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PubMed This is a summary of 8 peer-reviewed journal articles Updated
Genetics

The Genetics Behind the Syndrome

At a Glance

Cat-Eye Syndrome is a rare genetic condition caused by extra genetic material on chromosome 22. This usually appears as a small extra piece called a marker chromosome. While most cases happen spontaneously, about 25% are inherited from a parent who may carry the genetic change with no symptoms.

Understanding the genetics of Cat-Eye Syndrome (CES) can feel like learning a new language. At its simplest, your child’s body has received a few “extra pages” of genetic instructions. While most people have two copies of each chromosome, children with CES have more than that for a specific section of chromosome 22.

The “Extra Piece”: Small Supernumerary Marker Chromosomes

In most cases of CES, the extra genetic material exists as a small supernumerary marker chromosome (sSMC) [1].

  • Supernumerary means “extra.”
  • Marker chromosome means it is a tiny, separate piece of a chromosome that exists alongside the usual 46 chromosomes [2].

In CES, this tiny extra chromosome is usually a mirrored copy of the top part of chromosome 22 (the 22pter-q11.2 region) [3][4]. Because this extra piece contains two copies of those genes, and your child already has their two normal copies of chromosome 22, they end up with a total of four copies of those specific genes. This is called partial tetrasomy [1][5]. If they have only one extra copy instead of two, it is called partial trisomy [1].

Excess vs. Deletion: CES and DiGeorge Syndrome

You may have heard of 22q11.2 Deletion Syndrome (also known as DiGeorge Syndrome). While both conditions involve the same general neighborhood on chromosome 22, they are fundamentally different:

  • DiGeorge Syndrome is a deletion, meaning a piece of genetic material is missing [6].
  • Cat-Eye Syndrome is an excess, meaning there is extra genetic material [7].

Crucially, the extra chromosome piece in CES includes a unique central structure (the centromere and pericentromeric region), which makes it distinct from other simple genetic duplications (like 22q11.2 Microduplication Syndrome). This unique structure dictates how the syndrome presents itself [3].

The Role of Mosaicism

Up to 40% of people with CES have mosaicism [8]. This means the extra marker chromosome is not in every single cell of their body. Some cells have the extra piece, while others have the typical 46 chromosomes.

It is important to know that genotype-phenotype correlation—the ability to predict symptoms based on genetic results—is poor in CES [4]. Even if the extra piece of chromosome is small, or if a child has a lower percentage of mosaic cells, it does not reliably guarantee “milder” symptoms [1][4]. The presentation remains unpredictable.

How It Happens: De Novo vs. Inherited

Parents often ask if they did something to cause this. The answer is no. Genetic changes like this happen spontaneously during the very early stages of cell division.

  • De Novo (New): Roughly three-quarters of CES cases are “de novo,” meaning the change happened by chance for the first time in the child, and neither parent carries the marker [1].
  • Inherited: About one-quarter of cases are inherited from a parent [1].
  • The “Silent” Carrier: Because CES is so variable, a parent can carry the extra marker chromosome in a mosaic form and have almost no symptoms [1][7]. They may only find out they have it after their child is diagnosed.

Genetic counseling is highly recommended for families to understand these patterns and what they mean for the future [4].

