Sindromul X- fragil

1. Sindromul X- fragil (Sindromul Martin-Bell)

2. Sd. X- fragil se manifesta cu precadere la barbati printr-un retard mental grav

3. Sindromul X- fragil • Se datorează unei mutaţii dinamice( = din generatie in generatienumarul de repetitii trinucleotidice poatecreste) • In gametogeneza materna (cu precadere) are loc amplificarea secventei de 3 baze azotate [codon]. • Denumirea este data de faptul ca initial, prin analiza citogenetica cromozomul X parea sa prezinte o ruptura la nivelultelomerului bratului q, adica s-a presupus existenta la acest nivel a unui “situs fragil” (zona cz. cu rupturi frecvente) • Astazi, se stie ca, aspectul citogenetic este datorat multiplicarii excesive a codonuluiCGG la nivelulpromotoruluigenei FMR1(Fragile X Mental Retardation) • La baieţii cu o astfel de amplificare, unde codonul CGG este deci supranumerar, apare un retard psihomotor grav. • Sindromul X-fragil este cea mai cunoscuta cauza de autism

4. Situsul fragil, indicat prin sageti in imaginea alaturata (d), este de fapt, un codon amplificat anormal DISMORFISMUL CRANIO-FACIAL este evident: macrocefalie, fataalungita, fruntelunga si proeminenta, urechimari si proeminente, maxilareproeminente RM=retard mental

5. Diagnosticul clinic este certificat prin analize moleculare, ce evidentiaza prezenta repetitiilor trinucleotidice.

6. Tabloul clinic al sindromuluiasociazaretardul mental de la moderat la sever, macroorhidia (postpubertara, cu testiculeavandvolumul > 40ml) si dismorfismulcranio-facial specific. http://www.nfxf.org/html/facial.htm In majoritateacazurilorsindromul este cauzat de expansiuneatrinucleotidicainstabila CGG de la nivelulgenei FMR1 si metilareaanormala, care produce supresiatranscriptiei si consecutivnivelescazute ale proteinei FMRP la nivel cerebral. Sindromul X- fragil determinaaproape o jumatate din cazurile de retard mental si este ca frecventa a douacauza de handicap psihicdupasindromul Down. In OMIM pe langa sd. X-fragil, apare ca entitate de sine-statatoare si FXTAS (fragile Xtremor/ataxia syndrome) produs de o premutatie urmata de cresterea sintezei de FMR1.

7. In analiza Southern blot alaturata ADNul migreaza de la minus la +; cu cat fragmentul de ADN este mai mare, cu atat mai greu se va deplasa in gelul de agaroza. Interpretati desenul alaturat, apoi rasturnati raspunsul de mai jos. Interpretare: I1 prezinta premutatia (pe singurul sau cz. X), pe care fiica sa, II2, o mosteneste de aceeasi marime;II2 are 2 cz. X deci unul cu premutatia de la tata si unul cu gena FMR1 normala, mostenit de la mama, care nu mai este reprezentata in arborele genealogic; III1 are mutatiacompleta, adica sd. X-fragil, amprentaADNului sau aproape ca nu s-a deplasat de la punctul de start. II1 are un cz. X cu genanormala

9. Femeile cu sindromul X-fragil • Femeilepurtatoare de premutatiepot prezenta: • insuficientaovariana, menopauzainstalandu-se inainte de varsta de 40 de ani, • menopauzaprematura, instalatainainte de varsta de 45 de ani sau • o disfunctieovariana ( fertilitateredusa) in general • Scadereafertilitatii este corelata cu cresterea de FSH si altemodificarihormonale. • Femeilepurtatoare ale mutatieicomplete nu au riscurile de mai sus. Marimeamutatiei nu se coreleaza cu gravitateaclinicadatoritainactivariicz.X(cromatinasexuala) • Sfatul genetic dupa o testareprenatalapozitivanu poateprezice cu fidelitateafectareaintelectuala, comportamentala sau psihologica in cazulfetelor cu mutatiacompleta. Tulburarile se manifesta de la foarteusoarepana la retard sever si autism.

