the KIT defects most frequently found in pediatric patients: KIT Del419 (NM_000222.2(KIT):c.1255_1257del, p.Asp419del), KIT ITD501-502 (NM_000222.2(KIT):c.1500_1505dup, p.Ser501_Ala502dup) and KIT ITD502- 503 (NM_000222.2(KIT):c.1503_1508dup, p.Ala502_Tyr503dup) and in brown, the KIT K509I mutant found in several familial cases of the disease. For a complete overview of the various KIT mutations found in pediatric and adult mastocytosis patients, see Valent et al.7 Del, deletion; ITD, internal tandem dupli- cation.
and MCPV-1.1 through MCPV-1.4.16 While LAD, LUVA KIT WT
and ROSA cells express KIT wild-type (WT), HMC- 1.1, HMC-1.2 and ROSAKIT D816V cells harbor KIT activating mutations,15,17 and MCPV-1 are RAS-mutated cells.16 Although these molecular aberrancies do not recapitulate all the characteristics of neoplastic MC found in advanced SM, these last four cell lines are currently the best avail- able models for identifying molecular targets and defining the effects of several interventional (targeted) drugs cur- rently used to treat advanced SM.
21
exons 2, 8 and 9 encoding for the extracellular domain or
patients) (Figure 1).
Other less frequent mutations affect
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