Cat Eye Syndrome (CES) is a rare congenital disorder that impacts multiple organ systems. Although named for a specific eye anomaly, the clinical presentation is highly diverse. CES’s genetic cause involves a change in chromosome structure rather than a single gene mutation. This complexity explains why its inheritance pattern is more complex than standard Mendelian rules of dominant or recessive traits.
Defining Cat Eye Syndrome: Key Clinical Features
Cat Eye Syndrome is formally known as Schmid-Fraccaro syndrome. Its physical manifestations, or phenotype, can vary significantly among individuals. The condition is often characterized by a classic triad of features, although not every person with CES will exhibit all three. The feature that gives the syndrome its name is the ocular coloboma, a defect in the iris that can make the pupil appear elongated like a cat’s eye, which is present in about half of all cases.
Other common physical signs include preauricular pits or tags, which are small skin growths or depressions located near the outer part of the ear. Anal atresia is another frequently reported feature, representing an abnormal obstruction or absence of the anal opening. Furthermore, CES can involve abnormalities in major organs, such as congenital heart defects and malformations of the kidneys or urinary tract. The severity of the syndrome is highly variable, ranging from very mild symptoms that may go undiagnosed to severe, life-threatening malformations.
The Chromosomal Basis of Cat Eye Syndrome
The cause of Cat Eye Syndrome lies in a chromosomal abnormality: a duplication of genetic material from chromosome 22. Specifically, individuals with CES possess an extra piece of genetic material derived from the short arm and a proximal part of the long arm of chromosome 22, the region known as 22q11. This extra material is often present as a small supernumerary marker chromosome (sSMC).
The presence of this extra chromosome segment means that the genes within the 22q11 region are present in an increased dosage. In the classic presentation of CES, this leads to a partial tetrasomy of the 22q11 region, meaning the affected individual has four copies of this segment instead of the normal two. Because the condition is caused by an extra piece of a chromosome, a dosage issue, it does not fit neatly into the simple categories of Mendelian single-gene inheritance. The increased gene dosage leads to an overproduction of proteins that disrupt normal embryonic development, resulting in the syndrome’s characteristic features.
Inheritance Patterns and Genetic Counseling
When Cat Eye Syndrome is inherited from a parent, it generally follows an autosomal dominant pattern. This description is used because inheriting just one copy of the extra marker chromosome is sufficient to cause the syndrome.
The majority of cases, approximately 90%, are de novo, meaning the chromosomal abnormality arose spontaneously in the affected individual and was not inherited from either parent. In these de novo instances, the parents are genetically normal, and the chance of the syndrome recurring in a future child is very low, typically less than one percent.
Parental testing is crucial because in the rare inherited cases, a parent may carry the extra marker chromosome but be mildly affected or even asymptomatic due to the condition’s high degree of variable expressivity. When a parent is known to carry the sSMC, the risk of passing the syndrome on to each child increases significantly to 50%. Genetic counseling is therefore an important step, as a specialist can determine the specific recurrence risk based on parental karyotyping and the precise nature of the chromosomal change.