FMN1

FMN1 is a formin-family protein whose locus is essential for limb and kidney development and which contributes to cancer cell mechanics (PMID:9848078, PMID:34559979). The mouse Fmn gene spans ~400 kb with at least 24 exons and produces differentially processed transcripts in embryonic limb bud, adult brain, and kidney, including abundant circular RNAs generated by back-splicing; gene-targeted deletion of exon 4 or 5 abolishes circular transcript production without eliminating linear isoforms and produces variably penetrant renal agenesis, implicating these circular transcripts in kidney development (PMID:9848078, PMID:9119367). In glioblastoma cells, FMN1 protein localizes to the microtubule lattice and promotes mechanical cohesion, increasing cell stiffness, traction forces on substrate, and stochastic invasive migration; ectopic FMN1 raises these fitness parameters in both cancer and normal cells and its expression is induced by mechano-chemical signals (PMID:34559979). In humans, homozygous deletion of FMN1 causes a limb deformity and renal phenotype identical to the mouse limb deformity (ld) model (PMID:20610440). Beyond these findings, the biochemical actin- or microtubule-regulatory activity of the FMN1 protein has not been reconstituted in the available corpus.

1. 1997 Medium

   Establishing the architecture of the Fmn locus was the prerequisite for relating its transcripts to developmental phenotypes, defining a large multi-exon gene with tissue-specific isoforms.

   Evidence Genomic library screening, clone characterization, and probe hybridization mapping the mouse Fmn gene

   PMID:9119367

   Open questions at the time

   • No protein product characterized functionally
   • Roles of individual isoforms not assigned
   • No molecular activity defined for the encoded protein
2. 1998 Medium

   Whether the locus's non-linear transcripts had a developmental function was unknown; exon-specific deletion showed that circular Fmn RNAs are required for normal kidney development independent of linear mRNA.

   Evidence RT-PCR, ribonuclease protection, and exon 4/5 knockout mice with renal phenotyping

   PMID:9848078

   Open questions at the time

   • Mechanism linking circular transcripts to kidney development is correlative
   • Molecular targets of the circular RNAs unidentified
   • Penetrance is variable
3. 2010 Low

   Whether the mouse phenotype translated to humans was open; a homozygous human FMN1 deletion reproduced the limb deformity/renal phenotype, establishing FMN1 loss as causally relevant in humans.

   Evidence Array CGH genomic deletion mapping with clinical phenotyping, compared to the ld mouse model

   PMID:20610440

   Open questions at the time

   • Deletion also disrupts GREM1 regulatory elements, confounding attribution to FMN1 alone
   • No biochemical or cellular experiment on FMN1 protein performed
   • Mechanism of developmental defect not resolved
4. 2018 Low

   To place FMN1 in a developmental signaling context, genetic evidence linked FMN1 loss-of-function and point mutations to disrupted BMP-antagonistic signaling in limb patterning.

   Evidence Clinical genetics and mutation analysis with pathway review of WNT/BMP cross-talk

   PMID:30569497

   Open questions at the time

   • Pathway placement is inferred from genetics, not direct biochemistry
   • No demonstration that FMN1 protein acts on BMP components
   • Locus-level confounding with GREM1 persists
5. 2021 Medium

   The cell-biological function of FMN1 protein was undefined; mechanobiology in glioblastoma showed FMN1 acts from the microtubule lattice to confer mechanical cohesion and invasive fitness.

   Evidence Live-cell imaging, traction force and atomic force microscopy, gain/loss-of-function, and patient sample validation

   PMID:34559979

   Open questions at the time

   • Microtubule-based mechanism not reconstituted in vitro
   • Direct molecular activity (actin nucleation, MT binding) not biochemically defined
   • Single-lab mechanism

• The biochemical activity of the FMN1 protein and how it bridges its microtubule-associated mechanical role with its developmental requirement in limb and kidney remain unresolved.
• No in vitro reconstitution of FMN1 enzymatic/cytoskeletal activity
• Mechanistic connection between circular RNA function and protein function unclear
• Direct molecular partners of FMN1 not defined