1969-1978

(1) Lubs, H. A. A Marker X Chromosome. Am J Hum Genet1969, 21 (3), 231–244. (LANDMARK PAPER)

This paper was the first to identify a secondary constriction site on the X chromosome that they believed to be associated with intellectual disability. This second constriction site ended up being the fragile site that holds the FMR1 gene defective in Fragile X Syndrome.

(2) Harvey, J.; Judge, C.; Wiener, S. Familial X-Linked Mental Retardation with an X Chromosome Abnormality. J Med Genet1977, 14 (1), 46–50.

This paper studied four families with intellectually disabled individuals in several generations. Cytogenetics revealed an abnormal X chromosome in all affected males and some non-intellectually disabled female carriers. This was one of the first papers to confirm that the abnormal X chromosome was passed on through generations.

1979-1988

(3) Sutherland, G. R. Heritable Fragile Sites on Human Chromosomes I. Factors Affecting Expression in Lymphocyte Culture. Am J Hum Genet1979, 31 (2), 125–135. (LANDMARK PAPER)

This paper studied the effects of tissue medium composition on heritable fragile sites. One of those sites was the fragile site in Fragile X Syndrome. While this paper did not explicitly talk about Fragile X Syndrome, it did study the site affected on the X chromosome that leads to Fragile X Syndrome and found folic acid, folinic acid, and thymidine can inhibit its expression and pH can also affect expression.

(4) Sutherland, G. R. Heritable Fragile Sites on Human Chromosomes II. Distribution, Phenotypic Effects, and Cytogenetics. Am J Hum Genet1979, 31 (2), 136–148.

This paper documented families and individuals that had various fragile sites and studied the cytogenetics and phenotypes of the fragile sites. They found that the fragile site in Fragile X Syndrome is sometimes only expressed in a small number of metaphases. They also observed that an intellectual disability is only observed in hemizygotes and not homozygotes.

(5) Howard-Peebles, P. N.; Stoddard, G. R.; Mims, M. G. Familial X-Linked Mental Retardation, Verbal Disability, and Marker X Chromosomes. Am J Hum Genet1979, 31 (2), 214–222.

This paper studied cytogenetics and verbal evaluations of families with X-linked intellectual disabilities. They found that not all families with a non-specific X-linked intellectual disability have the marker X chromosome and affected individuals also had a generalized language disability that could be from poor tongue control.

(6) Mareni, C.; Migeon, B. R. Fragile X Syndrome: Search for Phenotypic Manifestations at Loci for Hypoxanthine Phosphoribosyltransferase and Glucose-6-Phosphate Dehydrogenase. Am J Hum Genet1981, 33 (5), 752–761.

This paper studied cells from individuals with X-linked intellectual disability to try and determine the cause of Fragile X Syndrome. They analyzed glucose-6-phosphate dehydrogenase and hypoxanthine phosphoribosyltransferase, which are two enzymes within the fragile site, and found that they are most likely not involved in Fragile X Syndrome, but other loci near them and an increased susceptibility to mutations may be the cause.

1989-1998

(7) Oberlé I.; Rousseau F.; Heitz D.; Kretz C.; Devys D.; Hanauer A.; Boué J.; Bertheas M. F.; Mandel J. L. Instability of a 550-Base Pair DNA Segment and Abnormal Methylation in Fragile X Syndrome. Science1991, 252 (5009), 1097–1102. https://doi.org/10.1126/science.252.5009.1097. (LANDMARK PAPER)

The authors of this paper elucidated the methylation patterns that affect the expression of Fragile X Syndrome. Males who carry the mutation pass on a 150-400 bp insertion to their daughters unchanged or with slight differences in the size of the insertion. In these males the mutated allele was unmethylated, while the allele on the inactive X chromosome of their daughters was methylated. The allele was fully methylated in males with Fragile X Syndrome. These findings provided a new, accurate, easier way to diagnose Fragile X Syndrome carriers.

