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When Amino Acids Don’t Get Along:
Neonatal Alloimmune Thrombocytopenia
Wauwatosa West SMART Team
Shazia Ali, Danielle Perszyk, Ben Schrank
Advisor: Mrs. Case
Mentors: Drs. Debra and Peter Newman, Blood Research Institute
3D Structure of the GPIIIa PSI Domain
Abstract
Platelets, also called thrombocytes, are required to control bleeding. Alloimmune thrombocytopenia is a
disease that results when an individual makes antibodies that bind to proteins on another individual’s
platelets. Neonatal Alloimmune Thrombocytopenia (NATP) occurs when a mother makes antibodies
that bind to her baby’s platelets. In this disease, the mother's antibodies on the fetal platelets can cause
them to be cleared by the immune system or prevent them from working properly, resulting in severe
bruising and hemorrhaging. Once the antibodies are gone, the baby’s platelets will then function properly
and initiate clotting. The baby is in danger when the antibodies are present at or before birth because of
the possibility of intracranial bleeding can lead to severe brain damage.
Red: Beta Sheets
A major target for antibodies in NATP is the glycoprotein IIb-IIIa, (GPIIb-IIIa), which is made up of two
subunits, GPIIb and GPIIIa, and is expressed only on platelets. NATP most commonly occurs when a
mother has the amino acid proline (Pro or P) and her baby has a leucine (Leu or L) at position 33 of the
GPIIIa subunit.
Scientists have struggled for years to solve the structure of the region within the GPIIIa subunit that
contains the L33P polymorphism. They have recently determined that it folds into a structure called a PSI
domain. We have built a model of the PSI domain of GPIIIa, which possesses a leucine at position 33.
The ability to visualize the structure adopted by the PSI domain of GPIIIa will hopefully enable scientists
to use their knowledge of that structure to find successful treatments for NATP.
The Molecule as Associated with the Disease
The PSI domain of the GPIIIa subunit of GPIIb-IIIa can adopt two different structures. One structure
occurs when there is a proline at position 33 and the other occurs when there is a leucine at that
position. Antibodies can recognize the structural difference, causing platelets to be cleared by the
immune system, but this recognition occurs in only one direction. For example, in NATP, if a mother
(♀) has only prolines and the father (♂) donates a proline to the fetus, there will be no problem. The
shape of the molecule will be the same in the mother and fetus. Therefore, the mother’s immune
system will accept it. However, if a mother has only prolines and the father donates a leucine to the
fetus, the fetus will have a proline-leucine dimorphism. The platelets in the fetus may not be able to
survive, as the mother may make antibodies against the leucine-containing forms of GPIIIa, causing
the fetal platelets to be cleared or inactivated.
Statistics:
ƒ 300,000 platelets/mL of blood
ƒ 80,000 GPIIb-IIIa receptors per platelet
ƒ ♂- proline/leucine & ♀- proline/proline = could result in disease if father donates
leucine
ƒ ♂- leucine/leucine & ♀- proline/proline = will result in disease (if other conditions are
in the mother)
ƒ leucine has a 85% frequency, proline has a 15% frequency
Blue: Alpha Helices
Yellow: Disulfide bonds
connecting cysteines;
enhance structure of
molecule, responsible for
holding the folded shape
together.
NATP Baby
present
Platelets
Either a proline (P) or leucine
(L) can occupy position 33.
The specific amino acid
determines the shape of the
molecule, which can be
recognized by an antibody.
The Disease
Neonatal Alloimmune Thrombocytopenia occurs with a frequency of 1 in 5000 to 1 in 1200 live births.
An infant is diagnosed with NATP if it has less than 150,000 platelets per microliter of blood. In about
10% of the cases the disease is classified as severe, in which there are less than 50,000 platelets per
microliter of blood. About 60% of infants with NATP are firstborns. Symptoms that the affected infants
may suffer include:
ƒ Petechiae, purpura, or overt bleeding at birth
ƒ Central nervous system hemorrhage
ƒ Intracranial hemorrhages characterized by severe neurologic sequelaie, porencephaly, and optic
hypoplasia
Treatment
Hypothesized structure
when leucine is present
Hypothesized structure
when proline is present
Conclusion
For many decades, scientists had sought the structure of the region within the platelet glycoprotein
GPIIb-IIIa responsible for NATP. With the new discovery of the PSI domain structure of GPIIIa,
scientists can now design drugs that could impair the antibody’s ability to attack foreign platelets,
thereby reducing the negative side effects of NATP. Our model also helped clarify some structural ideas
that had been previously unexplained. In one instance, our mentors observed that the leucine on the
bottom kink turned inward instead of outward, an observation unclear in the 2-D models on the
computer. To model something that was so new to the scientific community was a truly inspiring
experience and we hope that scientists will be able to use our model to better understand the structural
foundation of the disease Neonatal Alloimmune Thrombocytopenia.
To treat an infant with NATP requires increasing its platelet count. In severe cases, a platelet transfusion
is needed. These platelets can come from various sources:
ƒ The mother’s platelets that, by a saline solution, are washed free of antibody-containing plasma
Bibliography
ƒ Mother’s siblings’ platelets
ƒ Regional blood centers
To prevent birth trauma in subsequent pregnancies, the mother should have a Cesarean section.
Furthermore, on the day before delivery, maternal platelets can be obtained to transfuse to the neonate.
Newman, Peter J., et al., Quantitative Platelet Disorders, pp 441-451.
Xiong, J.-P., et al., Journal of Biological Chemistry, 297:40252-40254, 2004.
Xiao, T., et al., Nature, 432:59-67, 2004,
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Supported by the National Institutes of Health (NIH) – National Center for
Research Resources Science Education Partnership Award (NCRR(NCRR-SEPA)
This is the sequence of the first 54 amino acids, which comprise the PSI domain of the GPIIIa subunit. The
pairs of cysteines participating in disulfide bond formation are color-coded and connected by arrows. The
location of the polymorphism at amino acid 33 is colored red. Cysteine 13 participates in a long-range
disulfide bond with cysteine 435, which is present in an unmodeled portion of the GPIIIa subunit.