Impact of the COVID-19 Pandemic on Childhood Lead Testing and Blood Lead Levels.

OBJECTIVE: To evaluate the effect of the COVID-19 pandemic on childhood lead testing and blood lead levels. METHODS: A retrospective analysis of lead tests and results was performed across 3 urban medical centers during the pre-COVID-19 (March 10, 2019-March 9, 2020) and COVID-19 (March 10, 2020-March 10, 2022) periods. Interrupted time series analysis with quasi-Poisson regression was used to evaluate changes in lead testing between study periods. The relationship between sociodemographic features with detectable (≧2 µg/dL) and elevated (≧3.5 µg/dL) blood lead levels (BLLs) was assessed with multivariable logistic regression. RESULTS: Among a total of 16,364 lead tests across 10,362 patients, weekly testing rates significantly decreased during COVID-19 (relative risk (RR) 0.64, 95% (confidence interval) CI 0.53-0.78). Census tracts with the greatest proportion of pre-1950s housing had a stronger association with detectable BLLs during the COVID-19 period (pre-COVID-19 adjusted odds ratio (aOR) 1.73, 95% CI 1.35-2.20; aOR 2.58, 95% CI 2.13-3.12; interaction P value .014). When limited to 1 year following COVID-19 (March 10, 2020-March 10, 2021), the association between both elevated BLLs (pre-COVID-19: aOR 1.49, 95% CI 0.87-2.53; COVID-19: aOR 3.51, 95% CI 1.98-6.25; interaction P value .032) and detectable BLLs with pre-1950s housing were greater during the COVID-19 period (pre-COVID-19: aOR 1.73, 95% CI 1.35-2.20; COVID-19: aOR 2.56, 95% CI 1.95-3.34; interaction P value .034). CONCLUSIONS: The COVID-19 pandemic led to a significant reduction in lead surveillance and magnified the effect of known risk factors for lead exposure. Concerted clinical, public health, and community advocacy are needed to address care gaps and excess cases of lead poisoning.

TRF1 and TRF2 use different mechanisms to find telomeric DNA but share a novel mechanism to search for protein partners at telomeres.

Human telomeres are maintained by the shelterin protein complex in which TRF1 and TRF2 bind directly to duplex telomeric DNA. How these proteins find telomeric sequences among a genome of billions of base pairs and how they find protein partners to form the shelterin complex remains uncertain. Using single-molecule fluorescence imaging of quantum dot-labeled TRF1 and TRF2, we study how these proteins locate TTAGGG repeats on DNA tightropes. By virtue of its basic domain TRF2 performs an extensive 1D search on nontelomeric DNA, whereas TRF1's 1D search is limited. Unlike the stable and static associations observed for other proteins at specific binding sites, TRF proteins possess reduced binding stability marked by transient binding (∼ 9-17 s) and slow 1D diffusion on specific telomeric regions. These slow diffusion constants yield activation energy barriers to sliding ∼ 2.8-3.6 κ(B)T greater than those for nontelomeric DNA. We propose that the TRF proteins use 1D sliding to find protein partners and assemble the shelterin complex, which in turn stabilizes the interaction with specific telomeric DNA. This 'tag-team proofreading' represents a more general mechanism to ensure a specific set of proteins interact with each other on long repetitive specific DNA sequences without requiring external energy sources.

DNA polymerase δ stalls on telomeric lagging strand templates independently from G-quadruplex formation.

