Symptoms & Diagnosis

ZTTK syndrome presents with a highly variable range of features across multiple body systems. Most individuals have intellectual disability, global developmental delay, and speech and language difficulties, but the severity and combination of features differs significantly between individuals. Most variants are de novo — not inherited from either parent.

Core Symptoms — Dingemans et al. 2022 (n=52)

The following features were identified in the most comprehensive study of ZTTK syndrome to date (Dingemans et al. 2022, 52 patients with truncating SON variants). Percentages show how many patients in the study had each symptom. Because the known population is small, these figures should be interpreted with caution — milder cases discovered through wider diagnostic access may shift the picture over time.

• Intellectual disability: Mild to severe intellectual disability — 100%
• Speech delay: Delayed speech development; some individuals are non-verbal — 100%
• Motor delay: Delayed or inability to crawl, walk, or move independently — 96%
• Facial morphology: No confirmed common face phenotype identified yet — 95%
• Hypotonia: Low muscle tone (floppy tone) from infancy — 70%
• Feeding difficulties: Difficulty keeping food down, swallowing coordination challenges — 64%
• Epilepsy: Seizures can emerge in childhood; type and severity vary — 50%
• Autism features: Autism spectrum-like behaviors — 13%
• Cleft palate: Structural cleft palate — 2%

Note: The Dingemans 2022 data covers truncating variants only (frameshift, nonsense, deletions). Preliminary evidence suggests missense variant patients may have a milder phenotype. The foundation is actively studying this. Patients may be seen by 10 or more specialists across their lifetime.

Organ System Involvement

ZTTK is multisystemic. The following body systems are commonly involved. Periodic re-evaluation is recommended even when initial assessments are normal, as presentation evolves with age.

• Neurology — Brain abnormalities, epilepsy, global developmental delay, intellectual disability, autism-like features
• Ophthalmology — Cortical visual impairment, strabismus, vision loss
• Gastroenterology — Feeding difficulties, GERD, failure to thrive
• Immunology — Low vaccine response, immune deficiency, increased infection susceptibility
• Cardiology — Heart defects, arrhythmias, valve abnormalities
• Craniofacial — Cleft palate, laryngeal clefts, facial asymmetry
• ENT — Repeated infections, hearing loss, swallowing difficulties
• Endocrinology — Growth issues, thyroid disease, metabolic abnormalities
• Orthopedics — Scoliosis, hypermobility, mobility equipment needs
• Pulmonology — Respiratory infections, airway issues
• Urology — Ureter reflux, kidney complications
• Dermatology — Eczema, nail dysplasia, infection susceptibility
• Hematology — Abnormal blood counts, clotting issues

How ZTTK Is Diagnosed

Diagnosis is confirmed through genetic testing identifying a pathogenic variant in the SON gene. Because ZTTK was only described in 2016, many affected individuals were initially diagnosed with "unspecified intellectual disability" or received no genetic diagnosis at all. The expansion of whole-exome sequencing (WES) and whole-genome sequencing (WGS) has significantly improved identification rates.

Clinicians evaluating patients with intellectual disability, developmental delay, and multi-system involvement should include SON gene testing in the diagnostic workup. A GeneReviews entry and draft Standard of Care document are available for clinical reference — see the For Clinicians page.

Resources for Newly Diagnosed Families

If your child has recently received a ZTTK diagnosis, the foundation's Newly Diagnosed guide walks through the first steps — connecting with the ZTTK community, building a specialist team, and participating in research. You are not alone: hundreds of families around the world are on this journey.