The work, published in Frontiers in Microbiology, combines two complementary strategies: carefully tuned fermentation conditions that coax more product out of the cells, and targeted genetic rewiring of the biosynthetic machinery itself. Romidepsin, also known by its laboratory designation FK228, is a complex cyclic depsipeptide whose chemical synthesis remains impractical at scale, which makes microbial fermentation the only realistic production route. That constraint has kept the drug expensive and its supply chain fragile, so any genuine improvement in titer carries weight well beyond the laboratory bench.

Romidepsin’s biological activity stems from its ability to block histone deacetylases, enzymes that remove acetyl groups from histone proteins and thereby silence stretches of chromatin. By inhibiting these enzymes, the drug reopens tightly packed DNA, reactivates tumor suppressor genes, halts proliferation and pushes cancer cells into apoptosis. That mechanism earned it FDA approval, but producing it has been an upstream headache. The molecule is assembled by a hybrid trans-acyltransferase polyketide synthase and non-ribosomal peptide synthetase assembly line that stitches together cysteine, valine, dehydrothreonine derived from threonine, and malonyl-CoA through sequential condensation, macrocyclization and oxidative modification.

Earlier attempts to improve supply followed a winding path. Researchers identified the related thailandepsin gene cluster in Burkholderia thailandensis in 2011, and fermentation optimization of that system informed later work. Heterologous expression of the complete romidepsin gene cluster in Escherichia coli achieved only about 19 milligrams per liter, comparable to the native strain. Genome mining then revealed a highly homologous cluster in B. thailandensis MSMB43, which after minor modification produced 168.5 milligrams per liter. In 2022, the Shandong group took a decisive step: they deleted the native thailandepsin cluster from B. thailandensis E264 and replaced it with an attB integration site, creating a chassis strain called KOGC1 into which a hybrid romidepsin gene cluster could be slotted. That strain yielded 581 milligrams per liter, then the record, but still short of industrial requirements.

The new study began with physiology rather than genetics. The team grew the KOGC1 strain carrying an intact romidepsin gene cluster in M9 medium and systematically varied fermentation parameters using a sequential single-factor design. Romidepsin accumulation proved strictly growth-associated: titers climbed after 24 hours, peaked near 628 milligrams per liter at 60 hours and then declined, a pattern the authors attribute to end-product inhibition or nutrient depletion. A 60-hour harvest window was therefore fixed. Inoculum size mattered too; while 0.5 to 2 percent inocula performed similarly, a 5 percent inoculum depressed production, likely through high cell density and premature metabolite accumulation, so 1 percent was adopted.

The most striking fermentation lever was an adsorbent resin. Because romidepsin can be toxic to its own producer, the team added XAD-16 macroporous resin to the culture 12 hours after inoculation, letting the resin sequester the compound extracellularly as it formed. The authors speculate that this in-situ removal alleviates end-product toxicity and feedback inhibition, though they caution that direct evidence from intracellular and extracellular quantification, adsorption kinetics or cell viability assays was not obtained. Screening resin concentrations identified 4 percent weight per volume as optimal, roughly tripling the titer relative to resin-free controls. The team then supplemented the three amino acid precursors of the assembly line. Valine alone gave a marginal 6 percent gain, but feeding cysteine, valine and threonine together produced a synergistic effect, lifting the final titer to 743.5 milligrams per liter, a 28 percent increase over the 581 milligram baseline.

Genetic engineering delivered the second leap. Earlier work had shown that the AraC-family regulator TdpR acts as a positive activator of the cluster, and that including the tdpR gene boosted romidepsin yield 1.7-fold. The Shandong team reasoned that cranking up TdpR expression with strong promoters would push the whole pathway harder. Using RedEx technology, a method for seamless DNA insertion and deletion in large multimodular gene clusters, they inserted seven strong constitutive promoters upstream of tdpR. The promoters, drawn from a library of 50 natural promoters characterized in B. thailandensis E264 by luciferase reporter assays, were ranked by apparent strength from P46 down to P11.

