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Keywords:

  • cartilage;
  • bone;
  • tumor;
  • exostoses;
  • osteochondroma;
  • heparan sulfate

Abstract

  1. Top of page
  2. Abstract
  3. SKELETAL GROWTH AND DISEASE
  4. THE THIRD INTERNATIONAL MHE RESEARCH CONFERENCE: A MEETING REPORT
  5. REFERENCES

On October 29, 2009, researchers and physicians gathered at the Sheraton Four Points Hotel in Boston for 4 days to discuss a disease called multiple hereditary exostoses (MHE). MHE is an autosomal dominant disease that is associated with mutations in two enzymes that are required for heparan sulfate (HS) synthesis. Children with the disease form numerous benign bone tumors (osteochondromas) and have >2% chance of developing chondrosarcoma. The aim of the meeting was to generate new ideas for the diagnoses, treatment, and cure of this disease. Discussions ranged from orthopedic surgical treatment and patients' personal experiences to fundamental questions in skeletal biology and the precise molecular role that HS plays in developmental signaling pathways. Developmental Dynamics 239:1901–1904, 2010. © 2010 Wiley-Liss, Inc.


SKELETAL GROWTH AND DISEASE

  1. Top of page
  2. Abstract
  3. SKELETAL GROWTH AND DISEASE
  4. THE THIRD INTERNATIONAL MHE RESEARCH CONFERENCE: A MEETING REPORT
  5. REFERENCES

The growth plate is a specialized structure that is thought to have evolved to allow rapid lengthening of bone in a process called endochondral ossification. Growth plates are found on the ends of the long bones (such as the femur) where cartilage meets bone (Fig. 1). In this region, chondrocytes (the cells that make cartilage matrix) exit from a resting state and form long chains of proliferating cells. After going through several rounds of division, the chondrocytes exit the proliferation zone, secrete more matrix, become hypertrophic, and soon after become apoptotic. The remaining cartilage matrix serves as a template for bone deposition by osteoblasts (the cells that make bone matrix) to form the spongy (trabecular) bone that supports the bone marrow. Thus, lengthening of the trabecular bone is achieved through successive rounds of chondrocyte division, matrix secretion, apoptosis, and replacement by bone.

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Figure 1. Illustration of bone, cartilage, and growth plate.

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The skeleton is encased in a layer of cells that is called the perichondrium (over cartilage) or periosteum (over bone). These tissues are responsible for the deposition of much of the hard bone that forms on the outside (cortical bone) to give rise to a tubular shape. Cortical bone is first deposited as a thin “bone bark” that forms a cuff around the growth plate. The osteoblasts that form the bone bark originate within the perichondrium. Perichondral cells in this region, known as the groove of Ranvier, also differentiate into chondrocytes that contribute to the growth plate. Not only must the timing processes within the growth plate be exquisitely regulated, but also growth plate progression must be coordinated with formation of cortical bone. Many signalling pathways are involved in orchestrating these processes, and it is now clear that misregulation of bone growth can result in changes in height, bone integrity, and the formation of tumors.

MHE is a disease that affects >1:50,000 people and causes several skeletal abnormalities to occur during childhood (Bovee,2008). Patients are short, have bowed bones, and have a high incidence of a benign tumor called an osteochondroma. Osteochondromas are cartilage-capped outgrowths of the bone that are first seen near the groove of Ranvier. It is not yet clear whether they originate in the growth plate itself or in the surrounding perichondral layer. As the main growth plate passes by, the osteochondroma (which has its own growth plate) continues to grow out perpendicular to the bone. Growth of the osteochondroma ceases when adulthood is reached; however, there is an increased possibility of the development of chondrosarcoma. Osteochondromas are thought to have some similarity to enchondromas, which are cartilage tumors that form within the growth plate. Patients with Ollier disease have multiple enchondromas. Ollier disease has recently been shown to be associated with mutations in a parathyroid hormone (PTH) receptor (Hopyan et al.,2002). A third disease, metachondromatosis, results in the formation of enchondromas and osteochondroma-like outgrowths on the hands and feet. This disease is very rare and has not been mapped on the human genome.

In the mid-1990s, MHE patients were shown to harbor mutations in either Exostosin-1 or Exostosin-2 (EXT1 or EXT2; Ahn et al.,1995; Stickens et al.,1996; Wuyts et al.,1996). In the intervening years, these genes have been shown to encode glycosyltransferases that polymerize HS (McCormick et al.,1998) and mutations in these genes have been identified and analyzed in worms, flies, zebrafish, and mice (Lin,2004). These model systems led to an explosion in our understanding of the roles of HS during developmental processes and in particular in mediating extracellular interactions during signalling. All of these studies have led us to a much greater understanding of the underlying causes of MHE, but have yet to provide a cure for patients. On October 29, 2009, 42 clinicians, patients, and basic science researchers gathered for 4 days to discuss advances in this field, to strategize and come up with new ideas. Below is a summary of the meeting highlights.

