Beyond the rainbow: a review of advanced lineage tracing methodologies for interrogating the initiation, evolution, and recurrence of brain tumors

The mammalian forebrain is perhaps the pinnacle of evolution and one of the most complex structures in known existence. The origin of this complexity and diversity partly lies in dynamic behavior of progenitors during embryonic neural development, all of which is under the control of regulatory mechanisms that ensure all the elements end up in the right place at the right time. Historically, dye-base, histochemical, enzymatic, or fluorescent lineage tracing techniques have been used deconvolute developmental dynamics in tissues and cells. Technical limitations resulted from a restrictive number of fluorophores, the half-life of the dyes, or the ability to deconvolute mixed population. These limitations often impede larger scale lineage tracing using these methods in spatial and temporal contexts. Genetic barcoding techniques have been used for decades to explore clonal investigations and have now evolved with high-throughput sequencing methods to allow for impressive insights into population and even organism level lineage relationships. In this review we will discuss the contemporary progression of lineage tracing methodologies and how they were applied to answer questions around molecular and cellular mechanisms of gliogenesis and neurogenesis. We will also discuss recent advances in computational biology, single-cell sequencing, and in situ-based lineage tracing methodologies. Incorporation of these methods into toolset of lineage tracing promise to enable a higher-resolution, multimodal view of neural lineages during development and disease processes that highjack developmental signaling such as brain tumor development and recurrence-where traditional developmental hierarchies become more plastic and less predictable. Given the dismal prognosis of high-grade brain tumors like glioblastoma multiforme, a better understanding of the lineage relationships leading to disease heterogeneity and recurrence is desperately needed to formulate efficacious approaches to treatment. Here we discuss the past, present, and future of lineage tracing at the intersection of development and disease.