As noted above, the fundamentally distinct nature of FMR1
expression in the premutation range (high mRNA, relatively preserved FMRP levels), and the full mutation range (reduced/absent mRNA, FMRP), led to the idea of an RNA-mediated pathogenesis of FXTAS, based on the “RNA toxicity” model for myotonic dystrophy (DM).11,12
For the latter disorder, one or more proteins, including muscleblind-like 1(MBNL1
), is bound excessively (sequestered) by an expanded, non-coding C(C)UG repeat element. For MBNL1
, specific domains of clinical involvement, e.g., myotonia and insulin resistance, arise from the altered splicing of specific mRNAs due to functional insufficiency of the sequestered protein. FXTAS pathogenesis is believed to operate through a similar mechanism, whereby the expanded CGG-repeat element in the 5′ non-coding portion of FMR1
mRNA sequesters protein(s) that affect diverse pathways, leading to clinical involvement (). For FXTAS, the fundamental challenge at this point is two-fold; namely, to establish that the FMR1
mRNA is necessary and sufficient to produce all features of the neurodegenerative disorder and, predicated on the correctness of the model, to identify the specific proteins involved in mediating the RNA-triggered pathogenesis.
At present, several proteins have been identified that both interact with the CGG repeat and are potential mediators of downstream cellular dysregulation. Among these proteins are Sam68,13
a splicing modulator whose absence in the mouse leads to motor dysfunction;14,15
an RNA-binding protein whose targeted disruption in mouse leads to tremor and gait disturbances, and early loss of viability;18
and hnRNP A2,17,19
a multifunctional heterogeneous nuclear RNA binding protein thought to be involved with mRNA metabolism and transport. For Sam68, there is evidence of functional impairment in the brains of individuals with FXTAS, with disease-specific alterations in splicing of ATP11B
mRNAs. There is also recovery of control isoform ratios in cultured cells harboring expanded CGG-repeat alleles upon over-expression of exogenous Sam68 protein.13
Similarly, overexpression of Pur α protein in a CGG-repeat Drosophila
model reverses a neurodegenerative (eye) phenotype,16,19
even though there has not, as yet, been any demonstration of the functional significance of Pur α in rescuing a CGG-repeat-induced neurodegenerative phenotype in a mammalian system.
More recently, Muslimov et al.20
presented evidence based on rat neuronal transfection experiments, that binding of hnRNP A2 to the expanded CGG-repeat RNA interferes with its binding to the non-coding RNA, BC1, which normally facilitates dendritic transport of mRNAs that are required for synaptic function. In addition, they showed that overexpression of hnRNP A2 partially reversed the impaired dendritic transport of an mRNA known to be targeted for dendritic transport. These intriguing observations point to a possible role for hnRNP A2 in the pathogenesis of FXTAS. However, many dendritically-transported mRNAs occur via mechanisms that are likely to be independent of BC1.21
Thus, to assess what role, if any, that hnRNA A2 and/or BC1 play in FXTAS, it will be essential to study their roles in vivo
, e.g., with knock-in (KI) mice.
Finally, Sellier et al.22
reported preliminary findings that the expanded CGG-repeat RNA sequesters DiGeorge syndrome critical region 8 (DGCR8) protein (haploinsufficient in DiGeorge syndrome/22qdel syndrome).23,24
DGCR8, along with the RNase III Drosha, constitutes the microRNA (miR) nuclear processing complex that cleaves primary miRNAs (pri-miRs residing within portions of larger RNA transcripts) into precursor miRNAs (pre-miRs) in the miR biogenesis pathway.25
The importance of the observation by Sellier et al.22
is that substantial miR dysregulation (low miRs and elevated levels of pri-miRs) is observed in the brains of FXTAS cases.
Although one or more of these proteins, when sequestered, may mediate the “toxicity” of the FMR1
mRNA, there is still little understanding as to the downstream consequences of their actions. The problem is particularly daunting for both the DGCR8 protein, where reductions of multiple miRs would be expected to influence the expression of hundreds of proteins and many pathways, and for hnRNP A2, which is likely to facilitate the transport of many mRNAs of diverse function. Thus, even though we know of specific aspects of cellular dysfunction, e.g., mitochondrial abnormalities,26,27
altered lamin A nuclear architecture,28
and reduced telomere length,29
we do not know how they are linked. The challenge is now to “connect the dots” between the basic sequestration mechanism and the downstream processes that lead to disease.