Impact of the Human Cell Atlas on medicine

Single-cell atlases promise to provide a 'missing link' between genes, diseases and therapies. By identifying the specific cell types, states, programs and contexts where disease-implicated genes act, we will understand the mechanisms of disease at the cellular and tissue levels and can use this understanding to develop powerful disease diagnostics; identify promising new drug targets; predict their efficacy, toxicity and resistance mechanisms; and empower new kinds of therapies, from cancer therapies to regenerative medicine. Here, we lay out a vision for the potential of cell atlases to impact the future of medicine, and describe how advances over the past decade have begun to realize this potential in common complex diseases, infectious diseases (including COVID-19), rare diseases and cancer.

The legacy of the Human Genome Project

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A Synthesis Concerning Conservation and Divergence of Cell Types across Epithelia

Advances in single-cell RNA-seq (scRNA-seq) and computational analysis have enabled the systematic interrogation of the cellular composition of tissues. Combined with tools from developmental biology, cell biology, and genetics, these approaches are revealing fundamental aspects of tissue geometry and physiology, including the distribution, origins, and inferred functions of specialized cell types, and the dynamics of cellular turnover and differentiation. By comparing different tissues, such studies can delineate shared and specialized features of cell types and their lineage. Here, we compare two developmentally related murine epithelia, the airway and the small intestinal epithelia, which are both derived from the embryonic endodermal gut tube. We examine how airway and intestine generate and functionalize common archetypal cell types to fulfill similar shared physiologic functionalities. We point to cases in which similar cell types are repurposed to accommodate each tissue's unique physiologic role, and highlight tissue-specific cells whose specializations contribute to the distinct functional roles of each organ. We discuss how archetypal and unique cell types are incorporated within a cellular lineage, and how the regulation of the proportions of these cell types enables tissue-level organization to meet functional demands and maintain homeostasis.

The Human Tumor Atlas Network: Charting Tumor Transitions across Space and Time at Single-Cell Resolution

Crucial transitions in cancer-including tumor initiation, local expansion, metastasis, and therapeutic resistance-involve complex interactions between cells within the dynamic tumor ecosystem. Transformative single-cell genomics technologies and spatial multiplex in situ methods now provide an opportunity to interrogate this complexity at unprecedented resolution. The Human Tumor Atlas Network (HTAN), part of the National Cancer Institute (NCI) Cancer Moonshot Initiative, will establish a clinical, experimental, computational, and organizational framework to generate informative and accessible three-dimensional atlases of cancer transitions for a diverse set of tumor types. This effort complements both ongoing efforts to map healthy organs and previous large-scale cancer genomics approaches focused on bulk sequencing at a single point in time. Generating single-cell, multiparametric, longitudinal atlases and integrating them with clinical outcomes should help identify novel predictive biomarkers and features as well as therapeutically relevant cell types, cell states, and cellular interactions across transitions. The resulting tumor atlases should have a profound impact on our understanding of cancer biology and have the potential to improve cancer detection, prevention, and therapeutic discovery for better precision-medicine treatments of cancer patients and those at risk for cancer.

Toward a Common Coordinate Framework for the Human Body

Understanding the genetic and molecular drivers of phenotypic heterogeneity across individuals is central to biology. As new technologies enable fine-grained and spatially resolved molecular profiling, we need new computational approaches to integrate data from the same organ across different individuals into a consistent reference and to construct maps of molecular and cellular organization at histological and anatomical scales. Here, we review previous efforts and discuss challenges involved in establishing such a common coordinate framework, the underlying map of tissues and organs. We focus on strategies to handle anatomical variation across individuals and highlight the need for new technologies and analytical methods spanning multiple hierarchical scales of spatial resolution.

The Pediatric Cell Atlas: Defining the Growth Phase of Human Development at Single-Cell Resolution

Single-cell gene expression analyses of mammalian tissues have uncovered profound stage-specific molecular regulatory phenomena that have changed the understanding of unique cell types and signaling pathways critical for lineage determination, morphogenesis, and growth. We discuss here the case for a Pediatric Cell Atlas as part of the Human Cell Atlas consortium to provide single-cell profiles and spatial characterization of gene expression across human tissues and organs. Such data will complement adult and developmentally focused HCA projects to provide a rich cytogenomic framework for understanding not only pediatric health and disease but also environmental and genetic impacts across the human lifespan.

Uncovering a clinical portrait of sluggish cognitive tempo within an evaluation for attention-deficit/hyperactivity disorder: A case study

Despite the burgeoning scientific literature examining the sluggish cognitive tempo (SCT) construct, very little is known about the clinical presentation of SCT. In clinical cases where SCT is suspected, it is critical to carefully assess not only for attention-deficit/hyperactivity disorder (ADHD) but also for other comorbidities that may account for the SCT-related behaviors, especially internalizing symptoms and sleep problems. The current case study provides a clinical description of SCT in a 7-year-old girl, offering a real-life portrait of SCT while also providing an opportunity to qualitatively differentiate between SCT and ADHD, other psychopathologies (e.g. depression, anxiety), and potentially related domains of functioning (e.g. sleep, executive functioning [EF]). "Jessica" was described by herself, parents, and teacher as being much slower than her peers in completing schoolwork, despite standardized testing showing Jessica to have above average intelligence and academic achievement. Jessica's parents completed rating scales indicating high levels of SCT symptoms and daytime sleepiness, as well as mildly elevated EF deficits. More research is needed to determine how to best conceptualize, assess, and treat SCT, and Jessica's case underscores the importance of further work in this area.

Poly(ADP-ribose) regulates stress responses and microRNA activity in the cytoplasm

Poly(ADP-ribose) is a major regulatory macromolecule in the nucleus, where it regulates transcription, chromosome structure, and DNA damage repair. Functions in the interphase cytoplasm are less understood. Here, we identify a requirement for poly(ADP-ribose) in the assembly of cytoplasmic stress granules, which accumulate RNA-binding proteins that regulate the translation and stability of mRNAs upon stress. We show that poly(ADP-ribose), six specific poly(ADP-ribose) polymerases, and two poly(ADP-ribose) glycohydrolase isoforms are stress granule components. A subset of stress granule proteins, including microRNA-binding Argonaute family members Ago1-4, are modified by poly(ADP-ribose), and such modification increases upon stress, a condition when both microRNA-mediated translational repression and microRNA-directed mRNA cleavage are relieved. Similar relief of repression is also observed upon overexpression of specific poly(ADP-ribose) polymerases or, conversely, upon knockdown of glycohydrolase. We conclude that poly(ADP-ribose) is a key regulator of posttranscriptional gene expression in the cytoplasm.

Methods for purification of proteins associated with cellular poly(ADP-ribose) and PARP-specific poly(ADP-ribose)

Poly(ADP-ribose) (pADPr) is a posttranslational modification that regulates protein function through two major mechanisms: covalent modification of acceptor proteins and noncovalent binding of proteins to pADPr. pADPr is synthesized by a family of enzymes called poly(ADP-ribose) polymerases (PARPs) that are themselves major targets of pADPr modification. Here, we outline two methods for the purification of pADPr-binding proteins via pADPr purification under native conditions: purification of cellular pADPr and pADPr covalently linked to specific PARPs. Together, these methods provide complementary approaches to the identification of noncovalent pADPr-protein interactions in the cell.