10. Se observa caracteristica acestui tip particular de transmitere si anume BARBATII PURTATORI (I3, IV 1 si IV2). Acestia au premutatia si deci boala nu se manifesta la intensitatea maxima, adica nu au un retard mental grav

11. La fel cum transmiterea acestui sindrom este particulara. Cu siguranta este o transmitere X-lincata, gena FMR1 fiind pe cz. X. Uniiautoriconsidera transmiterea X-lincatadominanta(http://ghr.nlm.nih.gov/condition/fragile-x-syndrome)iaraltii X-lincatarecesiva(http://www.genetics.edu.au/pdf/factsheets/fs42.pdf) Poatecel mai corect este sa se adopteparerea ca, bolile determinate de gene de pe cz. X sunt “doar” X-lincate, cum propuneadeja in 2004 Dobyns de la Universitatea Illinois din Chicago (Am J Med Genet A. 2004 Aug 30;129A(2):136-43.Inheritance of most X-linked traits is not dominant or recessive, just X-linked.)

12. In acest arbore genealogic este specificatnumarul de repetitii trinucleotidice ale fiecareipersoane. Pe langa prezenta femeilor si barbatilorpurtatori de PM, un caz special ilreprezinta IV5, care are mutatiacompleta, dar nu este bolnava!

13. Urmatoarele sindroame sunt cele mai frecvent intalnite boli cauzate de mutatii dinamice.

14. Pentru cine nu se asazadoar pe carti, cateva date suplimentare in continuare, dar in original

15. FMR1 gene contains 17 exons spanning 38 kb. http://bioquest.org/icbl/icbl_details.php?product_id=3783

16. FMRP - The highest levels were observed in neurons, while glial cells contained very low levels.

17. Text la imaganterioara • Expansion of CGG repeats in the fragile X mental retardation 1 (FMR1) gene that encodes FMRP underlies fragile X syndrome (FRAXA). Repeats that contain >200 copies (full mutation) lead to loss of FMRP expression. FMRP contains two domains that bind RNA: the KH2 domain and the RGG box. The Ile304Asn mutation in the KH2 domain, which prevents FMRP from binding targets that contain the kissing complex motif, gives rise to a severe mental retardation phenotype. a | Abnormal dendritic spine morphology in patients with FRAXA. An increased density of long, immature dendritic spines indicates that FMRP has a role in synaptic maturation and pruning, possibly through its regulation of gene products that are involved in synaptic development. FMRP might also have a regulatory role in activity-dependent translation at the synapse. Stimulation of postsynaptic metabotropic glutamate receptors (mGluRs) results in increased protein synthesis and subsequent internalization of -amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors, which is important in the expression of long-term depression. FMRP, which is also upregulated by mGluRs, serves to dampen this process. The absence of FMRP in FRAXA results in over-amplification of this response. b | FMRP modulates the translation of its targets, probably through its association with the RNA-induced silencing complex (RISC). FMRP is transported to dendritic spines, together with its associated RNAs and proteins. mRNP, messenger ribonucleoprotein particle; NES, nuclear export signal; NMDA, N-methyl-D-aspartate; NLS, nuclear localization signal • http://www.nature.com/nrg/journal/v6/n10/full/nrg1691.html

18. http://www.nature.com/nrn/journal/v6/n5/fig_tab/nrn1667_F1.htmlhttp://www.nature.com/nrn/journal/v6/n5/fig_tab/nrn1667_F1.html

19. Text la imag. ant • Fragile X mental retardation protein (FMRP) enters the nucleus and could function through two possible mechanisms. In the first (1), FMRP could interact with other proteins, with itself (for example, the FMRP-homologous proteins FXR1P and FXR2P), and with RNA/mRNA to form a ribonucleocomplex that is probably involved in mRNA export from the nucleus to the cytoplasm. Once in the cytoplasm, a 'core' complex, containing FMRP and some of its nuclear partners, would interact with cytoplasm-specific proteins (such as cytoplasmic FMRP-interacting protein 1 (CYFIP1), CYFIP2 and Staufen) and move along dendrites to the synapses, transporting RNA/mRNA and, later, regulating synaptic protein synthesis. In the second mechanism (2), FMRP could be involved in the nuclear RNA interference pathway that is associated with small, non-coding RNAs (short hairpin RNAs or shRNAs) and specific nuclear partners (that is, nucleolin and Y-box binding protein 1 (YB1)). miRNA, microRNA; ncRNA, non-coding RNA.