(8) Verkerk, A. J. M. H.; Pieretti, M.; Sutcliffe, J. S.; Fu, Y.-H.; Kuhl, D. P. A.; Pizzuti, A.; Reiner, O.; Richards, S.; Victoria, M. F.; Zhang, F.; Eussen, B. E.; Ommen, G.-J. B. van; Blonden, L. A. J.; Riggins, G. J.; Chastain, J. L.; Kunst, C. B.; Galjaard, H.; Caskey, C. T.; Nelson, D. L.; Oostra, B. A.; Warren, S. T. Identification of a Gene (FMR-1) Containing a CGG Repeat Coincident with a Breakpoint Cluster Region Exhibiting Length Variation in Fragile X Syndrome. Cell1991, 65 (5), 905–914. https://doi.org/10.1016/0092-8674(91)90397-H. (LANDMARK PAPER)

This paper is one of the most important landmark papers in all the literature on Fragile X Syndrome because it identified the FMR1 gene as the gene that is hypermethylated in the known fragile site and varies in length among individuals.

(9) Hagerman, R. J.; Hull, C. E.; Safanda, J. F.; Carpenter, I.; Staley, L. W.; O’Connor, R. A.; Seydel, C.; Mazzocco, M. M. M.; Snow, K.; Thibodeau, S. N.; Kuhl, D.; Nelson, D. L.; Caskey, C. T.; Taylor, A. K. High Functioning Fragile X Males: Demonstration of an Unmethylated Fully Expanded FMR-1 Mutation Associated with Protein Expression. American Journal of Medical Genetics1994, 51 (4), 298–308. https://doi.org/10.1002/ajmg.1320510404.

The severity of Fragile X Syndrome falls on a spectrum, and this paper elucidated some of the causes of the varying phenotypes. High functioning Fragile X males were found to have different FMR1 mutations and methylation patterns compared to intellectually disabled Fragile X males. Incomplete methylation of the fully expanded mutation and lack of methylation of the full mutation, which is mitotically unstable, were observed and allowed for some production of FMR1 protein, which lessened intellectual disabilities and led the authors to believe that methylation of the FMR1 gene may affect the phenotype more than the CGG repeats. Full mutations of over 200 CGG repeats are usually completely methylated and mitotic instability is usually observed in premutations of 120-200 CGG repeats.

(10) Usdin, K.; Woodford, K. J. CGG Repeats Associated with DNA Instability and Chromosome Fragility Form Structures That Block DNA Synthesis in Vitro. Nucleic Acids Res1995, 23 (20), 4202–4209.

This paper found that the CGG repeats in the FMR1 gene inhibit DNA synthesis by forming intrastrand tetraplexes.

(11) Feng, Y.; Gutekunst, C.-A.; Eberhart, D. E.; Yi, H.; Warren, S. T.; Hersch, S. M. Fragile X Mental Retardation Protein: Nucleocytoplasmic Shuttling and Association with Somatodendritic Ribosomes. J. Neurosci.1997, 17 (5), 1539–1547. https://doi.org/10.1523/JNEUROSCI.17-05-01539.1997.

This paper performed localization studies of FMRP to find that it is commonly found in neuronal nucleoplasm and within nuclear pores and co-fractionated with polysomes and rough endoplasmic reticulum. The authors also collected evidence of nucleocytoplasmic shuttling of FMRP. Dendritic localization of FMRP suggested its role in the translation of proteins involved in dendritic structure or function.

1999-2008

(12) Bardoni, B.; Schenck, A.; Louis Mandel, J. A Novel RNA-Binding Nuclear Protein That Interacts With the Fragile X Mental Retardation (FMR1) Protein. Human Molecular Genetics1999, 8 (13), 2557–2566. https://doi.org/10.1093/hmg/8.13.2557.

Prior to this paper, FMRP was only known to interact with its homologs, FXR1P and FXR2P, but this paper showed that nuclear FMRP interacting protein (NUFIP) also interacts with FMRP. It was found to colocalize with FMRP nuclear isoforms. Both the FMR1 and NUFIP genes have similar neuronal expression in the brain and have high expression in regions associated with cognitive function, suggesting an interaction between FMRP and NUFIP is important for neuronal function most likely through binding RNA.

