Throughout the age of genetics, researchers have largely been limited by either the number of genetic loci they could study, the number of cells that could be analyzed simultaneously, or both. Fluorescence-activated cell sorting (FACS) ¹, invented in the 1960s, allows researchers to quantify protein abundance in a huge number of single cells. However, the number of distinct labels that can be used on a single cell is very limited. Through the 1980s, microscopy still reigned as the most information-rich single- cell assay. Some studies combined microscopy with in situ probe hybridization to resolve the spatial distribution of specific sequences, but those traded throughput for spatial and sequence resolution ^2,3 .

Several papers kicked off single-cell transcriptome analysis in 1990, using Southern and Northern blot readout ^4,5 . Importantly, these studies found that amplifying a single cell’s genetic material was a crucial step for sufficient detection of transcripts. For almost all subsequent advances in sequencing technology, the single-cell version of each method came after first figuring out how to preserve the data quality of the original method in amplified single cells. The first giant leap in the capabilities of single-cell genomics came with the advent of the microarray for transcriptomics⁶, giving the first insights into coregulatory genetic networks in single cells on a genome-wide scale ⁷. Microarrays came with shortcomings, though; only known genes or sequences could be assayed, and very little isoform information could be gathered from each short probe sequence.

Everything changed with the rise of high-throughput sequencing. The problems of number of cells assayed at once, a priori knowledge of transcripts for quantification, and base pair-level resolution were all solved. The first single-cell RNA-sequencing analysis came out in 2009⁸ , and the first single-cell DNA-sequencing method followed in 2011⁹ .

Building upon these, the next generation of methods enrich for subsets of the genome or transcriptome to provide epigenetic or epitranscriptomic information. The global methylation profile of single-cell genomes can be resolved by RRBS ¹⁰, chromatin organization by single-cell Hi-C 11 , chromatin accessibility by single-cell DNaseI hypersensitivity sequencing 12 or ATAC-seq¹³,¹⁴ , and RNA modifications by several methods¹⁵.

To round out the central dogma of single-cell genomic technologies, mass cytometry¹⁶ has given the tried-and-true method of FACS a face-lift. Although not unbiased detection, mass cytometry increases the number of protein species detected in a single cell from around a dozen to upwards of one hundred. With an eye towards today and the future, current trends in single-cell sequencing technology development push the boundaries of what is possible in two directions: throughput and multi-omic measurement.

Researchers increasingly turn their attention to very heterogeneous tissues or rare cell populations for single-cell analysis. Many sequencing methods approach the theoretical limit of their detection sensitivity, so gains in statistical power rely on gains in the number of cells profiled. To fill this need, microfluidic drops have emerged as an efficient way to prepare huge numbers of sequencing libraries. RNA-seq methods such as inDrop¹⁷ and Drop-seq¹⁸ showed promise in the lab, and commercial entities such as

10X Genomics have fulfilled this promise and now provide similar methods as a service. Alternatively, combinatorial indexing uses the split-and-pool approach to molecularly barcode individual cells for pooled sequencing ^13,19 .

The combination of multiple technologies to simultaneously measure different species from the same cell may currently be the most active frontier in the field. All in the past couple years, we have seen combined genome/transcriptome²⁰ , methylome/transcriptome²¹ , proteome/transcriptome²² , chromatin accessibility/TCR sequence²³ , genome editing screens/transcriptome²⁴ ; the list goes on and on and is constantly expanding. The power to correlate changes in one form of genetic information with changes in other forms can be incredibly informative, as we gain a fuller understanding of intracellular processes and regulatory networks.

References

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