21. Text la imag. ant • At synapses, protein synthesis is initiated by different cellular stimuli, and this leads to an independent response of a single synapse that can influence synaptic plasticity. a | In a wild-type spine, stimulation of metabotropic glutamate receptors enhances the synthesis of fragile X mental retardation protein (FMRP), which could act to negatively regulate the translation of proteins that are involved in ionotropic receptor internalization during long-term depression and of proteins that regulate the cytoskeleton (such as microtubule-associated protein 1B (MAP1B), activity-regulated cytoskeletal-associated protein (ARC), arginine-binding protein 2 (ARGBP2), postsynaptic density protein 95 (PSD-95) and Rac1). This receptor-coupled signalling pathway might also be responsible for FMRP phosphorylation and the consequent release of mRNAs from translational inhibition and/or the activation of translation of other specific dendritic mRNAs. The correct balance between synthesis and degradation of these proteins would promote and maintain the mature shape of the synapse. b | In a spine of a patient with fragile X syndrome, or in the mouse model of the syndrome, the absence of FMRP would lead to an increase and/or decrease in the translation of protein regulators of the cytoskeleton, both of which might have an effect on the lengthening of dendritic spines. c | The absence of FMRP could also lead to an increase in the translation of proteins that are involved in ionotropic (AMPA (-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) and NMDA (N-methyl-D-aspartate)) receptor internalization (INT.) during hippocampal long-term depression, which could lead to fewer receptors being present on the postsynaptic membrane and to thinner spines. mGluR, metabotropic glutamate receptor

22. The figure shows a hypothetical mechanism through which the absence of fragile X mental retardation protein (FMRP) could lead to failure of synapse pruning and, as a consequence, dendrite pruning, in a typical spiny stellate neuron in a whisker barrel (centre). The model assumes that FMRP regulates the synthesis of structural proteins (for example, postsynaptic density protein 95 (PSD-95)) or signalling proteins that form part of a complex that is important for stabilizing and maturing developing synapses (see Fig. 4 for one possible conceptualization of this process). When FMRP is present, this stabilization complex (carried by the transport granule) is selectively targeted to active synapses (upper left), which results in selective maturation and stabilization of spines (upper right) and pruning of non-stabilized synapses. In the absence of FMRP (lower left), the stabilization complex is equally targeted to active and inactive synapses, which results in a weaker form of maturation and stabilization, and gives rise to greater numbers of synapses and an immature morphology (lower right). article: From mRNP trafficking to spine dysmorphogenesis: the roots of fragile X syndrome Claudia Bagni & William T. Greenough in Nature Reviews Neuroscience 6, 376-387 (May 2005

23. Fragile X mental retardation protein (FMRP) binds different neuronal mRNAs. Four mechanisms of target recognition have been characterized. FMRP could recognize a G-quartet structure (a) or a poly(U) stretch (b) in the mRNA. Alternatively, FMRP could bind indirectly to the mRNA through either the small non-coding RNA brain cytoplasmic RNA 1 (BC1) (c) or microRNAs (miRNAs) (d). eIF2C2, eukaryotic translation initiation factor 2C, 2

24. In loc de tratament al sd. X

25. Factorul major care determinaprezentasauabsentasindromului X-fragilestenumarul de repetitii CGG in gena FMR1 de pe Xq27.3.  • In mod tipic, numarul de repetitii > 200, declanseazametilareainsulelorCpG din regiuneapromotora genei.  • Ca urmare, productia de FMRP (fragile X mental retardation protein) esteoprita. Absentaproteinei are ca rezultataparitiasindromului X-fra • Totusi, existasicazuri, ce nu se incadreazanici in premutatiisinici in mutatii complete. Un exempluestemozaicul,celuleleavand grade diferite de metilareurmare a unuinumarvariat de repetii CGG. Impactulboliidepinde in astfel de cazuri de procentul de celule care suntafectatesi de tipul de tesutimplicat • Exceptional ,existapersoane cu > 200 repetitii CGG, dar la care gena FMR1 nu estemetilata. Gama de simptomeestefoartelarga in astfel de cazuri, iarcaracteristicilesindromului X-frasuntmultmaiputin severe.

26. Pentrucei care au reusitsaajungasaciteasca tot pana la sfarsit (adicaaici) de curs: