Frequently Asked Questions about:
Laboratory Testing for Identifying Osteogenesis Imperfecta (OI) among children evaluated for "Unexplained Fractures"
What is osteogenesis imperfecta (OI)?
OI is a genetic disorder characterized by fragile bones that break easily, often with little or no apparent cause. There are currently six types of OI recognized that range in severity from mild (OI type I) to a form that is lethal in the newborn period (OI type II). More types are being defined by careful clinical and genetic studies.
The clinical diagnosis of OI rests on several features in addition to fractures. These features include a family history of the disorder, which is not always positive as many children have OI as a result of new mutations, blue or grey colored sclerae (the whites of the eyes), discolored and fragile teeth (dentinogenesis imperfecta), and bone deformity. As it may be difficult to make a diagnosis of the milder forms of OI in childhood, it is important that if the question of OI is raised the child be seen by a physician, such as a geneticist, who has had experience with the range of expression in OI.
What are "unexplained fractures"?
When a child or infant presents to medical care with fracture, there are instances when the cause of the fracture is unclear. When there is no definite traumatic event preceeding the fracture and the family is unable to recall an injury, the fracture is considered "unexplained".
Under the federal Child Abuse Prevention and Treatment Act (CAPTA), states are regulated to address concerns of child abuse. The statutes in each state are individualized and determine the grounds for intervention in protection of the well being of children in that state. Often "unexplained fractures" raise the possibility of child abuse and physicians are obligated to report to the state child protection agency.
How often will an infant with "Unexplained Fractures" be identified to have OI?
OI is a relatively uncommon disorder identified in about 1 per 10,000 - 20,000 births whereas child maltreatment occurs at a much higher frequency (2.4 per 1, 000 children between birth and 3 years of age were physically abused) (National Clearinghouse on Child Abuse and Neglect (NCCAN) statistics), which means that abuse as a cause of fracture is 24-48 times more likely as an explanation for fractures than the presence of OI. Given this statistic, unexplained fractures will sometimes result in the investigation of child abuse in families who have a child with OI.
Should all children with unexplained fractures have laboratory testing for OI?
No. If a geneticist or other physician experienced in the diagnosis and care of children with OI has examined the child with unexplained fractures and cannot readily exclude OI by physical examination, family history and review of x-rays, lab testing should be considered. However, when other information establishes non-accidental injury, OI testing may not be of value.
What sort of laboratory testing is available for OI?
There are two options for laboratory testing for OI, "DNA studies" of the type I collagen genes and "biochemical studies" of the collagen protein. At present, the DNA-based testing is believed to be slightly better at identifying children with OI. However, there are instances when a biochemical assay is necessary as well.
I. DNA Studies of the COL1A1 and COL1A2 Genes
Molecular genetic testing (DNA testing) is designed to determine if there are alterations in the sequence of either of the two genes (COL1A1 and COL1A2) that are responsible for synthesizing type I collagen. These tests are done by isolating DNA (genes) from any source of cells-usually blood, extracting DNA, then evaluating the nucleotide structure of the DNA by sequencing in an automated DNA sequencer.
What is the sensitivity of molecular DNA screening?
The sensitivity of COL1A1 and COL1A2 genomic screening is roughly 90-95% for the mild forms of OI that one might be looking to identify in the "unexplained fractures" population. The sensitivity in the more severe varieties may be better but is not exact.
How long does it take to complete genomic DNA testing?
In our laboratory, genomic sequencing of COL1A1 and COL1A2 may take 6-7 weeks.
II. Biochemical Studies - Analysis of type I collagen molecules made by cultured fibroblasts (from a skin biopsy).
Type I collagen is the major protein in bone and the vast majority of individuals with OI have mutations in one of the two genes that direct the synthesis of collagen in bone. A small biopsy - 2-3mm in diameter (about an eighth of an inch across)- is taken from the skin of an arm or leg (a small amount of local anesthetic is used to numb the region) and the cells are grown from the tissue. When cells have grown to an adequate number, the type I collagen molecules that they manufacture are collected and studied by a procedure called gel electrophoresis. This type of study can assess how much of the protein is made, whether the structure of the protein molecule is normal or abnormal, and how well it is chaperoned out of the cell (where it normally functions). Cells from individuals with OI have two distinct alterations in the type I collagen that they make, depending on the type of OI that the person has. In the mildest forms, the amount of type I procollagen produced, is decreased, while in the more severe forms of OI the amount made is normal and the structure of the collagen molecule is altered.
What is the sensitivity of biochemical testing in identifying children with OI?
Biochemical studies of cells identifies about 90% of children and adults with non-lethal forms of OI.
How long does biochemical testing take to complete?
The biochemical studies take about 2-3 months from the date of biopsy.
Because genetic testing of this nature is not 100% sensitive, if laboratory testing is normal, the diagnosis of OI cannot be entirely excluded.
The ability of both these tests to identify abnormalities depends on two factors:
- Whether OI in the child being examined results from a mutation in a collagen gene and
- Whether either or both tests can detect either the mutation itself (DNA testing) or the effects of the mutation on the amount and electrophoretic mobility of the chains of type I collagen.
Almost everyone with dominantly inherited OI (running through multiple generations in families) is thought to have mutations in one of the type I collagen genes. Most people who have new mutations (that is, have no family history of the disorder) are also thought to have such collagen gene mutations. It is becoming clear, however, that some forms of OI are probably not the result of mutations in these collagen genes.
Can genetic testing alone determine if the child has been abused or has OI?
No. On the basis of empiric data alone, a child with "unexplained fractures" is much more likely to have been abused than to have OI. OI is a rare disorder best diagnosed by a physician who has examined the child, considered the medical history, family history and social history, reviewed x-rays and laboratory testing. The laboratory testing is part of the total information considered by the child's doctor.
What does a negative test mean?
When a test is negative it means either that the child does not have a form of OI that results from mutations in the genes of type I collagen or that the type of mutation, or its effects on proteins, are not identified by that particular test. Other biological reasons for bone fragility are not excluded by the study.
It is often argued that if a test identifies, say, 90% of affected individuals and is negative in this situation, that the subject has a 10% chance of still having the disorder. This type of reasoning is not correct and the easiest way to test it is to ask a simple question: if I test 100 people off the street who appear to have no abnormalities other than having had a fracture in their life, do 10% still have OI? The answer is clearly no.
So, in testing children in whom the question of abuse has been raised, the issue that determines the likelihood that the child has OI is a combination of history of the fracture, history of the family, physical features, and outcome of the test. If there is a low suspicion that the child has OI to begin with, then the negative test simply confirms that idea, rather than leaving a 10% uncertainty that the child is affected.
References:
Steiner R, Pepin M, Byers PH Studies of collagen synthesis and structure in the differentiation of child abuse from osteogenesis imperfecta. J Pediatr. 1996 Apr;128(4):542-7.