THE THIRD INTERNATIONAL MHE RESEARCH CONFERENCE: A MEETING REPORT

  1. Top of page
  2. Abstract
  3. SKELETAL GROWTH AND DISEASE
  4. THE THIRD INTERNATIONAL MHE RESEARCH CONFERENCE: A MEETING REPORT
  5. REFERENCES

Skeletal Engineering

The meeting began with the orthopedic surgeons Dror Paley (Paley Advanced Limb Lengthening Institute, West Palm Beach, FL) and Scott Kozin (Shriners Hospital for Children, Philadelphia, PA) describing their experience in diagnosing and treating this disease. Osteochondromas form predominantly on the physes of long bones, pelvis, ribs, scapula, and vertebrae and begin to appear as early as 2 years of age. Osteochondromas often impinge on nerves or can severely limit movement. In these cases, surgical removal is the only option and this can be difficult especially if the osteochondroma is on a vertebra or has enveloped a nerve. One of the poorly understood aspects of this disease is that osteochondroma growth is often associated with severe bowing of long bones that can restrict movement and ultimately result in dislocation of a joint. The relationship between bowing and osteochondromas is still unclear, but it appears to involve unequal growth of two paired bones such as the radius and ulna. Correction usually involves surgically removing a wedge of bone, then attaching an external fixator that gradually (over a period of months) increases the distraction gap between the two resected ends of the bone leading to lengthening and straitening of the bone. One surprising observation during surgical dislocation of the hip, is the occasional presence of minute osteochondromas on the surface of the femoral head. The presence of these outgrowths on articular cartilage needs to be reconciled with current models for the origins of osteochondromas (see below).

Genotype-Phenotype Correlations

The next session focused on human genetics with Wim Wuyts (University Hospital of Antwerp, Belgium) and Luca Sangiorgi (Rizzoli Orthopaedic Institute) presenting their genotype–phenotype correlation studies. Although early studies suggested that there may be a third locus associated with MHE, it now appears that nearly all patients tested are heterozygous for mutations in either EXT1, approximately 60% of the cases, or EXT2, approximately 40% of the cases. Their analysis of data collected worldwide (Jennes et al.,2009) found that both women and patients carrying alleles in EXT2 tend to have less severe symptoms. Marion Kusche-Gullberg (University of Bergen, Norway) then presented in vitro data that suggested that the reduced expressivity associated with EXT2 mutations in humans may reflect differences in the activity of the two enzymes: Although both enzymes are glycosyltransferases that act to polymerize HS, EXT2 has significantly lower enzymatic activity than EXT1 and may be more important during formation and transport of the HS polymerase complex. Marion also went on to explain that reduced HS polymerase activity does not result in fewer chains being made, rather that the average chain length is shortened. This chain shortening may have differential effects on developmental signaling pathways that rely on HS. For example, short chains can still facilitate fibroblast growth factor 10 (FGF10) binding to its receptor, but are unable to mediate FGF2's interaction with its receptor (Osterholm et al.,2009).

Going Rogue

One of the controversial issues discussed was what causes skeletal cells to “go rogue” in the first place. One model is that osteochondromas arise due to reduced EXT gene dosage that results in reduced HS synthesis. This change then allows chondrocytes to occasionally escape normal developmental constraints to give rise to an osteochondroma. The alternative model is that osteochondromas originate from cells that have lost the second copy of the heterozygous EXT gene and then undergo clonal expansion (loss of heterozygosity or LoH). Elena Pedrini (Rizzoli Orthopaedic Institute, Italy) presented a genotypic analysis of 35 MHE patients that found that only 5 (14%) had a second mutation detectable in the osteochondroma. These data support the model that in the majority of cases, reduced gene dosage is the underlying cause. This model was also supported by the finding that, although mice heterozygous for null alleles of Ext1 or Ext2 do not reliably form osteochondromas, the double heterozygotes (Ext1−/+; Ext2−/+) display stereotypic osteochondromas along their long bones. These data, the outcome of a collaboration between the laboratories of Maurizio Pacifici (Thomas Jefferson University, USA) and Jeff Esko (University of California, USA), suggests that an environment conducive for osteochondroma formation is created when HS synthesis dips below a certain threshold.

However, the advocates of the LoH model were undeterred: First up was Judith Bovée (Leiden University Medical Centre, The Netherlands) whose laboratory has isolated mesenchymal stem cells from EXT+/− patients and has shown that chondrogenesis and HS synthesis (chain length and structure) are unaffected by reduced gene dosage. This argues somatic genetic changes are necessary for osteochondroma formation. Further support for the LoH model came from mice generated independently in the laboratories of Yu Yamaguchi (Burnham Institute for Medical Research, USA), Maurizio Pacifici, and Mario Capecchi (University of Utah). The lines use various Cre recombinase drivers to generate Ext knockouts in skeletal cells, thus experimentally inducing LoH. All the mutant mice displayed osteochondroma formation. An additional finding from these studies is that experimentally induced mouse osteochondromas are usually not clonal and recruit wild-type cells from the neighboring tissue (Jones et al.,2010). Thus, the identification of homozygous mutant cells in human osteochondromas may be complicated by the infiltration of heterozygous cells into the tumor. This may explain why LoH is not identified in many patients. Another unanswered question is the exact origin of osteochondromas: do they begin within cartilage, perichondral, or other skeletal tissues? Several studies have previously suggested the groove of Ranvier (a highly proliferative region of the perichondrium) as the starting point for osteochondromas (Ogden,1976; Delgado et al.,1985; Porter and Simpson,1999; Mansoor and Beals,2007). By using different tissue specific drivers for recombination, these laboratories aim to answer this question. However, finding lines that are exclusively expressed in one tissue, such as the perichondrium, may be tricky.

