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PubMed This is a summary of 24 peer-reviewed journal articles Updated
Clinical Genetics · Neurodevelopmental Delay–Intellectual Disability–Skeletal Defects Syndrome

Finding the Specific Genetic Cause

At a Glance

This umbrella label can reflect several rare genetic syndromes, so identifying the specific gene is the key diagnostic step. Exome or genome sequencing and chromosomal microarray may help, but testing can remain inconclusive and inheritance depends on the gene involved.

Moving from a descriptive label to a specific genetic diagnosis is often the most critical step in the journey. While a term like “neurodevelopmental delay-intellectual disability-skeletal defects syndrome” describes what is happening, a genetic diagnosis aims to tell you why [1]. By identifying the specific gene involved, doctors can move from general observations to a personalized care plan, keeping in mind that sometimes testing does not immediately find a cause [2].

The Umbrella of Ultra-Rare Syndromes

Several distinct genetic conditions fit under this descriptive umbrella. Although they share symptoms, they are caused by changes in different “instruction manuals” (genes) within the body, each with their own unique features and evidence base:

  • ZTTK Syndrome (Zhu-Tokita-Takenouchi-Kim Syndrome): Caused by changes in the SON gene. It often involves low muscle tone (hypotonia), seizures, and variable brain differences seen on an MRI, such as periventricular nodular heterotopia (clumps of neurons that didn’t migrate to the right spot during brain development) [3][4].
  • MTSS2-related IDDOF: This syndrome involves intellectual disability, eye (ocular) anomalies, and distinctive facial features. A reported clue is ptosis (drooping eyelids) and a delayed bone age, where the bones appear “younger” on X-rays than the individual’s actual age [5][6].
  • Hackman-Di Donato Syndrome: Linked to the NKAP gene, this condition is reported to present with intellectual disability and musculoskeletal abnormalities, sometimes including a marfanoid habitus—a body type characterized by being tall and thin with long limbs and fingers (arachnodactyly), along with joint hypermobility and scoliosis [7][8].
  • Rauch-Steindl Syndrome (RSS): Caused by changes in the NSD2 gene. Individuals often have growth failure that begins before birth (prenatal-onset growth restriction), short stature, and specific facial differences [9][10].
  • HUWE1-related Disorders: These can range from mild to severe and may include delayed speech, global developmental delay, small hand/foot abnormalities, and skeletal anomalies [11][12].

The Biological Link: Brain and Bone

It may seem strange that one gene can affect both the brain and the skeleton. This happens because many of these genes are involved during embryogenesis (the earliest stages of development in the womb) [13].

For example, the SON gene helps the body “cut and paste” genetic instructions (a process called RNA splicing) [13]. If this process is disrupted, the body may not correctly build the scaffolding for migrating brain cells or the foundation for growing bones [14][15]. Similarly, genes like NSD2 and HUWE1 are involved in turning other genes on or off [16][17]. When these processes are faulty, multiple systems—like the neurological and skeletal systems—develop differently at the same time [18].

How These Conditions Are Inherited

Understanding how a condition is passed down can help you understand the risks for other family members, and a genetic counselor should interpret these results. Most of these syndromes occur in one of three ways:

  1. Autosomal Dominant: Only one changed copy of the gene (from either parent) is needed. However, in many ultra-rare cases, the change is de novo, meaning it happened for the first time in the individual and was not inherited from either parent [19][20].
  2. X-Linked: The gene is located on the X chromosome. Because males typically have only one X chromosome, they are often more severely affected. Females, who have two X chromosomes, may have milder symptoms or no symptoms at all, though this is variable [21][22].
  3. Autosomal Recessive: An individual must inherit two changed copies of the gene (one from each parent) to be affected. The parents are usually “carriers.”

Importantly, a healthy parent carrying the same variant does not automatically rule it out as the cause. Factors like reduced penetrance, variable expression, mild parental disease, X-linked effects, or recessive carrier status can complicate interpretation.

The Diagnostic Process

To tell these syndromes apart, clinical geneticists look for specific “clues” during an exam:

  • Facial Features: The shape of the nose, the distance between the eyes, or the thickness of the lips can point toward one gene over another [17][23].
  • Growth Patterns: Doctors measure whether growth issues started before birth (common in NSD2) or appeared later [9].
  • Specific Imaging: An MRI of the brain or X-rays of the hands and spine can reveal hidden markers, like specific bone shapes or brain structures, that identify a particular syndrome [4][10].

Ultimately, testing is selected by a genetics professional. While Exome Sequencing (WES) or Genome Sequencing (WGS) are often used to find single-letter changes, Chromosomal Microarray (CMA) remains an important tool for finding missing or extra pieces of DNA [20][24].

Common questions in this guide

Which genetic tests can help find the cause of developmental delay and skeletal differences?
Exome sequencing and genome sequencing examine many genes for DNA changes, while chromosomal microarray looks for missing or extra pieces of DNA. A clinical genetics professional chooses the test or combination based on the person’s features and prior results.
What rare syndromes can cause this combination of developmental and skeletal findings?
Examples include ZTTK syndrome caused by SON changes, MTSS2-related IDDOF, Hackman-Di Donato syndrome linked to NKAP, Rauch-Steindl syndrome linked to NSD2, and HUWE1-related disorders. These conditions overlap but have different patterns of brain, growth, facial, eye, and skeletal findings.
Does a healthy parent rule out a genetic variant as the cause?
No. A parent may have no obvious symptoms because the variant has variable effects, causes mild disease, is on the X chromosome, or the parent is a carrier for a recessive condition. A genetics professional should interpret parent testing and the family’s results.
What does de novo mean on a genetic test?
A de novo variant is a DNA change that occurred for the first time in the affected person and was not inherited from either parent. Parent testing can help determine whether a finding is de novo or was passed through the family.
What is a variant of uncertain significance?
A variant of uncertain significance, or VUS, is a DNA change whose connection to health is not yet clear. It does not by itself confirm the cause of a person’s symptoms, and its interpretation may change as more evidence becomes available.
What if genetic testing does not find an explanation?
Testing does not always identify a cause immediately. Ask the genetics team what the result means, what types of DNA changes were assessed, and whether further evaluation may be useful.
Which health checks may be needed after a gene is identified?
Recommended monitoring depends on the gene and the person’s features. It may include a brain MRI, an echocardiogram, and follow-up for scoliosis or other orthopedic concerns, but the treating genetics team should personalize the plan.

Questions For You

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Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What specific genetic test was used to find this variant (e.g., Exome Sequencing or Chromosomal Microarray)?
  2. 2.Is the variant classified as 'pathogenic,' 'likely pathogenic,' or a 'variant of uncertain significance (VUS)'?
  3. 3.Was the variant inherited from a parent, or is it 'de novo' (a new change)?
  4. 4.Should we schedule specific baseline screenings, such as a brain MRI or echocardiogram, based on this gene?
  5. 5.Are there any specific skeletal or orthopedic issues, like scoliosis, that we should monitor for as they grow?

References

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This page is for informational purposes only and does not constitute medical advice. A clinical geneticist or genetic counselor should interpret your genetic report and discuss testing and care for your situation.