Key Proteins Less Abundant in Brain Tissue of Late-stage FXTAS Patients

An analysis of proteins in brain tissue from men with advanced fragile X-associated tremor/ataxia syndrome (FXTAS) found an overall decrease in the abundance of key proteins compared with samples from those without this disease, a study reported.

These findings provide insight into mechanisms of FXTAS, and identified potential biomarkers for disease progression and treatment, the researchers said. 

The study, “Human Cerebral Cortex Proteome of Fragile X-Associated Tremor/Ataxia Syndrome,” was published in the journal Frontiers in Molecular Biosciences.

FXTAS is a neurodegenerative disorder characterized by a series of motor and cognitive difficulties, mainly affecting men over 50 years of age. 

FXTAS is caused by the expansion of between 50 and 200 CGG repeats in the FMR1 gene, which encodes the fragile X mental retardation protein (FMRP). (Of note, G stands for guanine and C for cytosine, two of the four building blocks of DNA.)

In contrast to fragile X syndrome, in which the FMR1 gene has more than 200 CGG repeats leading to FMRP deficiency, recent evidence suggests that FXTAS mutations lead to an overproduction of FMR1 messenger RNA (mRNA), the molecule that carries the DNA instructions to make proteins, and normal or slightly lower levels of FMRP. 

To learn more about how FXTAS mutations lead to disease, researchers at the University of California Davis School of Medicine investigated overall protein abundance in brain tissue from men who died with end-stage FXTAS.

Their analysis included tissue from eight men with late-stage FXTAS (average age, 82) and six men (average age, 69) without any known neurological disease, serving as controls. Female cases were excluded to avoid the variable effects of two X chromosomes.

As expected, the average CGG repeat length in these FXTAS patients was 104.1, and 25 in the control group. 

Using mass spectrometry analysis to identify and quantify proteins in complex mixtures, 414 proteins were identified that were produced at levels that differed from controls — called differentially abundant proteins (DAPs). 

After adjusting for age between the two groups, the analysis singled out four proteins produced at significantly higher levels in patients, and 12 proteins with significantly lower levels. “The majority of proteins with significant levels of differential expression are biased toward decreased abundance in FXTAS,” the researchers wrote.

The greatest decreases were observed for the proteins TNC, CD38, and PSAT1, “proteins typically increased in other neurodegenerative diseases,” the team added. 

Proteins with the greatest increased abundance included novel neurodegeneration-related proteins and the small ubiquitin-like modifier 1/2 protein (SUMO1/2), involved in regulating a variety of cellular processes.

In an analysis of the 303 proteins with lesser abundance, 232 DAPs were associated with  specific functions, including protein binding, molecular breakdown (hydrolysis), RNA binding, DNA building block (nucleotide) binding, and gene regulation. Of the 122 proteins with greater abundance, 47 proteins were associated with functions such as RNA binding, oxidation, iron-binding, protein cofactor binding, antioxidant activity, and programmed cell death.

Next, protein analysis results were compared with a list of known FMRP-associated mRNAs to determine whether they were differentially expressed as proteins in FXTAS tissues. 

Of the 54 mRNAs that encode for proteins known to associate with FMRP, 37 (69%) were identified in FXTAS, which included a decrease in mRNA for the RHOA protein of 11%, a 12% decrease in mRNA for the RAC1 and CTNNB1 proteins, and a 50% increased abundance in mRNA for KCNC1 protein, related to brain cell activity. 

Of the 53 proteins known to associate with FMRP directly, 25 were identified in the FXTAS tissues. However, none of the differences reached statistical significance upon further analysis. 

As altered calcium signaling is known to be associated with FXTAS, 262 (37%) proteins related to calcium were identified in FXTAS. However, none demonstrated significantly different abundance compared with controls. 

Two proteins with nearly significant lower levels included VCAN and DSG1. Aside from binding calcium, these proteins function in cell-to-cell binding, membrane and cellular trafficking of molecules, and cell growth signaling.

Finally, the team compared DAPs found in FXTAS to those previously identified in tissue from people with Alzheimer’s and Parkinson’s disease, two other neurodegenerative disorders.

This analysis revealed PSAT1 and TNC were DAPs shared between Alzheimer’s disease and FXTAS, while RNF214 and PSAT1 were common to Parkinson’s disease and FXTAS. Proteins PSAT1, TNC, and RNF214, however, were decreased in abundance in FXTAS relative to these other diseases. A common differential production of PSAT1 alone was evident among all three disorders. 

“MS [mass spectrometry] analysis of male human cortex detected differential abundance of at least 16 proteins in late-stage FXTAS patients compared to controls,” the researchers concluded. “Furthermore, many DAPs shared among other neurodegenerative disorders displayed opposite directional abundance in FXTAS when compared to the same proteins” in Alzheimer’s and Parkinson’s. 

“These DAPs provide unique insight into mechanisms of FXTAS pathology, potential therapeutic targets, and candidate biomarkers for disease progression,” they added.