The results contained a instructive surprise. Although the reporter assays had ranked P16 relatively low, in the actual chromosomal context P16 drove the highest romidepsin titer of all, reaching 1,109.4 milligrams per liter, a 90 percent increase over the previous KOGC1 baseline and a 49 percent gain over the fermentation-optimized intermediate. P46 and P33 also performed well, yielding 984.6 and 870.2 milligrams per liter respectively. Quantitative PCR confirmed that transcript levels of tdpR and two biosynthetic genes, depE and tdpJ, were elevated in the best-performing strains, with P16 showing the highest mRNA levels. Notably, the chromosomal transcription ranking did not match the reporter-based promoter strength ranking, a discrepancy the authors flag as evidence that surrogate reporter data poorly predict native in-locus transcription, a lesson for anyone designing promoter swaps in secondary metabolism.

The team also probed regulatory cross-talk. The romidepsin cluster dep and the thailandepsin cluster tdp are highly conserved in gene content and share a common assembly line, yet their dedicated regulators are strikingly divergent: TdpR is an AraC-type protein while DepR from C. violaceum is LysR-type, with no significant sequence homology. When the researchers cloned depR into the romidepsin expression vector and integrated it into KOGC1, the resulting strain maintained detectable romidepsin production comparable to the TdpR control. The authors suggest DepR may support the pathway through mechanisms distinct from TdpR rather than acting as a direct functional analog, though whether it binds the cluster’s promoters directly or works through an indirect regulatory cascade remains an open, testable hypothesis.

Perhaps as important as the headline titer is what did not happen over time. Large exogenous biosynthetic gene clusters often destabilize, deleting or rearranging under the metabolic burden they impose, which has blocked many natural product platforms from industrial translation. The team passaged three representative engineered strains for five generations without any antibiotic selection and found no significant titer fluctuation from the first through the fifth generation. Colony PCR on randomly picked colonies at generations one, three and five confirmed that the full refactored cluster remained intact, with no evidence of deletion, rearrangement or cassette loss. The authors note this demonstrates short-term genotypic and phenotypic stability only, and that bioreactor-scale and extended-culture performance remain to be assessed.

Honest caveats accompany the achievement. All experiments were conducted in 65-milliliter flask cultures, so the platform must be viewed as a foundation for future scale-up rather than an immediately industrial process. Late-stage broth viscosity, worsened by the resin, complicates downstream extraction, and the team suggests that optimized cell lysis protocols or continuous-flow separation systems will be needed to minimize product loss. Quantitative yield differences among engineered strains did not reach statistical significance under the current replication scheme, although the P16 strain’s titer was consistently reproduced across multiple independent fermentation batches. The authors also emphasize that direct metabolomic evidence for precursor limitation and product toxicity is still lacking, and they propose CRISPR-Cas-based genome stabilization and multi-omics profiling as next steps. Even with those qualifications, the study delivers the most efficient romidepsin production platform yet described and a generalized framework for stabilizing large, unwieldy gene clusters, work that could eventually make this potent anticancer agent cheaper, more abundant and more accessible to the patients who need it.

Subject of Research: Metabolic engineering of the FK228 (romidepsin) biosynthetic gene cluster for high-yield production in Burkholderia thailandensis

Article Title: Engineering the FK228 biosynthetic gene clusters for high-yield production

Article References: Wang, M., Jia, X., Zhang, Y., Ma, J., Zhang, Y., Fu, J., & Li, R. (2026). Engineering the FK228 biosynthetic gene clusters for high-yield production. Frontiers in Microbiology, 17, Article 1933794. https://doi.org/10.3389/fmicb.2026.1933794

Image Credits: AI Generated

DOI: 10.3389/fmicb.2026.1933794

Keywords: FK228, romidepsin, HDAC inhibitor, Burkholderia thailandensis, biosynthetic gene cluster, promoter engineering, heterologous expression, XAD-16 resin, precursor feeding, TdpR, DepR, metabolic engineering

HDACs are enzymes found in virtually all eukaryotic cells. Their principal job is to remove acetyl chemical groups from histones, the spool-like proteins around which DNA is wound. When histones are heavily acetylated, chromatin is loose and genes can be read; when HDACs strip those acetyl groups away, chromatin condenses and gene transcription is silenced. The enzymes also modify non-histone proteins, influencing protein stability and activity directly. Based on their similarity to yeast proteins, HDACs are sorted into four classes: Classes I, II, and IV are zinc-dependent enzymes, while Class III, the sirtuins, depend on the metabolic cofactor NAD+. Class I members HDAC1, 2, 3, and 8 operate within multiprotein complexes and have been linked to cancer, inflammation, and infection. Class II enzymes shuttle between nucleus and cytoplasm, with HDAC6 in particular targeting non-histone substrates such as alpha-tubulin and molecular chaperones, thereby regulating autophagy and cellular architecture.

