Genetics and Rett syndrome: As easy as apple pie! Victoria Mok Siu M.D., FRCPC, FCCMG ORSA conference Ottawa April 24, 2016 Objectives Review chromosomes and genes Understand s Explore the reasons behind the wide variation in features of Rett syndrome Understand the challenges of gene-based therapy Chromosome to gene to protein Rett syndrome is a GENETIC condition because it is caused by a change in a gene Rett syndrome is usually NOT INHERITED The Cookbook analogy 46 chromosomes, ~22,000 genes MeCP2 1
Variations in our genes make us each unique! DNA Gene Protein TV program analogy commercial DNA exon intron splice site promoter TV program RNA Commercial free program after splicing! protein amino acids Over 99% of time, Rett syndrome is due to a new typo (de novo ) If the arose at the time of formation of the egg or sperm, risk to have another child with Rett syndrome is the same as the population risk Many roles of MeCP2 methylation Turns genes off Organizes genes Turns genes on Brings genes together 2
Many roles: Spectrum of features in individuals with RTT Classic Rett syndrome Variant Rett syndrome Mild learning disabilities (females) or neonatal encephalopathy and syndromic or nonsyndromic intellectual disability (males) Why the differences? 1. Mutations are like typos 1. The 2. Boys vs girls 3. X-inactivation 4. Tissue distribution 5. Other genes What is the change? A substitution A girl with Rett syndrome nutmeg cinnamon not too bad! R133C The amino acid arginine (Arg or R) At position 133 in the protein Has become a cysteine (Cys or C) pommes pommes de terre Preserved speech variant (phenotype-genotype correlation) 3
Premature stop A girl with Rett syndrome R168X The amino acid arginine (R) At position 168 in the protein Has become a stop codon (X) Missing the last step: Bake in oven at 350F An addition of a letter A girl with Rett syndrome: Mix the sugar and apples in the bowl. Mix the sugar and dapple si nth ebow l. Results in a shift of the reading frame (frameshift) and everything afterwards does not make sens. In the cdna sequence c. 1189insA At position 1189 An A has been inserted Mutations are like typos 2. Boys vs. girls Where is the change? The Y chromosome contains very few important genes! Binding sites are important 4
3. X-inactivation Females don`t need to use all the genes on both X chromosomes! early in embryonic life, one X in each cell in a female is inactivated (Barr body) through methylation (=an epigenetic modification) X-inactivation In some cells, the X with the normal gene is active and in others, the X with the is active What is the ratio in the brain, the gut, the blood cells? This will presumably determine the degree to which these systems are affected 4. Mosaicism for MeCP2 The may have arisen AFTER conception and only be present in a percentage of cells The presence of some cells with 2 normal MeCP2 genes may result in less severe features 5
Why does the occasional family have more than one child with Rett syndrome? Gonadal (germline) mosaicism No No a may have arisen only in the reproductive organ of a parent (gonad) no effect on the individual but risk for recurrence in offspring Gonadal mosaicism 5. Other genes with RTT features Looks like apple pie but this is peach pie! Genetic counselling aids Greenwood Genetic Centre CDKL5 The same phenotype (appearance) but a different gene CDKL5 features sound familiar? Seizures early onset before 5 months- difficult to treat Absent speech Hand wringing Slowing of head growth Small cold feet Toothgrinding Air swallowing Heart long QT, heartrate too fast or too slow X-linked 6
MeCP2 duplication The gene products interact Hypotonia Progressive spasticity No speech Recurrent respiratory infections Seizures Affects males more severely than females Angelman syndrome Unsteady gait Frequent laughing, smiling, and excitability. Microcephaly Seizures FOXG1 s Onset of Rett syndrome features from birth No regression Usually don t show loss of milestones Pitt-Hopkins syndrome intellectual disability hyperventilation recurrent seizures (epilepsy) distinctive facial features. typically do not develop speech fetal fingertip pads Diagnosis of Rett syndrome and related disorders Sequencing of specific gene Targeted gene panels Rett and atypical Rett Epilepsy panels Developmental delay panels 7
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