(13) Peier, A. M.; McIlwain, K. L.; Kenneson, A.; Warren, S. T.; Paylor, R.; Nelson, D. L. (Over)Correction of FMR1 Deficiency with YAC Transgenics: Behavioral and Physical Features. Human Molecular Genetics2000, 9 (8), 1145–1159. https://doi.org/10.1093/hmg/9.8.1145.

The authors of this paper wanted to observe the effects of introducing a yeast artificial chromosome containing the FMR1 gene into FMR1 knockout mice. Knockout mice with no YAC and therefore no FMRP had increased anxiety, and overexpression of FMRP caused hypoactivity and high levels of anxiety. Overexpression of FMRP may also reduce viability. The adverse side effects of too much FMRP prohibits genetic correction of FMRP deficiency in Fragile X Syndrome patients.

(14) Hagerman, R. J.; Leehey, M.; Heinrichs, W.; Tassone, F.; Wilson, R.; Hills, J.; Grigsby, J.; Gage, B.; Hagerman, P. J. Intention Tremor, Parkinsonism, and Generalized Brain Atrophy in Male Carriers of Fragile X. Neurology2001, 57 (1), 127–130. https://doi.org/10.1212/WNL.57.1.127. (LANDMARK PAPER)

This was the first paper describing Fragile X-associated tremor/ataxia syndrome (FXTAS), which is when there are 55-200 CGG repeats putting a person in the premutation range before being diagnosed with Fragile X Syndrome. People in the premutation range of CGG repeats have higher levels of FMR1 mRNA, but lower levels of FMRP, causing a phenotype of action tremors with executive function deficits and slowly progressive neurodegenerative disorders.

(15) Brown, V.; Jin, P.; Ceman, S.; Darnell, J. C.; O’Donnell, W. T.; Tenenbaum, S. A.; Jin, X.; Feng, Y.; Wilkinson, K. D.; Keene, J. D.; Darnell, R. B.; Warren, S. T. Microarray Identification of FMRP-Associated Brain MRNAs and Altered MRNA Translational Profiles in Fragile X Syndrome. Cell2001, 107 (4), 477–487. https://doi.org/10.1016/S0092-8674(01)00568-2. (LANDMARK PAPER)

The authors coimmunoprecipitated the FMRP ribonucleoprotein complex and the mRNAs it associated with, many of which are involved in neuronal function, to identify which ones selectively associate with FMRP-mRNP complexes. Nearly 70% of the mRNAs had a G quartet structure, which is an FMRP target.

(16) Alberts, B.; Johnson, A.; Lewis, J.; Raff, M.; Roberts, K.; Walter, P. Neural Development. Molecular Biology of the Cell. 4th edition2002.

A review paper on the development of neurons.

(17) Huber, K. M.; Gallagher, S. M.; Warren, S. T.; Bear, M. F. Altered Synaptic Plasticity in a Mouse Model of Fragile X Mental Retardation. Proc Natl Acad Sci U S A2002, 99 (11), 7746–7750. https://doi.org/10.1073/pnas.122205699. (LANDMARK PAPER)

This paper showed that metabotropic glutamate receptor-dependent long-term depression, a form of protein synthesis-dependent synaptic plasticity, is enhanced in the hippocampus of FMR1 knockout mice. This is a landmark paper because it inspired other papers to explore the use of mGluR5 inhibitors to reverse the effects of Fragile X Syndrome.

(18) Zalfa, F.; Giorgi, M.; Primerano, B.; Moro, A.; Di Penta, A.; Reis, S.; Oostra, B.; Bagni, C. The Fragile X Syndrome Protein FMRP Associates with BC1 RNA and Regulates the Translation of Specific MRNAs at Synapses. Cell2003, 112 (3), 317–327. https://doi.org/10.1016/S0092-8674(03)00079-5.

This paper found that BC1, a non-coding RNA, interacts with the FMRP protein and has complementary sequences to important mRNAs in synapses. FMRP is known to inhibit translation at the initiation step of these mRNAs, so the authors claim that BC1 helps FMRP target the mRNAs it regulates. Synaptic dysfunction is a known result of Fragile X Syndrome, so making the connection between regulation of synaptic mRNAs, BC1, and FMRP provides more information on the mechanistic details of FMRP.

