The DNA Library: Understanding Genetics and Diagnosis

Understanding the genetic “blueprint” of Kabuki syndrome can help demystify the diagnosis. While the physical features are what doctors often notice first, the underlying cause is almost always found in how the body manages its genetic instructions.

The Two “Types” of Kabuki Syndrome

Kabuki syndrome is caused by a change (mutation) in one of two specific genes. These genes act like “master switches” or “volume knobs” for other genes in the body ^[1].

Type 1: KMT2D (The Most Common)

• Frequency: This gene is responsible for approximately 75% of Kabuki syndrome cases ^[2].
• Inheritance: It follows an autosomal dominant pattern. This means only one copy of the changed gene is needed to cause the syndrome ^[3].
• Origin: In most individuals, this is a de novo mutation—a random occurrence that was not passed down from either parent ^[4].

Type 2: KDM6A (The Rarer Type)

• Frequency: This gene accounts for about 5% to 10% of cases ^[2].
• Inheritance: This is X-linked. Because the gene is located on the X chromosome, the inheritance pattern and way the syndrome appears can differ slightly between males and females ^[3].
• Origin: This can be a new (de novo) mutation, but importantly, it can also be inherited from a parent, often a mother who may have milder or no obvious symptoms. Therefore, parental genetic testing is critical for family planning ^[5].

What These Genes Actually Do: “Epigenetics”

Think of your DNA as a massive library of books. Every cell has the same books, but a heart cell needs to read different chapters than a brain cell. Epigenetic regulators—like the proteins made by the KMT2D and KDM6A genes—are the “librarians” ^[1].

They use chemical tags to mark which “books” (genes) should be open and which should stay closed ^[6]. When these “librarians” aren’t working correctly, the DNA library becomes disorganized. Genes that should be “turned up” might be “turned down,” leading to the multisystem features of Kabuki syndrome ^[7][8].

How a “Definitive Diagnosis” is Made

In 2019, an international group of experts created a standard checklist to ensure individuals are diagnosed accurately ^[2]. A person of any age can receive a definitive diagnosis if they have a history of infantile hypotonia (low muscle tone as a baby) and developmental delay, plus one of the following:

1. A Genetic Confirmation: A confirmed pathogenic (disease-causing) variant in either the KMT2D or KDM6A gene ^[2].
2. Specific Physical Features: If genetic testing is not available or is inconclusive, a diagnosis can be made if a person has “typical dysmorphic features” ^[2]. This includes:
   • Long palpebral fissures: Long openings for the eyes with the outer third of the lower eyelid turned slightly outward ^[2].
   • And at least two of these: Arched and broad eyebrows, a depressed nasal tip, large or cupped ears, or persistent pads on the fingertips ^[2].

When Sequencing is Unclear: DNA Methylation

Sometimes, standard genetic sequencing finds a “typo” in the gene, but the lab isn’t sure if it actually causes the syndrome. This is called a Variant of Uncertain Significance (VUS) ^[4].

In these cases, doctors can use a newer technology called DNA methylation profiling (often called an episignature) ^[9]. This test looks for a specific chemical pattern—a “molecular fingerprint”—on the DNA that is unique to Kabuki syndrome. This test has shown 100% accuracy in identifying the syndrome, providing a “yes” or “no” even when traditional sequencing is fuzzy ^[9][10].

Checklist: Your Genetic Test Report

When you look at the report, ensure these specific details are present so you can share them with specialists:

• [ ] Gene Name: Does it say KMT2D or KDM6A? ^[4]
• [ ] Variant Classification: Is it labeled “Pathogenic,” “Likely Pathogenic,” or “Variant of Uncertain Significance”? ^[4]
• [ ] c. notation (DNA Change): A string of numbers and letters (e.g., c.1234C>T) describing the exact location of the change ^[11].
• [ ] p. notation (Protein Change): Describes how the protein was altered (e.g., p.Arg412Ter) ^[11].
• [ ] Zygosity: Usually “Heterozygous,” meaning the change is on only one of the two copies of the gene ^[4].
• [ ] Inheritance: Does it note if the mutation is “de novo” (new) or if parents were tested? ^[4]