Ano letivo 2012-2013Biologia celular e molecular II

Cellular and molecular mechanisms in Osteogenesis

imperfecta

Luís Baptista, turma 2 Tânia Alves, turma 1Tiago Barbosa, turma 2Tiago Capela, turma 2

Osteogenesis Imperfecta • Osteogenesis imperfecta (OI) is a congenital bone disorder.

• Caused by mutations affecting type I collagen.

CollagenThe molecule Fibrous proteins found in all multicellular animals

Triple-stranded helical structure

Repeating sequence of three amino acids glycine-X-Y (X and Y are often proline and hydroxyproline)

28 types of collagen identified and described

90% of the collagen in the body is of type I

Main types found in connective tissue: types I, II, III, V and VI

Collagen fibrils. These collagen fibrils are present in the joint capsule tissue that surrounds the knee.

Collagen formation

What genes are related to osteogenesis imperfecta?

Mutations in the COL1A1, COL1A2, CRTAP, and LEPRE1 genes, as well as others.

COL1A1 and COL1A2 genes

The COL1A1 gene is located on the long (q) arm of chromosome 17 at position 21.33.

The COL1A2 gene is located on the long (q) arm of chromosome 7 at position 22.1.

CRTAP and LEPRE1 genes

The CRTAP gene is located on the short (p) arm of chromosome 3 at position 22.3.

The LEPRE1 gene is located on the short (p) arm of chromosome 1 at position 34.1.

How do people inherit Osteogenesis imperfecta?

• Most cases of cases have an autosomal dominant pattern of inheritance;

• Sporadic mutations;

• Less commonly, osteogenesis imperfecta has an autosomal recessive pattern of inheritance.

Molecular defectsOI type I

• Over 90% of patients have mutations that reduce the synthesis of pro-α1 chains to about one half;

• Mutations that reduce the synthesis of pro-α2 chains produce slightly more severe phenotypes;

OI types II, III and IV

• Mutations that produce structurally abnormal pro-α chains that have compromised assembly or abnormal folding of the triple helix;

“Procollagen suicide”One abnormal chain interacts with two normal chains

Folding is prevented

Accumulation of unfolded chains in the ER

Enzimatic degradation

Unfolded protein response (UPR) is estimulated

• Intracellular efects

• Intracellular efects

Glycine’s replacementReplacement of glycine by a bulkier amino

acid

Production of poorly formed and unstable triple helices

Disruption to helix folding

Increasing of posttranslational hydroxylation and glycosylation of lysines

• Intracellular efects

• Intracellular efects

N-propeptide’s persistenceAbnormal pro-α chains prevent normal processing of

the N-propeptides

N-propeptides persist

Incorrect fibril assembly

• Extracellular efects

• Extracellular efects

Collagen containing helix mutation can form insoluble aggregates in the ER that are degraded by the autophagosome-endosome system;

Glycine substitutions in the N-terminal region tend to produce milder phenotypes;

• Rare substitutions of charged amino acids or a branched amino acid in X- or Y- positions are lethal;

What determines the patients’ phenotype?

Hypothesis: gradient of phenotypic severity

Symptoms

Type I (Mild)

Type II (Perinatal Lethal)

Type III (Progressive Deforming)

Type IV

Diagnosis

• X rays

• Laboratory testing

• Dual Energy X-ray Absorptiometry (DXA)

• Bone Biopsy

• Diagnosis is primarily based on clinical evidence;

• The most frequent exams used to identifying the disease are:

TreatmentsCurrently prescribed

• orthopaedic surgery ;

• scoliosis management ;

• rehabilitation, including water therapy and physical activity ;

• Develop healthy lifestyle diet and exercise habits ;

TreatmentsCurrently being investigated:

Pharmacologic Therapy

Cellular and Genetic Therapy

Bisphosphonates

Teriparatide

Growth Hormone

Inhibition of receptor activator of nuclear factor-kappaB ligand (RANKL)

Bone marrow transplantation

Gene therapy

Treatments that are ineffective and are no longer prescribed:

• vitamin C

• sodium fluoride

• magnesium

• anabolic steroids

• calcitonin

Bone formation and bone resorption

Pharmacologic therapy

Synthetic analogues of pyrophosphate, like Pamidronate, Risedronate, Alendronate and zoledronic acid ;

Biphosphonates

Inhibit osteoclast-mediated bone resorption on the endosteal surface of bone by:

Teriparatide (Forteo)

In same cases, mainly in chilhood, there is an increase in the incidence of osteosarcoma, a malignant bone tumor

A synthetic form of the natural human Parathyroid Hormone (PTH) ;

Intermittent administration of these drug would:

• Reduce the chance of getting a fracture

• Increase bone mineral density and bone strength

Growth Hormone(GH)The effects of GH on bone cells are mediated through the functional GH receptors (GHRs) on osteoblast cells ;

But, it also stimulates the insulin-like growth factor type 1 (IGF-1) production in liver and bone

Increase in bone turnover and formation

Osteoblast formation is increased and osteoblast apoptosis is inhibited

Inhibition of receptor activator of nuclear factor-kappaB ligand (RANKL)

The first FDA-approved RANKL inhibitor was Denosumab

Key factor for osteoclast differentiation and activation ;

It’s a ligand for osteoprotegerin(OPG) ;

It’s a member of the tumor necrosis factor (TNF) cytokine family ;

Cellular and Genetic Therapy

contains both hematopoietic stem cells and mesenchymal stem cells (MSCs)

Bone marrow transplantation

latter precursors of osteoblasts

Cellular and Genetic TherapyGene Therapy

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