Genetics 2 - Chromosomes: Structure and Abnormalities Flashcards
learning outcomes
chromosome structure
Chromatin = DNA and protein
Protein - histones - tightly packing
1m of DNA into each of cells
Important for structure and control of transcription of DNA
chromatin structure
Bead - nucleosome - 150bp
DNA wrapped around
Modification of histones influences transcription
- Phosphorylation
- Methylation
- acetylation
euchromatin
gene switched on
active (open) chromatin
unmethylated cytosines (white circles)
acetylated histones
heterochromatin
gene switched off
silent (condensed) chromatin
methylated cytosines (red circles)
deacetylated histones
cytogenetics
- sample nucleated cells you can grow easily - T cells/skin fibroblasts/bone marrow/chorionic villi or amniotic fluid (antenatal)
- get chromosomes into a form in which they can be observed and differentiated from each other (metaphase chromosomes - grow the cells to dividing stage)
- chemical preparation to arrest cells and lyse nuclei
- stain chromosomes (giemsa)
conventional cytogenetics
limitation
identify chromosomes by banding patterns
arrange homologs side by side
⇒ karyogram
described as a karyotype
46, XX female and 46, XY male
limitation = resolution
detects GROSS chr changes (> 5Mb)
international system for human cytogenetic nomenclature (ISCN)
molecular cytogenetics
MOA
what is it used for
probe hybridisation - visualisation of location of specific nucleotide sequence (< 5Mb)
USES
is sequence present and if so on which Chr
are 2 sequences close to each other
preimplantation genetic screening
Fluorescent In Situ Hybridisation
multiplex FISH (M-FISH)
metaphase chr
detects small Chr rearrangements
uses painting probes
paint whole Chr
structural chr abnormalities
categories
heritability
change in chr structure
caused by chr breakage with subsequent abnormal realignment
BALANCED: no gain/loss of genetic content ⇒ often not clinically relevant but relevant if they interrupt a gene with dominant loss of function
Or GOF via fusion of 2 unrelated segments - CML
UNBALANCED: gain or loss of genetic content ⇒ clinically relevant
structural Chr abnormalities are heritable only if in germ line
deletion
part of chr deleted
sever effects
large deletions are lethal
occurence of large deletions
example
> 5 Mb
visible under microscope (conventional cytogenetics)
e.g. crie-du-chat syndrome (5p)
smaller deletions occurence
how are they visualised
e.g.
microdeletions are not visible under light microscope
some visible with FISH
smaller ones using chr oligonucleotide arrays - microarrays
e.g. Smith-Magennis Syndrome del (17p11.2)
duplication
e.g.
chr section repeated (often sequentially)
less sever than deletion of Chr section
e.g. Potocki-Lupski syndrome dup(17p11.2)
copy number variants
Mbs
role in….
deletions/duplications of genomic sequence
1 Kb → Mbs (variable no of repeats of a particular unit of sequence)
major determinants of human genetic variation - 5-10% of genome is CNVs
can represent functional (disease causing) mutations as well as genomic polymorphisms of uncertain relevance
inversions
e.g.
2 breaks occur in chr
part of chr in between is rotated 180 degrees before re-joining
results in looped chr pairs in meiosis I
can cause recombinant chrs if cross over happens within loop
e.g. F8 inversions (intron 22) in haemophilia A
translocation
non-homologous chromosomes exchange segments
reciprocal translocation
2 chromosomes swap non-homologus sections
chr number and genetic content are unchanged
common
robertsonian translocation
break occurs at the short arm of acrocentric chromosomes (13, 14, 15, 21, 22, Y) and results in fusion chr
change in chr number (45) - short p arms lost
Down Syndrome - chr change
4% is translocation (familial) DS
rob(14;21) or rob(21;21) + 21
balanced translocations
no extra or missing material (reciprocal)
DNA just packaged incorrectly
phenotype usually normal
robertsonian translocations = balanced
robertsonian
balanced translocation
unbalanced translocation
pathogenic
extra or missing material
miscarriage or baby with multiple congenital abnormalities
parents with balanced reciprocal translocations can generate gametes with balanced or unbalanced chr complement
increased risk for all their children
chronic myeloid leukaemia (CML)
translocation of what chr
type of mutation
associated protein
reciprocal translocation between q ends of chr 9 and 22
t(9, 22)(q34.1;q11.2)
philadelphia chr is a somatic mutation - not inherited
BCR-ABL fusion protein with “always on” tyrosine kinase activity that promotes cellular proliferation
GOF mutation for haematopoietic stem cells
acute promyelocytic leukaemia (APML)
results in
presence of _____ in blood smears ⇒ APML
cancer of bone marrow/WBCs
abnormal accumulation of immature granulocytes (promyelocytes) in bone marrow