(19) Ramos, A.; Hollingworth, D.; Adinolfi, S.; Castets, M.; Kelly, G.; Frenkiel, T. A.; Bardoni, B.; Pastore, A. The Structure of the N-Terminal Domain of the Fragile X Mental Retardation Protein: A Platform for Protein-Protein Interaction. Structure2006, 14 (1), 21–31. https://doi.org/10.1016/j.str.2005.09.018. (LANDMARK PAPER)

This paper solved the crystal structure of the N-terminal domain of FMRP and proposed some functions of the domain.

(20) Singh, K.; Gaur, P.; Prasad, S. Fragile x Mental Retardation (Fmr-1) Gene Expression Is down Regulated in Brain of Mice during Aging. Mol Biol Rep2007, 34 (3), 173–181. https://doi.org/10.1007/s11033-006-9032-8.

Singh et al. found that FMRP is downregulated in FMR1 knockout mice to varying degrees depending on age.

2009-2018

(21) Sharma, A.; Hoeffer, C. A.; Takayasu, Y.; Miyawaki, T.; McBride, S. M.; Klann, E.; Zukin, R. S. Dysregulation of MTOR Signaling in Fragile X Syndrome. J. Neurosci.2010, 30 (2), 694–702. https://doi.org/10.1523/JNEUROSCI.3696-09.2010.

This paper showed that mTOR phosphorylation and activity are higher in FMR1 knockout mice, which may provide a link between overactive group I mGluRs and aberrant synaptic plasticity in the Fragile X Syndrome mouse model.

(22) Osterweil, E. K.; Krueger, D. D.; Reinhold, K.; Bear, M. F. Hypersensitivity to MGluR5 and ERK1/2 Leads to Excessive Protein Synthesis in the Hippocampus of a Mouse Model of Fragile X Syndrome. J Neurosci2010, 30 (46), 15616–15627. https://doi.org/10.1523/JNEUROSCI.3888-10.2010.

FMR1 knockout mice have elevated levels of protein synthesis, but inhibition of either mGluR5 or the ERK1/2 signaling pathway restores protein synthesis levels to wild-type levels. They found the increased protein synthesis in FMR1 knockout mice was due to an increased sensitivity of the protein synthesis machinery to mGluR5-ERK1/2 signaling, rather than an increase in the activation of the signaling pathway. The authors also found that hypersensitivity to the ERK1/2 signaling pathway contributed to the audiogenic seizure susceptibility phenotype of Fragile X Syndrome.

(23) Phan, A. T.; Kuryavyi, V.; Darnell, J. C.; Serganov, A.; Majumdar, A.; Ilin, S.; Raslin, T.; Polonskaia, A.; Cynthia Chen; Clain, D.; Darnell, R. B.; Patel, D. J. Structure-Function Studies of FMRP RGG Peptide Recognition of an RNA Duplex-Quadruplex Junction. Nature Structural & Molecular Biology2011, 18 (7), 796–804. https://doi.org/10.1038/nsmb.2064.

This paper solved the crystal structure of an arginine-glycine rich peptide from FMRP bound to a guanine-rich RNA.

(24) Darnell, J. C.; Van Driesche, S. J.; Zhang, C.; Hung, K. Y. S.; Mele, A.; Fraser, C. E.; Stone, E. F.; Chen, C.; Fak, J. J.; Chi, S. W.; Licatalosi, D. D.; Richter, J. D.; Darnell, R. B. FMRP Stalls Ribosomal Translocation on MRNAs Linked to Synaptic Function and Autism. Cell2011, 146 (2), 247–261. https://doi.org/10.1016/j.cell.2011.06.013. (LANDMARK PAPER)

The authors of this paper provided a link between Fragile X Syndrome and autism spectrum disorders by showing many of the mRNAs that FMRP binds encode plasticity-related proteins, and FMRP represses their translation during elongation by stalling ribosomal translocation.

(25) Michalon, A.; Sidorov, M.; Ballard, T. M.; Ozmen, L.; Spooren, W.; Wettstein, J. G.; Jaeschke, G.; Bear, M. F.; Lindemann, L. Chronic Pharmacological MGlu5 Inhibition Corrects Fragile X in Adult Mice. Neuron2012, 74 (1), 49–56. https://doi.org/10.1016/j.neuron.2012.03.009.

