B3GNT5

B3GNT5 encodes the β-1,3-N-acetylglucosaminyltransferase known as Lc3 synthase, the committed enzyme that transfers GlcNAc in a β1,3 linkage onto lactosylceramide to generate lactotriosylceramide (Lc3Cer), the obligate precursor of all lacto- and neolacto-series glycosphingolipids (PMID:19014510). Germline knockout in mice causes pre-implantation lethality or, with alternative targeting strategies, postnatal B-cell deficiency with loss of splenic germinal centers, reproductive defects, and early death, establishing that lacto/neolacto-series gangliosides are essential for B-cell development and fertility (PMID:19014510, PMID:21087515). In cancer cells, B3GNT5 loss abolishes surface SSEA-1 expression and cancer stem cell properties, alters EMT markers and RTK signaling, and increases chemoresistance, while B3GNT5 protein stability itself depends on its own N-glycosylation (PMID:35526049, PMID:40847295). B3GNT5 expression is post-transcriptionally repressed by miR-30a-5p and miR-136-5p, which directly target its 3′-UTR and modulate downstream ERK/AKT signaling in trophoblast and liver cancer cells (PMID:32575164, PMID:34859256).

1. 2008 High

   Identifying B3GNT5 as the gene encoding Lc3 synthase resolved which enzyme initiates lacto-series glycosphingolipid biosynthesis and showed that this activity is essential for pre-implantation embryo survival.

   Evidence Gene knockout in mouse ES cells with enzymatic activity assay, GSL profiling, and germline knockout embryo genotyping

   PMID:19014510

   Open questions at the time

   • Mechanism of pre-implantation lethality not defined at the molecular level
   • No crystal structure or catalytic-site mutagenesis of B3GNT5
   • Expression pattern in adult tissues only partially mapped by ISH
2. 2010 High

   Two independent knockout strategies demonstrated that lacto/neolacto-series gangliosides generated by B3GNT5 are specifically required for B-cell homeostasis, germinal center formation, and reproductive function, revealing tissue-specific physiological consequences of Lc3 synthase loss.

   Evidence Two independent B3gnt5 knockout mouse lines with flow cytometry, histology, and phenotypic characterization

   PMID:21087515

   Open questions at the time

   • Molecular mechanism by which lacto-series GSLs support B-cell survival or germinal center maintenance is unknown
   • Whether B-cell defects are cell-intrinsic was not tested by conditional knockout or transplantation
   • Reproductive failure mechanism undefined
3. 2020 Medium

   Demonstrating that miR-30a-5p directly targets the B3GNT5 3′-UTR established the first post-transcriptional regulatory mechanism for B3GNT5, linking it to ERK/AKT signaling control in trophoblast cells.

   Evidence Dual-luciferase 3′-UTR reporter assay with miR-30a-5p mimic/inhibitor; phosphorylation Western blots in JAR and BeWo cells

   PMID:32575164

   Open questions at the time

   • Whether miR-30a-5p regulation of B3GNT5 occurs in non-trophoblast contexts is untested
   • Pathway between B3GNT5 protein and ERK/AKT activation not delineated
4. 2021 Medium

   Identification of a lncRNA MIR4435-2HG / miR-136-5p / B3GNT5 ceRNA axis in liver cancer revealed a second independent miRNA that represses B3GNT5 and showed that B3GNT5 levels are modulated by competing endogenous RNA networks.

   Evidence Dual luciferase reporters for sponge and target interactions; rescue experiments with miR-136-5p inhibitor in liver cancer cells

   PMID:34859256

   Open questions at the time

   • Functional readout limited to proliferation/migration; GSL-level changes not measured
   • In vivo relevance of ceRNA axis not tested
5. 2022 Medium

   Knockout of B3GNT5 in basal-like breast cancer cells proved that this enzyme is the sole source of SSEA-1 surface antigen and supports cancer stem cell properties, while also revealing that B3GNT5 protein stability is governed by its own N-glycosylation.

   Evidence B3GNT5 knockout in BLBC lines; flow cytometry for SSEA-1; mammosphere/colony formation assays; N-glycosylation site mutagenesis; in vivo tumorigenesis

   PMID:35526049

   Open questions at the time

   • Specific N-glycosylation sites critical for stability not fully mapped
   • Whether SSEA-1 loss itself drives the cancer stem cell phenotype versus other lacto-series GSL changes is not resolved
   • Single lab finding
6. 2025 Medium

   Mass spectrometry-confirmed accumulation of GlcCer and LacCer upon B3GNT5 depletion in HeLa cells directly validated its position downstream of LacCer in the GSL pathway and linked lacto-series GSL depletion to altered EMT, reduced RTK activation, and chemoresistance.

   Evidence CRISPR partial depletion; mass spectrometry of GSLs; Western blot for EMT markers and RTK phosphorylation; chemoresistance assays in HeLa cells

   PMID:40847295

   Open questions at the time

   • Partial rather than complete depletion; residual B3GNT5 activity may complicate interpretation
   • Whether EMT and chemoresistance phenotypes are directly caused by specific GSL species or by global GSL remodeling is unclear
   • Single cell line

• The structural basis for B3GNT5 substrate specificity, the identity of the GSL species that mediate B-cell and reproductive phenotypes, and whether B3GNT5 loss-of-function causes human disease remain unknown.
• No crystal or cryo-EM structure of B3GNT5
• No conditional knockout to determine cell-intrinsic requirements in specific tissues
• No human Mendelian disease linked to B3GNT5 mutations
• Mechanism connecting lacto-series GSL loss to RTK signaling and EMT not resolved