Genetics 2 - Chromosomes: Structure and Abnormalities Flashcards by Michelle Doody

learning outcomes

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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

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chromatin structure

Bead - nucleosome - 150bp

DNA wrapped around

Modification of histones influences transcription

Phosphorylation
Methylation
acetylation

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euchromatin

gene switched on

active (open) chromatin

unmethylated cytosines (white circles)

acetylated histones

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heterochromatin

gene switched off

silent (condensed) chromatin

methylated cytosines (red circles)

deacetylated histones

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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)

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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)

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international system for human cytogenetic nomenclature (ISCN)

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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

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Fluorescent In Situ Hybridisation

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multiplex FISH (M-FISH)

metaphase chr

detects small Chr rearrangements

uses painting probes

paint whole Chr

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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

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deletion

part of chr deleted

sever effects

large deletions are lethal

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occurence of large deletions

example

> 5 Mb

visible under microscope (conventional cytogenetics)

e.g. crie-du-chat syndrome (5p)

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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)

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duplication

e.g.

chr section repeated (often sequentially)

less sever than deletion of Chr section

e.g. Potocki-Lupski syndrome dup(17p11.2)

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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

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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

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translocation

non-homologous chromosomes exchange segments

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reciprocal translocation

2 chromosomes swap non-homologus sections

chr number and genetic content are unchanged

common

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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

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Down Syndrome - chr change

4% is translocation (familial) DS

rob(14;21) or rob(21;21) + 21

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balanced translocations

no extra or missing material (reciprocal)

DNA just packaged incorrectly

phenotype usually normal

robertsonian translocations = balanced

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robertsonian

balanced translocation

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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