| Aspect | Key findings | Evidence type/method | Source |
|---|---|---|---|
| Identity | ERVMER34-1 encodes HEMO, a human endogenous MER34 Env-like protein from the MER34 locus on chromosome 4q12; literature consistently maps HEMO to the human ERVMER34-1 gene/protein corresponding to UniProt Q9H9K5. | Primary gene/protein characterization; locus mapping; comparative annotation | Heidmann et al., 2017, *PNAS*, doi: https://doi.org/10.1073/pnas.1702204114 (pqac-00000007, pqac-00000008) |
| Structure/domains | HEMO is a 563-aa Env-like precursor with an N-terminal signal peptide; SU contains a CWLC motif; TM contains an immunosuppressive-domain-like region, a C-X6-CC motif, a 23-aa hydrophobic transmembrane domain, and a C-terminal cytoplasmic tail. The canonical furin cleavage motif is mutated to CTQG, and an adjacent hydrophobic fusion peptide is absent. | Sequence/domain analysis from primary paper | Heidmann et al., 2017, *PNAS*, doi: https://doi.org/10.1073/pnas.1702204114 (pqac-00000008) |
| Processing/shedding | HEMO is synthesized as a classical Env precursor; the mature shed form begins at residue 27 after signal peptide cleavage. A major secreted glycosylated species runs at ~48 kDa; MS mapped C-terminal truncation/cleavage mainly at Q432 and R433 (about 4:1 ratio). Full-length SU-TM is ~58 kDa and mainly cell-associated. Efficient shedding requires membrane anchoring; mutants truncated before the TM are not shed. A furin-engineered mutant (H-fur+) yields a smaller ~37-kDa SU-like product. | Transient transfection, Western blot, PNGase F deglycosylation, mass spectrometry, mutagenesis | Heidmann et al., 2017, *PNAS*, doi: https://doi.org/10.1073/pnas.1702204114 (pqac-00000010, pqac-00000006, pqac-00000014) |
| Localization | HEMO is extracellularly shed and detectable in placental blood and maternal circulation during pregnancy. In first-trimester placenta, immunostaining is strongest in villous cytotrophoblasts (CTs) and extravillous trophoblasts (EVTs), with more diffuse syncytiotrophoblast staining, consistent with release toward maternal blood. | Immunohistochemistry, WGA enrichment, Western blot of placental blood/tissue | Heidmann et al., 2017, *PNAS*, doi: https://doi.org/10.1073/pnas.1702204114 (pqac-00000009, pqac-00000010, pqac-00000014); Shimode, 2023, *Biomolecules*, doi: https://doi.org/10.3390/biom13101482 (pqac-00000017, pqac-00000021) |
| Expression (placenta) | Placenta is the dominant normal expression site. qRT-PCR across 20 tissues and 16 cell lines used placenta values as means from 12 first-trimester placentas; RNA-seq also showed significant placental expression with limited normal-tissue expression outside placenta, especially kidney. Placental samples analyzed included first-trimester tissues at 8–12 weeks gestation. | qRT-PCR, RNA-seq reanalysis, RACE, placental tissue profiling | Heidmann et al., 2017, *PNAS*, doi: https://doi.org/10.1073/pnas.1702204114 (pqac-00000008, pqac-00000009, pqac-00000006) |
| Expression (embryo/ESC/iPSC) | HEMO is expressed from the eight-cell stage through blastocyst and maintained in derived ESCs (reported across passages 0–10). It is reactivated during CD34+ cell reprogramming to iPSCs in parallel with OCT4; a shed ~48-kDa HEMO band is detectable in iPSC supernatants. RNA-seq panels included 124 single-cell embryo/ESC samples and 28 reprogramming/iPSC-related samples. | Single-cell and bulk RNA-seq analyses; Western blot of iPSC supernatants | Heidmann et al., 2017, *PNAS*, doi: https://doi.org/10.1073/pnas.1702204114 (pqac-00000009, pqac-00000011, pqac-00000016, pqac-00000018) |
