Turner Syndrome

Abstract

Turner syndrome (TS) is a sex chromosome disorder affecting phenotypic females who have one intact X chromosome and a completely or partially missing second sex chromosome. It was first described approximately a century ago by Seresevskij, Ullrich and Turner. However, the cytogenetic basis of TS was only reported by Ford in 1959 following Tjio and Levan's optimisation of chromosome visualisation. TS karyotypes include classic monosomy X (40%-50%); monosomy X mosaicism (3%-25%); isochromosome X (10%-18%); ring X (10%-16%); mosaicism for monosomy X and a normal or structurally abnormal Y chromosome (6%-12%); deletion Xp (< 5%) and unbalanced X-autosome translocation (< 2%). While parental age does not affect the complete loss of one X chromosome, the paternal X chromosome is absent in three-quarters of patients with TS. Clinically, detecting the parental origin of the remaining X chromosome is not currently useful in routine TS care. Recurrence risk is low for phenotypically normal parents with a child diagnosed with TS. Pregnancy loss is the outcome for the majority (~99%) of TS cases; however, prenatal ultrasound findings for foetuses with TS may include abnormalities like cystic hygroma and hydrops. Postnatal phenotype for patients with TS includes short stature, delayed puberty, ovarian dysgenesis, hypergonadotropic hypogonadism, infertility, cardiac defects, endocrine, metabolic and autoimmune disorders. This review aims to outline clinical indications for testing, describe test methodologies, provide genetic test result examples that highlight complex TS karyotype diagnoses, summarise clinical management options and discuss the phenomenon of 'normal' sex chromosome loss with advancing age.

Keywords: Turner syndrome; clinical management of Turner syndrome; cytogenetics; genetic basis of Turner syndrome; genetic testing; indications for testing Turner syndrome; monosomy X; normal sex chromosome loss.