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Increased nucleolar organizer regions in osteoclast nuclei of Paget's bone disease
D Chappard1, N Retailleau/Gaborit, R Filmon
1LHEA Laboratoire d'Histologie-Embryologie, Faculté de Médecine and CHU d'Angers, France. Daniel.Chappard@univ-angers.fr
Abstract:
The etiology of Paget's disease of bone is still unknown but several studies have reported a viral origin. At the electron microscopic level, characteristic nuclear and cytoplasmic inclusions have been found and mimic paramyxoviral nucleocapsids in osteoclasts (Oc). Sarcomatous degeneration is observed in 2% of pagetic patients. Nuclear organizer regions are parts of the nucleolus involved in the synthesis of ribosomes, and they contain nonhistone proteins that can be silver stained (AgNORs) on the interphasic nuclei. AgNOR number is known to correlate with the proliferative activity of the cell populations, whether normal or malignant. Cancer cells have an increased demand for (robosomal) rRNA and correlations have been found between AgNORs and proliferative antigens. We have adapted the AgNOR staining method to undecalcified bone biopsies at the light and TEM levels. Bone sections from 10 pagetic patients (without a previous bisphosphonate treatment) were stained for AgNORs. 10 patients having metabolic bone diseases associated with an increased Oc number (i.e., primary and secondary hyperparathyroidism) were used as controls. AgNORs appeared as black dots within the nucleoli of Oc nuclei and were easily numbered. A maximum of two or three dots could be seen in Oc nuclei from control subjects. In pagetic Oc, AgNOR number was greatly increased (6.80 +/- 2.57 dots vs. 2.12 +/- 1.07 in controls). TEM study also showed AgNOR in the nucleoli of pagetic patients' Oc. The viral inclusions within the nuclei appeared faintly stained and could not be confused with AgNORs. The large number of AgNORs in the nuclei of pagetic Oc reflects the need for a greater abundance of ribosomes. With the pagetic Oc being highly active, the cytoplasmic synthesis of proteins is maximized (probably hydrolases involved in the matrix breakdown). An increase in AgNORs does not reflect the proliferative activity of the cell because Oc are made by the fusion of precursors. It is postulated that: (a) other mRNAs (of viral/oncogene origin) could be actively transcribed in pagetic patients and require more numerous ribosomes; or (b) a viral genome/oncogene promotes alteration of the nuclear/nucleolar mechanism.
Insights
Paget's disease of bone shows significantly increased AgNORs in osteoclasts, indicating heightened ribosome synthesis. This suggests a potential viral or oncogene influence on cellular mechanisms in pagetic bone cells.
Area of Science:
- Bone biology
- Cellular pathology
- Virology
Background:
- Paget's disease of bone etiology is unknown, with suspected viral origins.
- Osteoclasts (Oc) in Paget's disease exhibit inclusions resembling paramyxoviral nucleocapsids.
- Sarcomatous degeneration occurs in 2% of Paget's patients.
Purpose of the Study:
- To investigate the role of Nuclear Organizer Regions (AgNORs) in Paget's disease osteoclasts.
- To correlate AgNOR number with cellular activity in Paget's disease bone biopsies.
- To differentiate AgNOR staining from viral inclusions in osteoclasts.
Main Methods:
- Adapted AgNOR staining for light and transmission electron microscopy (TEM) on undecalcified bone biopsies.
- Analyzed bone sections from 10 Paget's disease patients (untreated with bisphosphonates).
- Used bone sections from 10 patients with metabolic bone diseases (hyperparathyroidism) as controls.
Main Results:
- AgNORs, visualized as black dots in nucleoli, were significantly increased in Paget's osteoclasts (6.80 ± 2.57) compared to controls (2.12 ± 1.07).
- TEM confirmed AgNOR presence in osteoclast nucleoli of Paget's patients.
- Viral inclusions were distinct from AgNORs and faintly stained.
Conclusions:
- The elevated AgNOR count in Paget's osteoclasts reflects an increased demand for ribosomes, likely for enhanced protein synthesis (e.g., matrix-degrading hydrolases).
- Increased AgNORs do not indicate proliferation but suggest active transcription of non-ribosomal mRNAs (viral/oncogene) or altered nucleolar mechanisms.
- Findings support a potential viral or oncogene-driven alteration in nuclear/nucleolar function in Paget's disease.