CNS expression of murine fragile X protein (FMRP) as a function of CGG-repeat size.
Large expansions of a CGG-repeat element (>200 repeats; full mutation) in the fragile X mental retardation 1 (FMR1) gene cause fragile X syndrome (FXS), the leading single-gene form of intellectual disability and of autism spectrum disorder. Smaller expansions (55-200 CGG repeats; premutation) result in the neurodegenerative disorder, fragile X-associated tremor/ataxia syndrome (FXTAS). Whereas FXS is caused by gene silencing and insufficient FMR1 protein (FMRP), FXTAS is thought to be caused by 'toxicity' of expanded-CGG-repeat mRNA. However, as FMRP expression levels decrease with increasing CGG-repeat length, lowered protein may contribute to premutation-associated clinical involvement. To address this issue, we measured brain Fmr1 mRNA and FMRP levels as a function of CGG-repeat length in a congenic (CGG-repeat knock-in) mouse model using 57 wild-type and 97 expanded-CGG-repeat mice carrying up to ~250 CGG repeats. While Fmr1 message levels increased with repeat length, FMRP levels trended downward over the same range, subject to significant inter-subject variation. Human comparisons of protein levels in the frontal cortex of 7 normal and 17 FXTAS individuals revealed that the mild FMRP decrease in mice mirrored the more limited data for FMRP expression in the human samples. In addition, FMRP expression levels varied in a subset of mice across the cerebellum, frontal cortex, and hippocampus, as well as at different ages. These results provide a foundation for understanding both the CGG-repeat-dependence of FMRP expression and for interpreting clinical phenotypes in premutation carriers in terms of the balance between elevated mRNA and lowered FMRP expression levels.
Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Davis, CA, USA.
Anna Lisa
AL
Ludwig
Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Davis, CA, USA.
Glenda M
GM
Espinal
Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Davis, CA, USA, MIND Institute, University of California, Davis, Health System, Sacramento, CA, USA and.
Dalyir I
DI
Pretto
Department of Neurological Surgery, University of California, Davis, School of Medicine, Davis, CA, USA.
Amanda L
AL
Jamal
Department of Neurological Surgery, University of California, Davis, School of Medicine, Davis, CA, USA.
Gloria
G
Arque
Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Davis, CA, USA, MIND Institute, University of California, Davis, Health System, Sacramento, CA, USA and.
Flora
F
Tassone
MIND Institute, University of California, Davis, Health System, Sacramento, CA, USA and Department of Neurological Surgery, University of California, Davis, School of Medicine, Davis, CA, USA.
Robert F
RF
Berman
Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Davis, CA, USA, MIND Institute, University of California, Davis, Health System, Sacramento, CA, USA and pjhagerman@ucdavis.edu.
Paul J
PJ
Hagerman
D000818:Animals; D002531:Cerebellum / Q000378:metabolism*; D004195:Disease Models, Animal; D005260:Female; D051860:Fragile X Messenger Ribonucleoprotein 1 / Q000235:genetics* / Q000378:metabolism*; D005600:Fragile X Syndrome / Q000235:genetics / Q000473:pathology*; D005625:Frontal Lobe / Q000378:metabolism*; D005786:Gene Expression Regulation; D006624:Hippocampus / Q000378:metabolism*; D006801:Humans; D008297:Male; D051379:Mice; D009928:Organ Specificity; D012333:RNA, Messenger / Q000378:metabolism; D019680:Trinucleotide Repeat Expansion
D051860:Fragile X Messenger Ribonucleoprotein 1; D012333:RNA, Messenger