The Analysis of Pendolino (peo) Mutants Reveals Differences in the Fusigenic Potential among Drosophila Telomeres

Abstract

Drosophila telomeres are sequence-independent structures that are maintained by transposition to chromosome ends of three specialized retroelements (HeT-A, TART and TAHRE; collectively designated as HTT) rather than telomerase activity. Fly telomeres are protected by the terminin complex (HOAP-HipHop-Moi-Ver) that localizes and functions exclusively at telomeres and by non-terminin proteins that do not serve telomere-specific functions. Although all Drosophila telomeres terminate with HTT arrays and are capped by terminin, they differ in the type of subtelomeric chromatin; the Y, XR, and 4L HTT are juxtaposed to constitutive heterochromatin, while the XL, 2L, 2R, 3L and 3R HTT are linked to the TAS repetitive sequences; the 4R HTT is associated with a chromatin that has features common to both euchromatin and heterochromatin. Here we show that mutations in pendolino (peo) cause telomeric fusions (TFs). The analysis of several peo mutant combinations showed that these TFs preferentially involve the Y, XR and 4th chromosome telomeres, a TF pattern never observed in the other 10 telomere-capping mutants so far characterized. peo encodes a non-terminin protein homologous to the E2 variant ubiquitin-conjugating enzymes. The Peo protein directly interacts with the terminin components, but peo mutations do not affect telomeric localization of HOAP, Moi, Ver and HP1a, suggesting that the peo-dependent telomere fusion phenotype is not due to loss of terminin from chromosome ends. peo mutants are also defective in DNA replication and PCNA recruitment. However, our results suggest that general defects in DNA replication are unable to induce TFs in Drosophila cells. We thus hypothesize that DNA replication in Peo-depleted cells results in specific fusigenic lesions concentrated in heterochromatin-associated telomeres. Alternatively, it is possible that Peo plays a dual function being independently required for DNA replication and telomere capping.

Telomere fusion in Drosophila: The role of subtelomeric chromatin

“…Our recent studies on the Drosophila pendolino (peo) gene 33 have provided strong support for our hypothesis on the different evolutionary dynamics of terminin and non-terminin telomere proteins. Strong peo mutants display in average five telomeric fusions/cell and often exhibit multicentric linear chromosomes that resemble little "trains".…”

“…In these mutants, the vast majority of telomeric fusions involve the XR, the Y and the 4 th chromosome telomeres, suggesting that mutations in peo preferentially affect Ha-telomeres and that this effect is partially masked in strong peo mutants where most telomeres are fused. 33 A preferential involvement of Hatelomeres in fusion events is a peculiar feature of peo mutants and has not been observed in the other Drosophila mutants we characterized in the past (eff, Su(var)205, cav, mre11, rad50, nbs, tefu, woc, moi and ver) ( Fig. 2B; see also ref.…”

“…peo encodes a conserved E2 variant ubiquitin-conjugating enzyme that is enriched in many polytene chromosome bands and interbands, and is therefore a non-terminin protein. 33 We have shown that AKTIP and Ft1, the human and mouse orthologues of Peo, are required for telomere maintenance. 34 Strikingly, Peo and AKTIP directly bind unrelated terminin and shelterin components, respectively.…”

“…Moreover, peo mutant brains do not differ from control brains in the relative frequency of nuclei in early, mid or late S phase of the cell cycle, suggesting that heterochromatin replicates normally in a peo mutant background. 33 These data suggest that the wild type function of peo is required for general DNA synthesis and not for completion of specific subphases of DNA replication. Thus, it is unlikely that mutations in peo specifically affect replication of Ha-telomeres, which presumably replicate at the end of the S phase, together with the bulk of heterochromatin.…”

Mice with reduced expression of the telomere‐associated protein Ft1 develop p53‐sensitive progeroid traits

“…Human AKTIP, mouse Ft1, and Drosophila Pendolino (Peo) are orthologous ubiquitin E2 variant proteins involved in telomere maintenance (Burla et al., 2015; Cenci, Ciapponi et al., 2015). AKTIP mediates proper telomere replication, binds telomeric DNA and the shelterins TRF1 and TRF2, and interacts with the DNA replication machinery components PCNA and RPA70.…”