Testing for Recessive OI - Types VII and VIII
Parental Status | Biochemical Test | Molecular Test | Samples Needed | Methodology | Eligibility | Contact Us | References
Background: It has long been suspected that there was a recessive form of severe OI. Two genes have recently been identified (1-4) as responsible for most of these cases. The two genes encode for proteins that are part of a complex that is involved in the 3-hydroxylation of type I collagen at one site in each alpha1 chain, a1(I)Pro986.
The first gene, responsible for Type VII OI, is CRTAP. It codes for a protein named cartilage associated protein that is synthesized by several cell types in bone and also in other tissues. Investigators first realized the importance of CRTAP for bone development when the knock-out Crtap mouse was made at Baylor University by Roy Morello and Brendan Lee (1). These investigators also found that type VII OI, a moderately severe form of OI described by Francis Glorieux and colleagues (5) in a First Nations family in Canada, was caused by a partially functional CRTAP mutation. Investigators at the BEMB, NIH found mutations that totally inactivate CRTAP (null mutations) in three infants with a lethal form of OI.
The second gene, responsible for Type VIII OI, is LEPRE1. It codes for a protein with two names, Leprecan and Prolyl 3-Hydroxylase 1 (P3H1), because it was isolated independently for both its occurrence in matrix as a proteoglycan (leprecan) and its intracellular role in collagen modification (P3H1). This is the protein in the modifying complex that contains the enzymatic 3-hydroxylation activity. Mutations in LEPRE1 have included both lethal and severe forms of OI. There appears to be a particular mutation in LEPRE1 that is of West African origin since it occurs in contemporary West Africans as well as African-Americans.
Both Types VII and VIII OI have a recessive pattern of inheritance. This means that each parent is a carrier of the mutation and passes this on to their child, who has two copies of the mutation. Two copies of the same mutant gene are required for an individual to have the clinical condition.
Parental Status: The parents of children with types VII and VIII OI are true carriers of the condition present in their affected child. The mutation is present in one copy of the gene in each of their cells. As a couple they have a 25% risk of an affected child with each pregnancy. Their unaffected children have a 2/3 risk of being a carrier themselves.
Biochemical Test: We can test indirectly for the biochemical effects of mutations in CRTAP and LEPRE1 on type I collagen. The absence of either of these proteins, which participate in collagen modification, leads to overmodification of collagen synthesized by skin cells in culture.
Molecular Test: This test determines the specific gene mutation(s) by direct sequencing of the coding regions of the two genes and the adjacent non-coding regions. An affected child may have a different mutation in the same gene from each of his/her parents.
Samples Needed: The optimal sample would be a dermal punch biopsy (skin biopsy) since a rapid screening test is possible from skin cells, and both the biochemical and molecular tests could be done on the cells grown from this sample. Molecular testing alone requires DNA from a blood sample. If mutations are identified, parental carrier status can be confirmed and sibling carrier status tested on DNA from a blood sample. Please note that no specimen will be accepted without prior authorization. A consent form can be obtained by request only. Contact information.
Skin Biopsy Sample Blood Sample Top of Page
Methodology
Skin Biopsy: Dermal fibroblasts are grown to confluence in culture. Total RNA from patient fibroblasts is examined using real-time reverse transcription PCR, which quantitates the relative CRTAP and LEPRE1 mRNA transcript levels compared to a control cell line. Fibroblast collagen protein is studied on gel electrophoresis to determine whether the collagen shows the typical overmodification caused by recessive OI.
Genomic DNA: The seven exons of CRTAP and/or 15 exons of LEPRE1 and their flanking intronic sequences are amplified by PCR using patient genomic DNA from skin or blood cells. These PCR products are sequenced using automated dideoxy termination sequencing. Mutations found on sequencing are confirmed using restriction enzyme digestion. This restriction digestion can be further utilized to screen parents and siblings for carrier status.
Who is Eligible: Individuals with lethal to moderate forms of OI are candidates for the recessive forms. There are certain clinical features that are common to the currently identified individuals with types VII and VIII OI, such as white sclerae, and other characteristic features on X-rays. Prior exclusion of structural mutations in type I collagen, preferably by molecular testing is necessary. We may also request copies of radiographs. We will test both genes without charge for individuals appropriate for this testing.
How to Contact Us: We would like to have a telephone conversation with you and/or your physician before accepting samples. Please initiate our discussion by e-mail to oiprogram@mail.nih.gov or by contacting the BEMB clinical phone line at 301-496-0741.
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- Morello, R., et al. CRTAP is required for prolyl 3-hydroxylation and mutations cause recessive osteogenesis imperfecta. Cell 2006; 127(2): 291-304.
- Cabral WA. et al. Deficiency of prolyl 3-hydroxylase (Leprecan) causes a novel recessive metabolic disorder of bone resembling lethal/severe osteogenesis imperfecta. American Society of Human Genetics annual meeting 2006; Abstract #279.
- Barnes AM, et al. Recessive lethal form of osteogenesis imperfecta caused by null mutations in CRTAP. American Society of Human Genetics annual meeting 2006; Abstract #280.
- Vranka JA, Sakai LY, Bachinger HP. Prolyl 3-hydroxylase 1, enzyme characterization and identification of a novel family of enzymes. J Biol Chem 2004; 279: 23615-21.
- Ward LM, et al. Osteogenesis imperfecta type VII: an autosomal recessive form of brittle bone disease. Bone 2002; 31(1): 12-8.