Osteogenesis Imperfecta

Question 1

What is Osteogenesis imperfecta and how is it characterised?

Answer 1

Collective term for a heterogeneous group of connective tissue syndromes

Characterized primarily by liability to fractures throughout life

Question 2

Clinical features of Osteogenesis imperfecta?

Answer 2

Bone fractures (brittle bones) with little trauma
Loose joints
Weak teeth (dentinogenesis imperfecta)
Blue sclera, or a bluish colour in the white of the eye.
Bowed legs and arms
Short stature
Hearing loss
Estimated incidence approximately 1 per 20,000 births

Question 3

What is blue sclera caused by?

Answer 3

blue sclera, caused by thinness of collagen fibers of thesclerathat allow visualization of the underlying uvea.

Question 4

What genetic testing is used?

Answer 4

Genomic sequencing - Next generation

Question 5

How is the patient tested properly?

Answer 5

By sequencing over 30 different genes

Question 6

Why would WHole genome sequencing not be used?

Answer 6

A whole genome sequence would be over the top. We have less than 50 genes to analyse.

Whole genome sequence only becomes a cost effective tool once we being to need 100s of genes sequenced at once.

Question 7

What happens in OI

Answer 7

Defects in collagen

Question 8

Why is collagen important?

Answer 8

Collagen fibres form major component of bones and extra cellular matrix

Question 9

WHat collagen genes are involved in 90% of cases?

Answer 9

COL1A1 & COL1A2 collagen genes

Question 10

What other gene are invovled?

Answer 10

Autosomal recessive genes e.g. BMP1
X-linked genes e.g. PLS3
Other types in genes involved in collagen processing
Have related but different symptoms

Question 11

How man COL1 associated OI types? How are they inherited?

Answer 11

These are the four COL1 associated OI types.
These are all inherited in an autosomal dominant manner.

Question 12

COL1A1 and COL1A2 OI Type 1

Answer 12

Type 1, classic OI, is the most common and most mild. With classic OI features of bone fractures and blue sclerae.

Question 13

COL1A1 and COL1A2 OI Type 2

Answer 13

Type 2, lethal OI, is lethal in pregnancy. The fetus will have severe skeletal abnormalities and fractures. The rib cage collapses and internal organs damaged.

Question 14

COL1A1 and COL1A2 OI Type 3

Answer 14

Type 3, progressively deforming OI, is severe but not lethal. Its features will be apparent at birth with multiple fractures, very short stature and wheelchair bound.

Question 15

COL1A1 and COL1A2 OI Type 4

Answer 15

Type 4, common variable OI, is similar but has dentinogenesis imperfecta.

Question 16

WHat other OI type worth remebering?

Answer 16

OI type 5 and is caused by a single variant, a c.-14C>T in the IFITM5.
This variant is within the 3’ non coding DNA before the first exon and interefreres with correct transcription of the gene.
It is inherited in an autosomal dominant manner.

Question 17

Why is an NGS analysis necessary?

Answer 17

Because there are many genes can be involved in causing one type of disorder,

Question 18

COL1A1
c.[600G>A];[=]
p.[(Gly200Val)];[(=)]

Answer 18

COL1A1 gene
Change of nucleotide G at position 600 for an A in one allele
Glycine at position 200 changed for a Serine
Missense amino acid change
The second allele is wild type

Question 19

What information do we need to do an variant analysis?

Answer 19

So we need to know the frequency of the variant in the normal population – is it seen in the normal population?

Whether the gene that the variant is found in is consistent with the features the patient has, or are the features seemingly unrelated to this gene?

Is the variant located in an important part of the gene (mutation hotspot) or a part of the gene with no known function attached?

Has the variant previously been reported in an affected patient? It could be a well established variant.

Is the amino acid change predicted to effect the protein structure and / or function?

Meaning how different are the amino acids in terms of size and properties?

Question 20

Case study - Variant analysis
Frequency
Not reported in normal control population genome database (Gnomad)

Answer 20

ACGS Variant Classification Guidelines:
PM2 - Absent from controls (or at extremely low frequency if recessive).

Question 21

Whats the most common mechnanism of OI disorder?

Answer 21

Changing a Gly of the Gly-X-Y repeat

Question 22

Whats the role of glycine in collagen?

Answer 22

Glycine in collagen is vital for structural integrity of the helix.
Every third amino acid is in the centre of the helix, this is glycine.
Glycine is the smallest amino acid.
The only amino acid that can be located here and maintain the helix.

Question 23

What happens if you swap glycine for anyother amino acid?

Answer 23

Swap Glycine for any other amino acid and the structure of the triple helix is disrupted.

