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Protein-Based Biomarker Could Enable Precision Medicine For Depression

By Deborah Borfitz

January 25, 2022 | It has proven exceedingly difficult to identify reliable and reproducible genetic markers for depression, but it now appears possible to efficiently leverage a biochemical hallmark to create a blood test, according to Mark Rasenick, Ph.D., professor of physiology and biophysics and psychiatry at the University of Illinois at Chicago and as well as co-founder and chief scientific officer of Pax Neuroscience.

The basis of the test is the four-decade-old discovery that depression is tied to low levels of the intracellular molecule adenylyl cyclase, made in response to neurotransmitters such as serotonin and epinephrine, which is now understood to be a problem in the movement of an intermediary protein called Gs alpha (Gsα). It is getting stuck in fatty sections of cell membranes called “lipid rafts.”

With their newly published proof-of-concept study in Molecular Psychiatry (DOI: 10.1038/s41380-021-01399-1), the researchers have moved one step closer to developing an easy-to-measure, protein-based biomarker test to diagnose depression and the efficacy of drug therapy in individual patients, Rasenick says. Their goal is nothing short of precision medicine for depression.

In the study, which involved 108 subjects (including 59 healthy controls), the biomarker effectively differentiated between the 49 participants with major depressive disorder and the controls by measuring the coupling between Gsα and adenylyl cyclase in human platelets. Nineteen of the depressed participants received antidepressants for six weeks and those who responded to the treatment had significantly higher adenylyl cyclase responses at the end of the study than the non-responders.

This is the first-ever test that might indicate therapeutic response with a single biomarker, says Rasenick, who is also a research career scientist at Jesse Brown VA Medical Center. The ideal expectation is that the test could be used to determine if antidepressant therapies are working as soon as one week after beginning treatment—which, not coincidentally, is the average circulation lifespan of platelets in the blood.

If that pans out, it could increase the market for antidepressants, Rasenick continues. Estimates from the Sequenced Treatment Alternatives to Relieve Depression (STAR*D) trial of the National Institute of Mental Health indicate that only about one-third of people treated with antidepressants get better after the first round of therapy and roughly the same proportion were treatment-resistant after 14 weeks of therapy.

“There is no indication that there is anything biologically different about [the nonresponders] so … perhaps it is because failure begets failure and both the doctor and patient kind of give up,” says Rasenick. “It doesn’t mean these people remain profoundly depressed; it just means they didn’t achieve remission.”

Depression is typically diagnosed by primary care physicians, he notes, and they could also utilize a test like this to differentiate it from a multitude of other conditions, including diabetes and hypothyroidism, which can present with some of the same symptoms.

With further research, Pax Neuroscience hopes to develop a marketable screening test that might not only confirm a biological underpinning for a depressive disorder but predict response as well.

The Backstory

Much effort has gone toward creation of a blood test for diagnosing depression without making a lot of headway, says Rasenick. The focus has been on genetics, including use of whole genome sequencing, gene expression profiling, and messenger RNA. Several studies have uncovered genes of interest, but there is little overlap among them.

Two recent genome-wide association studies using very large databases, and published in high-profile journals, shed mixed light on the topic, Rasenick continues. One implicated 19 genes and the other identified 16 genes as bellwethers of how well individuals respond to antidepressants—and there was zero overlap.

As Rasenick tells it, the alternative tactic being used here took flight after he met M.D./Ph.D. student David Menkes in 1983 and they began talking about the theory that antidepressant action was tied to beta-adrenergic receptors. It didn’t seem feasible to look at beta-adrenergic receptors, since their activity cycle would be erased if they started grinding up mouse brains, so Rasenick suggested they look instead at G-protein coupling to the enzyme adenylyl cyclase. This resulted in a 1983 paper published in Science (DOI: 10.1126/science.6849117).

“It has taken until now to really parse that out and understand what is happening at the biochemical level to turn that into what could very well be a trackable blood test that can be done in high throughput,” Rasenick explains. Step one was to demonstrate in rats that different classes of antidepressant drugs all prompt an increase in adenylyl cyclase by the G-protein, G alpha, as reported in Molecular Pharmacology (1989, PMID: 2511428).

