Genetics 3 - Haemoglobinopathies and Mutation Flashcards
learning outcomes
where is the H antigen located
what is it responsible for
H locus on chr 19 - fucosyltransferase
responsible for synthesis of a sequence of monomers (saccharides and related) on RBC surface molecules
what determines the ABO group
ABO locus chr 9
glycosyltransferase
codominance
2 alleles of the same gene which code for proteins with different specific functions are co-expressed (both alleles are expressed completely) in (compound) heterozygote individuals - rare
whereas incomplete dominance is a blending of traits, in co-dominance an additional phenotype is produced
e.g. AB blood group
O blood group
unmodified H antigen
A blood group
addition of N acetyl-galactosamine
B blood group
addition of N acetyl-glucosamine
AB blood group
addition of N acetyl-galactosamine and N acetyl-glucosamine
6 genotypes of ABO blood group antigen
homozygous
AA - A
BB - B
OO - O
heterozygous
AO - A
BO - B (O = null mutation)
AB - AB (co-dominant expression)
Hb tetramer - adults
2 x α globins - 141 AAs
2x non-α globins - usually β globins - 146 AAs
humans are diploid = 23 pairs of chromosomes
2 copies (often different alleles) of each gene
Hb genes have BIALLELIC EXPRESSION
both paternal and maternal alleles are expressed - both alleles need to be working for normal Hb synthesis
function = carry O2
β and α gene cluster - on which chromosomes
In order of how they are expressed from development to adulthood
changes in globin synthesis in embryonic development
https://www.youtube.com/watch?v=vhB0oNLYIqo
HbFF
HbA
HbA2
HbS - sickle cell anaemia
what happens to free Hb
catabolised and excreted (renal)
how is Hb prevented from being lost
packaged in erythrocytes
conc of Hb in RBCs
320-350g/L of cytoplasm
close to limit of solubility of Hb in physiological solution
Hb - what is and isn’t soluble
globin chains (monomers) - not soluble
tetramer - highly soluble
what happens when Hb exceeds solubility limit
polymerisation and precipitation
distorted RBC shape and impaired function
RBC lysis
release of Hb
once transcription of globin genes is activated
lots of Hb is made
how is Hb gene expression coordinated
by chromatin restructuring
correct proportion of α and β chains requires co-ordinated gene expression from 2 chr
safety valve (protease) for degradation of α chains can correct some excess of α globins
- finite capacity - easily overloaded
region responsible for regulation of Hb synthesis
LOCUS CONTROL REGION
1000s of bps upstream of β globin gene cluster
Required for expression of non-alpha globin genes and enhances expression of link genes at distal reg. sites by recruiting chromatin modifying co-activator and transcription complexes
HS - hypersensitive site
Short regions of chromatin sensitive to cleavage binding nucleases
LCR - cis acting reg. region
Encoded on same molecule it is acting on
HS 40 - cis acting - same gene structure it regulates
transcriptional regulation
cis acting sequences (acting from the same molecules)
⇒ act on the DNA strand on which they are encoded
do not encode for peptides
promoters/enhancers/silencers
haemoglobinopathies
normal quantities of globins that have abnormal sequences
causes globin chain polymerisation and misshapen RBCs
e.g. sickle cell disease
due to mutation
thalassemias
normal globin chain sequences but the different chains are not in correct proportions