Sugars and Signaling

Another hotly debated issue was whether osteochondromas form as a result of disruption of a particular developmental signaling pathway. Previous studies in mice and zebrafish have suggested that Indian Hedgehog (IHH) and noncanonical WNT signaling are involved in osteochondroma formation (Koziel et al.,2004; Koyama et al.,2007; Clement et al.,2008). IHH is a major regulator of skeletogenesis, and its diffusion is restricted by heparan sulfate in growing bones. Andrea Vortkamp (University Duisburg-Essen, Germany) presented evidence that Hedgehog signaling is still active in mouse osteochondromas made up of Ext1−/− cells. And Patrick Allard from the Tabin Laboratory (Harvard Medical School, USA) presented his identification and characterization of the Cardin-Weintraub (HS binding) domain in IHH, which is conserved from fish to man.

The link to different signaling pathways has also been made by a series of elegant human studies. Pancras Hogendoorn (Leiden University Medical Centre, The Netherlands) presented analysis of primary cilia orientation in normal and osteochondroma growth plates. Whereas chondrocytes in normal growth plates project their cilium parallel to the longitudinal growth axis, in osteochondromas the cilia are highly disordered. As primary cilia play a crucial role in Hedgehog signaling, this result suggests that reception of IHH may be altered in osteochondromas. Fred Kaplan (University of Pennsylvania, USA) presented his analysis of an extremely rare (1 in 2 million people) disease called fibrodysplasia ossificans progressiva (FOP). FOP causes muscle to progressively turn into bone and has been shown to be caused by a single amino acid change that results in activation of a bone morphogenetic protein (BMP) receptor (Kaplan et al.,2009). What links this disease with MHE is that FOP patients also have a high incidence of osteochondroma formation, suggesting that the BMP pathway may also play a role in MHE. Henry Kronenberg (Harvard Medical School, USA) presented his laboratory's evidence that the PTH pathway acts postnatally in mice to inhibit chondrocyte apoptosis. This finding sheds light on how mutations in the PTH receptor could give rise to enchondromas in Ollier disease patients (see above) by interfering with the normal process of apoptosis during endochondral ossification. This also raises the possibility that the PTH pathway may be affected during osteochondroma formation as well as progression to chondrosarcoma.

More Than Just a Bone Disease

One of the new themes for the field is whether MHE is purely a skeletal disease, or should be considered a syndrome that affects other tissues and processes within the body. For years patients have complained of other problems ranging from digestion problems to childhood behavioral difficulties. However, these complaints have been by nature anecdotal and, therefore, largely ignored by the research community. Hudson Freeze (Burnham Institute for Medical Research) presented his analysis of a condition called protein losing enteropathy (PLE), which results in loss of plasma proteins due to leakiness of the intestine. Intriguingly, HS is lost on the basolateral surface of the intestinal epithelia, and HS injection in patients reduces permeability and halts protein leakage (Liem et al.,2008). Preliminary studies suggest that Ext1−/+ mice may also have increased intestinal permeability, indicating that similar studies should be performed with MHE patients. Jeff Esko (University of California, USA) presented his work on the role of HS in reducing atherosclerosis by promoting lipoprotein clearance (Stanford et al.,2010), and Yu Yamaguchi presented analysis of the behavioral effects of HS deficiency. His lab has generated mice with a tissue-specific knockout in mature neurons. By putting mice through a series of tests, they determined that the only behavioral changes are reduced physical fear and increased social fear. These traits are reminiscent of Autism, a condition that has been suggested to be associated with MHE (Bolton et al.,1995; Ishikawa-Brush et al.,1997; Li et al.,2002).

As the meeting drew to a close, there was a palpable feeling of optimism as Craig Eaton (President of The MHE Research Foundation) gave his thanks to all of the speakers as well as to the organizers, Sarah Ziegler (Vice President of The MHE Research Foundation) and Yu Yamaguchi. The meeting was supported by the NIH, the Burnham Institute for Medical Research, the New York State Department of Health, St Mary's Medical Centre, The CDG Family Network, and the Paley Advanced Limb Lengthening Institute. The next meeting is scheduled for November 3–6, 2011, to be hosted by Maurizio Pacifici and once again co-organized by The MHE Research Foundation. With the announcement that NIH grants totaling more than 2 million USD had recently been awarded for MHE research, as well as new initiatives being discussed, expectations are high, and we have a lot to look forward to next time around.

REFERENCES

  1. Top of page
  2. Abstract
  3. SKELETAL GROWTH AND DISEASE
  4. THE THIRD INTERNATIONAL MHE RESEARCH CONFERENCE: A MEETING REPORT
  5. REFERENCES
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