BPN14770 drug: innovative treatment for cognitive function in adult male patients with Fragile X Syndrome

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A new drug discovered through a research collaboration between the University at Buffalo and Tetra Therapeutics took a major step toward becoming a first-in-class treatment for Fragile X Syndrome, a leading genetic cause of autism.

The drug, BPN14770, achieved positive topline results in a phase 2 clinical study.

The innovative treatment improved cognitive function in adult male patients with Fragile X Syndrome.

Fragile X Syndrome – a genetic disorder for which there is no cure – is the most commonly known cause of inherited intellectual disability, according to the Centers for Disease Control and Prevention.

“We are very excited about the results of this study,” said Mark Gurney, Ph.D., founder and chief executive officer of Tetra Therapeutics.

“In addition to being safe and well tolerated, treatment with BPN14770 led to significant cognitive improvement, specifically in the language domains, and we also saw a clinically meaningful benefit in overall daily functioning.

These findings validate our approach to treating this disease through a mechanism that addresses a core deficit in the disorder.”

The research was conducted at Rush University Medical Center by principal investigator and pediatric neurologist Elizabeth Berry-Kravis, MD, Ph.D. Funding was provided by the FRAXA Research Foundation, a nonprofit dedicated to financing Fragile X Syndrome research.

Preclinical investigation of BPN14770 was completed through a collaboration between UB School of Pharmacy and Pharmaceutical Sciences faculty members James M. O’Donnell, Ph.D., dean and professor, and Ying Xu, MD, Ph.D., research associate professor, and biotechnology company Tetra Therapeutics.

The drug inhibits the activity of phosphodiesterase-4D, an enzyme that plays a key role in memory formation, learning, neuroinflammation and traumatic brain injury.

Previous studies found that BPN14770 has the potential to promote the maturation of connections among neurons, which are impaired in patients with Fragile X Syndrome.

“The collaboration with Tetra Therapeutics has been interesting and productive, combining our lab’s expertise in preclinical pharmacology and theirs in drug discovery and development,” said O’Donnell. “Seeing years of research lead to a successful trial for treatment of this serious genetic disorder is quite rewarding.”

BPN14770’s potential to improve cognitive and memory function could also translate to treatments for Alzheimer’s disease, developmental disabilities, traumatic brain injury and schizophrenia.


The expansion of the trinucleotide CGG above normal range (greater than 54 repeats) in the non-coding region of the Fragile X Mental Retardation 1 (FMR1) gene (Fig. 1) is responsible for the development of the fragile X- associated disorders in those carrying the premutation (55–200 CGG repeats), including fragile-X associated tremor/ataxia syndrome (FXTAS)(1, 2), fragile X-associated primary ovarian insufficiency (FXPOI) (3) and fragile X-associated neuropsychiatric disorders (FXAND)(4); and the presence of fragile X syndrome (FXS) in those carrying the full mutation (greater than 200 CGG repeats).

This review details the clinical presentation and neuropathology of the two entities affecting normal brain function: FXTAS, a neurodegenerative disease that commonly develops during the seventh decade of life in 40% of premutation male carriers and 16% of female carriers (5); and FXS, a neurodevelopmental disorder found in 1:7000 males and 1:11000 females (6) causing intellectual disability and Autism Spectrum Disorder (ASD) in more than half of those affected (7).

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Figure 1.
FMR1 GGG repeat length and gene expression.
Overview of the relationship between CGG repeat length (A,B,C) within the FMR1 gene (left column) and its effects on FMR1 mRNA (middle) and FMRP protein synthesis (right). A) FMR1 alleles bearing less than 45 CGG repeats are considered in the normal range. B) CGG repeat expansion into the premutation range (containing 55–200 CGG repeats) causes an upregulation in FMR1 mRNA transcripts. For most premutation cases FMRP levels (black shapes) are not altered, although some individuals may show a modest reduction. Additionally, RAN translation of FMR1 mRNA produces toxic FMRpolyG protein species (red shapes). C) CGG repeat expansion into the full mutation range (200+ repeats) causes hypermethylation of the FMR1 gene, resulting in full transcriptional and translational silencing. Figure Key: FMR1 Gene: Open reading frame indicated with solid blue, non-coding 5’ and 3’ regions indicated with shaded black pattern. CGG repeat is located in the 5’ untranslated region (red shaded in A and B, white shaded in C to represent hypermethylation of the gene). FMR1 mRNA transcripts indicated with curved blue lines. FMRP protein represented as black shapes and FMRpolyG is represented as red shapes.

