We evaluated the relative frequencies of mutations in the APC and MUTYH genes in a large number of individuals who had undergone genetic testing. Our results help further inform the evolution in the understanding of the genetic epidemiology of the classic hereditary colorectal cancer syndrome, FAP, and shed some light on the important differences in disease patterns between carriers of APC mutations versus those with biallelic MUTYH mutations.
The clinical syndrome of FAP was first reported in 1847. In 1975, Bussey described the clinical characteristics of patients with hundreds to thousands of colorectal polyps 17
. In 1991, the adenomatous polyposis coli (APC
) gene was cloned and found to be mutated in FAP patients 18-20
. MAP was described in 2002 when Al-Tassan et al. noted biallelic germline mutations in the base excision repair gene MUTYH
in a family with recessive inheritance of multiple colorectal adenomas and CRC 3
Previous studies (predating the discovery of MAP) have reported widely varying prevalence of pathogenic APC
mutations among individuals with a classic polyposis phenotype (52% to 82%) likely due to varying mutation analysis techniques and patient selection 21-26
. However, these studies primarily involved small cohorts that were geographically and ethnically homogeneous. After the discovery of MUTYH, APC
mutation-negative probands with classic FAP were screened for MUTYH
mutations. These relatively small studies reported MUTYH
mutation prevalence rates ranging from 7.5% to 20% in classic polyposis 6, 8
The results of our study, in which all individuals were tested for both APC and MUTYH mutations, indicate that there is significant heterogeneity in mutation prevalence even among individuals with a classic polyposis phenotype. Among individuals with ≥1000 adenomas, 80% (95%CI 71-87) had a pathogenic APC mutation, and MUTYH played a minor role (2%, 95%CI 0.2-6). The distribution and prevalence of mutations was markedly different, however, in individuals with 100-999 adenomas (still considered classic polyposis) - only 56% (95%CI 54-59) were APC carriers, and a higher proportion (7%, 95%CI 6-8) had biallelic MUTYH mutations. No pathogenic APC or MUTYH mutations were detected in 18% (95%CI 12-26) of individuals with ≥1000 adenomas and 35% (95% CI 33-38) with 100-999 adenomas, potentially attributable in part to genes that have not been identified.
In contrast, in the 3253 individuals with attenuated polyposis, prevalence rates of pathogenic APC
mutations were similar (10%, 95%CI 9-11 and 7%, 95%CI 6-8 respectively). This MUTYH
prevalence rate is lower than prior reports from smaller cohorts of attenuated polyposis patients, where estimates have ranged from 22% to 29% 6-9, 27-29, 11, 30
We did not evaluate the genotype-phenotype correlation among individuals with APC mutations as has been previously reported, as this study aimed to highlight the clinical characteristics associated with a pathogenic mutation in either of the two familial polyposis genes (APC or MUTYH) and the differences in these characteristics between mutation carriers. Ten or more adenomas and young onset adenomas (< 50 years) were associated with a mutation in either gene (APC or MUTYH). There was an incremental increase in the odds of a mutation with an increasing number of adenomas and earlier age at adenoma diagnosis. Individuals with ≥ 10 adenomas and young onset adenomas (prior to 50 years) were significantly more likely to have an APC mutation. The presence of ≥ 10 adenomas was associated with a pathogenic MUTYH mutation but in contrast to individuals with an APC mutation, the odds of a mutation did not incrementally increase with earlier age at diagnosis and were highest between 30-49 years.
The study population is both a weakness and strength. This was not a population-based study, and subjects had undergone testing based on a personal or family history suggestive of a polyposis syndrome by health care providers who may have had variable expertise in genetic evaluation; therefore prevalence estimates, particularly in the groups with fewer numbers of individuals must be interpreted with caution due to potential ascertainment and referral bias 31
. Nonetheless, this cohort is representative of individuals for whom genetic testing for APC
genes should be considered and reflects the characteristics of the population at risk. We did not verify the pathology of polyps or the clinical data provided on the test order form. Although data were provided by health care providers whose specific specialty or training was not reported on the form, other studies using similar methods of data collection for cohorts tested for familial CRC syndromes have been externally validated, suggesting that the data are likely to be accurate, and are likely not to vary between the groups being compared 32-33
. We also used multiple imputation techniques for missing data so as to minimize selection bias which has been demonstrated to be particularly important in genetic association studies, where missing data may be distributed differentially and may generate spurious associations 34
. However, results obtained from using both complete case data and imputed data were similar.
The test order form did not elicit a history of hyperplastic polyps which have been reported in small cohorts with MAP 35
. However, only a small percentage of patients with MAP present with hyperplastic polyposis and adenomatous polyps and CRC remain the most common clinical presentation. Targeted sequence analysis was performed to detect the two most common MUTYH
mutations Y179C and G396D and full MUTYH
gene sequencing was performed in a small percentage of individuals. It is however known that Y179C and G396D mutations account for the vast majority of mutant alleles in individuals of Northern American and European ancestry that comprised the majority of our study subjects 8, 36-38
. The use of MUTYH
gene rearrangement analysis and allele-specific APC
analysis which have recently been reported, but are not widely available commercially, may result in a small improvement in the yield of testing 39
Through evaluation of the phenotypic differences between mutation carriers in this large study, a pattern has emerged. Overall, in individuals with multiple adenomas, the APC mutation rate progressively increases with increasing polyp burden whereas the MUTYH mutation rate remains relatively constant across different categories. Furthermore, the prevalence of APC mutations varies significantly among individuals with classic polyposis (≥ 1000 adenomas: 80%, 95%CI 71-87; 100-999 adenomas: 56%, 95%CI 54-59). In contrast, biallelic MUTYH mutations are rare in individuals with ≥ 1000 adenomas and their prevalence is relatively constant among individuals with < 1000 adenomas. Our evaluation of individuals who underwent genetic testing due to a personal or family history suggestive of a familial polyposis syndrome suggests that genetic evaluation for APC and MUTYH mutations may be considered in individuals with 10 or more adenomas. However, our results are derived from a selected cohort of high-risk individuals, and need to be validated in larger populations of unselected patients. The mutation probabilities presented may assist providers in their decision to recommend genetic evaluation and counsel patients undergoing genetic testing. However, it remains important to also consider the limitations of genetic testing at the present time- a third of patients with a classic FAP phenotype are found to not carry a mutation in either the APC or MUTYH gene. Such individuals should undergo periodic re-evaluation as other susceptibility genes are identified.