The enigmatic meiotic dense body and its newly discovered component, SCML1, are dispensable for fertility and gametogenesis in mice.

Meiosis is a critical phase in the life cycle of sexually reproducing organisms. Chromosome numbers are halved during meiosis, which requires meiosis-specific modification of chromosome behaviour. Furthermore, suppression of transposons is particularly important during meiosis to allow the transmission of undamaged genomic information between generations. Correspondingly, specialized genome defence mechanisms and nuclear structures characterize the germ line during meiosis. Survival of mammalian spermatocytes requires that the sex chromosomes form a distinct silenced chromatin domain, called the sex body. An enigmatic spherical DNA-negative structure, called the meiotic dense body, forms in association with the sex body. The dense body contains small non-coding RNAs including microRNAs and PIWI-associated RNAs. These observations gave rise to speculations that the dense body may be involved in sex body formation and or small non-coding RNA functions, e.g. the silencing of transposons. Nevertheless, the function of the dense body has remained mysterious because no protein essential for dense body formation has been reported yet. We discovered that the polycomb-related sex comb on midleg-like 1 (SCML1) is a meiosis-specific protein and is an essential component of the meiotic dense body. Despite abolished dense body formation, Scml1-deficient mice are fertile and proficient in sex body formation, transposon silencing and in timely progression through meiosis and gametogenesis. Thus, we conclude that dense body formation is not an essential component of the gametogenetic program in the mammalian germ line.

Photoreceptor Progenitor mRNA Analysis Reveals Exon Skipping Resulting from the ABCA4 c.5461-10T→C Mutation in Stargardt Disease.

PURPOSE: To elucidate the functional effect of the ABCA4 variant c.5461-10T→C, one of the most frequent variants associated with Stargardt disease (STGD1). DESIGN: Case series. PARTICIPANTS: Seventeen persons with STGD1 carrying ABCA4 variants and 1 control participant. METHODS: Haplotype analysis of 4 homozygotes and 11 heterozygotes for c.5461-10T→C and sequence analysis of the ABCA4 gene for a homozygous proband. Fibroblasts were reprogrammed from 3 persons with STGD1 into induced pluripotent stem cells, which were differentiated into photoreceptor progenitor cells (PPCs). The effect of the c.5461-10T→C variant on RNA splicing by reverse-transcription polymerase chain reaction was analyzed using PPC mRNA. In vitro assays were performed with minigene constructs containing ABCA4 exon 39. We analyzed the natural history and ophthalmologic characteristics of 4 persons homozygous for c.5461-10T→C. MAIN OUTCOME MEASURES: Haplotype and rare variant data for ABCA4, RNA splice defects, age at diagnosis, visual acuity, fundus appearance, visual field, electroretinography (ERG) results, fluorescein angiography results, and fundus autofluorescence findings. RESULTS: The frequent ABCA4 variant c.5461-10T→C has a subtle effect on splicing based on prediction programs. A founder haplotype containing c.5461-10T→C was found to span approximately 96 kb of ABCA4 and did not contain other rare sequence variants. Patient-derived PPCs showed skipping of exon 39 or exons 39 and 40 in the mRNA. HEK293T cell transduction with minigenes carrying exon 39 showed that the splice defects were the result of the c.5461-10T→C variant. All 4 subjects carrying the c.5461-10T→C variant in a homozygous state showed a young age of STGD1 onset, with low visual acuity at presentation and abnormal cone ERG results. All 4 demonstrated severe cone-rod dystrophy before 20 years of age and were legally blind by 25 years of age. CONCLUSIONS: The ABCA4 variant c.5461-10T→C is located on a founder haplotype lacking other disease-causing rare sequence variants. In vitro studies revealed that it leads to mRNA exon skipping and ABCA4 protein truncation. Given the severe phenotype in persons homozygous for this variant, we conclude that this variant results in the absence of ABCA4 activity.

Meiotic DNA break formation requires the unsynapsed chromosome axis-binding protein IHO1 (CCDC36) in mice.

DNA double-strand breaks (DSBs) are induced by SPO11 during meiosis to initiate recombination-mediated pairing and synapsis of homologous chromosomes. Germline genome integrity requires spatiotemporal control of DSB formation, which involves the proteinaceous chromosome axis along the core of each meiotic chromosome. In particular, a component of unsynapsed axes, HORMAD1, promotes DSB formation in unsynapsed regions where DSB formation must occur to ensure completion of synapsis. Despite its importance, the underlying mechanism has remained elusive. We identify CCDC36 as a direct interactor of HORMAD1 (IHO1) that is essential for DSB formation. Underpinning this function, IHO1 and conserved SPO11-auxiliary proteins MEI4 and REC114 assemble chromatin-bound recombinosomes that are predicted activators of DSB formation. HORMAD1 is needed for robust recruitment of IHO1 to unsynapsed axes and efficient formation and/or stabilization of these recombinosomes. Thus, we propose that HORMAD1-IHO1 interaction provides a mechanism for the selective promotion of DSB formation along unsynapsed chromosome axes.

On the development of extragonadal and gonadal human germ cells.

Human germ cells originate in an extragonadal location and have to migrate to colonize the gonadal primordia at around seven weeks of gestation (W7, or five weeks post conception). Many germ cells are lost along the way and should enter apoptosis, but some escape and can give rise to extragonadal germ cell tumors. Due to the common somatic origin of gonads and adrenal cortex, we investigated whether ectopic germ cells were present in the human adrenals. Germ cells expressing DDX4 and/or POU5F1 were present in male and female human adrenals in the first and second trimester. However, in contrast to what has been described in mice, where 'adrenal' and 'ovarian' germ cells seem to enter meiosis in synchrony, we were unable to observe meiotic entry in human 'adrenal' germ cells until W22. By contrast, 'ovarian' germ cells at W22 showed a pronounced asynchronous meiotic entry. Interestingly, we observed that immature POU5F1+ germ cells in both first and second trimester ovaries still expressed the neural crest marker TUBB3, reminiscent of their migratory phase. Our findings highlight species-specific differences in early gametogenesis between mice and humans. We report the presence of a population of ectopic germ cells in the human adrenals during development.