Mitochondrial DNA mosaicism in normal human somatic cells.

Somatic cells accumulate genomic alterations with age; however, our understanding of mitochondrial DNA (mtDNA) mosaicism remains limited. Here we investigated the genomes of 2,096 clones derived from three cell types across 31 donors, identifying 6,451 mtDNA variants with heteroplasmy levels of ≳0.3%. While the majority of these variants were unique to individual clones, suggesting stochastic acquisition with age, 409 variants (6%) were shared across multiple embryonic lineages, indicating their origin from heteroplasmy in fertilized eggs. The mutational spectrum exhibited replication-strand bias, implicating mtDNA replication as a major mutational process. We evaluated the mtDNA mutation rate (5.0 × 10-8 per base pair) and a turnover frequency of 10-20 per year, which are fundamental components shaping the landscape of mtDNA mosaicism over a lifetime. The expansion of mtDNA-truncating mutations toward homoplasmy was substantially suppressed. Our findings provide comprehensive insights into the origins, dynamics and functional consequences of mtDNA mosaicism in human somatic cells.

Whole-genome sequences reveal zygotic composition in chimeric twins.

While most dizygotic twins have a dichorionic placenta, rare cases of dizygotic twins with a monochorionic placenta have been reported. The monochorionic placenta in dizygotic twins allows in utero exchange of embryonic cells, resulting in chimerism in the twins. In practice, this chimerism is incidentally identified in mixed ABO blood types or in the presence of cells with a discordant sex chromosome. Here, we applied whole-genome sequencing to one triplet and one twin family to precisely understand their zygotic compositions, using millions of genomic variants as barcodes of zygotic origins. Peripheral blood showed asymmetrical contributions from two sister zygotes, where one of the zygotes was the major clone in both twins. Single-cell RNA sequencing of peripheral blood tissues further showed differential contributions from the two sister zygotes across blood cell types. In contrast, buccal tissues were pure in genetic composition, suggesting that in utero cellular exchanges were confined to the blood tissues. Our study illustrates the cellular history of twinning during human development, which is critical for managing the health of chimeric individuals in the era of genomic medicine.

Widespread somatic L1 retrotransposition in normal colorectal epithelium.

Throughout an individual's lifetime, genomic alterations accumulate in somatic cells1-11. However, the mutational landscape induced by retrotransposition of long interspersed nuclear element-1 (L1), a widespread mobile element in the human genome12-14, is poorly understood in normal cells. Here we explored the whole-genome sequences of 899 single-cell clones established from three different cell types collected from 28 individuals. We identified 1,708 somatic L1 retrotransposition events that were enriched in colorectal epithelium and showed a positive relationship with age. Fingerprinting of source elements showed 34 retrotransposition-competent L1s. Multidimensional analysis demonstrated that (1) somatic L1 retrotranspositions occur from early embryogenesis at a substantial rate, (2) epigenetic on/off of a source element is preferentially determined in the early organogenesis stage, (3) retrotransposition-competent L1s with a lower population allele frequency have higher retrotransposition activity and (4) only a small fraction of L1 transcripts in the cytoplasm are finally retrotransposed in somatic cells. Analysis of matched cancers further suggested that somatic L1 retrotransposition rate is substantially increased during colorectal tumourigenesis. In summary, this study illustrates L1 retrotransposition-induced somatic mosaicism in normal cells and provides insights into the genomic and epigenomic regulation of transposable elements over the human lifetime.

Enhanced ocular efficacy of topically-delivered dorzolamide with nanostructured mucoadhesive microparticles.

Dorzolamide eye drops are widely prescribed to reduce intraocular pressure (IOP) in the treatment of ocular hypertension and glaucoma. However, in an eye drop formulation, dorzolamide is rapidly cleared from the preocular space, hence requiring multiple daily administrations. Here, we sought to increase the preocular retention of dorzolamide using nanostructured, mucoadhesive microparticles (MUCO_NM) as carriers for topical delivery to the eye. MUCO_NM were prepared by freeze-milling dorzolamide-loaded, electrospun nanofibers composed of poly(lactic-co-glycolic acid) and polyethylene glycol. The microparticles were embedded in a rapidly-dissolving tablet of polyvinyl alcohol. To assess in vivo efficacy, the MUCO_NM were administered topically to the eyes of rabbits, and IOP was measured and compared to that in eyes treated with Trusopt®, a marketed eye drop of dorzolamide. The MUCO_NM showed a 35% greater maximum IOP decrease and a>2-fold increase in the duration of the IOP decrease, compared to Trusopt®. This enhanced efficacy was comparable to that obtained with a single administration of 4 drops of Trusopt® or 2 administrations of Trusopt® at a 4-h interval. Our findings suggest that this MUCO_NM preparation is a promising carrier for topical delivery of dorzolamide to the eye, with enhanced drug efficacy and the potential to reduce administration frequency.