Monozygous twins share a common genotype. However, most monozygotic twin pairs are not identical; several types of phenotypic discordance may be observed, such as differences in susceptibilities to disease and a wide range of anthropomorphic features. There are several possible explanations for these observations, but one is the existence of epigenetic differences. To address this issue, we examined the global and locus-specific differences in DNA methylation and histone acetylation of a large cohort of monozygotic twins. We found that, although twins are epigenetically indistinguishable during the early years of life, older monozygous twins exhibited remarkable differences in their overall content and genomic distribution of 5-methylcytosine DNA and histone acetylation, affecting their gene-expression portrait. These findings indicate how an appreciation of epigenetics is missing from our understanding of how different phenotypes can be originated from the same genotype.DNA methylation ͉ epigenetics ͉ histones H uman monozygotic (MZ) twins account for 1 in 250 live births (1). The origin of MZ twins is attributed to two or more daughter cells of a single zygote undergoing independent mitotic divisions, leading to independent development and births. They are considered genetically identical, but significant phenotypic discordance between them may exist. This quality is particularly noticeable for psychiatric diseases, such as schizophrenia and bipolar disorder (2). MZ twins have been used to demonstrate the role of environmental factors in determining complex diseases and phenotypes, but the true nature of the phenotypic discordance nevertheless remains extremely poorly understood. In this context, differences in the placenta, amniotic sac, and vascularization of the separate cell masses or even mosaicism in genetic and cytogenetic markers in MZ may exist (3), although the published studies are very few in number. Thus, the real causes for MZ twin discordance for common diseases and traits remain to be established. Epigenetic differences may be an important part of the solution to this puzzle. Indeed, epigenetic profiles may represent the link between an environmental factor and phenotypic differences in MZ twins. Cloned animals provide another example of how epigenetics may explain phenotypic differences in beings that have identical genetic sequences. In this case, inefficient epigenetic reprogramming of the transplanted nucleus is associated with aberrations in imprinting, aberrant growth, and lethality beyond a threshold of faulty epigenetic control (4). MZ twins are another phenomenon in which epigenetics can ''make the difference.'' To address this possibility, we have profiled the epigenetic patterns related to global and locus-specific DNA methylation and histone H3 and H4 acetylation in the largest series of MZ twins for which molecular studies have been reported. Materials and MethodsSubjects. Eighty volunteer Caucasian twins from Spain were recruited in the study, including 30 male and 50 female subje...

Purpose: Pheochromocytomas (PCC) and paragangliomas (PGL) are genetically heterogeneous neural crest-derived neoplasms. Recently we identified germline mutations in a new tumor suppressor susceptibility gene, MAX (MYC-associated factor X), which predisposes carriers to PCC. How MAX mutations contribute to PCC/PGL and associated phenotypes remain unclear. This study aimed to examine the prevalence and associated phenotypic features of germline and somatic MAX mutations in PCC/PGL.Design: We sequenced MAX in 1,694 patients with PCC or PGL (without mutations in other major susceptibility genes) from 17 independent referral centers. We screened for large deletions/duplications in 1,535 patients using a multiplex PCR-based method. Somatic mutations were searched for in tumors from an additional 245 patients. The frequency and type of MAX mutation was assessed overall and by clinical characteristics.Results: Sixteen MAX pathogenic mutations were identified in 23 index patients. All had adrenal tumors, including 13 bilateral or multiple PCCs within the same gland (P < 0.001), 15.8% developed additional tumors at thoracoabdominal sites, and 37% had familial antecedents. Age at diagnosis was lower (P ¼ 0.001) in MAX mutation carriers compared with nonmutated cases. Two patients (10.5%) developed metastatic disease. A mutation affecting MAX was found in five tumors, four of them confirmed as somatic (1.65%). MAX tumors were characterized by substantial increases in normetanephrine, associated with normal or minor increases in metanephrine.Conclusions: Germline mutations in MAX are responsible for 1.12% of PCC/PGL in patients without evidence of other known mutations and should be considered in the genetic work-up of these patients.