MUTYH-associated polyposis (MAP)
MUTYH-associated polyposis (MAP), caused by biallelic mutations in MUTYH (formerly known as MYH), is characterized by a greatly increased lifetime risk of colorectal cancer (43% to almost 100% in the absence of timely surveillance). Although typically associated with ten to a few hundred colonic adenomatous polyps that are evident at a mean age of about 50 years, colonic cancer develops in some individuals with biallelic MUTYH mutations in the absence of polyposis (Wang et al. 2004). Duodenal adenomas are found in 17%-25% of individuals with MAP; the lifetime risk of duodenal cancer is about 4%. Also noted are a modestly increased risk for rather late-onset malignancies of the ovary, bladder, and skin, and some evidence for an increased risk for breast and endometrial cancer. More recently, thyroid abnormalities (multinodular goiter, single nodules, and papillary thyroid cancer) have been reported in some studies. Some affected individuals develop sebaceous gland tumors.
Biallelic mutations in MUTYH have been found to account for approximately 10% of polyposis patients, but <1% of all colorectal cancer (Halford et al. 2003; Wang, Baudhuin et al. 2004). The largest population study to date indicates that approximately 0.2% of all colorectal cancer is caused by biallelic mutations in MYH (Webb et al. 2006). It was demonstrated in the same study that monoallelic MUTYH mutations are not associated with an increased risk of colorectal cancer. The MAP phenotype is similar AFAP, with extra-colonic manifestations consisting of duodenal polyps but not intra-abdominal desmoids although occasionally patients may have up to several thousand polyps. Among Caucasians approximately 80% of mutations in MUTYH causing MAP are Y165C or G382D (Sieber, Lipton et al. 2003).
The E466X mutation is a common founder mutation among Pakistani populations. The family tree above shows 2 Pakistani brothers who were affected with colorectal cancer and polyps in their 30s due to this founder mutation. Y90X is a founder mutation in Indian populations (Sieber, Lipton et al. 2003).
Around 25–30% of polyposis cases with more than 20 polyps and without evidence of a dominant inheritance pattern, in whom genetic analysis has not identified an APC mutation, are due to bi-allelic mutations in the base excision repair (BER) gene, MUTYH. Polyps can be exclusively adenomatous or mixed adenomatous/hyperplastic. Since the mode of inheritance is autosomal recessive, lack of vertical transmission of the polyposis phenotype in the family should raise the possibility of MUTYH-associated polyposis (MAP). Siblings are at 25% risk of carrying bi-allelic deleterious mutations. Children of a bi-allelic carrier are at high risk if the other parent also carries at least one mutant allele. Large, systematic studies of MUTYH mutation frequency in colorectal cancer cases and controls suggest penetrance in bi-allelic carriers is very high, and probably >90%.
The heterozygote carrier frequency in the UK is around 2% and around 1:10 000 homozygous or compound heterozygotes for two MUTYH mutations. The proportion of polyposis syndromes due to MUTYH in clinical practice is less clear because studies have so far focused on selected research case series of multiple polyps that have been screened negative for APC mutations. In one study 4% of multiple polyp cases (3–100) and 8% of APC mutation negative polyposis cases carry MUTYH mutations.
Damaged DNA is repaired by several mechanisms, one of which involves a family of enzymes involved in base-excision repair (BER). The MUTYH gene (also known as MYH) encodes a DNA glycosylase involved in the repair of the oxidative lesion 8-oxoguanine, a by-product of cellular metabolism and oxidative damage of DNA.
The products of three BER repair genes, OGG1, MTH1 and MYH work together to prevent 8-oxo-G induced mutagenesis. Mutations in MUTYH cause an autosomal recessive colorectal cancer and polyposis syndrome MYH-associated polyposis (MAP; OMIM 608456) (Al-Tassan et al. 2002). Somatic mutations in the APC gene in polyps from individuals affected with MAP are almost invariably G to T transversions (Sieber et al. 2003), and it was by understanding the underlying DNA repair mechanism of this mutation, base-excision repair, that MUTYH was identified as a candidate-predisposition
gene. G to T transversion mutations were also identified in KRAS in codon 12 (Lipton et al. 2003). The adenoma to carcinoma pathway in MAP does not involve BRAF V600E, SMAD4 or TGFBIIR mutations, or microsatellite instability, and the cancers are near-diploid (Lipton, Halford et al. 2003). Thus, tumours with germline MUTYH mutations tend to follow a distinct pathway.
Colorectal surveillance & screening
Treatment of manifestations: Suspicious polyps identified on colonoscopy should be removed until polypectomy alone cannot manage the large size and density of the polyps, at which point either subtotal colectomy or proctocolectomy is performed. Duodenal polyps showing dysplasia or villous changes should be excised during endoscopy. Abnormal findings on thyroid ultrasound examination should be evaluated by a thyroid specialist to determine what combination of monitoring, surgery, and/or fine needle aspiration (FNA) is appropriate. Surveillance: Individuals with biallelic MUTYH germline mutations: Evaluation of relatives at risk: Offer molecular genetic testing for the familial mutations to all siblings of an individual with genetically confirmed MAP in order to reduce morbidity and mortality through early diagnosis and treatment.
Counselling: MAP is inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being a carrier with a small increased risk of CRC, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk family members and prenatal diagnosis for pregnancies at increased risk are possible if the disease-causing mutations in the family have been identified.
UK Recommendations: Large bowel surveillance colonoscopy every 2–3 years is recommended from age 25 years for patients who are biallelic MUTYH carriers (or homozygous carriers of other BER gene defects). Colonoscopy is the preferred modality because of the likelihood of polyps requiring polypectomy.
Experience is limited because the role of MUTYH and other BER genes has only relatively recently been demonstrated. Hence, available evidence comes from pooled descriptive experience and opinion. However, there is a substantial colorectal cancer risk for those who are bi-allelic carriers. Although indirect evidence suggests colonoscopic surveillance and polypectomy may be effective in colorectal cancer control, this has yet to be definitively determined. Indeed, we are not aware in the literature to date of any control subjects with bi-allelic MUTYH mutations who have reached the age of 55 years without developing colorectal cancer or polyposis . Hence, the risk may be sufficiently high to merit at least considering prophylactic colectomy and ileorectal anastomosis or even proctocolectomy and ileo-anal pouch if dense rectal polyposis is a feature. The patient should be counselled about the limited evidence available to guide decisions on either surveillance or pre-emptive surgical strategies
- Hereditary Colorectal Cancer Syndromes (familycancerclinic.wordpress.com)
- Polyposis (familycancerclinic.wordpress.com)
- Hyperplastic Polyposis Syndrome (familycancerclinic.wordpress.com)
- Hereditary mixed polyposis syndrome (HMPS) (familycancerclinic.wordpress.com)
- Familial adenomatous polyposis (FAP) (familycancerclinic.wordpress.com)