Question 24

Describe the collagen triple helix

Answer 24

Collagen helix - intertwining of the 3 collagen COL1A chains (2x COL1A1, 1x COL1A2)
Forms helix with every third residue (Gly) located in the centre of the triple helix
Here there is only space for glycine, the smallest amino acid
Therefore the amino acid sequence of the α-chain is characterised by the Gly-X-Y repeat

Question 25

Whats the normal assembly of collagen type 1?

Answer 25

Two COL1A1s and one COL1A2 are produced from the genes and then intertwined in a helix.

Question 26

Describe effect of qualitative structural mutation - Gly triple helix mutations

Answer 26

Disrupts the Gly-X-Y sequence
Disturbs collagen structure
Result in impaired collagen function,
OI types II-IV

Question 27

What are qualitative defects?

Answer 27

Qualitative differences are those that are characterized by differences in processes, mechanisms, and structures

Qualitative defects describe the situation where mutations result in the synthesis of a structurally abnormal alpha collagen that then forms, or try’s to form, a helix with structurally normal alpha collagens.
This causes a disturbed helical folding resulting in impaired helical function.

Question 28

WHat mutations can cause a qualitative defect.

Answer 28

Mutations such as glycine changes, C-terminal mutations, inframe variants and some splice changes can cause a qualitative defect.

(Qualitative is defect of structure or function)

Question 29

Examples of changes that cause qualitative defects?

Answer 29

Mutations such as glycine changes, C-terminal mutations, inframe variants and some splice changes can cause a qualitative defect.

Question 30

What mutations are type 2 causing?

Answer 30

Those mutations closer to the C-terminus tend to be more severe, type 2 OI causing.

Question 31

What do qualitative defects tend to cause?

Answer 31

Type 2 to type 4

Question 32

According to the ACGS 2020 variant classification, what can be used and why?

Answer 32

classification PM1 strong can be used. This is because the Gly of the Gly-X-Y repeat is considered to a mutation hot spot.

Question 33

WHat are quantitative defects?

Answer 33

Quantitative defects are where the mutation causes a reduction in the amount of collagen.

Question 34

Describe quantitative defects

Answer 34

Mutations that lead to premature truncation – nonsense, frameshifts

Result of these mutations is a quantitative defect in collagen type I,

Only about half of the normal amount of the protein is produced (reduced quantity)

But the collagen type I protein that is produced has a normal structure

Collagen fibre maintains some functionality

Tends to cause mild OI type I

Question 35

Variant assessment
The Glycine of the Gly-X-Y triplet repeat is changed for a valine
Predicted to disrupt the collagen helix
Common mechanism of disease for OI

Answer 35

ACGS Variant Classification Guidelines:

PM1_strong - Located in a mutational hot spot and/or critical and well-established functional domain (e.g. active site of an enzyme)

Question 36

COL1A1
c.[600G>A];[=]
p.[(Gly200Val)];[(=)]

Answer 36

Not seen in normal population database PM2
Glycine vs valine PP3
Located in collagen triple helix repeat PM1_Strong
Variant database: reported to occur in another patient with OI type 1. PP5
COL1A1 gene consistent with patients clinical symptoms PP4

Question 37

COL1A1
c.[600G>A];[=]
p.[(Gly200Val)];[(=)]

Pathogen or bengin?

Answer 37

Class 5 - Pathogenic

Question 38

Treatment

Answer 38

Treatment with bisphosphonates
Increases bone strength/density, reduces fractures
Osteoclast cells break down/dissolve old bone, absorb the bisphosphonate drug.
Osteoclast activity is slowed down.
This reduces bone breakdown, increases density.
No cure.

Question 39

What condition is COL1A1 related Osteogenesis

Answer 39

COL1A1 related Osteogenesis is an autosomal dominant condition

Question 40

What does autosomal mean?

Answer 40

“Autosomal” means that the gene is located on one of the autosome chromosomes

Question 41

What does dominant mean?

Answer 41

Dominant” means that a pathogenic variant in a single copy of the disease-associated gene is enough to cause the disease

Question 42

Autosomal dominant inheritance

Answer 42

A parent with one gene mutated will pass this or the normal copy to offspring, therefore there is a 50% chance of their offspring inheriting the mutation and being affected with the disorder.

Question 43

What further testinc ould be done?

Answer 43

Relative testing

Question 44

Familial testing

Answer 44

Family members with variant can start treatment early
Inform life choices - Know to avoid contact sports!
Prenatal diagnosis available.

Family members without variant
Reassured that they do not have OI
No need to test their offspring