Fragile X Syndrome
Clinical Aspects

FXS is caused by the lack or deficiency of the FMR1 protein FMRP in both males and females with a full mutation. With CGG repeats of >200 there is typically silencing of FMR1 through methylation. The subsequent lack of FMRP, a regulator of translation, leads to dysregulation of hundreds of proteins that affect synaptic plasticity and connectivity in the developing brain leading to intellectual disability (ID) and other clinical features of the syndrome (8–12).

Along with the variable presentation of ID, 60% of boys and 20% of girls with FXS are also diagnosed with ASD (13). The complexity of the clinical presentation is accentuated with a well reported psychiatric profile including general anxiety, social avoidance and hyperactive behaviors. These characteristics are commonly seen in those with and without the comorbid presentation of FXS and ASD (13–15).

Other comorbid conditions frequently diagnosed during childhood in FXS are seizures (16), recurrent otitis media, strabismus and obesity (17). A Prader-Willi like phenotype, with obsessive/compulsive behaviors, delayed puberty, small genitalia, hyperphagia and lack of satiation after meals leading to severe obesity, has also been described in less than 10% of boys with FXS (18–20).

The physical characteristics of FXS include an elongated face, broad forehead, high palate, prominent ears, hyperextensible finger joints, flat feet and macroorchidism (during and after puberty) (17). However, classic facial characteristics have differences inherent to age and ethnicity (21).

In addition to commonly recognized characteristics, patients can present with a variable presentation of connective tissue alterations. Their presence is attributed to FMRP dysregulation of essential components of the extracellular matrix including elastin. Phenotypic findings related to connective problems include soft velvet-like skin, joint hyperextensibility, particularly in the fingers, double jointed thumbs, flat feet with pronation, mitral valve prolapse, dilated aortic root, and occasional scoliosis (22).

After puberty, there is a tendency for improvement of the most problematic behaviors during childhood, including aggression, hyperactivity and irritability; however, behavior and comorbidities can also worsen in those exposed to exogenous neurotoxins (23). During adulthood, patients with FXS seem to have an increased risk of hypertension, obesity, gastrointestinal disorders, parkinsonism, mood disorders, anxiety and in some cases dementia (24, 25).

However, patients with FXS have a normal life span. The female phenotype differs from the males since they have the benefit of an unaffected X chromosome. Their cognition includes 30% with an IQ less than 70 (intellectual disability), 30% with an IQ in the borderline range (70–79) and 30% with an IQ in the normal range (above 80), but anxiety and attentional problems can occur in all groups. (7).

Diagnostic Criteria
The diagnosis of FXS can only be confirmed using genetic testing. Southern blot analysis reports an expansion of the CGG trinucleotide number greater than 200 repeats in the 5’ untranslated region in the FMR1 gene located on the X chromosome. The result is a full methylation of the gene and its subsequent silencing (17).

However, in some cases individuals can present with mosaicism, showing variability in CGG allele size and in methylation patterns within and between different cell lines. This particular genotype benefits the clinical phenotype by improving both the cognitive and behavioral profiles of males and female patients with FXS (26).

Cell and Molecular Pathology
The origin of all changes that lead to the molecular, pathological and clinical symptoms shown by individuals with FXS is the loss of functional FMRP (Fig. 1C). While CGG expansion leading to hypermethylation and functional silencing of FMR1 is by far the most common genetic cause of FXS, loss of a functional FMRP due deletions or point mutation can also occur (27).

While FMRP expression is ubiquitous, it is expressed at highest levels in the brain and testes (28, 29). FMRP expression has been detected in neurons, astrocytes, microglia, and oligodendrocyte precursor cells (11, 30, 31), and it is largely localized in the cytosol of neurons, in close association with ribosomes of the endoplasmic reticulum, and at high levels in dendritic spines (32).

FMRP can also appear in cytoplasmic granules that are transported to dendrites, axons, and pre-synaptic terminals in some neurons (33–38), enabling localized translation (39–41). FMRP granules are also present in axon growth cones during development, likely playing a role in axon guidance, circuit formation, and synaptogenesis (36, 38, 39).

FMRP is an RNA binding protein that regulates translation of numerous associated mRNAs. FMRP is largely considered a translational repressor that suppresses translation initiation and elongation of nascent proteins (reviewed extensively in 42, 43). FMRP also binds and regulates miRNA and miRNA machinery (43, 44), thus exerting translational control through a separate but complimentary molecular mechanism.

Accordingly, due to a loss of FMRP mediated translational repression, there is a modest (10–20%) but functionally significant elevation in FMRP-regulated proteins in FXS patients and in FMR1 KO mice (43).

