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Multiple endocrine neoplasia syndrome type 1 (MEN1) syndrome has benefited from the identification of the gene whose mutations account for the genetic susceptibility to develop endocrine tumors. Asymptomatic MEN1 mutant carriers need to be clearly recognized because the gene-related mutations confer a high risk of multiple primary cancers, occur at younger ages, and affect multiple family members who inherit the cancer-predisposing genetic mutation.
Multiple endocrine neoplasia syndrome type 1 (MEN1) syndrome is characterized by the occurrence of varying combinations of more than 20 endocrine and non-endocrine tumors. Endocrine tumors are represented mainly by the ‘classic’ P-triad originally described by Wermer: parathyroid, pituitary, and pancreatic tumors [1-4]. Tables 1 and and22 describe the endocrine and non-endocrine tumors associated with MEN1.
The familial form of MEN1 syndrome occurs with a significantly higher frequency (90% of cases) than the simplex form, where only one individual is affected within a family with no history of the disease (10% of cases). Familial MEN1 is defined in an individual who has at least one first-degree relative with one or more main endocrine tumors or involvement of only one organ and a MEN1 disease-causing germline mutation. MEN1 syndrome is inherited in an autosomal dominant manner, and each child of an affected individual has a 50% chance of inheriting the mutation .
MEN1 syndrome can be defined by the presence of two ‘classic’ endocrine tumors (parathyroid, pituitary, or tumors of the gastro-entero-pancreatic tract) in an affected subject. In Figure 1, an algorithmic summary of the possible diagnostic scenario is presented.
The MEN1 gene spans 9 kb and consists of 10 exons with a 1830-bp/1845-bp coding region encoding a novel 610/615-amino acid protein (two isoforms ), referred to as menin [11-13]. More than 1000 different germline MEN1 mutations, without evidence of hot-spot regions, have been described [14-18], mainly predicting absent or truncated menin. Approximately 1-3% of MEN1 germline mutations consist of large deletions detectable by Southern blot analysis or other gene dosage procedures (i.e., based on polymerase chain reaction) [15-18]. Polymorphic variants have also been described . Neither the finding of a tumor suppressor mechanism nor the identification of binding partners has established the ultimate pathways of menin action in normal tissues or in tumors .
Genetic counseling has a central role in the management of MEN1 patients and their closely related family members. MEN1 genetically predisposed subjects may benefit greatly from early identification by DNA analysis, especially at a presymptomatic stage [15,17]. Once a pathogenic MEN1 mutation has been identified in a proband, referral to a clinical geneticist is advised. Since it is recommended that adequate genetic counseling be given prior to DNA testing, presymptomatic testing in families with an identified MEN1 mutation should be performed within the context of genetic counseling  (Tables 3 and and44).
Subjects in whom the germline mutation has not been identified are at risk if they have inherited the MEN1 mutation from one affected parent or if they are the relatives of subjects clinically defined as suffering from MEN1.
Genetic testing should be offered to at-risk members of a family in which a germline MEN1 mutation has been identified in an affected relative . If molecular genetic testing is not possible or is not informative, individuals with a 50% risk (first-degree relatives of an individual with MEN1 syndrome) should undergo routine evaluation (Table 4). A DNA test for MEN1 may be offered to children within their first decade because tumors such as insulinoma and pituitary adenomas have developed in some children by the age of 5 years [2,20,21] (Table 4). Unfortunately, the great diversity and the lack of both mutational hot-spots and genotype-phenotype correlation make mutational screening time-consuming, arduous, and expensive . Currently, a DNA test identifying an individual as a mutant gene carrier does not usually lead to immediate medical or surgical treatment, but it does suggest that precocious and frequent clinical screening should be carried out. Since we are still unable to predict tumor penetrance and malignancy individually, lifelong follow-up of MEN1 carriers is strongly recommended to prevent tumor morbidity.
Approximately 90% of MEN1 individuals have an affected parent. However, the family history may appear negative because of (a) failure to recognize the disorder in family members, (b) early death of the parent before the onset of symptoms, or (c) late onset of the disease in the affected parent . The risk to the siblings of the proband depends on the genetic status of the proband’s parents. If a parent of the proband is affected or has a disease-causing mutation, the risk to the siblings is 50%. If the disease-causing mutation found in the proband cannot be detected in the DNA of either parent, two possible explanations exist: (a) germline mosaicism in a parent or (b) a de novo mutation in the proband . Each child of an individual with MEN1 has a 50% chance of inheriting the mutation. The risk to other family members depends on the status of the proband’s parents. If a parent is found to be affected or to have a disease-causing mutation or both, his or her family members are at risk .
When neither parent of a proband with MEN1 has the disease-causing mutation or clinical evidence of the disorder, it is likely that the proband has a de novo mutation (approximately 10%). However, explanations such as alternate paternity or maternity (i.e., with assisted reproduction), undisclosed adoption, or secretiveness within the family could also be considered .
