Purine nucleoside phosphorylase (PNP) deficiency
by Jody L. Duke
Purine nucleoside phosphorylase (PNP) deficiency is a very rare genetic
disease. It is an autosomal recessive disorder, so both the father and the
mother must pass a defective allele on to their child.
PNP is an enzyme encoded by a gene on chromosome 14 that is 9000 bases long.
This gene has 6 exons and 5 introns. When PNP is assembled, three PNP subunits
join together to form a trimer. Each subunit has its own bonding site, so
they can work independently even while joined together. Consequently, a
person heterozygous for this disorder can still function normally because
they have a normal allele to code for an active protein.PNP is supposed
to free the base guanine from its sugar and phosphate so that it can be
used again. When PNP is defective, the guanines with phosphate and sugar
still attached (called dGTP) build up and inhibit another enzyme, called
ribonucleotide reductase, from working. When ribonucleotide reductase is
inactive, DNA synthesis can't occur and cells can't replicate.
The lack of cell replication is especially critical in the immune system.
T-cells in the immune system are needed to fight infections, and B-cells,
when activated by T-cells, engulf and destroy pathogens. When PNP is defective,
T-cells can't replicate and therefore can't activate B-cells or combat infections,
causing a condition known as SCID (severe combined immune deficiency). Children
with PNP deficiency are highly prone to infections, autoimmune disorders,
neurological impairment, and cancer. They are more susceptible to cancer
because they don't have T-cells to secrete biochemicals to kill the cancer
cells. These children usually die in their teens or earlier of infection
or cancer. However, if PNP deficient children are protected from infections
and carcinogens, their chances of surviving longer will be increased.
PNP deficiency was initially described in 1975, and since then 32 other
patients with the disorder have been documented. Since this disease is very
rare, not much is known about it. We are just beginning to learn the genetic
basis of PNP deficiency.
In the gene for PNP that is 9000 bases long, four point mutations are known
to be linked to the disorder. These mutations have been described in papers
by Lucy Andrews and several colleagues. In an allele of one PNP deficient
patient, a missense mutation in exon 2 at amino acid position 51 changed
the sequence AGT to GGT, which changed the amino acid from serine to glycine.
These two amino acids are similar in size and shape, so this mutation doesn't
affect the enzyme. This may be a mutation that occurs throughout the population
but it doesn't have a visibly different phenotype. Another mutation was
shown in exon 4 at amino acid position 128. GAT is switched to GGT, causing
aspartic acid to change to glycine. This mutation affects the charge of
the molecule and therefore renders the protein inactive. A third mutation
was found at amino acid position 234 when CGA was changed to CCA, exchanging
a proline for an arginine. These two amino acids are differently charged
and position 234 is in a very tight turn. Proline changes the entire structure
of the protein so it is now inactive.
These three mutations were described in a patient in 1992. A fourth mutation
was described in a patient in 1995, yet this mutation was not describe in
the patient studied earlier. This mutation is a switch from G to T in the
very last base of exon 2. When the intron between exons 2 and 3 was spliced,
the splicing machinery did not stop at exon 2. Instead is followed all the
way through to intron 1. This means that exon 2 was mistakingly spliced
out and now the PNP protein is lacking that exon. Also, a more serious problem
is created. The G to T switch occurs at the first base in a codon. This
first base is indeed part of exon 2, but the remaining two bases of the
codon were in exon 3. With exon 2 gone and that first base of the codon
missing, the rest of the gene is frameshifted, rendering it ineffective.
Most of these discoveries of mutations leading to PNP deficiency occurred
using standard genetic techniques. The DNA of the PNP deficient patients,
the patient's parents, and a normal subject were isolated. Then the DNA
was cut with restriction enzymes, denatured into single strands, and amplified
using PCR. The amplified DNA was sequenced by gel electrophoresis and blotting
techniques. Several methods, like the Bethesda Research Laboratories dsDNA
sequencing kit, were used to accomplish the sequencing.
The discovery of these different mutations has a very important implication.
PNP deficiency is clearly caused by different defects in the gene. It may
be difficult to detect with genetic testing because it is unclear how many
and which kind of mutations we are searching for. Surely, even more causes
of PNP deficiency will be documented in the future, making detection even
Andrews, Lucy G., Michelle R. Aust, and Michael Barrett. "Molecular
Analysis of Mutations in a Patient with Purine Nucleoside Phosphorylase
Deficiency." American Journal of Human Genetics 51 (1992): 763-72.
Andrews, Lucy G., and Louise M. Markert. "Exon Skipping in Purine Nucleoside
Phosphorylase Deficiency mRNA Processing Leading to Severe Immunodeficiency."
The Journal of Biological Chemistry 267 (1992): 7834-8.
Klug, William S., and Michael R. Cummings. Concepts of Genetics. New Jersey:
Prentice Hall, 1994.
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