Lesson 23 Flashcards
what four ways can DNA methylation cause human disorders?
- damage on DNA that cause tumors or genetic diseases
- aberrant silencing of gene expression causing genetic disorders
- aberrant expression of specific genes (cancer)
- diseases associated with imprinting Beck-Wiederman syndrome or Angelman syndrome
what is one of the of the main damages to DNA that causes tumors or genetic disease?
damage resulting from deamination
what do we mean when we say “aberrant gene expression”?
quite a large number of genes are affected by the process → oftentimes a mutation in a reader or writer
what is Rett syndrome (RTT)?
a generic and progressive neurological disorder (rare) that mainly affects females
describe the early, post natal phase of RTT:
appears symptom free: there are no issues detected - only detectable through prenatal genetic testing
describe the second phase of RTT:
there a developmental stagnation followed by a regression phase → during this phase they start to be disinterested in the world outside (unresponsive) and can become violent (screaming) or lose all skills they have previously developed
what happens to the brain during the regression phase?
the growth of the brain slows down and patient suffer from microcephaly
what happens after the regression phase?
the disease stabilizes but severe neurological damage is already done and many are unable to express themselves or communicate
what are some of the main clinical features of RTT?
severe ID, loss oc expressive language, loss of hands use, stereotypical hand movements. apraxia, breathing abnormalities, epilepsy, scoliosis, bone fragility, heart issues
what is the first and second hallmark of RTT?
regression followed by loss of the use of their hands
what chromosome is RTT linked to?
X → only in females
describe the specific gene mutated in RTT:
gene is X-linked and encodes for methyl-CpG-binding protein 2 (MeCP2), a reader of DNA methylation that brings histone acetylases on DNA → is a transcriptional repressor that binds methylated DNA and silences gene
expression reading methylated CpGs without sequence specificity
describe MeCP2:
highly abundant nuclear factor that is ubiquitously expressed (particularly in neural tissue) and in mature neurons appears t function as a chromatin condensing factor
what type of disorder is RTT:
mongenic
what disorders to MeCP2 mutations cause in males?
neonatal encephalotomy (severe intellectual disability, seizures, and breathing issues) and Rett syndrome (mother has mutation on one X and passes it to son, much less severe but presents symptoms)
what type of mutation is RTT associated with?
loss of function
what is an example of MeCP2 gain of function mutation?
MeCP2 duplication sundrome
what is MeCP2 duplication syndrome?
the subjects have 2 or 3 copies of the wild type gene (more copies the worse the phenotype) - resembles RTT → intellectual disability, bad seizures, respiratory infections due to the fact that they cannot swallow properly due to breathing issues
who is the carrier for MeCP2 duplicate syndrome?
the mother
what three things is the clinical severity of RTT dependent on?
- x-activation patterns
- MeCP2 mutations
- presence of modifier genes
what are the two main types of MeCP2 mutations?
- premature stop codon and framshifts
- missense mutations in the TRD or CTD or methyl binding domain
why is it easier to study KO MeCP2 in male mice?
they are hemixygous, therefore everything is more homogenous → the onset is earlier and more severe
what is an issue with many pre-clinical studies being performed only on male mice?
because of the hormones, female mice behave diversely depending on which period they are in and this poses an issue for understanding the disorder
what occurs in the brains of KO MeCP2 mice?
there is a reduced volume and it is believed this reduction is not given by the loss of cells, but is given by the fact that the neurons that are without MECP2 are smaller in soma, less arborized, they do have less spine in synaptic contacts and all of these leads to a brain
in which the number of cells seems to be the same with respect to a normal one, but everything has a higher densities, and this leads to a smaller brain
what happens if there are less spines in the mouse brain?
there are going to be less synapse → this is a synaptogenesis disorder = there are less contacts and networks are not functioning properly
what is there an imbalance between in KO MeCP2 brains?
imbalance between excitatory and inhibitory activity → imbalance changes depending on the brain district so there are regions that are hyperoxidated and regions that are hypooxidized
what did BA inject at high dosages to go from no MeCP2 to normal MeCP2 expression in mice?
tamoxifen
what are two dangers of transducing the new gene into the affected cells?
- the issue of getting good viruses with good diffusion in the brain and to be able to control the level of those dosages
- the general toxicity of the high dosage of MeCP2 the the virus could induce
what are the goal of potential pharmacological approaches?
meant to modulate the molecular pathways disrupted by the impairment of MeCP2 activity
what is MeCP2 also relevant for besides wild type neurons?
astrocytes → it has been proven that astrocytes secrete factor that are detrimental for the neurons, so we can try to understand what those facts are and how to isolate and delete them
if a drug is able to be created for RTT, when will it most likely need to be given?
the patients entire life
how do stem cells act in neurons?
function through bio standard mechanisms → if we transplant stem cells in a mouse that is affected by stroke or multiple sclerosis (conditions in which
there are stimuli that attract the cells), stem cells go to the brain and start to secrete factors that are useful for the brain itself
what is lacking in Rett syndrome?
it is well known that a neutrophil factor is lacking →BDNF: but you cannot provide BDNF because it doesn’t cross the blood brain barrier
if we treat KO neurons with NPCs the phenotype is rescued, what does this mean?
shows that NPCs are able to secrete something that has a positive effect on defective neurons → could be a good cure but in particular can be helpful for identifying which pathways might be effective
what is Rett syndrome most likely caused by?
subtle activation or dysregulation of different genes (those patents who have too much “transcriptional noise” in their brains) → defined as a dominant disorder
what is ICF syndrome?
immunodeficiency, centromere instability, and facial anomalies syndrome → rare genetic recessive disorder in which the mutated gene is mainly DNMT3B
how are these patents diagnosed?
they present with multiple recurrent infections (the number of B cells is incredibly reduced) and it is possible to find aberrant chromosome shape
what is DNMT3B?
a de novo methyl transferase that methylates the centromeric and peri-centromeric DNA, particularly chromosomes 1, 9, and 16
how is ICF inherited?
inherited from the parents → from two healthy carriers and gain two mutated alleles
where are the majority of the mutations in ICF patients?
most are in the catalytic domain: these are hypomorphic DNMT each they are function but greatly reduced - sometimes there are also mutations in the amino terminal portion of the protein
what region is sometimes mutated in ICF patients?
PWWP → important in recognizing some specific post translational modification of histones
what do we know for sure about ICF?
- there is a general reduction of methylation, so is
not only the pericentromeric heterochromatin; in general, there is a reduction of 40% in these patients
and this is occurring everywhere (in every region) - we can see a decrease of
methylation in different regions, and this leads to spurious transcription
→ we have a disorder in which we have an unscheduled transcription, the activation of non-coding RNAs and
microRNAs, transcriptional start sites that should not be used are activated: so, we have different defects in gene expression that leads to this phenotype
how does chromatin tie into epigenetic pathways that cause cancer?
chromatin can express different transcriptional factors, DNA sequence mutations can affect those transcriptional factors and different epigenetic mechanisms might go to the affect the gene expression leading to cancer
what two contradictory features does cancer generally have?
hypomethylation and hypermethylation
where does hypomethylation in cancer usually affect?
generally affects regions where methylation mostly occurs = the regions relevant for genomic defense
what two things can hypomethylation induce?
can either induce activation, and this is dangerous for genome stability, or it can activate sequences that can start to jump around or produce viruses
where can hypomethylation occur?
oncogenes
