SYCE1

SYCE1 is a central-element structural protein of the meiotic synaptonemal complex (SC) required for homologous chromosome synapsis, and its loss disrupts meiotic synapsis in mouse (PMID:19247432). The structural core of human SYCE1 (aa 25–179) homodimerizes through an anti-parallel curved coiled-coil of ~20 nm, and the full-length molecule extends beyond 50 nm with elongated C-termini, consistent with a physical strut that tethers central-element components (PMID:30607510). SYCE1 engages SIX6OS1 (C14ORF39) through multivalent contacts—an N-terminal interface that opens the SYCE1 core dimer to form a 1:1 complex plus a distinct downstream interface—and both interfaces are required for SC assembly and synapsis, while a separate interaction with SYCP1 contributes to complex integrity (PMID:32917591, PMID:34718620). Disruption of either the SIX6OS1 or SYCP1 interaction by patient-derived variants impairs SC assembly and meiosis (PMID:34718620). Human pathogenic alleles modeled in humanized mice—including p.R241* and the c.197-2A>G splice mutation—abolish SYCE1 protein and reduce transcript to minimal levels through transcript degradation, producing failure of homologous synapsis, meiotic arrest before pachytene, increased apoptosis, and infertility in both sexes, establishing SYCE1 as causative for premature ovarian insufficiency and meiotic arrest (PMID:32402064, PMID:39909494). A frameshift allele (p.F230fs) lowers expression and mislocalizes SYCE1 to the cytoplasm, indicating the C-terminal region is needed for nuclear targeting (PMID:35023261).

1. 2009 High

   Establishing whether SYCE1 is required for meiotic synapsis was the first step in defining its in vivo role, and the null mouse showed it is essential while linking SC structural proteins to recombination machinery.

   Evidence Mouse knockout with co-immunoprecipitation linking SC proteins and RAD51

   PMID:19247432

   Open questions at the time

   • Did not resolve SYCE1's own structure or direct binding partners
   • Mechanism of how synapsis initiates from recombination sites not defined for SYCE1 itself
2. 2019 High

   How SYCE1 contributes architecturally to the SC was unknown; biophysical analysis showed it homodimerizes into an elongated anti-parallel coiled-coil strut, providing a structural basis for tethering central-element components.

   Evidence Solution biophysics (MALS, SAXS) on human SYCE1 core and full-length protein

   PMID:30607510

   Open questions at the time

   • No high-resolution atomic structure
   • Binding partners contacted by the extended C-termini not mapped in this study
3. 2020 High

   The molecular basis of SYCE1 integration into the SC was clarified by demonstrating multivalent SIX6OS1 binding that remodels the SYCE1 core dimer, with both interfaces functionally required for synapsis.

   Evidence Mouse genetics with targeted mutations plus biochemical binding and co-localization assays

   PMID:32917591

   Open questions at the time

   • Stoichiometry within the assembled SC lattice not fully defined
   • Relationship of the SIX6OS1 interface to the SYCP1 contact not resolved
4. 2020 High

   Whether a specific human SYCE1 nonsense allele causes ovarian insufficiency was tested by humanizing the mutation in mouse, showing loss of protein via transcript degradation and synapsis failure.

   Evidence CRISPR/Cas9 humanized mouse, immunofluorescence, qRT-PCR, meiotic spread cytology

   PMID:32402064

   Open questions at the time

   • Did not address partial-function or missense alleles
   • Did not separate transcript decay from protein instability mechanistically
5. 2022 Medium

   How disease variants act at the molecular level was addressed by showing POI and NOA variants disrupt SYCE1 binding to SYCP1 or SIX6OS1, tying clinical phenotypes to specific interaction interfaces.

   Evidence Co-immunoprecipitation/pulldown of wild-type versus patient-variant SYCE1

   PMID:34718620

   Open questions at the time

   • No orthogonal structural or in vivo confirmation of disrupted interactions
   • Quantitative effect on SC assembly not measured
6. 2022 Medium

   A frameshift variant revealed that the SYCE1 C-terminus is required for correct nuclear targeting, since the mutant mislocalized to the cytoplasm with reduced expression.

   Evidence In vitro transfection of WT and mutant constructs in human cell lines, immunofluorescence, Western blot

   PMID:35023261

   Open questions at the time

   • Cell-based overexpression only, not validated in meiotic cells in vivo
   • Nuclear import signal not mapped
7. 2022 Low

   A potential non-meiotic role was explored by overexpression in somatic gonadal cells, where SYCE1 activated steroidogenic genes and increased androgen synthesis.

   Evidence Overexpression/knockdown of Syce1/Syce3 in primary mouse Sertoli and Leydig cells with steroidogenic gene and hormone assays

   PMID:35697131

   Open questions at the time

   • Single-lab cell-based overexpression without in vivo validation
   • No direct mechanistic link between SYCE1 and steroidogenic gene regulation
   • Function distinct from canonical meiotic role not independently confirmed
8. 2025 High

   Whether a splice-site human allele recapitulates infertility was confirmed in humanized mice, extending the loss-of-function/transcript-degradation mechanism to meiotic arrest and apoptosis in both sexes.

   Evidence CRISPR/Cas9 humanized mouse, immunofluorescence, RT-PCR, cytology, TUNEL apoptosis assay

   PMID:39909494

   Open questions at the time

   • Does not distinguish which downstream events trigger apoptosis
   • Timing of arrest relative to recombination checkpoints not dissected

• The atomic-resolution architecture of the assembled SYCE1–SIX6OS1–SYCP1 central-element lattice and the molecular basis of any non-meiotic SYCE1 functions remain open.
• No high-resolution structure of the assembled central element
• Non-meiotic steroidogenic role unvalidated in vivo
• Nuclear import determinants not mapped