Understanding Koolen-de Vries Syndrome (Genetics & Basics)

Receiving a diagnosis of Koolen-de Vries Syndrome (KdVS) can feel overwhelming, but understanding the biology behind it is a powerful first step in advocating for your child. Also known as 17q21.31 microdeletion syndrome, KdVS is a rare genetic condition that affects many systems in the body ^[1][2].

Importantly, children with KdVS can live long, fulfilling lives, and the condition does not typically affect life expectancy ^[1]. While it is a lifelong journey, knowing the “why” behind your child’s challenges—such as developmental delays, a friendly personality, or unique facial features—can help you focus on the best ways to support them ^[1][3].

The KANSL1 Gene: The “Master Controller”

The root cause of KdVS lies in a specific gene called KANSL1, located on chromosome 17 ^[4][3]. Think of this gene as an architect that helps build and maintain the “wiring” of the brain and other organs. Specifically, the KANSL1 protein helps regulate how other genes are turned on or off through a process called epigenetic regulation ^[5][6].

When the KANSL1 gene is not working correctly, the body cannot produce enough of this essential protein. This leads to haploinsufficiency—a medical term meaning that having only one working copy of a gene (instead of the usual two) isn’t enough for the body to function normally ^[1][7].

Two Ways the Diagnosis Happens

While all children with KdVS share similar traits, the genetic “typo” can happen in one of two ways:

1. Microdeletion: A small piece of chromosome 17 (the 17q21.31 region) is missing. This missing piece includes the KANSL1 gene and often several neighboring genes ^[8][7].
2. Gene Variant (Mutation): The chromosome is intact, but there is a specific “spelling error” or tiny “loss-of-function” change within the KANSL1 gene itself that stops it from working ^[3][5].

Both versions result in the same syndrome because the lack of a working KANSL1 gene is the main driver of the condition ^[1][7]. However, children with a full microdeletion may sometimes have additional health concerns, like heart defects, because those neighboring missing genes also play a role in development ^[3][7].

It Is Not Your Fault: The ‘De Novo’ Event

One of the most important things for parents to hear is that KdVS is almost always a de novo event ^[9][10]. De novo is Latin for “from the beginning.” This means the genetic change happened randomly in the egg or sperm, or very early during conception.

• It was not caused by anything you did, ate, or were exposed to during pregnancy.
• It is typically not inherited from either parent ^[9][2].
• Because it is a random event, the risk of having another child with KdVS is usually very low (typically estimated at less than 1%) ^[9].

Understanding Haplotypes (H1 and H2)

In the 17q21.31 region where KdVS occurs, humans carry one of two structural patterns called haplotypes, labeled H1 and H2 ^[11][12]. The H2 haplotype is an “inversion,” meaning a segment of DNA is flipped backward. While carrying the H2 pattern is common and does not cause health problems on its own, research shows that the random microdeletions that cause KdVS almost always occur on a chromosome that has this H2 structure ^[11].

Importantly, carrying the H2 pattern is completely normal and out of anyone’s control. Passing down an H2 chromosome does not mean you caused the microdeletion ^[11].

How KdVS is Confirmed

Geneticists use specific tools to find these changes:

• Chromosomal Microarray (CMA): This test looks for missing or extra pieces of DNA. It is the standard tool for finding the 17q21.31 microdeletion ^[9][8].
• Whole Exome Sequencing (WES) or Whole Genome Sequencing (WGS): These tests “read” the DNA letter-by-letter. They are used to find the specific KANSL1 gene variants that a microarray might miss ^[13][14].

Current medical trends are shifting toward using WGS as a first-tier test because it can detect both deletions and individual gene errors at the same time ^[15][16].

Learn More About Managing KdVS