by Rami Khoury
Osteopetrosis is a rare metabolic bone disease characterized by generalized increase in skeletal mass. It is inherited in a number of mammalian species, including man, and result from congenital defect in the development or function of osteoclast. Several forms of osteopetrosis have been discovered with overlapping clinical and radiographic features. The disorder is inherited in two forms. The first is autosomal recessive with manifestations in the newborn and a progressive course leading to death in early age. The second is autosomal dominant which is usually milder disorder with delayed manifestations that could carry with the patient into adult life. In each of the cases, consequent impairment of bone resorption prevents formation of bone marrow cavities, causing delayed or absent tooth eruption and results often in abnormally shaped bone. There are different forms of mutation for each of the autosomal recessive and dominant disease. The different forms of mutations are derived from changes in different genes that code for proteins on the signaling pathway of osteoclast synthesis. In order to understand the osteoclast pathway one must understand what are osteoclast and how they are synthesized. Osteoclast are cells in the bone matrix responsible for degrading old bone, while concurrently osteoblasts lay down new bone to fill in the excavated tunnel. The osteoclasts originate, like macrophages, from hemopoietic stem cells in the bone marrow. These precursor cells are released as monocytes into the bloodstream and collect at sites of bone resorption. There activation is influenced by factors secreted by the osteblasts, and other factors. Osteoblastic stromal cells play an important roll in modulating the differentiation of osteoclast proginators in two different ways: one is the production of osteoclast inducing factor (OIF), and two is cell-to-cell recognition between osteoclast progenitors and osteoblastic stromal cells.
A number of OIF as well as systemic hormones induce osteoclast differentiation. They are classified into three categories in terms of the signal transduction: Vitamin D receptor-mediated signals [1 alpha,25(OH)2D3]; protein kinase A-mediated signals (PTH, PTHrP, PGE2, and IL-1); and gp130-mediated signals (IL-6, IL-11, oncostatin M, and leukemia inhibitory factor). [Ref 4]. All of these OIF appear to act on osteoblastic cells to induce osteoclast differentiation factor, which recognizes osteoclast progenitors and prepares them to differentiate into mature osteoclasts. An example of OIF is the mutation in the Csfm gene leading to osteopetrosis. A study in homozygous mice for the recessive mutation show that fibroblasts cells are defective in the production of functional macrophage colony-stimulating factor (M-CSF). An interesting discovery in this research was that they found the Csfm mRNA to be present at normal levels. This suggests that the mutation is within the Csfm gene itself. After sequencing the Csfm complementary DNA prepared from op/op fibroblast, a single base pair insertion was found in the coding region of the Csfm gene that generates a stop codon 21 base pair down stream. Thus the pathological changes in this mutant results from the absence of M-CSF. [Ref 1].
The second type of mutation is responsible for osteoclast cell-to-cell adhesion and recognition. To demonstrate this mutation, experimental gene disruption in mice of the c-src proto-oncogene encoding a non receptor tyrosine kinase was performed. The results were that osteoclast were present but inactive. This is due to an impaired ability of osteoclasts to adhere to the bone surface and form bone-resorbing ruffled borders. Osteopontin (OPN), a secreted phosphoprotein, mediates osteoclast adherence to the bone matrix. It was found in the same study that mice with the c-src mutation expressed OPN mRNA and protein at a significantly reduced levels as compared with normal mice. This suggest a roll for the proto-oncogene c-src in the regulation of OPN gene expression. Even more interesting, it was found that OPN gene expression can be induced by treatment with epidermal growth factor (EGF) and 12-O-tetradecoanoyl phorbol-13-acetate (TPA). Thus indicating that c-src and EGF may regulate OPN gene expression through a common signaling pathway. [Ref 5].
There are many studies being conducted on how osteopetrosis work. The key element to understand this disease is to first learn how the osteoclast are regulated and function. Many different mutation have already been discovered that produce osteopetrosis. There does not seem to be a constant cause, although its found that osteopetrosis is detected much more in close sibling marriages that are related in blood. Its also known that whether the disease is inherited as recessive or dominant, the phenotype and onset of expression varies. In the first example where OIF play a roll, the disease can be cured by transplantation of the hemopoietic cells. In other cases, bone marrow transplantation has also been successful in curing human osteopetrosis. This together with the variability in the age of onset and severity of clinical aspects, suggests that a multiplicity of genetic mutation may cause the human disease.
An example of the dominant mutation is presented here by a pedigree of the Neumans family:


As seen in Jack Neumans case, the dominant mutation is not always expressed. This form of the mutation is referred to as incomplete penetrance. Even though Jack carries the mutation he does not express it. Since Nikkei doesn't carry the mutant, then Jack must of past on the mutation, and some how its penetrant in his descendants.
An example of the recessive mutation is presented by the pedigree of Barbers family. In which no one in three generations showed any sign of osteopetrosis, however when two cousins in the third generation were married, two out of three of their offspring expressed the phenotype of osteopetrosis. Its interesting to notice that two out of three express the mutation. Normally we would have expected the classical 3:1 ratio. However, like the dominant form, osteopetrosis is derived from a multiple forms of mutations.

1. The murine mutation osteopetrosis is the coding region of the macrophage colony
stimulating factor gene. Nature. Vol. 345, May 1990.
2. Osteopetrosis in Src-deficient mice is due to an autonomous defect of osteoclasts. Cell
Biology. Vol. 90, May 1993.
3. Chipping away at marble-bone disease. The New England Journal of Medicine. Vol.
332, June 15, 1995.
4 Modulation of osteoclast differentiation by local factors. School of Dentistry, Showa
University, Tokyo, Japan.
5 Recent developments in the understanding of pathophysiology of osteopetrosis.
Department of Pathophsiology, University of Berne, Switzerland.

Return Case Studies in Virtual Genetics 1996-1997