When the analysis method was further refined, Rasenick and his colleagues learned the underlying issue was that Gsα was getting stuck in the lipid raft and demonstrated it using cellular models that were much easier to manipulate, he continues. The finding was then verified in a post-mortem investigation in humans, as reported in the Journal of Neuroscience (2008, DOI: 10.1523/JNEUROSCI.5713-07.2008).

In 2010, Pax Neuroscience launched with a handful of patents to begin the work of turning a hypothesis into a market-ready blood test. The company’s name references the Latin word for peace, Rasenick says, adding that he considers the stigma against depression and other forms of mental illness an issue of social justice.

The chief product of Pax Neuroscience is MoodMarkDx, the biomarker-based blood test currently done via Western blotting. But the test could be done at high throughput in a standard 384- or 1536-well format if the assay was licensed and blood samples were sent by different facilities to a central location for processing.

Drug companies in the past have used MoodMarkDx on cultured cells as an inexpensive way to help with go/no-go decision-making on their pipeline products, Rasenick says. The immediate goal of Pax Neuroscience now is to find the funds to do a much larger test to confirm the assay can reliably separate depressed from nondepressed individuals as well as quickly determine who is responding to antidepressant treatment—a critical regulatory step that would allow its use beyond facilities regulated under the Clinical Laboratory Improvement Amendments of 1988 (CLIA).

Preliminary data suggests that Gsα gets stuck in lipid rafts due to modification of the protein, he notes. Data from cells and post-mortem analysis in rats also suggest that at least one of the anchors for Gsα is modified in depression such that it no longer restrains the protein.

The better those molecular details are understood, the more effective researchers expect they will be at developing newer assay methods—and perhaps even novel compounds with antidepressant activity.

Current Screenings

The reason none of the genetic approaches to diagnosing depression have borne fruit may be because depression is a phenotype where any two patients may have few symptoms in common, says Rasenick. Multiple different etiologies could result in what is generically called depression.

While it is unknown if more Gsα in lipid rafts causes depression, he adds, “it certainly looks like it is a marker that is consistent with depression. And for a biomarker, do we really need to have it relate to the actual cause of the disease… [if] many different factors converge on a certain point [where the Gsα accumulation happens]?”

Primary care physicians would presumably welcome such a tool. Depression screening is currently done using a question such as whether patients feel depressed or have lost pleasure in doing things that once brought delight. “It is not remotely quantifiable.”

Psychiatrists and psychologists ask an entire panel of questions to derive a number for scoring depression, but the exercise is subjective, continues Rasenick. In contrast, the MoodMarkDx test has been shown to produce less than 10% week-to-week variation from blood draws on the same subject prior to the initiation of treatment.

Many depression specialists will order cytochrome P450 blood tests to help determine how patients’ metabolize a drug before prescribing it, he notes, which of course says nothing about how well an antidepressant might work. About 80% of antidepressants are in any case written by primary care physicians, including gynecologists who often serve in that role for women, where such screenings are not typically done but on-site blood draws are commonplace.

Clinical Trials

The impact of the novel biomarker test could be potentially huge. One in five people either were, are, or will be depressed at some point in their life, says Rasenick. At present, it is estimated that only about half of all individuals who are depressed are being treated—in part because it is viewed as an “all in their head” weakness rather than an illness detectable in the blood. Social stigma against mental illness plays a significant role as well, he adds.

Treatment wouldn’t necessarily require drugs, Rasenick says. “In fact, we predict that people who go to psychotherapy will show the same change [as those treated with antidepressants].” After all, people variably do and don’t get better when exposed to similar quantities of the same drugs.

Rasenick reports that he and his colleagues (notably, Pax Medical Director Steven Targum, M.D.) are now in discussions with a company developing a new antidepressant about using their screening assay in soon-to-launch clinical trials for the drug. Standard validated psychometric evaluations will allow a comparison between treatment responders from nonresponders relative to their MoodMarkDx results and the trajectory of clinical ratings.

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