A blue ring nebula from a stellar merger several thousand years ago.

Stellar mergers are a brief but common phase in the evolution of binary star systems1,2. These events have many astrophysical implications; for example, they may lead to the creation of atypical stars (such as magnetic stars3, blue stragglers4 and rapid rotators5), they play an important part in our interpretation of stellar populations6 and they represent formation channels of compact-object mergers7. Although a handful of stellar mergers have been observed directly8,9, the central remnants of these events were shrouded by an opaque shell of dust and molecules10, making it impossible to observe their final state (for example, as a single merged star or a tighter, surviving binary11). Here we report observations of an unusual, ring-shaped ultraviolet ('blue') nebula and the star at its centre, TYC 2597-735-1. The nebula has two opposing fronts, suggesting a bipolar outflow of material from TYC 2597-735-1. The spectrum of TYC 2597-735-1 and its proximity to the Galactic plane suggest that it is an old star, yet it has abnormally low surface gravity and a detectable long-term luminosity decay, which is uncharacteristic for its evolutionary stage. TYC 2597-735-1 also exhibits Hα emission, radial-velocity variations, enhanced ultraviolet radiation and excess infrared emission-signatures of dusty circumstellar disks12, stellar activity13 and accretion14. Combined with stellar evolution models, the observations suggest that TYC 2597-735-1 merged with a lower-mass companion several thousand years ago. TYC 2597-735-1 provides a look at an unobstructed stellar merger at an evolutionary stage between its dynamic onset and the theorized final equilibrium state, enabling the direct study of the merging process.

NSD1 mutations deregulate transcription and DNA methylation of bivalent developmental genes in Sotos syndrome.

Sotos syndrome (SS), the most common overgrowth with intellectual disability (OGID) disorder, is caused by inactivating germline mutations of NSD1, which encodes a histone H3 lysine 36 methyltransferase. To understand how NSD1 inactivation deregulates transcription and DNA methylation (DNAm), and to explore how these abnormalities affect human development, we profiled transcription and DNAm in SS patients and healthy control individuals. We identified a transcriptional signature that distinguishes individuals with SS from controls and was also deregulated in NSD1-mutated cancers. Most abnormally expressed genes displayed reduced expression in SS; these downregulated genes consisted mostly of bivalent genes and were enriched for regulators of development and neural synapse function. DNA hypomethylation was strongly enriched within promoters of transcriptionally deregulated genes: overexpressed genes displayed hypomethylation at their transcription start sites while underexpressed genes featured hypomethylation at polycomb binding sites within their promoter CpG island shores. SS patients featured accelerated molecular aging at the levels of both transcription and DNAm. Overall, these findings indicate that NSD1-deposited H3K36 methylation regulates transcription by directing promoter DNA methylation, partially by repressing polycomb repressive complex 2 (PRC2) activity. These findings could explain the phenotypic similarity of SS to OGID disorders that are caused by mutations in PRC2 complex-encoding genes.

NCKAP1 Disruptive Variants Lead to a Neurodevelopmental Disorder with Core Features of Autism.

NCKAP1/NAP1 regulates neuronal cytoskeletal dynamics and is essential for neuronal differentiation in the developing brain. Deleterious variants in NCKAP1 have been identified in individuals with autism spectrum disorder (ASD) and intellectual disability; however, its clinical significance remains unclear. To determine its significance, we assemble genotype and phenotype data for 21 affected individuals from 20 unrelated families with predicted deleterious variants in NCKAP1. This includes 16 individuals with de novo (n = 8), transmitted (n = 6), or inheritance unknown (n = 2) truncating variants, two individuals with structural variants, and three with potentially disruptive de novo missense variants. We report a de novo and ultra-rare deleterious variant burden of NCKAP1 in individuals with neurodevelopmental disorders which needs further replication. ASD or autistic features, language and motor delay, and variable expression of intellectual or learning disability are common clinical features. Among inherited cases, there is evidence of deleterious variants segregating with neuropsychiatric disorders. Based on available human brain transcriptomic data, we show that NCKAP1 is broadly and highly expressed in both prenatal and postnatal periods and demostrate enriched expression in excitatory neurons and radial glias but depleted expression in inhibitory neurons. Mouse in utero electroporation experiments reveal that Nckap1 loss of function promotes neuronal migration during early cortical development. Combined, these data support a role for disruptive NCKAP1 variants in neurodevelopmental delay/autism, possibly by interfering with neuronal migration early in cortical development.

An absence of ex-companion stars in the type Ia supernova remnant SNR 0509-67.5.

A type Ia supernova is thought to begin with the explosion of a white dwarf star. The explosion could be triggered by the merger of two white dwarfs (a 'double-degenerate' origin), or by mass transfer from a companion star (the 'single-degenerate' path). The identity of the progenitor is still controversial; for example, a recent argument against the single-degenerate origin has been widely rejected. One way to distinguish between the double- and single-degenerate progenitors is to look at the centre of a known type Ia supernova remnant to see whether any former companion star is present. A likely ex-companion star for the progenitor of the supernova observed by Tycho Brahe has been identified, but that claim is still controversial. Here we report that the central region of the supernova remnant SNR 0509-67.5 (the site of a type Ia supernova 400 ± 50 years ago, based on its light echo) in the Large Magellanic Cloud contains no ex-companion star to a visual magnitude limit of 26.9 (an absolute magnitude of M(V) = +8.4) within a region of radius 1.43 arcseconds. (This corresponds to the 3σ maximum distance to which a companion could have been 'kicked' by the explosion.) This lack of any ex-companion star to deep limits rules out all published single-degenerate models for this supernova. The only remaining possibility is that the progenitor of this particular type Ia supernova was a double-degenerate system.