versão On-line ISSN 1683-9803
Introduction: Ring chromosomes are an uncommon chromosomal disorder characterized by loss of the ends of the chromosome followed by fusing of the two ends to form a circle. The phenotype and clinical characteristics of the patient show a direct relationship with which chromosome is involved and the amount of genetic material lost from the ends. This abnormality can even be present without having a clinical effect. However, ring chromosome 20 syndrome is characterized by mental retardation, behavioral disorders, dysmorphism, and refractory epilepsy involving various types of seizures. It is caused by changes at the 20q13 region of chromosome 20 and is the third type of epilepsy known to be of genetic origin. Case Report: Two female infants of 9 months of age presenting due to psychomotor retardation and dysmorphic features. Product of a monochorionic and monoamniotic twin pregnancy delivered by cesarean, both incubated for 24 days due to low birth weight; Twin 1, weighing 1,600 gm, presented a preauricular tag and café-au-lait spots on the legs; head raising at 7 months. Twin 2, weighing 1.570 gm; head raising at 6 months. The mother reports Twin 2 experienced hyperextension convulsions without causal diagnosis. Both otherwise present identical phenotypes with dolichocephaly, hypertelorism, narrow palpebral fissures with epicanthic folds. CT, echocardiogram, and ophthalmic exams normal for age. General hypotonia. Family history: Mother and father in second marriage, both 44 years of age, not consanguine. The mother has eight children by her first husband, and the father one child. Cytogenetic study of both showed a mosaic ring chromosome 20: 45,XX,-20/46,XX, r(20) in the 5% and 95% respectively of the metaphases analyzed. No previous reports of ring chromosome 20 in monozygotic twins appear in the literature. Discussion: The possible clinical implications are under discussion since the abnormality could compromise the two genes related to epileptic channelopathies (CHRNA4 and KCNQ2).
Palavras-chave : chromosomes; human; Pair 20; ring chromosomes.