Clinical characteristics and genetic features of patients with Wilson's disease in southwestern China.

Telomeres cap the ends of linear chromosomes and provide a molecular barrier for the human genome. Following each cell division, progressive telomere shortening erodes that barrier and threatens the stability of essential genetic material. Critically short, or dysfunctional telomeres induce replicative senescence and/or cell death and ultimately, lead to cellular aging. To overcome the replicative senescence associated with critically short telomeres, most cancer cells activate one of two known mechanisms of telomere elongation, reactivation of the enzyme telomerase or activation of the alternative lengthening of telomeres (ALT) pathway []. A small minority of tumors lack both mechanisms and continue to proliferate even as their telomeres continue to shorten (i.e. ever-shorter telomere phenotype; EST) [, ]. While the prevalence of EST in human cancer is unclear, reactivation of telomerase accounts for cellular immortalization in approximately 90% of human cancers while the alternative lengthening of telomeres (ALT) pathway is active in approximately 10% of all cancers. However, the prevalence of ALT increases to 80% in tumors of mesenchymal origin, including osteosarcoma [, ]. Nevertheless, the etiology surrounding the activation of ALT in osteosarcoma remains unclear. Reactivation of telomerase (hTERT) commonly occurs through promoter mutations, gene amplification, or epigenetic changes and promotes telomere elongation via the action of the telomerase reverse transcriptase enzyme [–]. In contrast, the ALT pathway does not rely on a single enzyme, but catalyzes telomere elongation via homologous recombination [, ]. ALT positive tumors frequently harbor functionally inactivating mutations in genes involved in chromatin remodeling and DNA damage repair, including (α-thalassemia/mental retardation syndrome X-linked), (death-domain associated protein), (SLX4 interacting protein), or (SWI/SNF Related, Matrix Associated, Actin Dependent Regulator of Chromatin, Subfamily A-like 1) [–]. However, functional inactivation of these proteins alone is insufficient to induce activation of the ALT pathway suggesting that additional defects likely contribute to the process [–]. Recent studies have demonstrated that while loss of ATRX in cell culture promotes some phenotypes associated with ALT, combining knockout with knockout of the RNA component of the telomerase catalytic core , generates bona fide ALT activity []. These data suggest that perhaps functional inactivation of the telomerase holoenzyme may precede activation of ALT in vivo. The telomerase catalytic core consists of the hTERT reverse transcriptase and the hTR RNA template and together this minimal core complex is sufficient to promote telomerase activity in vitro []. However, large scale purification strategies have identified that the telomerase holoenzyme is a much larger ribonucleoprotein complex []. Structurally, the hTR RNA consists of a pseudoknot containing the template region bound by hTERT and an H box (consensus ANANNA) and ACA box (H/ACA) motif that acts as a scaffold for recruitment of the small nucleolar ribonucleoproteins (snoRNP) DKC1, NHP2, NOP10, and GAR1. This H/ACA domain also harbors a Cajal body box, or CAB motif that is bound by the protein TCAB1 [, ]. Together, these proteins facilitate the processing and trafficking of the hTR transcript within the nucleus to ensure assembly and localization of an active telomerase enzyme at telomere ends []. Loss of function of DKC1, NHP2, NOP10, or TCAB1 leads to defects in hTR maturation and as a result, loss of telomerase activity. Moreover, germline mutations in DKC1, NHP2, NOP10, and TCAB1 have been identified in patients with the telomere length disorder, dyskeratosis congenita (DC). DC is a rare disease pathology that is defined by a genetic predisposition to critically short telomeres due to deficiencies in telomerase activity []. These findings highlight the importance of the telomerase holoenzyme, not just the catalytic core, in the establishment and maintenance of telomere elongation via telomerase.