What the new review makes clear is that HDAC family members are not uniformly disrupted in endometriosis; instead, each subtype follows its own pattern, and those patterns appear to map onto different stages and features of disease. HDAC1 is broadly upregulated. Studies of endometriotic stromal cell lines show elevated HDAC1 mRNA and protein compared with normal cells, and immunohistochemistry of human lesions reveals strong staining in ovarian, skin, and gastrointestinal implants, with ovarian lesions showing the highest levels. Experiments with exosomes derived from ovarian endometrioma tissue found that the long non-coding RNA HOTAIR upregulates HDAC1 by suppressing microRNA-761, and that these exosomes promote ectopic lesion growth in mice. Genetic work has even linked HDAC1 single nucleotide polymorphisms to endometriosis risk in South Indian women, and in mouse models HDAC1 staining intensity rises as lesions progress. In deep infiltrating endometriosis, the most aggressive form, HDAC1 levels correlate positively with the degree of fibrosis.

HDAC2 tells a more complicated story. Its expression is elevated in some cell models and in skin lesions and eutopic endometrial stroma, yet lower in ovarian and peritoneal implants, and one study found HDAC2 staining actually declines as mouse lesions progress while being significantly lower in deep infiltrating disease than in ovarian endometrioma. Functionally, however, silencing HDAC2 in endometriotic cells activates the HNF4A/ARID1A axis, curbing proliferation and invasion while promoting apoptosis, and reduces lesion size in animals. HDAC3, by contrast, is downregulated, particularly in women with infertility. In baboon models of the disease, HDAC3 expression falls as disease advances, and mice lacking HDAC3 specifically in the uterus show implantation failure and defective decidualization, tied to de-repression of collagen genes and impaired progesterone signaling. HDAC8, meanwhile, is upregulated and strongly linked to fibrosis: a specific HDAC8 activator accelerated lesion growth and scarring in mice, while the inhibitor PCI-34,051 dose-dependently suppressed lesions, eased fibrosis, and reduced pain.

Among the Class II and III enzymes, HDAC6 stands out for its dependence on lesion subtype and cellular context. A tissue microarray analysis of 168 endometriotic lesions confirmed that HDAC6 is significantly elevated in both epithelium and stroma of deep infiltrating disease and positively correlated with fibrosis, but unchanged in ovarian endometrioma. In cases lacking ARID1A expression, epithelial HDAC6 was higher still, suggesting a possible route toward malignant transformation. On the sirtuin side, SIRT1 is persistently expressed throughout the menstrual cycle in patients, whereas in healthy endometrium it appears only during menstruation. Mice engineered to overexpress Sirt1 in the uterus show implantation failure and downregulation of progesterone target genes, and the SIRT1/BCL6 axis has been implicated in the progesterone resistance that characterizes the disease. SIRT3, which governs mitochondrial energy metabolism and antioxidant defense, is reduced in ovarian endometrioma tissue, while SIRT7 is upregulated in ectopic lesions and drives a metabolic shift from mitochondrial respiration toward glycolysis through the TUFM/SIRT7 axis, promoting proliferation and lesion growth in mice.

These expression anomalies feed into five interconnected pathological pathways. HDACs promote transcription of pro-inflammatory cytokines such as TNF-alpha, IL-1 beta, and IL-6, and weaken immune surveillance that would otherwise clear ectopic cells; exosomal miR-22-3p from peritoneal macrophages, for example, suppresses SIRT1 and activates the pro-invasive NF-kappaB pathway. HDAC1 and HDAC2 silence cyclin-dependent kinase inhibitors such as p21 and p57, pushing cells through the G1/S checkpoint, while SIRT1 upregulation is linked to reduced p53-mediated apoptosis. Invasion and migration are enhanced through epithelial-mesenchymal transition: HDAC1 suppresses RHOB, HDAC2 modulates the HNF4A/ARID1A axis, and HDAC6 remodels the cytoskeleton via alpha-tubulin deacetylation. HDAC1 also activates STAT3 to promote angiogenesis, and under hypoxia HDACs bind HIF-1 to drive adhesion, proliferation, and vessel-forming genes. Finally, HDACs regulate aromatase and estrogen receptor alpha, boosting local estrogen, and mediate epigenetic silencing of progesterone receptor B, the molecular basis of progesterone resistance.