not enough Hb (anaemia) and/or abnormal accumulation of globin subunits (toxic)
caused by mutation
sickle cell anaemia cause
what does it result in
caused by mutation in β globin gene sequence
under conditions of low O2 tension
polymerisation of Hb
distortion of RBC shape and function
obstruction of small BVs
intravascular haemolysis
splenomegaly
genotypes of sickle cell
codon for glutamic acid becomes a codon for valine
gene map locus 11p15.5
1 mutated allele = HbAS ⇒ sickle cell trait
2 mutated alleles = HbSS ⇒ sickle cell anaemia (no normal HbA)
how to diagnose sickle cell
Hb electrophoresis
based on differing charge of the different Hb tetramers and their differing migration patterns in an electric field
thalassaemias
most common genetic disorders of Hb
inherited condition characterised by defects in the balanced biosynthesis of normal Hb globin chains
results in:
- not enough Hb (anaemia)
- abnormal accumulation of globin subunits
gene for β globulin
HBB on chr 11
2 copies expressed - 1 on each chr
2 types of β-thalassaemia
MINOR
heterozygous mutation
1 defective gene copy
MAJOR
homozygous/compound heterozygous mutations
both gene copies defective
how many HBB variants have been described
where are there problems
transcription (promoter)
processing of mRNA
translation of mature mRNA
post-translation integrity of β globin
β-thalassaemia minor (β-thal trait)
2 types
genotypes
symptoms
heterozygous for defective β globulin expression
either β0 (absent) or β+ (reduced)
GENOTYPES: β/β0, β/β+
clinically asymptomatic or mild symptoms
mild microcytic anaemia
small and hypochromic RBCs
β-thalassaemia major
both copies of chr 11 affected
homozygous for defective β globulin expression
either β+ (reduced) or β0 (absent)
GENOTYPES:
β0/β0,
β+/β+,
β0/β+ (compound heterozygote)
serious illness requiring lifelong transfusions
genes for α globulins
Hbα1
Hbα2
on chr 16
all 4 copies expressed - 2 on each chr
4 types of α-thalassaemia
- silent carrier - 1 defective locus
- α-thalassaemia trait - 2 defective loci
- HbH disease - 3 defective loci
- α-thalassaemia major/HbBart - 4 defective loci ⇒ hydrops faetalis
α+ thalassaemia
1 defective locus
α-thalassaemia minima or silent carrier
clinically asymptomatic
α0-thalassaemia
2 defective loci
α-thalassaemia minor/trait
often clinically asymptomatic but mild cryptic symptoms
mild hypochromic microcytosis
mild anaemia
α thalassaemia trait - asian/mediterranean vs african populations
A/M - common deletion of both copies of gene from 1 chr 16 (cis deletion) with 1 normal chr 16
African - 1 gene missing from each of 2 copies of chr 16 (trans deletions)
severe α-thalassaemias - HbH disease
HbH disease
3 defective loci
most common in Asian heritage
need 1 chr with no α gene
β chain excess and production of HbH (4 x β globins)
severe α-thalassaemias - α-thalassaemia major
HbBart
4 defective loci
hydrops faetalis
no α produced
get production of 4 γ tetramers
no O2 released to tissues and foetus dies
most often gene deletions
things to remember
learning outcomes
which of the following blood types are least likely for parents of a girl with blood type O
AB and B
if girl is blood group O, only 1 possible genotype - OO homzygous
so she must have gotten O allele from both parents
production of DNA
DNA → protein
mutation
permanent heritable change in nucleotide sequence of a gene or chr
change in:
genomic DNA - g.
complimentary (coding) DNA - c.
protein - p.
