Priapism in SCD: Clinical and Genetic Correlations.

Priapism is a complication of sickle cell disease (SCD) that occurs due to obstruction of the corpora cavernosa of the penis. We have studied priapism in relation to several clinical and genetic factors in 249 adult male patients with SCD, 92 (37%) of whom reported a positive history of priapism. The mean age of male patients without a history of priapism was 35.2 years (± 10.8 years) compared with a mean age of 36.4 years (± 11.3 years) in male patients with a positive history of priapism. Because of the possible relationship with nitric oxide biology, we examined the co-occurrence of priapism with proteinuria, leg ulcers and stroke. Of the males with a positive history of priapism, 20% also had a history of 2+ or greater proteinuria, compared to a presence of 2+ or greater proteinuria in only 10% of males without a history of priapism (p=0.03). Similarly, 34% of males with a positive history of priapism also had a history of leg ulcers, compared to the presence of leg ulcers in 22% of males without priapism (p=0.03). No statistically significant association between the occurrence of priapism and stroke was observed. In an effort to identify genetic risk factors for priapism, we examined 262 single nucleotide polymorphisms (SNPs) in a total of 56 genes, primarily involved in red blood cell adhesion and inflammation pathways. Chi Square tests of association were constructed for the genotypes of each SNP with two clinical categories: patients with a positive history of priapism and patients without a history of priapism. When the frequency of rare homozygotes was less than five individuals, we combined these rare homozygote individuals with heterozygote individuals for analysis. All p-values were uncorrected for multiple testing. We found associations with 12 SNPs in 8 genes: SLC4A2 (p=0.003); ITGAV (p=0.004 and p=0.02 for two different SNPs); F13A1 (p=0.004 and p=0.02 for two different SNPs); AQP1 (p=0.01 and p=0.04 for two different SNPs); TGFBR2 (p=0.01 and p=0.02 for two different SNPs); ADRB2 (p=0.03); MGC (p=0.04); and ARG2 (p<0.05). These genes are involved in a variety of functions, including adhesion, coagulation, signal transduction, NO biology and immune response. We examined 21 non-coding SNPs in the Klotho gene, but we did not find an association between priapism and Klotho, as was recently reported by Nolan and colleagues (2005). The only possible trend for association we observed in Klotho was at marker rs1888057 (p=0.07); we did not observe association with the SNP (rs2249358) (p=0.82) Nolan and colleagues found associated with priapism. These data support our over-arching hypothesis that genetic factors mediate the variability and risk of developing organ-specific complications of SCD. A better understanding of the genetic factors that contribute to the occurrence of complications such as priapism should ultimately lead to a better understanding of SCD pathophysiology as well as to improved treatment for patients with SCD.