X-linked Opitz G/BBB Syndrome: MID1 Gene Sequencing

X-linked Opitz G/BBBsyndrome is a congenital midline malformation syndrome characterized by facialanomalies, laryngo-tracheo-esophageal defects, and genitourinary abnormalities.Facial anomalies seen in X-linked Opitz G/BBB syndrome include ocularhypertelorism, prominent forehead, widow's peak, broad nasal bridge, andanteverted nares. Genitourinary abnormalities include hypospadias,cryptorchidism, and hypoplastic/bifid scrotum. The condition is geneticallyheterogeneous, as there is an autosomal dominant form as well.

Hypertelorismand hypospadias are the most frequent manifestations of X-linked Opitz G/BBBsyndrome, being present in almost all individuals. Developmental delay andmental retardation are observed in about 50% of affected males. Cleft lip and/or palate are present in approximately 50% of affected individuals. Other malformations present in fewer than 50% of individualsinclude congenital heart defects, imperforate or ectopic anus, and midline brain defects(Dandy-Walker malformation and agenesis or hypoplasia of the corpus callosumand/or cerebellar vermis). Monozygotic twinning is unusually frequent infamilies of individuals with X-linked Opitz G/BBB syndrome, and may be amanifestation of the defect; unusually severe cases with early lethality occuras twins. Wide clinical variability occurs even among members of the samefamily. Female carriers usually manifest only ocular hypertelorism. Theprevalence of X-linked Opitz G/BBB syndrome ranges from one in 50,000 to one in100,000 males.

Thediagnosis of X-linked Opitz G/BBB syndrome is established most often byclinical findings. MID1 (Xp22) is the only gene currently known to be associated with X-linked Opitz G/BBB syndrome. Sequence analysis of the MID1 gene detects mutations in15%-45% of males with clinically diagnosed Opitz G/BBB syndrome. Deletions andduplications in and of the MID1 genehave also been reported. The cohorts tested for MID1 mutations ofteninclude simplex cases (i.e., individuals with no family history of Opitz G/BBB syndrome), who therefore cannot be determined to have either theX-linked form or the autosomal dominant form. The detection rate is higher in individuals with clear X-linkedinheritance. De novo mutations havebeen reported.

For patients with suspected X-linked Opitz G/BBBsyndrome, sequence analysis is recommended as the first step in mutation identification. For patients in whom mutations are not identified by full gene sequencing, deletion/duplication analysis is appropriate.

Click here for the GeneTests summary on this condition.

This test is indicated for:

• Confirmation of a clinical/biochemical diagnosis of X-linked Opitz G/BBB syndrome
• Carrier testing in adult females with a family history of X-linked Opitz G/BBB syndrome

PCR amplification of 9 exons contained in the MID1 gene is performed on the patient's genomic DNA. Direct sequencing of amplification products is performed in both forward and reverse directions, using automated fluorescence dideoxy sequencing methods. The patient's gene sequences are then compared to a normal reference sequence. Sequence variations are classified as mutations, benign variants unrelated to disease, or variations of unknown clinical significance. Variants of unknown clinical significance may require further studies of the patient and/or family members. This assay does not interrogate the promoter region, deep intronic regions, or other regulatory elements, and does not detect large deletions.

• Deletion/duplication analysis of the MID1 gene by CGH array is available for those individuals in whom sequence analysis is negative (ZD).
• A CGH array-based test for deletion/duplication analysis of 64 different X-linked intellectual disability genes is available (OL).
• Custom diagnostic mutation analysis (KM) is available to family members if mutations are identified by targeted mutation testing or sequencing analysis.
• Prenatal testing is available to adult females who are confirmed carriers of mutations. Please contact the laboratory genetic counselor to discuss appropriate testing prior to collecting a prenatal specimen.

Requisition Forms

• Comprehensive Molecular Test Requisition Form
• Autism/Intellectual Disability/Developmental Delays Test Requisition Form

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