CTEP, an mGlu5 inhibitor, was found to reverse Fragile X Syndrome phenotypes in early adulthood mice. Learning and memory deficits, hypersensitivity to auditory stimuli, elevated ERK and mTOR activity and increased dendritic spine density were reversed. Locomotor activity was increased, and macroorchidism was partially reversed.

(26) Arikkath, J. Molecular Mechanisms of Dendrite Morphogenesis. Frontiers in Cellular Neuroscience2012, 6.

This is a review paper that discusses mechanisms that affect dendrite morphology.

(27) Atluri, V. S. R.; Kanthikeel, S. P.; Reddy, P. V. B.; Yndart, A.; Nair, M. P. N. Human Synaptic Plasticity Gene Expression Profile and Dendritic Spine Density Changes in HIV-Infected Human CNS Cells: Role in HIV-Associated Neurocognitive Disorders (HAND). PLoS One2013, 8 (4), e61399. https://doi.org/10.1371/journal.pone.0061399.

This paper studied the effects of downregulating synaptic plasticity genes on dendritic spine formation.

(28) Gkogkas, C. G.; Khoutorsky, A.; Cao, R.; Jafarnejad, S. M.; Prager-Khoutorsky, M.; Giannakas, N.; Kaminari, A.; Fragkouli, A.; Nader, K.; Price, T. J.; Konicek, B. W.; Graff, J. R.; Tzinia, A. K.; Lacaille, J.-C.; Sonenberg, N. Pharmacogenetic Inhibition of EIF4E-Dependent Mmp9 MRNA Translation Reverses Fragile X Syndrome-like Phenotypes. Cell Reports2014, 9 (5), 1742–1755. https://doi.org/10.1016/j.celrep.2014.10.064.

It was noted that individuals with Fragile X Syndrome have more phosphorylated eukaryotic initiation factor 4E (eIF4E), which is used in the Ras/ERK/Mnk pathway to stimulate translation. This excess of phospho-eIF4E leads to an excess of metalloproteinase 9 (MMP-9) protein, which the authors found to induce Fragile X Syndrome phenotypes. Genetic inhibition and pharmacological inhibition using cercosporamides, inhibitors of Mnk1/Mnk2 kinases, reduced eIF4E phosphorylation and decreased Mmp9 mRNA translation, which reversed Fragile X Syndrome phenotypes.

2019-Present

(29) Griffiths, K. K.; Wang, A.; Wang, L.; Tracey, M.; Kleiner, G.; Quinzii, C. M.; Sun, L.; Yang, G.; Perez-Zoghbi, J. F.; Licznerski, P.; Yang, M.; Jonas, E. A.; Levy, R. J. Inefficient Thermogenic Mitochondrial Respiration Due to Futile Proton Leak in a Mouse Model of Fragile X Syndrome. The FASEB Journal2020, 34 (6), 7404–7426. https://doi.org/10.1096/fj.202000283RR.

Griffiths et al. found that a deficiency in Coenzyme Q causes a proton leak and open cyclosporine-sensitive channel. Supplying FMR1 knockout mice with CoQ fixed the leak and normalized phenotypes associated with Fragile X Syndrome.

(30) Licznerski, P.; Park, H.-A.; Rolyan, H.; Chen, R.; Mnatsakanyan, N.; Miranda, P.; Graham, M.; Wu, J.; Cruz-Reyes, N.; Mehta, N.; Sohail, S.; Salcedo, J.; Song, E.; Effman, C.; Effman, S.; Brandao, L.; Xu, G. N.; Braker, A.; Gribkoff, V. K.; Levy, R. J.; Jonas, E. A. ATP Synthase C-Subunit Leak Causes Aberrant Cellular Metabolism in Fragile X Syndrome. Cell2020, 182 (5), 1170-1185.e9. https://doi.org/10.1016/j.cell.2020.07.008. (LANDMARK PAPER)

The authors of this paper found a leak in the mitochondrial inner membrane that FMRP typically fixes by increasing the translation of the beta subunit of ATP synthase, which increases mRNA translation rates and allows for synapse maturation. Individuals with Fragile X Syndrome don’t have FMRP, so they cannot fix this leak, which results in the abnormal synapses that characterize Fragile X Syndrome.

