SPB Deficiency in Humans and Knockout Mice

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SP-B Deficiency in Humans and Knockout Mice

• Carreen Attaway
• October 6, 2004

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Lung Surfactant

• A mixture of lipids and proteins needed to reduce
  surface tension at the air-liquid interface
• DPPC is the main phospholipid
• Four other main components SP-A, SP-B, SP-C, and
  SP-D

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Lung Surfactant

• Synthesized by alveolar type II cells and stored
  in secretory granules called lamellar bodies
• Antiadhesive function for alveolar reexpansion

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Surfactant Proprotein B

• 381 amino acid monomeric proprotein
• 42 kDa
• 7 Cysteine residues

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Surfactant Protein B

• 79 amino acid homodimer
• 18 kDa
• 3 intramolecular and 1 intermolecular disulfide
  bridges

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Surfactant Protein B

• Length of 9.5 kilobases, containing 11 exons on
  chromosome 2
• Hydrophobic leader sequence

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Surfactant Protein B

• Functions
• Membrane binding
• Membrane lysis
• Membrane function
• Promotes lipid adsorption
• Formation of tubular myelin

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Surfactant Protein B

• Surface active
• 5 amphiphilic alpha helices
• Lowers surface tension
• Disrupts attractive forces between water molecules

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SP-B Deficiency

• First recognized as a cause of lung disease in
  1993
• Infants usually born full-term, but quickly
  develop respiratory disease
• Typical symptoms include tachypnea, grunting,
  flaring, retractions, and hypoxemia.

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SP-B Deficiency

• Mutation termed 121ins2 accounts for 60-70 of
  abnormal alleles
• Base-pair insertion
• Frameshift mutation
• Premature stop codon
• Decrease in SP-B mRNA

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SP-B Deficiency

• Other mutations
• Missense
• Deletions
• Insertions
• ProSP-B, but no mature SP-B

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SP-B Deficiency

• SP-B replacement therapies can only temporarily
  improve survival
• Failure due to requirement for proteolytic
  fragments
• Only effective treatment so far is lung
  transplantation

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SP-B Deficiency

• Prior to surfactant replacement therapy

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SP-B Deficiency

• Three hours later
• Following surfactant replacement therapy

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Knockout Mice

• Homozygous inactivity leads to death
• Disruption of surfactant homeostasis
• Alveolar proteinosis
• Only SP-B knockouts result in death

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Knockout Mice

• Study 1-Yusen, Cohen, Hamvas
• Heterozygous mice
• Mild air-trapping
• Diminished lung compliance
• Heterzygous humans
• Normal lung compliance

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Knockout Mice

• Study 2-Cole
• Leads to lethal adult respiratory distress
• At least 25 of normal SP-B production
  recommended for successful replacement therapy

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Knockout Mice

• Surfactant responsiveness
• Inhibited SP-B expression
• Surfacant unresponsiveness
• Regulatory or structural genetic defects in SP-B
  expression
• Disruption of the alveolocapillary membrane

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Conclusion

• SP-B is essential for successful fetal-neonatal
  pulmonary transition
• Knockout SP-B studies in mice may help further
  our understanding of SP-B deficiency in humans