Common questions in this guide

What causes Cat-Eye Syndrome?
Cat-Eye Syndrome is caused by having extra genetic material on a specific section of chromosome 22. In most cases, this appears as a tiny extra piece called a small supernumerary marker chromosome (sSMC).
Is Cat-Eye Syndrome inherited from the parents?
About 75% of the time, the genetic change happens spontaneously during early cell division and is not inherited. However, in roughly 25% of cases, the extra chromosome is inherited from a parent who may be a 'silent' carrier with few or no symptoms.
What is the difference between Cat-Eye Syndrome and DiGeorge Syndrome?
While both conditions involve the same general area of chromosome 22, DiGeorge Syndrome is caused by missing genetic material (a deletion). In contrast, Cat-Eye Syndrome is caused by having extra genetic material (an excess).
What does mosaicism mean in Cat-Eye Syndrome?
Mosaicism means that the extra genetic material is present in some of your child's cells, but not all of them. Up to 40% of people with Cat-Eye Syndrome have this mosaic pattern, though it does not reliably guarantee milder symptoms.
Can genetic testing predict how severe my child's Cat-Eye Syndrome will be?
Unfortunately, predicting the severity of symptoms based solely on genetic test results is very difficult in Cat-Eye Syndrome. The physical and developmental presentation remains unpredictable regardless of the specific genetic details.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Is my child's extra genetic material considered a 'Type I' or 'Type II' marker chromosome, and what does that mean for them?
  2. 2.Since we know this can be inherited from a parent with very few symptoms, should we undergo 'FISH' or 'microarray' testing to see if one of us is a mosaic carrier?
  3. 3.How does the presence of the pericentromeric region on chromosome 22 affect my child's diagnosis compared to a simple duplication?

Questions For You

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References

References (8)
  1. 1

    Cat eye syndrome: Clinical, cytogenetics and familial findings in a large cohort of 43 patients highlighting the importance of congenital heart disease and inherited cases.

    Jedraszak G, Jobic F, Receveur A, et al.

    American journal of medical genetics. Part A 2024; (194(4)):e63476 doi:10.1002/ajmg.a.63476.

    PMID: 37974505
  2. 2

    Small supernumerary marker chromosomes and their correlation with specific syndromes.

    Jafari-Ghahfarokhi H, Moradi-Chaleshtori M, Liehr T, et al.

    Advanced biomedical research 2015; (4()):140 doi:10.4103/2277-9175.161542.

    PMID: 26322288
  3. 3

    A child with cat-eye syndrome and oculo-auriculo-vertebral spectrum phenotype: A discussion around molecular cytogenetic findings.

    Glaeser AB, Diniz BL, Santos AS, et al.

    European journal of medical genetics 2021; (64(11)):104319 doi:10.1016/j.ejmg.2021.104319.

    PMID: 34474176
  4. 4

    Clinical and molecular cytogenetic findings of cat eye syndrome and a 2-year-old patient with congenital aural atresia and hearing loss.

    Xu L, Cheng X, Tang L, et al.

    BMC pediatrics 2024; (24(1)):658 doi:10.1186/s12887-024-05136-9.

    PMID: 39402511
  5. 5

    Prenatal diagnosis and molecular cytogenetic identification of small supernumerary marker chromosomes: analysis of three prenatal cases using chromosome microarray analysis.

    Xue H, Chen X, Lin M, et al.

    Aging 2020; (13(2)):2135-2148 doi:10.18632/aging.202220.

    PMID: 33318309
  6. 6

    ADA2 deficiency due to a novel structural variation in 22q11.1.

    Grossi A, Cusano R, Rusmini M, et al.

    Clinical genetics 2019; (95(6)):732-733 doi:10.1111/cge.13518.

    PMID: 30920658
  7. 7

    Growth hormone deficiency and pituitary malformation in a recurrent Cat-Eye syndrome: a family report.

    Jedraszak G, Braun K, Receveur A, et al.

    Annales d'endocrinologie 2015; (76(5)):629-34.

    PMID: 26518262
  8. 8

    Dynamic nature of somatic chromosomal mosaicism, genetic-environmental interactions and therapeutic opportunities in disease and aging.

    Vorsanova SG, Yurov YB, Iourov IY

    Molecular cytogenetics 2020; (13()):16 doi:10.1186/s13039-020-00488-0.

    PMID: 32411302

This page explains the genetics of Cat-Eye Syndrome for educational purposes. Your pediatric geneticist or genetic counselor is the best source for interpreting your child's specific genetic report and advising on family planning.

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