Bloom's syndrome: Why not premature aging?: A comparison of the BLM and WRN helicases

Abstract

Genomic instability is a hallmark of cancer and aging. Premature aging (progeroid) syndromes are often caused by mutations in genes whose function is to ensure genomic integrity. The RecQ family of DNA helicases is highly conserved and plays crucial roles as genome caretakers. In humans, mutations in three RecQ genes - BLM, WRN, and RECQL4 - give rise to Bloom's syndrome (BS), Werner syndrome (WS), and Rothmund-Thomson syndrome (RTS), respectively. WS is a prototypic premature aging disorder; however, the clinical features present in BS and RTS do not indicate accelerated aging. The BLM helicase has pivotal functions at the crossroads of DNA replication, recombination, and repair. BS cells exhibit a characteristic form of genomic instability that includes excessive homologous recombination. The excessive homologous recombination drives the development in BS of the many types of cancers that affect persons in the normal population. Replication delay and slower cell turnover rates have been proposed to explain many features of BS, such as short stature. More recently, aberrant transcriptional regulation of growth and survival genes has been proposed as a hypothesis to explain features of BS.

Keywords: Aging; BLM; Bloom’s syndrome; Cancer susceptibility; Genomic instability; RecQ helicases.

Fig. 1

Domain structure of human BLM protein. (A) Schematic representation of BLM domains. The functional domains are the helicase domain; the RecQ C-terminal domain (RQC) comprising the Zn²⁺-binding motif (Zn) and winged helix (WH) with the -hairpin in pink; and the helicase and ribonuclease D C-terminal (HRDC) domain. The N-terminal strand exchange (SE) domain is in purple and the interaction domain with topoisomerase III (TopoIII ) is indicated. The nuclear localization signal (NLS) is in dark grey. (B) Crystal structure of human BLM_640-1298 in complex with ADP and a 3′-overhang DNA duplex. The colors are as in (A): the helicase domain is in blue, the Zn²⁺-binding motif is in green and yellow, the winged-helix (WH) domain is in orange with the -hairpin in pink, and the HRDC domain is in red. The ADP is shown in stick form and the DNA in cartoon form with the phosphate backbone in dark grey. The calcium and zinc ions are shown as light grey spheres. Reproduced with permission of the International Union of Crystallography (Swan et al., 2014).

Fig. 2

Homologous recombination repair of double strand breaks (DSB). DSB is resected from 5′ to 3′ to produce a 3′-single strand overhang, which invades the intact homologous chromatid, to form the D-loop. (A) In synthesis-dependent strand-annealing (SDSA), the invading strand dissociates and re-anneals with the complementary 3′-tail to produce non-crossing over (NCO) products. (B) Second-end capture results in the formation of double Holliday Junction (dHJ). dHJ can be resolved by BLM-TopoIII -RMI-mediated dissolution to give NCO or can be cleaved by HJ resolvases (triangles) to produce both NCO and crossing-over (CO), depending on the cleavage plan (plain or empty triangles). CO products will manifest as sister-chromatid exchanges. Newly synthesized DNA is depicted as dashed line in the same color as the template. The established and potential roles of BLM in this pathway are indicated (dashed arrows). See text for details.

Fig. 3

Model for replication fork stalling and restart. Replication fork stalls when it encounters an obstacle (yellow ball) leading to single-strand DNA exposure. Newly synthesized DNA strands can anneal to each other to produce a four-way Holliday junction (HJ) or reversed fork. Reversed fork can be converted back to a functional replication fork, allowing lesion bypass, or translesion synthesis, or repair. Reversed fork can also be cleaved, leading to the formation of one-ended DSB. The sister chromatid is used as a template for recombination-mediated restart of the replication fork. Depending on the HJ cleavage (triangles), homologous recombination (HR) process can lead to crossing-over (CO), visualized as sister chromatid exchanges (SCEs). Replication forks can also break directly when encountering DNA damage without fork regression. Newly synthesized DNA is depicted as dashed line in the same color as the template. The potential roles of BLM in stabilization and restart of stalled replication forks are indicated (dashed arrows). See text for details.

Fig. 4

Model for the origin of ultra-fine bridges (UFBs). (A) Fully replicated but still intertwined DNA duplexes give rise to UFBs in anaphase and are resolved by topoisomerase II. These catenanes structures are commonly observed at centromeres. (B) Late replication intermediates lead to UFBs in anaphase. These hemicatenane structures arise predominantly from fragile sites under replicative stress and are resolved by BLM-TopoIII -RMI1-2. Parental DNA molecule is depicted in blue and in red is the nascent strand synthesized in S phase (C) Visualization of UFB in anaphase. HeLa cells were fixed and stained for BLM (white) and DAPI (cyan). In interphase, BLM localizes to distinct nuclear loci (PML nuclear bodies) seen as discreet foci. In anaphase, BLM localizes to UFB that appears as a thin thread between the two masses of sister chromatids. Scale bar represent 10μm.