Multiple Epiphyseal Dysplasia (MED) and

Pseudoacoachondroplasia (PSACH)

Presenter: Michelle Connors

What is MED?

- Multiple Epiphyseal Dysplasia is a spectrum of disorders that effect the epiphysis which are the growing ends of bones.
- MED is an autosomal dominate condition with three known genes but it is believed that there could be as many as five genes responsible for the condition. This would account for the wide variation of phenotypes seen.
- Classically there are two forms of the disease:
        Fairbanks: The more severe form which is characterized by small fragmented epiphysis
        Ribbing: The less severe form  which is characterized by flat epiphysis.
Even though it is now known that there is more than just two variations of the disorder patients are still classified into one of these two overlapping categories.
- onset is usually during the early school years but in severe cases problems can become evident at the age of 2 while in other cases it may be the late teens to early twenties before problems arise.
-It is not just a human disease as some dog breeds have shown signs of MED in puppies.

Symptoms:

- Moderate short stature (Adult height ranges from 145- 170 cm)
- waddling gate and/or limp
- Pain and stiffness in the weight bearing joints, the knees and hips in particular.
- short stubby metacarpals/ metatarsals (in 80% of cases at least 2SD below the norm) and in some cases shortened fingers.
- back pain due to T12/L1 notching and a deformity in the ring apophysis
- early onset osteoarthritis (20s or 30s)
- development of coxa vara (hip problem) , genu varum, and genu valgum (Knee problems)
- shoulder pain and minimal loss of motion in 1/3 of cases.
- Normal life span
- Normal intelligence

Diagnosis

- Phenotypically very similar to Perth's Disease, Hyponthyroidism and spondylepiphyseal dysplasia but X-rays can give a clear diagnosis.
- The milder the case the later in life it is diagnosed, in some cases nothing will appear wrong with an epiphysis until damage had been done by improper growth. See X-ray
- Genetically related to Pseudoacoachondroplasia (PSACH) and milder forms of PSACH can are phenotypically identical to sever forms of the Fairbanks form of MED.
- PSACH and MED can be distinguished by looking at cartilage under an electron microscope. The cartilage is less disorganized in a patient with MED than in a PSACH patient and the inclusions of the rough endoplasmic reticulum. Inclusions are dialations of the rough ER that alternate being electron dense and being electron lucent. What causes inclusions is not known.


 

Treatment

- Weight Control should be emphasized early on as excess weight makes it harder on the leg joints.
- Refrain from high contact sports and high impact activities such as jogging. Swimming and bike ridding are preferred sports because they are weight free on the joints.
- Physiotherapy and maybe gate training
- Osteotomies around knee or hips.
- May need joint replacement surgery in adulthood. Typically the hips but sometimes the knees as well.
- No diet, pill or surgery can be a cure.

Brief History

- first described by H.A.T Fairbank in 1935
- 1937 S. Ribbing described "Hereditary multiple Epiphyseal disorder"
- in 1947 called "Dysplasia Epiphysialis Multiplex" by Fairbank
- 1995 first gene for MED is discovered (Briggs et al.)

MED vs. PSACH

- Like MED, PSACH in a disorder concerning the epiphysis.
- disproportional short stature, Joint laxity, and early onset osteoarthritis.
- Clinically similar to MED although typically more sever.
- Ultrastructural and biochemical features in the cartilage are similar in both MED and PSACH
- Both linked ot the COMP protein.

Proteins Involved

- Mutations in two different proteins have been shown to cause MED of PSACH.
- Cartilage Oligomeric Matrix Protein (COMP) is an important cartilage protein with a Ca++ binding site that helps keep calcium in our bones.
- The gene for COM is on the short arm of Chromosome 19.
- Type IX collagen is a nonfibrial consisting of 3 collegenous domains COL1-COL3 interrupted by 4 noncollagenous domains NC1-NC4.
- Collagen IX is a structural components of hyaline cartilage, inter vertebral disks and the vitreous body of the eye. Precise function is unknown but a roll in long term stability of the fibril network has been suggested.
- Collagen IX is made of three chains each made on coded by different genes. Two of these genes have been linked to mutations causing MED.  COL9A2 (on Chromosome 1) and COL9A3 (on Chromosome 20) are the two genes know to be  linked to MED Logically mutations in COL9A1 would also cause the disorder but no examples of this has been found yet.