When a disease-causing germline mutation has been identified in an affected family member, the genetic testing of at-risk asymptomatic individuals is appropriate for surveillance. When a known disease-causing mutation is not identified, linkage or haplotype analysis can be considered in families with more than one affected family member from different generations. Early detection of at-risk individuals affects medical management, and testing of asymptomatic individuals during childhood is beneficial .
Prenatal testing for MEN1 syndrome is not commonly requested, partly due to the lack of a universal consensus on performing such a diagnosis in MEN1. The disease-causing allele of an affected family member must be identified or linkage established in the family before prenatal testing can be performed .
The advantages of DNA analysis are that (a) it requires a single blood sample and (b) it does not need to be repeated since the analysis is independent of the age of the individual and provides an objective result. Approximately 45% of germline mutations detected by sequence analysis are small deletions, and approximately 15% are small insertions . The likelihood of detecting a MEN1 mutation is higher in individuals with more main P-triad tumors, especially from families with hyperparathyroidism and pancreatic islet tumors [22-24]. MEN1 genetic screening should also be offered to patients with primary hyperparathyroidism or gastrinomas after thorough investigation into the family history . Simplex MEN1 cases are less likely to test positive than familial cases, in part because some of these simplex cases may be caused by somatic mosaicism . Individuals who have a single MEN1-related tumor and no family history of MEN1 syndrome rarely have germline MEN1 mutations .
Intronic MEN1 mutations, such as SpaGVs (splicing-affecting genomic variants), have been recently reported. They are likely to be of significance in the 10% of MEN1 patients who do not have coding region mutations [26,27]. A new intron 3 mutation associated with PRL-oma (prolactin-secreting adenoma), decreased familial penetrance, and variable effects on MEN1 mRNA and menin have recently been described . The MLPA (multiplex ligation-dependent probe amplification) assay may detect large deletions (4%) as germline mutation in MEN1 . Through polymorphism analyses, gene dose assays, and nucleotide sequencing, a large germline deletion (approximately 29-kb pairs), spanning the whole MEN1 gene, has been identified in one patient with a positive family history for MEN1 whose germline MEN1 mutation was undetectable by conventional sequencing analysis . Moreover, genetically diagnosed patients already harbor manifestations at the time of diagnosis, confirming that screening for a MEN1 mutation should be done at an early age .
It is very important to consider that germline mutations in other genes may cause a MEN1-like disorder in MEN1 mutation-negative families, namely the AIP gene  and the four cyclin-dependent kinase inhibitor genes CDKN1A/p15, CDKN2C/p18, CDKN2B/p21, and CDKN1B/p27 [33-35]. Interestingly, several of the proteins encoded by these genes play a role within the same molecular pathway as the menin protein. Although germline mutations in these genes appear to be rare (probably explaining only a small fraction of the MEN1 mutation-negative families), it may still be important to consider analysis of these genes in such families.
MEN1 mutant gene carriers must be followed by periodic clinical tumor surveillance as well as surveillance of recurrence after treatment or progression of the disease. The knowledge about carrier status enables early diagnosis and intervention [2,17]. A prospective clinical study on MEN1 mutant gene carriers revealed that biochemical evidence of neoplasia could be identified an average of 10 years before the clinical evidence of the disease, allowing early surgery. Thus, genetically positive individuals should undergo a focused surveillance for early identification of potentially malignant neuroendocrine tumors accounting for morbidity and/or mortality related to MEN1 .
Importantly, a very recent study having as the primary endpoint the evaluation of the occurrence of non-functioning pancreatic tumors (PETs) in asymptomatic MEN1 children carriers revealed the presence of non-functioning PETs, providing the opportunity to perform clinical surveillance to unravel their growth . Thus, according to Triponez et al. , the possibility of precociously identifying asymptomatic MEN1 children carriers, as well as young adults with MEN1, may be helpful for the early identification of non-functioning PETs that otherwise may not be biochemically identified.
A multicenter study of more than 250 MEN1 gene carriers revealed that, as a result of differential tumor detection, MEN1 carriers born during the second half of the 20th century tend to have their tumors diagnosed earlier than carriers of the same age born in the first half , a known general phenomen (anticipation phenomenon) observed in several other inherited tumors.
The identification of many molecular partners interacting with menin has increased our knowledge of its pathophysiology. However, more studies are necessary to clarify the MEN1-dependent tumorigenesis and the role that menin has in the development of endocrine and non-endocrine tumors. In the near future, there are prospects for novel treatments based on DNA, RNA, or even other small molecules. A better understanding of the intricate molecular pathway networks related to menin will be helpful for designing novel therapeutic strategies.
The electronic version of this article is the complete one and can be found at: http://f1000.com/reports/m/2/14
The author declares that he has no competing interests.