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Boomerang dysplasia

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Alternate names

Dwarfism with short, bowed, rigid limbs and characteristic facies; Boomerang-like skeletal dysplasia

Definition

Boomerang dysplasia (BD) is a rare lethal skeletal dysplasia characterized by severe short-limbed dwarfism, dislocated joints, club feet, distinctive facies and diagnostic x-ray findings of underossified and dysplastic long tubular bones, with a boomerang-like bowing.

Epidemiology

Boomerang dysplasia is a rare disorder; its exact prevalence is unknown. Approximately 10 affected individuals have been identified.

Cause

  • Mutations in the FLNB gene cause boomerang dysplasia.
  • The FLNB gene provides instructions for making a protein called filamin B.
  • This protein helps build the network of protein filaments (cytoskeleton) that gives structure to cells and allows them to change shape and move.
  • Filamin B attaches (binds) to another protein called actin and helps the actin to form the branching network of filaments that makes up the cytoskeleton.
  • It also links actin to many other proteins to perform various functions within the cell, including the cell signaling that helps determine how the cytoskeleton will change as tissues grow and take shape during development.
  • Filamin B is especially important in the development of the skeleton before birth.
  • It is active (expressed) in the cell membranes of cartilage-forming cells (chondrocytes).
  • Cartilage is a tough, flexible tissue that makes up much of the skeleton during early development.
  • Most cartilage is later converted to bone (a process called ossification), except for the cartilage that continues to cover and protect the ends of bones and is present in the nose, airways (trachea and bronchi), and external ears.
  • Filamin B appears to be important for normal cell growth and division (proliferation) and maturation (differentiation) of chondrocytes and for the ossification of cartilage.

Gene mutations

  • FLNB gene mutations that cause boomerang dysplasia change single protein building blocks (amino acids) in the filamin B protein or delete a small section of the protein sequence, resulting in an abnormal protein.
  • This abnormal protein appears to have a new, atypical function that interferes with the proliferation or differentiation of chondrocytes, impairing ossification and leading to the signs and symptoms of boomerang dysplasia.

Inheritance

Autosomal dominant pattern, a 50/50 chance.

This condition is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. Almost all cases result from new mutations in the gene and occur in people with no history of the disorder in their family.

Signs and symptoms

  • Affected individuals are born with inward- and upward-turning feet (clubfeet) and dislocations of the hips, knees, and elbows.
  • Bones in the spine, rib cage, pelvis, and limbs may be underdeveloped or in some cases absent.
  • As a result of the limb bone abnormalities, individuals with this condition have very short arms and legs. Pronounced bowing of the upper leg bones (femurs) gives them a "boomerang" shape.
  • Some individuals with boomerang dysplasia have a sac-like protrusion of the brain (encephalocele).
  • They may also have an opening in the wall of the abdomen (an omphalocele) that allows the abdominal organs to protrude through the navel.
  • Affected individuals typically have a distinctive nose that is broad with very small nostrils and an underdeveloped partition between the nostrils (septum).
  • Individuals with boomerang dysplasia typically have an underdeveloped rib cage that affects the development and functioning of the lungs.
  • As a result, affected individuals are usually stillborn or die shortly after birth from respiratory failure.

For most diseases, symptoms will vary from person to person. People with the same disease may not have all the symptoms listed. 80%-99% of people have these symptoms

  • Abnormality of tibia morphology(Abnormality of the shankbone)
  • Aplasia/Hypoplasia of the fibula(Absent/small calf bone)
  • Growth hormone deficiency
  • Micromelia(Smaller or shorter than typical limbs)
  • Narrow chest(Low chest circumference)
  • Poorly ossified vertebrae
  • Severe short-limb dwarfism

30%-79% of people have these symptoms

5%-29% of people have these symptoms

  • Abnormality of the ulna

Diagnosis

Diagnosis can be confirmed from skeletal radiographs, chondro-osseous histopathology and genetic testing.

Distinctive radiographic findings are similar to AOI but, BD presents with a more severe deficiency in mineralization, with non-ossification of certain segments of limbs and vertebrates, and a boomerang-like shape of some long tubular bones.

Differential diagnosis

Treatment

  • Cervical spine instability in asymptomatic infants can be successfully managed with posterior arthrodesis.
  • Function can be stabilized (if not improved) in infants with myelopathic signs by a combination of anterior decompression and circumferential arthrodesis.
  • Hip dislocation in individuals with Larsen syndrome usually requires operative reduction. Scoliosis and clubfeet are managed in a routine manner.
  • Anesthetic agents that exhibit more rapid induction and recovery are preferred in those with laryngotrachiomalacia. When possible, cleft palate and hearing loss are best managed by multidisciplinary teams.[1][1].

References

  1. Robertson S. FLNB Disorders. 2008 Oct 9 [Updated 2020 Feb 13]. In: Adam MP, Ardinger HH, Pagon RA, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2020. Available from: https://www.ncbi.nlm.nih.gov/books/NBK2534/

NIH genetic and rare disease info

Boomerang dysplasia is a rare disease.


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