Anatomic patterns of relapse and progression following treatment with 131 I-MIBG in relapsed or refractory neuroblastoma.

OBJECTIVES: Patients with metaiodobenzylguanidine (MIBG)-avid relapsed or refractory neuroblastoma after initial therapy may exhibit transient responses to salvage treatment with iodine-131 metaiodobenzylguanidine (131 I-MIBG). It is unclear whether disease progression following 131 I-MIBG treatment occurs in previously involved versus new anatomic sites of disease. Understanding this pattern of relapse will inform the use of consolidation therapy following 131 I-MIBG administration. METHODS: Patients with relapsed or refractory metastatic MIBG-avid neuroblastoma or ganglioneuroblastoma, who received single-agent 131 I-MIBG, had stable or responding disease 6-8 weeks following 131 I-MIBG, but subsequently experienced disease progression were included. MIBG scans were reviewed to establish anatomic and temporal evolution of MIBG-avid disease. RESULTS: A total of 84 MIBG-avid metastatic sites were identified immediately prior to MIBG therapy in a cohort of 12 patients. At first progression, a total of 101 MIBG-avid sites were identified, of which 69 (68%) overlapped with pre-treatment disease sites, while 32 (32%) represented anatomically new disease areas. Eight of 12 patients had one or more new MIBG-avid sites at first progression. Of the 69 involved sites at progression that overlapped with pre-treatment disease, 11 represented relapsed sites that had cleared following MIBG therapy, two were persistent but increasingly MIBG-avid, and 56 were stably persistent. CONCLUSIONS: Previously involved anatomic disease sites predominate at disease progression following 131 I-MIBG treatment. Nevertheless, the majority of patients progressed in at least one new anatomic disease site. This suggests that consolidation focal therapies targeting residual disease sites may be of limited benefit in preventing systemic disease progression following 131 I-MIBG treatment of relapsed or refractory neuroblastoma.

Defective axonal transport of Rab7 GTPase results in dysregulated trophic signaling.

Retrograde trophic signaling of nerve growth factor (NGF) supports neuronal survival and differentiation. Dysregulated trophic signaling could lead to various neurological disorders. Charcot-Marie-Tooth type 2B (CMT2B) is one of the most common inherited peripheral neuropathies characterized by severe terminal axonal loss. Genetic analysis of human CMT2B patients has revealed four missense point mutations in Rab7, a small GTPase that regulates late endosomal/lysosomal pathways, but the exact pathological mechanism remains poorly understood. Here, we show that these Rab7 mutants dysregulated axonal transport and diminished the retrograde signaling of NGF and its TrkA receptor. We found that all CMT2B Rab7 mutants were transported significantly faster than Rab7(wt) in the anterograde direction, accompanied with an increased percentile of anterograde Rab7-vesicles within axons of rat E15.5 dorsal root ganglion (DRG) neurons. In PC12M cells, the CMT2B Rab7 mutants drastically reduced the level of surface TrkA and NGF binding, presumably by premature degradation of TrkA. On the other hand, siRNA knock-down of endogenous Rab7 led to the appearance of large TrkA puncta in enlarged Rab5-early endosomes within the cytoplasm, suggesting delayed TrkA degradation. We also show that CMT2B Rab7 mutants markedly impaired NGF-induced Erk1/2 activation and differentiation in PC12M cells. Further analysis revealed that CMT2B Rab7 mutants caused axonal degeneration in rat E15.5 DRG neurons. We propose that Rab7 mutants induce premature degradation of retrograde NGF-TrkA trophic signaling, which may potentially contribute to the CMT2B disease.