Table #1: MED gene summary

MED gene Protein Chromosome
EDM1 COMP 19
EDM2 COL9A2 1
EDM3 COL9A3 20

The COMP Gene

- On the short arm of Chromosome 19
- Mutations that occur in the area of the DNA that codes for the Ca++ binding site in the protein cause either MED or PSACH.
- Deletions or additions produce PSACH
- Substitutions typically cause MED but can cause PSACH as well thought not as frequently.
- The Fairbank's form of MED is more often linked to a COMP mutation than the Ribbing form.
 

Some known COMP mutations causing PSACH:

    - deletion of 12 nucleotides from exon 10 resulting in the loss of valine 513 - lysine 516
    - deletion of one of the five GAC repeats in exon 13 (GAC repeats in the 7th calmodulin-like repeat in exon 13 are a mutation hot spot.)
 

Some known COMP mutations causing MED:

    - Cysteine 371 substituted by serine
    - Missense mutations in exons 9, 10, 11, 13, and 14


 

The COL9A2 Gene

- The COL9A2 gene codes for the second chain of the Collagen IX protein and is on Chromosome 1
- No mutations in COL9A2 have been shown to cause PSACH but both form of MED have been linked to COL9A2
- There is only one know mutation in COL9A2 know to cause MED and that's in intron 2.
- But how can a mutation in an intron cause a disorder? Aren't the introns sliced out of mRNA before the protein is formed?
- This mutation is in the exon 3 splice donor site which is part of the intron that is recognized in splicing. without this site working properly Exon 3 is completely lost in the splicing causing a big mess in the finished protein.
 

The COL9A3 Gene

- COL9A3 codes for the third chain of the collagen IX protein and is on Chromosome 20
- COL9A3 was until recently not believed to be linked to any human disorders.
- It was know that there were some families where there were no mutations in the COMP or COL9A2 gene so there must be a third gene.
- How do you fine that gene? Where do you look?
- COL9A1 and COL9A3 were the perfect place to start.

Finding the third gene

- This was done by Petteri Paassilta, Jaana Lohiniva, Susanna Annunen, Jacky Bonaventure, Martine Le Merrer Lori Pai and Leena Ala-Kokkol in 1998
- First they need a family that has MED but does not have the two know genes for MED.  Blood samples were taken from 6 affected family members, 4 unaffected family members and 65 healthy volunteers that acted as a control.
- Linkage analysis was done to see if either the COMP, COL9A1, COL9A2 or COL9A3 genes were inherited with the conditions. COMP, COL9A1 and COL9A2 were all ruled out
- Polymerase Chain Reaction (PCR) was used to amplify al 32 exons and exon boundaries of COL9A3
- The exons and exon boundaries were then sequenced and compared.
- RNA was also compared using a retro virus (Epstein-Barr virus) that had it's RNA replaced with the RNA of an infected individual or one of the control group
- The RNA from both individuals was made into cDNA by the virus enzymes and DNA was then sequenced after undergoing PCR
 

Results

-The sequencing from the exons and exon boundaries showed an A/T transversion in the intron just before exon 3. This transversion could cause the deletion of exon 3 like it does in COL9A2. How can you tell for sure? The RNA results.
- When the RNA was run on an electrophoretic gel a clear difference was seen.
- In the healthy people only one 280-bp band was produced.
- In individuals affected with MED two band were produced. One was 280-bp (wild type) And the other was 240-bp (missing exon 3)
- This proves that an A/T transversion in the intron before exon 3 of the COL9A3 gene cases the MED phenotype.
- This is the first time any disorder has been linked to COL9A3.

Living with MED

    Part of the reason I am so interested in MED is because I myself have the Fairbank's form of the disorder. I was diagnosed at the age of 7 but I should have been diagnosed earlier. I am now 21 and I count myself lucky that I have not yet needed any surgery or osteotomies and I have had relatively few problems other than a knee cap that wouldn't behave and stay put when I was a child. When I was diagnosed as a child very little was know about MED and all that was know was clinical. Now with the speed of information in this high tech era quite a bit is know about this rare disease. Being well informed is the best advice I can give to some one who has been diagnosed with MED. Ask questions and do a little research on your own.
    The MED support group is a wonderful source of information and support. Any MED patient or parent of a MED child should contact the support group and get on their mailing list.

Any questions and/or comments? E-mail me:

connors_michelle@hotmail.com