Despite considerable efforts, there are still no valid, reliable and feasible peripheral/blood biomarkers that can diagnose MDD, classify MDD subtypes and measure treatment response, even in adult-onset MDD.23
Our research is unique in the attempt to discover a biomarker panel for early-onset MDD, a more severe disorder than adult onset. Our approach is also innovative in combining theoretical and atheoretical strategies in animal models of depression to identify a panel of transcripts in the blood that did distinguish subjects with early-onset MDD from those in the ND group. Moreover, a partly overlapping set of transcripts differentiated youths with MDD-only from those having MDD with comorbid Anxiety Disorders, providing the first panel of blood transcripts that might be useful for detecting these endophenotypes.
Previous research to identify panels of blood biomarkers has focused on serum factors46, 47
or on blood expression biomarkers using the complex approach of Convergent Functional Genomics developed by Le-Niculescu et al.22
The strategy recently being used to identify serum-based markers is to select them from biochemical domains previously associated with MDD.47
This approach has the advantage of focusing on the biologically functional protein end points. The Convergent Functional Genomics approach combined brain and blood expression data from a pharmacogenomic animal model with human blood and post-mortem expression data and human genetic linkage/association data. This elegant assemblage of multiple levels of information has the advantage that the markers thus selected could yield information about genetic vulnerability and related transcriptomic changes.
Our approach is not a duplication of these efforts described above, but rather one that is based on the atheoretical, or unbiased, exploratory exploitation of two theoretical animal models of depression. These models comprised the genetic and the environmental (chronic stress) components of MDD etiology, and thereby these candidate biomarkers for MDD highlight genetic vulnerability factors and their transcriptomic consequences, in addition to biological costs of a repeated stressor. The strengths of our approach include the uniqueness of the genetic animal model we employed, and the selection process of candidate markers. Namely, transcripts were selected as candidates when they showed significant same-directional differences in any of the brain regions examined and in the blood of the genetically ‘depressed' WMI compared with their genetically very close control, the WLI. This allowed us to verify that the candidate transcripts were relevant to brain functions and, therefore, some of these transcripts can be regarded as prospective novel drug targets. Another strength is that our chronic stress blood markers were compiled from a study of four genetically diverse strains, thereby providing a rather narrow, but powerfully informative, set of transcripts. These transcripts represent generalizable responsiveness to stress and signify the notion that stress marks all organisms, regardless of their vulnerability or resilience.
The first biomarker panel we define consists of 11 transcripts that differentiated youths with MDD from those without any disorder. These candidate markers are derived almost equally from the genetic and chronic stress models of depression. The genes expressing these transcripts belong to three broad functional categories: those involved in transcription, neurodevelopment and neurodegeneration. Genes with transcriptional regulatory functions include MAF
, which encodes a DNA-binding, leucine zipper-containing transcription factor, and the cytoplasmic cerebellar degeneration-related protein 2 antigen (CDR2), which harbors a helix-leucine zipper motif and interacts specifically with c-Myc.48
Genes that regulate, modify or interfere with neurodevelopment include RAPH1
(Ras association and pleckstrin homology domains 1
, also known as LPD
), which is intimately involved in proper neuronal migration.49
Tyrosine phosphatase PTP4A3,
also called PRL-3
, has oncogenic activity, but has also been reported to promote cell migration.50
encodes an enzyme, cytidine monophosphate N
-acetylneuraminic acid synthetase (also known as CMP-Neu5Ac synthetase), which regulates brain sialylation levels and, therefore, very likely affects brain development.51
Cell adhesion molecule 1
, also known as immunoglobulin superfamily, member 4
), is a synaptic adhesive molecule involved in neural cell adhesion processes and synaptogenesis.52
is thought to contribute to depressive-like behavior in a recent mouse genetic study,53
and has also been linked with social impairments and anxiety-like behavior.54, 55
A group of candidate biomarkers have known functions in processes that mediate neuronal damage. PSME1
(proteasome activator subunit 1
, also called PA28a
) activates the proteosomal hydrolysis of intracellular proteins. Inadequate glucose supply causes damage of neuronal cells, and PSME1
is highly responsive to hypoglycemic environment in neurons.56
, also called MIRL
or protectin, encodes a single-chain, glycosylphosphatidylinositol-anchored cell surface protein structurally homologous to snake venom neurotoxins.57
The protein has complement-inhibitory properties, but its capability to mediate complement-mediated damage to neurons is also recognized.58, 59, 60, 61
gene encodes catalase, a key antioxidant enzyme that serves as a defense against oxidative stress. Chronic unpredictable stress decreases CAT
expression in the mouse cerebral cortex and hippocampus;62
these effects may be mirrored by the decreased expression of this gene in the blood of both the chronic stress model and subjects with MDD. The expression of AMFR
(autocrine motility factor receptor
), otherwise known as GP78
, which encodes an endoplasmic reticulum membrane-anchored ubiquitin ligase, is increased by accumulation of neurodegenerative disease proteins, such as mutant huntingtin,63
SOD1 and ataxin-3.64
This increase in AMFR
expression may represent a protective response to enhance the removal of these disease proteins, and suggests that a decrease of AMFR/GP78 expression, as we found in MDD, might make the organism more vulnerable to these diseases.
