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persistent disparity occurring between Asian or Pacific Islanders and non-Hispanic black women, who have overall rates of preterm birth of 10.5 and 17.9 percent, respectively (CDC, 2005i). As discussed in Chapter 4, some evidence suggests that genetic factors may play a role in the disparities in preterm birth rates by race-ethnicity. However, the evidence by no means proves that genetic factors contribute to these disparities. The most direct way to study whether genetic factors vary among racial-ethnic groups is to find variants that influence susceptibility to the risk of preterm birth and then to assess whether these variants differ in frequency or effect across populations.

Allele Frequency

One possible reason for a genetic influence on racial disparities in preterm birth is that susceptibility variants may be present in one population but absent in others or may vary in frequencies across diverse populations. This may affect the number of individuals at increased risk for preterm birth. One obvious example is the unequal distribution of disease-associated alleles for certain recessive disorders, such as sickle cell disease or Tay-Sachs disease. One study examined a total of 179 African American women and 396 white women for the presence of functionally relevant allelic variants in cytokine genes (Hassan et al., 2003). African American women were found to be significantly more likely to carry allelic variants known to upregulate proinflammatory cytokines, and the ORs increased with the allele dose. The ORs for African American women compared with those for white women to have genotypes that up-regulate the proinflammatory cytokine variantss IL-1, IL1A-4845G/G, IL1A-889T/T, IL1B-3957C/C, and IL1B-511A/A ranged from 2.1 to 4.9. The proinflammatory cytokine genotype IL6-174G/G variant was 36.5 times (95% CI 8.8–151.9) more common among African American women than white women. The frequencies of genotypes known to down-regulate the antiinflammatory cytokine genotypes IL10-819T/T and IL10-1082A/A were elevated 3.5-fold (95% CI 1.8– 6.6) and 2.8-fold (95% CI 1.6–4.9), respectively, in African American women compared with those in white women. Except for the gamma interferon genotype, cytokine genotypes found to be more common in African American women were consistently those that up-regulate inflammation (Hassan et al., 2003; Ness et al., 2004).

Genetic Effects

Another possible reason for a genetic influence on racial-ethnic disparities in preterm birth rates is the variation in the effect of a given genetic variant between racial groups. However, data that can be used to either

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