Download Molecular basis of G6PD deficiency

Molecular basis of G6PD
deficiency
Dr Narazah Mohd Yusoff
Director, Human Genome
Center,
USM
Introduction
G6PD deficiency – one of the most common
inherited disorders, 400 million people affected
–tropical and subtropical countries
Epidemiological and in vitro studies - selection
advantage during Plasmodium falciparum
infection
Most affected individuals asymptomatic,
however risk of acute haemolysis
Genetics
1989 - more than 400 variants of G6PD
Different biochemical forms of the enzyme
exhibited, distinguished by different
electrophoretic mobility, kinetic properties.
Variants divided 5 classes according to the
residual enzyme activity based (WHO).
Mediterranean and African (A-) variants - by far
the most clinically significant.
Enzyme activity scarcely detectable in the
Mediterranean type but close to normal in the
African variant.
Genetics
Thus, latter variant is considered to be the
less severe form G6PD deficiency
More recently, the dev.- new gene cloning
techniques and complementary DNA
sequencing techniques - identify the
precise mutation of variants
 Many variants previously thought to be
unique have proved to be identical.
Genetics
Majority of the variants - from a single
point-mutation resulting in amino acid
substitution in gene encoding for G6PD
located at the Xq28 region on the tip of
the long arm of the X- chromosome.
The G6PD gene
TABLE 1 Molecular data on human G6PD
DNA
Size of gene (in kilobases)
Number of exons
13
Number of introns
12
mRNA
Size (in nucleotides) 2269
Protein
Number of amino acids
18.5
515
Molecular weight (in Daltons)
59,265
Subunits per molecule of active enzyme 2 or 4
Genetics
G6PD deficiency - genetically heterogeneous
condition.
This explains why G6PD deficiency is
predominantly a male syndrome.
Males possess only one copy of the gene, thus
they are either normal or G6PD deficient
However, females - can be either normal,
heterozygous or homozygous
It is often quite difficult to differentiate between
them based on the phenotypic expression alone
Genetics
Heterozygous females with G6PD deficiency two red blood cells populations, one normal and
one that enzyme deficient.
One copy of the gene governs the synthesis of
the normal G6PD and the other makes the
variant.
In the early embryonic stage of life one of the
two X-chromosomes is inactivated, hence each
cell afterwards synthesise only either the normal
or the deficient enzyme
Genetics
This phenomenon - insight into why female
heterozygotes appear to have a greater
resistance against P.falciparum malaria than do
deficient male hemizygotes.
The growth of malarial parasites - impaired
upon first passage from normal to G6PD
deficient RBC.
Through subsequent passages, they could adapt
and grow normally though by inducing their
own production of G6PD to protect it against
oxidative damage.
Genetics
In heterozygous G6PD deficient females
however, such adaptation is not possible
due to the presence of a mosaic of
deficient and normal red blood cells, thus
the protection against malaria
G6PD and its resistance to
malaria
Interesting - the incidence of G6PD deficiency relatively high in places where P. falciparum
malaria has been a life-threatening factor for
centuries, as in the Mediterranean but rare
elsewhere.
This distribution - typical of other main
genetically controlled traits, e.g. sickle cell
anaemia, thalassaemia, HbE and persistent
foetal haemoglobin (HbF).
Apparently, - a positive biological selective
pressure for the emergence of these mutations
G6PD and its resistance to
malaria
Several attempts -made to associate other
conditions with G6PD deficiency based on the
study of genetic locations.
 Many other genes located on X-chromosome
besides the G6PD gene.
 By examining genes located close to the Xq28
region, a possible linkage with the expression of
G6PD enzyme can be postulated.
Genetics
E.g the proximity of genes for red and green
pigments to Xq28 has lead to the suggestion of
a linkage between G6PD deficiency and
congenital colour blindness by Yucel.
A study was carried out in Turkey to test the
hypothesis.
It was found that that none of the colour blind
subjects were G6PD deficient and vice versa.
G6PD and its resistance to
malaria
Based on the results obtained Yucel concluded
that there was a linkage disequilibrium between
congenital colour blindness and G6PD
deficiency, which may be due to crossing over of
the closely situated genes during meiosis
 Nevertheless, this does not preclude
conclusively the possibility of a linkage between
G6PD deficiency and other clinically significant
conditions.
G6PD and its resistance to
malaria
The answer - the malarial parasites which show
a surprisingly high susceptibility to oxidative
stress require glutathione and the products of
the G6PD oxidative shunt for optimal growth.
Not unlike the sickle cell trait which utilises an
independent mechanism of protection against
malaria, G6PD deficiency creates an inhospitable
environment for the malarial parasites and
discourage lodging of the protozoa in the red
blood cells
Summary
G6PD deficiency – commonest
enzymopathy
Varied clinical manifestations
Genetic heterogeneity
Malaria protection
Public health problem