shortage of normal WBCs, RBCs and platelets
presence of promyelocytes containing multiple Auer rods on peripheral blood smear is highly suggestive of APML
characteristic cytogenetic abnormality in 95%
type of mutation (APML)
chr involved
reciprocal translocation between chr 15 and chr 17 t(15;17)q(24;21)
somatic mutation - not inherited
symptoms of APML
bruising
bleeding
susceptible to infection
how can APML be seen
on conventional karyotype
fusion protein of APML
PML-RARα
MCQ
things to remember
ploidy
no of complete sets of chr in a cell and hence the no of possible alleles for autosomal/pseudoautosomal genes
how is ploidy defined
in terms of genetic content (n) or chromosome number/amount of DNA (c)
N changes when type of genetic content changes, not with replication of chromatids
C changes when the chr number changes e.g. following DNA replication (S phase)
baby of a man and woman
meiosis in baby
homologous recombination in meiosis
when does recombination occur
during meiosis I
homologs align
crossover
separate the homologous chr
paired chromatids stay together
Haploid (1n) after meiosis I
MEIOSIS II
chromatids separate
4 cells - haploid gametes after meiosis II
2 with mixture
crossover in sex chromosomes
FEMALES
X chr pairing is similar to autosomes
MALES
X and Y chr do pair
crossing over occurs at a small section at tip of short arm (pseudo-autosomal region)
purpose of sex in biology
generate genetic diversity
how many possible chr combinations in gametogenesis
> 8 million
223 or 2n
numerical chr abnormalities
change in chr number
arise from errors in mitosis and meiosis
not inherited - de novo
euploidy
normal diploid chr complement (2n) of 46 chr (46, XX or 46, XY)
haploid
(n) normal chr no in the gametes of 23 chr (23, X or 23, Y)
aneuploidy
chr no varies from normal diploid complement
patau syndrome chr complement
47, XY, + 13
polyploidy
multiples of the haploid chr complement of > 2n
triploidy
triple chr complement (3n) e.g. 69, XXX
trisomy
3 copies of a chr
Edward’s syndrome
47, XX, +18
monosomy
1 copy of a chr
turner syndrome
46, X
only viable monosomy
interphase FISH
what does it analyse
what is there no need for
efficiency
what does it detect
analyse nuclear DNA
analyse amniocentesis directly
no need to grow cells
partial info in < 24 hours
detects common trisomies e.g. chr 18 (photo)
klinefelter syndrome
47, XXY
47, XXX
female
mostly normal
47, XYY
male
tall
mostly normal
patau syndrome
47, XY, + 13
translocation down syndrome
46, XY, rob(14;21)(q10;q10), +21
karyotype for CML
46, XX, t(9, 22)(q34.1;q11.2)
trisomy 21/Down’s
47, XY, +21
balanced robertsonian translocation
45, XX, rob(14;21)(q10;q10)
symptoms of Down’s
floppy - hypotonic
single crease across his palm
large tongue
atypical facial features
only autosomal trisomy compatible with survival into adulthood
variable degree of intellectual disability
increased risk of congenital defects of heart and digestive system
decline in cognitive function is common after 50
alzheimer’s in 50%
survival rate of Edward’s and Patau’s
rare to survive into adulthood
normal chr number
trisomy and monosomy
why is recurrence more likely in oogonia vs sperm
+ greater risk with older age of mother
long phase of meiosis in females
ageing of oocytes
sperm - turnover in meiosis I is quick
how could DS arise if there was a normal number of chromosomes (46 XY)
another way that trisomy/monosomy can occur, apart from NDJ
robertsonian translocation
break occurs at short arm of ACROCENTRIC chromosomes (13, 14, 15, 21, 22, Y) and results in a fusion chr and risk of trisomy/monosomy in children
4% of DS is translocation (familial) DS
rob(14;21) or rob(21;21)
how can a parent with robertsonian translocation have a healthy child
short arm - no important genetic info
RISK FOR OFFSPRING OF THESE CHILDREN
can a parent with a robertsonian translocation resulting in an isochromosome 21 have a healthy child
case
APML
can be diagnosed with FISH (preferrably) or conventional karyotyping
RAR gene - FISH and APML
retinoic acid receptor
controls differentiation of WBCs beyond promyelocyte (Chr 17)
PML - FISH and APML
functions in haematopoiesis and late erythropoiesis
tumour suppressor gene
chr 15
PML-RARα - FISH and APML
protein function
WBC stuck at promyelocyte stage and proliferate abnormally
what do probes normally do
what do they do in APML
probes normally hybridise to different chromosomes
in APML, both probes hybridise to just 1 chr (PML-RAR fusion) chr 17
treatment of acute promyelocytic leukaemia
ATRA (All Trans Retinoic Acid)
vit A derivative
interacts with RARα to induce cell differentiation and apoptosis
vit A derivatives can play a role in managing this type of leukaemia
help blood cells to differentiate
things to remember
