cancers Review ALT Positivity in Human Cancers: Prevalence and Clinical Insights Danny MacKenzie, Jr. † , Andrea K. Watters † , Julie T. To , Melody W. Young, Jonathan Muratori , Marni H. Wilkoff, Rita G. Abraham, Maria M. Plummer * and Dong Zhang *   Citation: MacKenzie, D., Jr.; Watters, A.K.; To, J.T.; Young, M.W.; Muratori, J.; Wilkoff, M.H.; Abraham, R.G.; Plummer, M.M.; Zhang, D. ALT Positivity in Human Cancers: Prevalence and Clinical Insights. Cancers 2021, 13, 2384. https:// doi.org/10.3390/cancers13102384 Academic Editor: Mario Del Rosso Received: 12 April 2021 Accepted: 11 May 2021 Published: 14 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Biomedical Sciences, College of Osteopathic Medicine, New York Institute of Technology, Old Westbury, NY 11568, USA; dmackenz@nyit.edu (D.M.J.); awatters@nyit.edu (A.K.W.); jto01@nyit.edu (J.T.T.); myoung08@nyit.edu (M.W.Y.); jmurator@nyit.edu (J.M.); mwilkoff@nyit.edu (M.H.W.); rabrah08@nyit.edu (R.G.A.) * Correspondence: mplummer@nyit.edu (M.M.P.); dzhang12@nyit.edu (D.Z.); Tel.: +1-516-686-1327(M.M.P.); +1-516-686-3872 (D.Z.); Fax: +1-516-686-3832 (M.M.P. & D.Z.) † These authors had equal contribution. Simple Summary: Since it was first described over two decades ago, the Alternative Lengthening of Telomeres (ALT) pathway has been well accepted to hold clinical significance in cancer development, cancer diagnosis, and cancer treatment. In this review, first, we discuss how the activation of this pathway is determined. Then, we provide up-to-date statistics on the cancers ALT activity is detected in. We discuss the relationships between ALT positivity and prognosis as well as the pathogenetics of ALT positive cancers. Finally, we evaluate pre-clinical and clinical investigations of potential therapies targeting ALT. Abstract: Many exciting advances in cancer-related telomere biology have been made in the past decade. Of these recent advances, great progress has also been made with respect to the Alternative Lengthening of Telomeres (ALT) pathway. Along with a better understanding of the molecular mechanism of this unique telomere maintenance pathway, many studies have also evaluated ALT activity in various cancer subtypes. We first briefly review and assess a variety of commonly used ALT biomarkers. Then, we provide both an update on ALT-positive (ALT+) tumor prevalence as well as a systematic clinical assessment of the presently studied ALT+ malignancies. Additionally, we discuss the pathogenetic alterations in ALT+ cancers, for example, the mutation status of ATRX and DAXX, and their correlations with the activation of the ALT pathway. Finally, we highlight important ALT+ clinical associations within each cancer subtype and subdivisions within, as well as their prognoses. We hope this alternative perspective will allow scientists, clinicians, and drug developers to have greater insight into the ALT cancers so that together, we may develop more efficacious treatments and improved management strategies to meet the urgent needs of cancer patients. Keywords: alternative lengthening of telomeres; cancers; ALT biomarkers; ATRX; DAXX 1. Introduction 1.1. Telomere and Telomere Maintenance Mechanisms Human telomeres consist of repetitive DNA sequences of (TTAGGG) n at the terminal ends of each linear chromosome. The normal length of human telomeres ranges between 10 kilobases (kb) and 15 kb. Despite the high fidelity of DNA replication machinery, DNA ends progressively shorten with each cell division in a process termed the “End Replication Problem” [1]. Though this loss occurs in most somatic cells, telomeres protect coding DNA from attrition through structural barriers. Condensed primarily as heterochromatin, telomeres are also associated with a variety of proteins, including a six-member protein complex called Shelterin. The primary functions of Shelterin include protecting telomeres, facilitating telomere synthesis, and modulating the DNA damage response (DDR) at Cancers 2021, 13, 2384. https://doi.org/10.3390/cancers13102384 https://www.mdpi.com/journal/cancers