Male monozygotic twins discordant for periventricular nodular heterotopia and epilepsy

Identical but not the same: the value of discordant monozygotic twins in genetic research

Monozygotic (MZ) twins show remarkable resemblance in many aspects of behavior, health, and disease. Until recently, MZ twins were usually called "genetically identical"; however, evidence for genetic and epigenetic differences within rare MZ twin pairs has accumulated. Here, we summarize the literature on MZ twins discordant for Mendelian inherited disorders and chromosomal abnormalities. A systematic literature search for English articles on discordant MZ twin pairs was performed in Web of Science and PubMed. A total number of 2,016 publications were retrieved and reviewed and 439 reports were retained. Discordant MZ twin pairs are informative in respect to variability of phenotypic expression, pathogenetic mechanisms, epigenetics, and post-zygotic mutagenesis and may serve as a model for research on genetic defects. The analysis of single discordant MZ twin pairs may represent an elegant approach to identify genes in inherited disorders.

Monozygotic twins with severe myoclonic epilepsy in infancy discordant for clinical features

Male monozygotic twins with genetically determined severe myoclonic epilepsy in infancy are described. Although seizure onset, clinical seizure symptomatology, and motor and mental development were almost identical until age 38 months, their clinical courses then became discordant. The emergence of myoclonus was delayed by 12 months in twin 1 compared with twin 2. Regression in language development, which is a common feature of severe myoclonic epilepsy in infancy, was obvious in twin 2 after the emergence of myoclonus, whereas twin 1 did not demonstrate any regression. The clinical-course discordance between twins was attributable to bacterial meningitis, which twin 1 developed at age 35 months. Bacterial meningitis may have affected the clinical course of severe myoclonic epilepsy in infancy in twin 1, resulting in delayed onset of myoclonus and more favorable language development in twin 1 than in twin 2, who did not experience bacterial meningitis.

Somatic mosaicism and variable expressivity

For more than 50 years geneticists have assumed that variations in phenotypic expression are caused by alterations in genotype. Recent evidence shows that 'simple' mendelian disorders or monogenic traits are often far from simple, exhibiting phenotypic variation (variable expressivity) that cannot be explained entirely by a gene or allelic alteration. In certain cases of androgen insensitivity syndrome caused by identical mutations in the androgen receptor gene, phenotypic variability is caused by somatic mosaicism, that is, somatic mutations that occur only in certain androgen-sensitive cells. Recently, more than 30 other genetic conditions that exhibit variable expressivity have been linked to somatic mosaicism. Somatic mutations have also been identified in diseases such as prostate and colorectal cancer. Therefore, the concept of somatic mutations and mosaicism is likely to have far reaching consequences for genetics, in particular in areas such as genetic counseling.

Causes of epilepsies: insights from discordant monozygous twins

Whereas some patients with epilepsy have known acquired or genetic causes, in many the cause is unknown. By analyzing monozygotic twins, discordant for epilepsy, subtle etiological factors may be detected. We analyzed 12 monozygotic, discordant twins for factors explaining discordancy. These factors were presence of major clinical risk factors, presence of possibly epileptogenic lesions on brain magnetic resonance imaging (MRI), and quantitative brain volume abnormalities. Major risk factors, with associated acquired lesions were found in 4 of 12 twins. An MRI lesion without a major risk factor was found in a further 4 of 12 twins. Two of these had unilateral malformations of cortical development, 1 had bilateral periventricular heterotopia, and 1 had focal atrophy. Significant twin-twin differences in MRI volumes without obvious MRI lesions or major risk factors were found in 2 of 12 twins. Both had larger volumes than their co-twins, and idiopathic generalized epilepsy. No clinical or MRI findings accounting for discordance for epilepsy were found in 2 of 12 twins. In 10 of 12 pairs a clinical or MR correlate of epilepsy was found; some of those were subtle and only apparent by twin-twin comparison. They may be due to occult acquired factors, such as prenatal insults, or to genetic abnormalities resulting from postfertilization genetic processes.