| Maternal blood abundance | HEMO is present at low levels in men and nonpregnant women but rises during gestation; peak concentration in pregnant blood was estimated at ~1–10 nM, approximately 1–2 orders of magnitude below peak hCG. | Comparative Western blot quantification of sera/plasma | Heidmann et al., 2017, *PNAS*, doi: https://doi.org/10.1073/pnas.1702204114 (pqac-00000009, pqac-00000014, pqac-00000016, pqac-00000019) |
| Tumor associations | Transcriptome screens found heterogeneous tumor expression with high-level signals in germ-line, liver, lung, breast, and ovary tumors. In ovarian cancer, expression showed histotype dependence: elevated in clear-cell carcinoma (n=60) and endometrioid cancers (n=96), but not clearly in serous (n=289) or mucinous (n=34) histotypes. HEMO protein was detected by IHC in clear-cell ovarian tumor cells. Dataset summaries included 1,033 normal and 2,315 neoplasm samples, plus 479 additional tumor samples. | Microarray/RNA-seq meta-analysis; ovarian tumor immunohistochemistry | Heidmann et al., 2017, *PNAS*, doi: https://doi.org/10.1073/pnas.1702204114 (pqac-00000009, pqac-00000016, pqac-00000020) |
| Regulatory mechanisms | Unlike many ERV genes, HEMO is transcribed from a non-LTR CpG-rich promoter. A 760-bp fragment around the start site showed strong promoter activity (>500-fold) in luciferase assays. CpG methylation inversely correlated with expression (methylated in 293T and BeWo; unmethylated in iPSC and CaCo-2), and 5-Aza-dC treatment derepressed transcription. | RACE-PCR, promoter luciferase assays, bisulfite methylation mapping, pharmacologic demethylation | Heidmann et al., 2017, *PNAS*, doi: https://doi.org/10.1073/pnas.1702204114 (pqac-00000008) |
| Evolutionary features | HEMO is described as the oldest captured full-length env in humans, with capture likely >100 Mya before the Laurasiatheria–Euarchontoglires split. The locus is highly degenerate as a provirus (no clear 5' LTR, truncated 3' LTR, degenerate pol), yet the env ORF is preserved in simians under purifying selection and retains shedding capacity. | Comparative genomics, synteny, phylogeny, selection analysis | Heidmann et al., 2017, *PNAS*, doi: https://doi.org/10.1073/pnas.1702204114 (pqac-00000007, pqac-00000008, pqac-00000013); Johnson, 2019, *Nat Rev Microbiol*, doi: https://doi.org/10.1038/s41579-019-0189-2 (pqac-00000000) |
| Hypothesized function | Direct function remains unresolved. Experimental evidence indicates HEMO has no detectable fusogenic activity. Because it localizes similarly to ERVH48-1/Suppressyn in CTs and EVTs and is shed into maternal blood, reviews have suggested it may act as a cell-fusion inhibitor or other secreted placental factor, but this remains hypothetical. No direct receptor-binding or immunosuppression assay for HEMO was reported in the cited contexts. | Negative functional assay for fusion; comparative interpretation in recent reviews | Heidmann et al., 2017, *PNAS*, doi: https://doi.org/10.1073/pnas.1702204114 (pqac-00000006, pqac-00000012); Shimode, 2023, *Biomolecules*, doi: https://doi.org/10.3390/biom13101482 (pqac-00000017, pqac-00000021) |
| Shedding mechanism | Shedding resembles proteolytic ectodomain release seen for some viral Env proteins and was inhibited dose-dependently in transfected cells by broad-spectrum ADAM/MMP inhibitors (Batimastat, Marimastat, GM6001; shown across ~0.1–10 µM), supporting metalloproteinase-mediated processing at the cell surface. | Inhibitor perturbation in transfected cells; Western blot readout | Heidmann et al., 2017, *PNAS*, doi: https://doi.org/10.1073/pnas.1702204114 (pqac-00000006, pqac-00000019) |


*Table: This table compiles evidence-backed findings for human ERVMER34-1/HEMO, including identity, structure, shedding, localization, expression, tumor associations, regulation, and proposed function. It highlights key quantitative details and the methods supporting each claim.*