As a graduate student in biochemistry at the University of Wisconsin Madison, Lynne Maquat developed an appreciation for designing experiments to understand the regulated series of steps that comprise biological processes. “I’m a mechanistic biochemist by training,” Maquat says. This training put her in good stead as she transitioned from working with bacteria during graduate school to unraveling the mystery of human diseases, including βo thalassemia, which is characterized by the absence of the beta-globin protein subunit of hemoglobin, as a postdoctoral researcher.
Patients with this disease have a faulty gene, with a signal, called a premature stop codon, that tells the cell to stop making the beta-globin protein subunit before it is finished. In theory, this should have meant the cell made a partial version of the beta-globin protein. Instead, it simply didn’t make any detectable protein. It was this mystery that Maquat set out to solve. “We were able to show that in these patients, the beta-globin gene was transcribed normally, the beta-globin RNA precursor was processed normally into beta-globin mRNA, but beta-globin mRNA was unstable,” Maquat says.
When Maquat expanded her focus to include other hemolytic anemias in her own lab at Roswell Park Cancer Institute, she found a similar pattern in those diseases as well. “In sequencing the disease-associated alleles, we found that these alleles also harbored either a frameshift mutation, or a nonsense mutation that resulted in the premature termination of translation,” Maquat says. Maquat continued her research on NMD, later moving her lab to the University of Rochester Medical Center, where she founded the Center for RNA Biology: From Genome to Therapeutics. At Rochester, she pursued her unexpected findings that mRNA processing in the nucleus of a cell was mechanistically connected to mRNA translation and degradation in the cytoplasm of that cell by a complex series of regulated steps, among other differences between NMD in human cells relative to yeast.
When it came to the mechanism of these diseases, what they had in common was that the mRNA had a premature stop codon, and the transcript was unstable, lasting for a far shorter time than its counterpart in patients without the disease. It was this observation, combined with further research, that led to mechanistic insights of nonsense-mediated mRNA degradation in humans. “This eliminates mRNAs that have the potential to encode a truncated, toxic protein,” Maquat says.
As Maquat’s career advanced, she continued her work on uncovering the mechanism of NMD, which proved to be a far richer topic than she had ever imagined, one that would fuel a lifetime of discovery, innovation and curiosity. “I could have never predicted how important this pathway is for our cells when I first started working on it, but for some reason, when I saw it again with a second disease, I felt like this was something I was meant to figure out” Maquat says.
Maquat was able to show that NMD plays an important role in ensuring the quality control of mRNAs produced in a cell, targeting mRNAs with premature stop codons, whether from a mutation or abnormal processing of precursors to mRNAs, for degradation. This pathway turned out to be an integral part of how cells eliminate mistakes, which are far more common than many people realize. “We estimate that a third of the mRNAs that are made in our cells have mistakes that are cleaned up by NMD,” Maquat says.
As Maquat and her team have also shown, NMD plays an important role in allowing cells to adapt to environmental stressors. This includes the cellular inhibition of NMD to promote the death of breast cancer cells during chemotherapy, and the cellular inhibition of NMD to promote muscle-cell differentiation during muscle-cell development. "The way that the cell inhibits NMD in response to stress is different depending upon the stressor,” Maquat says.
Looking back, Maquat is grateful for the opportunity to have spent so much of her time working to understand one of the major pathways of human gene regulation. “After decades of working on NMD, we’ve learned a lot about how cells regulate normal and disease-associated gene expression, and we’ve also learned a lot of other things, too. In hindsight, we now understand mechanisms by which RNA metabolism regulates human gene expression in ways that no one would have imagined and that have offered new cellular targets for the development of therapeutics,” Maquat says.
Maquat and her team recently discovered that NMD is hyper-activated in Fragile X Syndrome, which is the most common single gene cause of austism and intellectual disability. After finding that a small molecular inhibitor of NMD normalizes much of gene expression in patient-derived neuronal cells, they are now working to develop a therapeutic for this incurable disease.