SickKids

Paediatric Laboratory Medicine
Paediatric Laboratory Medicine
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Craniosynostosis Molecular Analysis

Background

Craniosynostosis occurs when the bones of a baby’s skull fuse together before the brain has stopped growing. Craniosynostosis can happen before the baby is born or during the first few months of life. Fusion at one or more locations and excessive growth at others leads to the distortion of the skull. This distortion may cause increased pressure and impaired blood flow in the brain, airway obstruction, impaired vision and hearing, learning difficulties and adverse psychological effects. Deformations in the skull may also be accompanied by abnormalities in the skeletal system, often in the hands and feet. Both genetic and environmental factors contribute to craniosynostosis. Most cases that are genetic in origin arise from new mutations. Genetic craniosynostosis syndromes show autosomal dominant inheritance, meaning that an affected individual has a 50 per cent chance of passing on the mutation to their offspring, who will also be affected.

Common craniosynostosis disorders may be caused by mutations in the fibroblast growth factor receptor 1, 2 or 3 (FGFR1, FGFR2, FGFR3) genes as well as a transcription factor gene called transcription factor TWIST. The diagnosis of non-syndromic craniosynostosis is specifically based on the identification of a mutation in the fibroblast growth factor receptor 3 (FGFR3) gene. The diagnosis of Apert, Crouzon, Pfeiffer and Saethre-Chotzen Syndromes is based on clinical findings, although there is considerable clinical overlap. DNA-based testing of FGFR1 (chromosome 8p11.2-p12), FGFR2 (chromosome 10q25-q26), FGFR3 (chromosome 4p16.3), and TWIST (chromosome 7p21-p22) genes may be helpful in establishing the diagnosis in questionable cases.

Reason For Referral

  • Individuals clinically suspected of being affected with craniosynostosis
  • pregnancies at risk due to abnormal ultrasound findings or a family history of craniosynostosis

Testing Methodology

Direct Mutation Analysis: Samples are analyzed by direct DNA sequencing to identify the common mutations in the relevant genes, based on the clinical diagnosis at referral:

Apert Syndrome

Patient samples are analyzed by direct DNA sequencing for the presence of the two most common mutations in the FGFR2 gene: p.Ser252Trp and p.Pro253Arg.
Approximately 98 per cent of patients with Apert syndrome will be detected by this test>

Non-Syndromic Craniosynostosis

Patient samples are analyzed for the presence of the defining mutation in the FGFR3 gene: c.749C>G p.Pro250Arg).
This mutation is sufficient for the diagnosis of non-syndromic craniosynostosis, but is also found in patients clinically diagnosed with Crouzon, Pfeiffer and Saethre-Chotzen syndromes.

Crouzon Syndrome

Patient samples are analyzed by direct DNA sequencing to detect mutations in exons 7 and 8 (previously exons IIIa and IIIc) of FGFR2. If a mutation in FGFR2 is not found, patient samples are then tested for the presence of the non-syndromic craniosynostosis mutation in FGFR3 (see above)
A mutation will be detected in approximately 50 per cent of patients clinically suspected to have Crouzon syndrome, who are referred for molecular testing. For example, of 10 people clinically thought to have Crouzon syndrome, this test will support the diagnosis in about 5.

Pfeiffer Syndrome

Patient samples are analyzed by direct DNA sequencing to detect mutations in 7 and 8 (previously exons IIIa and IIIc) of FGFR2. If a mutation in FGFR2 is not found, patient samples are then tested for the presence of the non-syndromic craniosynostosis mutation in FGFR3 (see above). For cases of familial Pfeiffer syndrome, direct DNA sequencing is used to detect the c.755C>G p.Pro252Arg) mutation in FGFR1.
A mutation will be detected in 60-70 per cent of patients clinically suspected to have Pfeiffer syndrome, who are referred for molecular testing. For example, of 10 people clinically diagnosed with Pfeiffer syndrome, this test will support the diagnosis in about 7.

Saetre-Chotzen Syndrome

Patient samples are analyzed by direct DNA sequencing to detect mutations in TWIST. If a mutation in TWIST is not found, patient samples are then tested for the presence of the non-syndromic craniosynostosis mutation in FGFR3 (see above).
A mutation will be detected in 60 per cent of patients clinically suspected to have Saethre-Chotzen syndrome, who are referred for molecular testing. For example, of 10 people clinically diagnosed with Saethre-Chotzen syndrome, this test will support the diagnosis in about 6.

Potential Outcomes & Interpretation of Test Results

Clinical Diagnosis

Gene(s) Tested

  • The detection of a mutation in the gene(s) indicated supports the associated clinical diagnosis. This individual is affected and may pass on the mutation to offspring.

  • When no mutation is detected in the gene(s) indicated, the associated clinical diagnosis is not supported by genetic testing. Since craniosynostosis has genetic and environmental causes, the absence of a mutation does not rule out the diagnosis

Apert Syndrome

FGFR2
(exons 7- previously exon IIIa)

Crouzon &
Pfeiffer Syndromes

1. FGFR2
(exons 7 & 8- previously exons IIIa & IIIc)
2. FGFR3(p.Pro250Arg)

Familial Pfeiffer
Syndrome

FGFR1
(p.Pro252Arg)

Saethre-Chotzen
Syndrome

1. TWIST
2. FGFR3 (p.Pro250Arg)

Non-syndromic
Craniosynostosis

FGFR3 (p.Pro250Arg)

Cautions

  • The genetic causes of craniosynostosis are heterogeneous, even when a clear clinical diagnosis is made. The Molecular Genetics Laboratory has devised this mutation analysis strategy based on laboratory findings and the published literature. Molecular testing will not detect all possible mutations leading to craniosynostosis.
  • It is often helpful to first identify the mutations(s) in an affected family member or in the parent of the affected family member. If the familial mutation can be identified in this way, the molecular test is conducted only for the familial mutation.
  • Test results should be interpreted in the context of clinical findings, family history and other laboratory data.
  • Chromosome analysis should be performed on all patients clinically suspected of being affected with Saethre-Chotzen syndrome prior to molecular analysis. These studies may identify patients with a translocation involving chromosome 7 or a cytogenetically detectable microdeletion of band 7p21-22.
  • This test was developed and its performance characteristics validated by the Molecular Genetics Laboratory at the Hospital for Sick Children. It has not been cleared or approved by the U.S. Food and Drug Administration. The FDA has validated that such clearance or approval is not necessary. This test is used for clinical purposes.

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