The second panel consists of 18 transcripts that distinguished youths who had MDD with comorbid anxiety disorders from those with MDD alone. Only six gene transcripts were in both panels, and thus the majority of these markers are unique to this diagnostic category. Furthermore, the panel differentiating MDD youths with and without comorbid anxiety disorders had a substantially higher number of genes derived from the chronic stress model than from the genetic model. These latter observations support the long-standing clinical impression that MDD with comorbid anxiety disorders is a unique phenotype. Moreover, it is possible that there are different etiologic factors involved in this endophenotype, for example, exposure to chronic stress particularly at the highly stressful period of adolescence. Most of these transcripts have no known function relevant to MDD or brain, including three of the four ‘genetic depression model' markers: FAM46A (family with sequence similarity 46, member A), NAGA (N-acetylgalactosaminidase) and ZNF291/SCAPER (S-phase cyclin A-associated protein in the ER). Among the ‘chronic stress model' markers, AHSP (α hemoglobin stabilizing protein), DGKA (diacylglycerol kinase, α), SLC4A1 (solute carrier family 4, anion exchanger, member 1) and KIAA1539 have either no brain-related functions to date or no known function.
Genes whose transcripts differentiated MDD with and without comorbid anxiety and which have known functions relevant to MDD or stress encode proteins involved in immunoregulation or neurodegeneration. IRF3
(interferon regulatory factor 3
), identified from the chronic stress animal model, has been established previously as a chronic psychological stress-responsive gene in human peripheral blood cells.65
, an intracellular Toll-like receptor, is an innate immunity receptor that activates inflammation and adaptive immunity. TLR7,
or its agonists, induce inflammatory responses in the periphery and in the brain,66
and greater expression of TLR7
are associated with poor functional outcome in ischemic stroke patients.67
Polymorphism in GCLM
(glutamate cysteine ligase modulatory subunit
), the first rate-limiting enzyme of glutathione synthesis, has been associated with depression and schizophrenia, although this association has not been confirmed.68
Nevertheless, glutathione deficits have been observed in several neurodegenerative and psychiatric disorders including Alzheimer's, Parkinson's or Huntington's diseases69
as well as in schizophrenia. GGA3
(golgi-associated, gamma adaptin ear containing ARF binding protein 3
) is involved in the pathogenesis of Alzheimer's disease.70
It is interesting to note that subjects with lower levels of GGA3
may be at risk for developing Alzheimer's disease,70
and that MDD is thought to be a triggering or precipitating factor in Alzheimer's disease. Several of the blood candidate markers whose expression differed between MDD and controls or between MDD subtypes are involved in different neurodegenerative processes, suggesting that MDD, and perhaps other psychiatric illnesses, can lead to neurodegeneration. These markers, therefore, could present a generic ‘neurodegenerative fingerprint' in the brain and blood. Alternatively, as Alzheimer's, Parkinson's and other neurodegenerative illnesses are known to show symptoms of depression before their regular presentation, depression might be a precipitating factor in these illnesses indicated by the presence of these biomarkers.
When we explored associations between CTQ Total scores and transcript levels, we found that the transcripts with potential clinical significance were all derived from the chronic stress animal model. These four transcripts, CMAS, IRF3, PSME1 and PTP4A3, share no close connections in their functions, but their altered expressions are likely to represent long-term consequences of maltreatment, as these youths may have experienced maltreatment for as long ago as 8–10 years before the time of data collection.
The current study's limitations lie in its relatively small samples sizes and the limited number of target transcripts we were able to pursue. Additionally, we recognize that the animal models only mirror some aspects of early-onset MDD and, therefore, the markers derived from them cannot be all inclusive. Nonetheless, the purpose of this type of study in biomarker research is to determine if there are sufficient data to proceed to a full-size study of the candidate transcripts in a large representative sample of young people with and without MDD.
In summary, we have taken a novel approach to identifying potential peripheral biomarkers for early-onset MDD. The main goal of this pilot study was to determine if blood transcripts from both a genetic and a chronic stress animal model of depression could lead to candidate blood biomarkers for early-onset MDD in human subjects. The pilot data presented here suggest that our approach leads to a clinically valid diagnostic panel of blood transcripts that can differentiate early-onset MDD from controls and MDD with from MDD without anxiety. The next step is to test our findings in a large sample of youths with MDD, comparing them with youths without any psychiatric disorder and youths diagnosed with other psychiatric disorders. Eventually, the effect of treatment on validated biomarkers panels can be established, allowing for further individualization of MDD treatment strategies.