Table 9. Practice Guidelines for Diagnosis and Colon Surveillance of Lynch Syndrome continued...
While familial clusters account for approximately 20% of all CRC cases in developed countries, the rare and highly penetrant Mendelian CRC diseases contribute to only a fraction of familial cases, which suggests that other genes and/or shared environmental factors may contribute to the remainder of the cancers. Two studies attempted to determine the degree to which hereditary factors contribute to familial CRCs.
The first study utilized the Swedish, Danish, and Finnish twin registries that cumulatively provided 44,788 pairs of same-sex twins (for men: 7,231 monozygotic [MZ] and 13,769 dizygotic [DZ] pairs; for women: 8,437 MZ and 15,351 DZ pairs) to study the contribution of heritable and environmental factors involved in 11 different cancers. The twins included in the study all resided in their respective countries of origin into adulthood (>50 years). Cancers were identified through their respective national cancer registries in 10,803 individuals from 9,512 pairs of twins. The premise of the study was based on the fact that MZ twins share 100% and DZ twins share 50% of their genes on average for any individual twin pair. This study calculated that heritable factors accounted for 35%, shared environmental factors for 5%, and nonshared environmental factors for 60% of the risk of CRC. For CRC, the estimated heritability was only slightly greater in younger groups than in older groups. This study revealed that although nonshared environmental factors constitute the major risk of familial CRC, heredity plays a larger-than-expected role.
The second study utilized the Swedish Family-Cancer Database, which contained 6,773 and 31,100 CRCs in offspring and their parents, respectively, from 1991 to 2000. The database included 253,467 pairs of spouses, who were married and lived together for at least 30 years, and who were used to control for common environmental effects on cancer risk. The overall SIR for cancers of the colon, rectum, and colon and rectum combined in the offspring of an affected parent was 1.81 (95% CI, 1.62–2.02), 1.74 (95% CI, 1.53–1.96), and 1.78 (95% CI, 1.53–1.96), respectively. The risk conferred by affected siblings was also significantly elevated. Because there was no significantly increased risk of CRC conferred between spouses, the authors concluded that heredity plays a significant role in familial CRCs; however, controls for shared environmental effects among siblings were absent in this study.
Ten percent to 15% of persons with CRC and/or colorectal adenomas have other affected family members,[370,371,373,374,375,379,380,381,382,383,384] but their findings do not fit the criteria for FAP, and their family histories may or may not meet clinical criteria for LS. Such families are categorized as having familial CRC, which is currently a diagnosis of exclusion (of known hereditary CRC disorders). The presence of CRC in more than one family member may be caused by hereditary factors, shared environmental risk factors, or even chance. Because of this etiologic heterogeneity, understanding the basis of familial CRC remains a research challenge.