Previous evidence indicates that telomeres resemble common fragile sites and present a challenge for DNA replication. The precise impediments to replication fork progression at telomeric TTAGGG repeats are unknown, but are proposed to include G-quadruplexes (G4) on the G-rich strand. Here we examined DNA synthesis and progression by the replicative DNA polymerase δ/proliferating cell nuclear antigen/replication factor C complex on telomeric templates that mimic the leading C-rich and lagging G-rich strands. Increased polymerase stalling occurred on the G-rich template, compared with the C-rich and nontelomeric templates. Suppression of G4 formation by substituting Li(+) for K(+) as the cation, or by using templates with 7-deaza-G residues, did not alleviate Pol δ pause sites within the G residues. Furthermore, we provide evidence that G4 folding is less stable on single-stranded circular TTAGGG templates where ends are constrained, compared with linear oligonucleotides. Artificially stabilizing G4 structures on the circular templates with the G4 ligand BRACO-19 inhibited Pol δ progression into the G-rich repeats. Similar results were obtained for yeast and human Pol δ complexes. Our data indicate that G4 formation is not required for polymerase stalling on telomeric lagging strands and suggest that an alternative mechanism, in addition to stable G4s, contributes to replication stalling at telomeres.

POT1-TPP1 regulates telomeric overhang structural dynamics.

Human telomeres possess a single-stranded DNA (ssDNA) overhang of TTAGGG repeats, which can self-fold into a G-quadruplex structure. POT1 binds specifically to the telomeric overhang and partners with TPP1 to regulate telomere lengthening and capping, although the mechanism remains elusive. Here, we show that POT1 binds stably to folded telomeric G-quadruplex DNA in a sequential manner, one oligonucleotide/oligosaccharide binding fold at a time. POT1 binds from 3' to 5', thereby unfolding the G-quadruplex in a stepwise manner. In contrast, the POT1-TPP1 complex induces a continuous folding and unfolding of the G-quadruplex. We demonstrate that POT1-TPP1 slides back and forth on telomeric DNA and also on a mutant telomeric DNA to which POT1 cannot bind alone. The sliding motion is specific to POT1-TPP1, as POT1 and ssDNA binding protein gp32 cannot recapitulate this activity. Our results reveal fundamental molecular steps and dynamics involved in telomere structure regulation.

Telomere proteins POT1, TRF1 and TRF2 augment long-patch base excision repair in vitro.

Human telomeres consist of multiple tandem hexameric repeats, each containing a guanine triplet. Guanosine-rich clusters are highly susceptible to oxidative base damage, necessitating base excision repair (BER). Previous demonstration of enhanced strand displacement synthesis by the BER component DNA polymerase ß in the presence of telomere protein TRF2 suggests that telomeres employ long-patch (LP) BER. Earlier analyses in vitro showed that efficiency of BER reactions is reduced in the DNA-histone environment of chromatin. Evidence presented here indicates that BER is promoted at telomeres. We found that the three proteins that contact telomere DNA, POT1, TRF1 and TRF2, enhance the rate of individual steps of LP-BER and stimulate the complete reconstituted LP-BER pathway. Thought to protect telomere DNA from degradation, these proteins still apparently evolved to allow selective access of repair proteins.

Telomeric protein TRF2 protects Holliday junctions with telomeric arms from displacement by the Werner syndrome helicase.

WRN protein loss causes Werner syndrome (WS), which is characterized by premature aging as well as genomic and telomeric instability. WRN prevents telomere loss, but the telomeric protein complex must regulate WRN activities to prevent aberrant telomere processing. Telomere-binding TRF2 protein inhibits telomere t-loop deletion by blocking Holliday junction (HJ) resolvase cleavage activity, but whether TRF2 also modulates HJ displacement at t-loops is unknown. In this study, we used multiplex fluorophore imaging to track the fate of individual strands of HJ substrates. We report the novel finding that TRF2 inhibits WRN helicase strand displacement of HJs with telomeric repeats in duplex arms, but unwinding of HJs with a telomeric center or lacking telomeric sequence is unaffected. These data, together with results using TRF2 fragments and TRF2 HJ binding assays, indicate that both the TRF2 B- and Myb domains are required to inhibit WRN HJ activity. We propose a novel model whereby simultaneous binding of the TRF2 B-domain to the HJ core and the Myb domain to telomeric arms promote and stabilize HJs in a stacked arm conformation that is unfavorable for unwinding. Our biochemical study provides a mechanistic basis for the cellular findings that TRF2 regulates WRN activity at telomeres.