The therapeutic logic follows directly. Histone deacetylase inhibitors, already established as anti-cancer drugs, can restore acetylation and reactivate silenced genes, and because epigenetic modifications are reversible, a single inhibitor can theoretically correct multiple pathological programs at once. Preclinical evidence is accumulating rapidly. Vorinostat, or SAHA, upregulated thioredoxin-binding protein-2 in endometrial stromal cells and promoted apoptosis through the TRX/TBP-2 system; in a mouse model of endometriosis-associated ovarian cancer it reduced tumor volume by inhibiting M2 macrophage polarization and lowering IL-10. Romidepsin, a Class I-specific inhibitor, inhibited HDAC activity in endometriotic epithelial cells with an IC50 of 6.5 nanomolar, induced histone acetylation, upregulated p21, and triggered caspase-dependent apoptosis. Trichostatin A reduced lesion area by 54.3 percent and relieved hyperalgesia in mice, suppressed NF-kappaB activation in endometriotic cells, which proved more than ten times more sensitive than normal cells, and induced apoptosis via NAG-1 in primary stromal cells.

Perhaps the most unexpected player is butyrate, a short-chain fatty acid produced by gut bacteria. Mouse studies showed endometriosis depletes butyrate-producing microbiota and colonic butyrate levels; fecal microbiota transfer confirmed that stool from diseased mice promotes lesion growth while normal stool inhibits it. Mechanistically, butyrate suppresses endometrial cell proliferation by activating GPR43 and GPR109A, inhibiting HDAC activity, and upregulating RAP1GAP, and it enhances ferroptosis sensitivity of ectopic cells through the FFAR2/PPAR-gamma/PINK1/Parkin axis. Serum butyrate in patients is reduced and inversely correlated with disease stage. Valproic acid, a widely used antiepileptic, also inhibits HDACs: in rats it dose-dependently shrank lesions and eased pain, and in human cells it silenced the CYP19 aromatase promoter, cutting estrogen synthesis, while reactivating the tumor suppressor C/EBPalpha. Selective HDAC8 inhibition with PCI-34,051 in a deep infiltrating disease model cut lesion weight by nearly two-thirds, reduced fibrosis, and relieved pain, hinting that subtype-selective drugs could avoid the off-target toxicity of pan-inhibitors.

Formidable obstacles remain before any of this reaches the clinic. No HDAC inhibitor is approved for endometriosis, and the known safety profile from oncology, including myelosuppression, thrombocytopenia, gastrointestinal symptoms, fatigue, and QT prolongation, is a serious concern for long-term use in benign disease. Reproductive safety is the thorniest issue: HDACs regulate gametogenesis, follicular development, implantation, and endometrial receptivity. Valproic acid carries a well-documented teratogenic risk of roughly 10 percent congenital malformation rate with pregnancy exposure, and embryonic exposure to butyrate or valproate can impair primordial germ cell development in mice. The review’s authors therefore call for subtype-selective inhibitors guided by single-cell expression maps, lesion-targeted delivery systems such as nanoparticles or exosomes to limit systemic exposure, combination strategies with hormonal, anti-inflammatory, or anti-angiogenic drugs, and biomarkers such as histone H3 acetylation states and circulating exosomal non-coding RNAs to stratify patients. If those pieces come together, epigenetic therapy could open the first genuinely new front against endometriosis in decades.

Subject of Research: The role of histone deacetylases and their inhibitors in the pathogenesis and treatment of endometriosis

Article Title: Research Progress of Histone Deacetylase and Its Inhibitors in Endometriosis

Article References: Xiong, S., Xu, J., Pu, J., Zhao, S., & Jiang, L. (2026). Research Progress of Histone Deacetylase and Its Inhibitors in Endometriosis. Reproductive Sciences. https://doi.org/10.1007/s43032-026-02184-7

Image Credits: AI Generated

DOI: 10.1007/s43032-026-02184-7

Keywords: endometriosis, histone deacetylase, HDAC inhibitors, epigenetics, vorinostat, romidepsin, trichostatin A, butyrate, valproic acid, fibrosis, progesterone resistance, reproductive health