Genetics and Pain Severity in Sickle Cell Disease

Sickle cell disease (SCD) is the most common genetic disease in the United States inherited
as an autosomal recessive disorder, where approximately 70,000 individuals have sickle cell
disease. Acute painful vaso-occlusive crisis (VOCs) is one of the common complications of SCD
that influences overall survival (Platt, Thorington et al. 1991). Pain, is also the most
common cause of SCD morbidity, which has a negative impact on quality of life of these
individuals and their families. There is significant inter-individual variation in the
frequency and course of severe VOCs that result in hospital based treatment, the reasons for
which have not been clearly elucidated. Vaso-occlusion of irreversibly sickle red cells
within the microcirculation is believed to be the proximate cause of painful VOCs, however it
is likely that other non-SCD related factors affecting pain perception and sensitivity to
pain will also contribute to individuals susceptibility to pain and therefore contribute to
the observed inter-individual variability in the course of VOC. Early identification of
individuals who are at high risk for developing severe pain related morbidity and chronic
pain syndromes is crucial since early multimodal interventions might have the potential to
minimize both the morbidity and mortality associated with VOCs.



Patients with SCD are hypothesized to have lower nitric oxide (NO) bioavailability due to NO
scavenging by cell free hemoglobin released into plasma during red cell hemolysis. NO
deficiency has been identified as a key factor in development vascular dysfunction in SCD. NO
has also recently been identified as a key mediator in processing nociceptive signals and
modulation of pain in non-SCD models. Thus, low NO is associated with lower pain perception
(Meller, Dykstra et al. 1992; Tegeder, Costigan et al. 2006). GTP cyclohydrolase (GCH1) is
the rate-limiting enzyme for synthesis of an essential cofactor for both NO production and
metabolism of aromatic amino acids, namely tetrahydrobiopterin (BH4). Therefore it is
hypothesized that genetic variants in the GCH1 gene will affect BH4 levels and which will
have a secondary impact on vascular dysfunction and sensitivity to pain in SCD.

The primary goal of this protocol is to establish patterns of sensitivity to experimental
pain among subjects with SCD compared to healthy African American controls. In addition, an
exploratory analysis will determine if increased sensitivity to experimental pain correlates
with the frequency and intensity of clinical pain in those with SCD. Once an expected pattern
of experimental pain phenotypes are established for a cohort with SCD, we will then further
explore the role of GCH1 genetic variants in experimental pain perception and vascular
function. If successful, a longer term secondary objective is to establish a sufficiently
large patient cohort with experimental pain phenotypes for future exploratory genetic studies
to investigate the role of other loci that might influence sensitivity to experimental pain
and vascular function in SCD.

- ELIGIBILITY CRITERIA FOR ALL PARTICIPANTS:

All study participants must be of self-described African or African American ancestry. They
will be at least 18 years old and must be able to provide informed, written consent for
participation.

INCLUSION CRITERIA:

Inclusion Criteria for Sickle Cell Patients

1. Diagnosis of sickle cell disease (documentation of SS, SC, S beta + thalassemia, S
beta + thalassemia by electrophoresis is required).

2. If taking chronic analgesics (NSAID, acetaminophen) or opioids, study subjects should
be on a stable dose for 4 weeks prior to recruitment.

Inclusion Criteria for Control Subjects

1. Hemoglobin AA genotype by HPLC or hemoglobin electrophoresis.

2. General good health defined as the absence of untreated major medical conditions (e.g.
uncontrolled systemic hypertension, etc.).

EXCLUSION CRITERIA:

Exclusion Criteria for Sickle Cell Patients

1. History of severe vaso-occlusive pain crisis resulting in either evaluation in an
emergency department or admission to a hospital during the two weeks prior to study
enrollment.

2. Acute pain at the time of enrollment defined as spontaneous recent onset pain with a
self rated score of 6 or higher on a scale of 0-10. (This is acute pain not the pain
that subjects function at on a daily basis.)

Exclusion Criteria for Control Subjects

1. Acute pain or injury at enrollment or a recent history of chronic pain (daily pain
reported for at least 6 months) in the past 3 years.

2. Major medical/psychiatric illness known to cause pain.

3. Sickle cell trait.