Smith-McCort Dysplasia
Gene: RAB33B
Protein: ras-related protein Rab-33B
Clinical Characteristics
- Spondyloepimetaphyseal dysplasia
- Normal birth parameters with progressive skeletal abnormalities
- Short stature
- Pectus carinatum
- Exaggerated lordosis
- Platyspondyly with double-hump or notched appearance of vertebrae
- Decreased joint mobility:
- Limited elbow extension
- Limited flexion of interphalangeal joints causing incomplete fist formation
Inheritance pattern: Autosomal recessive
What Can Be Learned From This Test
Testing is performed by sequencing all exons and surrounding intronic regions of the RAB33B gene. This will detect point mutations, small deletions and small insertions. It will not detect a partial or whole gene deletion or duplication.
A negative test does not rule out a diagnosis of Smith-McCort dysplasia (SMC). A mutation could be present in a region not sequenced or in another gene. Mutations in the DYM gene have also been associated with SMC.
Dyggve-Melchior-Clausen syndrome (DMC) is a disorder that is similar to SMC, but also includes intellectual disability and microcephaly. Mutations in the DYM gene have been associated with both phenotypes, SMC and DMC. Mutations identified to date in RAB33B have been identified in individuals with phenotypes consistent with SMC. At this time, it is not clear if some cases of DMC are due to RAB33B mutations.
Sample Requirements
Draw 2ml–4ml of blood in EDTA/purple-top tube (minimum of 1ml–2ml for infants).
Turnaround time: 10-14 business days
CPT Codes and Cost
Full Gene Sequencing
- Code: 81479
- Cost: $250
Known Variant Testing
- Code: 81479
- Cost: $225