G > A ⇒ G change to A
classifications of mutations (5)
- deletions
- insertions
- substitutions (missense, nonsense, splice site) - No change in number of bases - 1 base swapped out for another
- frameshifts - may arise from deletions or insertions
- dynamic mutation - tandem repeats e.g. HD
3 functional consequences of mutations
- loss of function - inactivating - protein has no/less function - often recessive
- gain of function - activating - increase in normal gene function e.g. increased gene expression or different and abnormal function - usually dominant
- silent mutations - multiple codons for the same base - redundancy
2 categories of silent mutations
- synonymous nucleotide change - no change in AA sequence - multiple codons for each AA
- non-synononymous nucleotide change - change in AA sequence that results in no change of function (AA produced is similar to the original one)
amorph
- effect on normal function
- heteroxygote pattern
- example
- complete loss
- recessive but dominant if haploinsufficient
- O blood type allele
hypomorph (reduced)
- effect on normal function
- heteroxygote pattern
- example
- partial loss
- recessive but dominant if haploinsufficient
- CFTR mutations in CF
hypermorph
- effect on normal function
- heteroxygote pattern
- example
- increased
- dominant
- EGFR oncogene in cancer
antimorph (dominant negative)
- effect on normal function
- heteroxygote pattern
- example
- antagonistic
- dominant
- FBN1 mutations in Marfan syndrome (Proteins that function in a mixed multimer - Collagen network can’t form)
neomorph
- effect on normal function
- heterozygote pattern
- example
- different/new
- dominant
- BCR-ABL fusion protein in CML
isomorph (silent)
no effect on normal function
no heterozygote pattern
haploinsufficient
a single copy of wild-type allele is not sufficient for normal phenotype
Hypomorph - reduced function - Hb does form a mixed multimorph, this 1 is NOT INHERITED IN A DOMINANT FASHION
Neomorph - fusion protein - new function - dominant
If it’s a fusion protein it is more than likely neomorph
case description of sickle cell anaemia
acute chest syndrome
sickle cell crisis
chronic pain
organ damage
swelling in hands and feet
bacterial infections
autosplenectomy by mid-childhood
require immunisation against common pathogens and prophylactic ABs
sickle cell prenatal diagnosis
chorionic villous sampling (9-10 weeks)
PCR amplify fragment of β globin gene
oligonucleotide probe hybridisation/sequencing
can also do amniocentesis - need to grow cells in lab first to do PCR
sickle cell screening
isoelectric screening
certain at risk mothers are screened
why is sickle cell anaemia incidence lower in African Americans than in Africans
Evolutionary pressure on Africans to inherit this because it protects them from malaria
evolution
pop. genetics - change in freq of an allele in a population over time
adaptation
a heritable trait that aids the survival and reproduction of an organism in its current environment
polymorphism
2 or more discontinuous (different) forms occur in a single population in the same place at the same time
single (panmitic) population means random (unrestricted) mating within the group
height is
NOT a polymorphism
balanced genetic polymorphism
simultaneous occurence in the same pop. of 2+ “discontinuous” genetic forms in “such proportions” that the frequency of occurence of the rarest of them cannot be explained just by recurrent mutation or immigration
such proportions = freq of at least 1% of alleles
something in the environment is acting to select for maintenance of equilibrium (balance) between the different forms in the population
i.e. natural selection
HbAS sickle cell trait
confers partial resistance to malaria
sickle cell and balanced genetic polymorphism
HbSS (sickle cell disease) = an inevitable consequence of selection pressure for the maintenance of the heterozygous state
if the heteroZ state was not advantageous you would expect the extinction of HbSS by -ve selection
sickle cell anaemia
HbSS
character/trait
clinically manifest phenotype
pattern of inheritance is recessive
usually parents do not have sickle cell anaemia
sickle cell trait
HbAS
different character/trait
cryptic phenotype
pattern of inheritance is dominant
parent almost always has HbAS
genetic context and thalassaemias
a specific β globin allele associated with different phenotype
depending on co-inherited modifying factors
level of expression of HbF
level of expression of α globin
* Sequence of globin genes is correct - Proportion being produced - imbalance
β globin gene mutations associated with β-thalassaemia
gene associated with β thalassaemias
HBB gene promoter - where transcription machinery binds
position is normally A
nutation A-G = no binding and no transcription
common in black people with thalassaemia
also occurs in chinese people with thalassaemia
disease associated with the point mutation differs in different ethnic groups
differences in ability to compensate by synthesis of HbF in response to erythroid stress
β thalassaemias and LCR deletions
genes may be fine but regulation of gene expression is critical to function LCR deletions
β globin gene is structurally normal
DNA sequence is normal for 500 bp 5’ to 3’
large 5’ deletion
far less β globin produced (no enhancer)
prenatal diagnosis of β thalassaemia
Hb electrophoresis of parent’s blood first
then CVS or amniocentesis and PCR
blood of baby won’t work
not expressing β globin to sufficient levels for clear delineation of hetero/homoxygous
abnormal face shape - β thalassaemia
physiological response that represents an effort to compensate for the physiological deficit associated with the inherited mutation
hypoxia - high EPO - bone marrow hyperplasia
increased haematopoiesis distorts bones
things to remember