Some key FMRP regulated mRNAs/proteins include second messenger proteins involved in mGluR1 and mGluR5 signal transduction (EIF4E and S6K), (34, 35, 45), GABAA and GABAB receptor subunits (46–48), numerous voltage gated ion channels (49–51), Bone morphogenic protein receptor 2 (BMPR2) (52), matrix metalloproteinase 9 (MMP9) (53), and amyloid precursor protein (APP) (54, 55). Many of these affected mRNA/protein species play a direct role in synaptic transmission.

FXS dendrites in the cortex and hippocampus show increased spine density and size, and reduced spine maturity (36, 56, 57). FMRP mediated suppression of the FMPR2-Cofilin pathway is necessary for normal dendrite formation and maturation – and accordingly disruption in this pathway through a loss of FMRP contributes to dendritic abnormalities in FXS.

Suppression of MMP9, which is upregulated in the FMR1 KO mouse, normalizes dendritic spine morphology and synapse formation (58). Additionally, loss of FMRP leads to excess soluble APP levels, which also contributes to a lack of dendrite maturation (55). Normalizing APP levels in FMR1 KO mice rescues alterations in synaptic spines, LTP deficits, and reduces audiogenic seizures (54).

The excitation/inhibition imbalance hypothesis has been proposed to explain how cellular and circuit-level alterations in excitatory/inhibitory signaling may lead to clinical symptomology in idiopathic ASD (59). Given the tremendous overlap in FXS and ASD symptomology, as well as the high rates of co-diagnosis in patients with FXS, this hypothesis is believed to largely apply to FXS as well.

One way in which excitation and inhibition balance may be disrupted in FXS is through dysregulation of glutamatergic and GABAergic signal transduction. FMRP binds and regulates second messenger proteins that mediate metabotropic glutamate receptor I family (mGluR1 and mGluR5) signal transduction.

When FMRP is absent, there is increased phosphorylation of two such downstream effectors – eukaryotic translation initiation factor 4E (EIF4E) and ribosomal protein S6 kinase (S6K), which leads to excess translation of mRNAs that are typically bound and regulated by FMRP (45, 60, 61).

There is an increase in synaptic long-term depression (LTD) in FMR1 KO mice, which is believed to be related to dysregulation in mGluR1 signaling. Another consequence of mGluR1 signaling dysfunction in FXS is a reduction in inhibitory retrograde endocannabinoid signaling by mGluR1+ dendrites (7, 62), which likely leads to increased glutamatergic signaling from upstream pre-synaptic glutamatergic neurons and increased excitatory tone. Deficits in GABA signaling have also been characterized in the FMR1 KO mouse, suggesting that a lack of inhibitory GABAergic tone could also lead to hyperexcitability in the FXS CNS.

Neuropathology
Structural MRI studies have identified a pattern of regional volume alterations in patients with FXS, characterized by an enlargement in the caudate nucleus (63–67) and lateral ventricles (63, 64, 66), and a reduction in cerebellar vermis (63, 66, 68).

Alterations in caudate and cerebellar vermis appear as early as one year of age (69), and persist into adulthood (63, 66). There also appears to be a moderate and region-specific alteration in cortical lobe grey matter volume, with modest reductions in temporal (63, 67) and frontal lobes (63, 65), and a modest increase in the parietal (63, 65) and occipital lobes (63, 65).

Although less consistent, volumetric reductions of amygdala (63, 70) and enlargement of hippocampus (71) have sometimes been observed. White matter volumetric alterations have also been detected, including increased white matter volume in the septal fornix (67), increased brainstem-hippocampus tract and cingulate-corpus callosum tract volume (65), and decreases in frontal lobe (65) and cerebellar white matter (67).

Most human FXS structural abnormalities are not recapitulated in the FMR1 KO mouse – striatal volume is unaltered (72) or reduced (73, 74), and there is no change in cerebellar vermis volume (72–74) or cortical lobe volume (72, 73).

The neuropathological correlates of these structural abnormalities in the human FXS brain are poorly characterized – there only exist a handful of such studies and all typically have very small sample sized (n ≤ 3 for all). The earliest and most well characterized finding demonstrated that there are alterations in dendrites and synaptic spines in the postmortem FXS brain.

More specifically, FXS cortical tissue in the occipital and temporal cortices have more dendritic spines (56), and these spines are longer and immature (57, 75). Ultrastructural analysis also shows a reduction in synaptic size at dendritic contacts (57). Cerebellar Purkinje cells are reduced in number (76, 77) and in dendritic arbor complexity (76), and the hippocampal structure presents with restricted hyperplasia in the CA1 region (76).

Structural and functional MRI studies have both been able to correlate abnormal activation patterns with specific symptom domains in FXS patients. For example, intellectual functioning, as indicated by IQ, is inversely correlated with caudate volume (65) and positively correlated with cerebellar vermis volume (63).

reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7027994/

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