LEMD2

The karyosphere (karyosome) is a structure that forms in the oocyte nucleus-germinal vesicle (GV)-at the diplotene stage of meiotic prophase due to the assembly of all chromosomes in a limited portion of the GV. In some organisms, the karyosphere has an extrachromosomal external capsule, the marker protein of which is nuclear F-actin. Despite many years of theories about the formation of the karyosphere capsule (KC) in the GV of the common frog Rana temporaria, we present data that cast doubt on its existence, at least in this species. Specific extrachromosomal strands, which had been considered the main elements of the frog's KC, do not form a continuous layer around the karyosphere and, according to immunogold labeling, do not contain structural proteins, such as actin and lamin B. At the same time, F-actin is indeed noticeably concentrated around the karyosphere, creating the illusion of a capsule at the light microscopy/fluorescence level. The barrier-to-autointegration factor (BAF) and one of its functional partners-LEMD2, an inner nuclear membrane protein-are not localized in the strands, suggesting that the strands are not functional counterparts of the nuclear envelope. The presence of characteristic strands in the GV of R. temporaria late oocytes may reflect an excess of SMC1 involved in the structural maintenance of diplotene oocyte chromosomes at the karyosphere stage, since SMC1 has been shown to be the most abundant protein in the strands. Other characteristic microstructures-the so-called annuli, very similar in ultrastructure to the nuclear pore complexes-do not contain nucleoporins Nup35 and Nup93, and, therefore, they cannot be considered autonomous pore complexes, as previously thought. Taken together, our data indicate that traditional ideas about the existence of the R. temporaria KC as a special structural compartment of the GV are to be revisited.

Regulation by the small modifier SUMO is heavily dependent on spatial control of enzymes that mediate the attachment and removal of SUMO on substrate proteins. Here we show that in fission yeast, delocalisation of the SUMO protease Ulp1 from the nuclear envelope results in centromeric defects that can be attributed to hyper-SUMOylation at the nuclear periphery. Unexpectedly, we find that while this localised hyper-SUMOylation impairs centromeric silencing, it can also enhance centromere clustering. Moreover, both effects are at least partially dependent on SUMOylation of the inner nuclear membrane protein Lem2. Lem2 has previously been implicated in diverse biological processes, including the promotion of both centromere clustering and silencing, but how these distinct activities are coordinated was unclear; our observations suggest a model whereby SUMOylation may serve as a regulatory switch, modulating Lem2 interactions with competing partner proteins to balance its roles in alternative pathways. Our findings also reveal a previously unappreciated role for SUMOylation in promoting centromere clustering.

Marbach-Rustad progeroid syndrome is an extremely rare disease caused by a heterozygous variant in the LEMD2 gene. To date, only two patients and one LEMD2 pathogenic variant have been reported in Marbach-Rustad progeroid syndrome. Here we describe the third case of Marbach-Rustad progeroid syndrome worldwide, which is also the first case in China. The proband was affected with premature birth, failed to thrive, facial abnormalities, feeding difficulties, skull defects and delayed motor milestones, but had a normal intelligence and speech. Whole exome sequencing (WES) initially did not find a phenotype-causing variant when the proband was 1 year of age. The reanalysis of WES data 4 years later revealed the proband harbored a de novo heterozygous c.1436C>T(p.Ser479Phe) variant in the LEMD2 gene, which is known responsible for Marbach-Rustad progeroid syndrome. Sanger sequencing confirmed the presence of this variant in the proband and absence in his parents and two elder sisters. Our study provides accurate clinical diagnosis for the proband and adds a new patient with Marbach-Rustad progeroid syndrome. Our study suggests the LEMD2 c.1436C>T(p.Ser479Phe) variant as a hotspot. Our work also indicates reanalysis of WES data of negative cases might identify pathogenic variant and improve diagnostic efficiency.