(31) Kurosaki, T.; Imamachi, N.; Pröschel, C.; Mitsutomi, S.; Nagao, R.; Akimitsu, N.; Maquat, L. E. Loss of the Fragile X Syndrome Protein FMRP Results in Misregulation of Nonsense-Mediated MRNA Decay. Nat Cell Biol2021, 23 (1), 40–48. https://doi.org/10.1038/s41556-020-00618-1.

The authors of this paper found that an absence of FMRP causes hyperactive nonsense-mediated mRNA decay (NMD), indicating that FMRP is a repressor of NMD and targets the NMD targets, most of which are involved with synaptic signaling pathways, through direct binding to UPF1. The authors found that small molecules were able to suppress hyperactive NMD and restore normal neuronal differentiation in Fragile X Syndrome cells.

(32) Raj, N.; McEachin, Z. T.; Harousseau, W.; Zhou, Y.; Zhang, F.; Merritt-Garza, M. E.; Taliaferro, J. M.; Kalinowska, M.; Marro, S. G.; Hales, C. M.; Berry-Kravis, E.; Wolf-Ochoa, M. W.; Martinez-Cerdeño, V.; Wernig, M.; Chen, L.; Klann, E.; Warren, S. T.; Jin, P.; Wen, Z.; Bassell, G. J. Cell-Type-Specific Profiling of Human Cellular Models of Fragile X Syndrome Reveal PI3K-Dependent Defects in Translation and Neurogenesis. Cell Rep2021, 35 (2), 108991. https://doi.org/10.1016/j.celrep.2021.108991.

This paper discovered that FMRP helps regulate PI3K signaling during early neural development. The absence of FMRP in Fragile X Syndrome causes excess protein synthesis, which creates more cells in a proliferative state rather than a differentiating state, which impairs neurogenesis.

(33) Fisher, P. R.; Allan, C. Y.; Sanislav, O.; Atkinson, A.; Ngoei, K. R. W.; Kemp, B. E.; Storey, E.; Loesch, D. Z.; Annesley, S. J. Relationships between Mitochondrial Function, AMPK, and TORC1 Signaling in Lymphoblasts with Premutation Alleles of the FMR1 Gene. Int J Mol Sci2021, 22 (19), 10393. https://doi.org/10.3390/ijms221910393.

This paper explored mitochondrial function and signaling in lymphoblasts containing the premutation allele for Fragile X Syndrome. For the first time, it was proven that TORC1 activity was decreased in lymphoblasts with the premutation, which caused an increase in non-mitochondrial respiration and AMPK activity and a decrease in reactive oxygen species. The increased AMPK activity led to hyperactivity of the mitochondria. An interesting observation was that TORC1 activity is elevated in cells with the full mutation but decreased in premutation cells indicating that the large number of CGG repeats that cause TORC1 activity to increase dominates in the full mutation.

(34) Fernandes, G.; Mishra, P. K.; Nawaz, M. S.; Donlin-Asp, P. G.; Rahman, M. M.; Hazra, A.; Kedia, S.; Kayenaat, A.; Songara, D.; Wyllie, D. J. A.; Schuman, E. M.; Kind, P. C.; Chattarji, S. Correction of Amygdalar Dysfunction in a Rat Model of Fragile X Syndrome. Cell Reports2021, 37 (2), 109805. https://doi.org/10.1016/j.celrep.2021.109805.

Individuals with Fragile X Syndrome have previously been found to have impaired recall of conditioned fear, which inspired the authors of this paper to study the role of the amygdala in Fragile X Syndrome, since most prior research was done on the hippocampus and neocortex. It was previously found that inhibiting mGluRs in the hippocampus and neocortex reverse some Fragile X Syndrome phenotypes, but this paper found that the opposite is true for the amygdala – activation of mGluRs reversed deficient synaptic transmission and plasticity, which reversed some Fragile X Syndrome phenotypes, such as bringing back conditioned fear. The authors call for a modification in the way mGluR inhibitors are used to treat Fragile X Syndrome to include circuit-specific differences.