Systemic mastocytosis, an unusual cause of osteoporosis

Bone mineral density, bone turnover markers and fractures in patients with indolent systemic mastocytosis

OBJECTIVE

We systematically assessed bone mineral density (BMD), bone turnover markers (BTM), and fractures in a large cohort of patients with Indolent Systemic Mastocytosis (ISM).

METHODS

Eighty-two patients (mean age 48 years, 37 women) with ISM were studied. BMD was measured by dual X-ray absorptiometry at the lumbar spine and proximal hip. The serum markers of bone turnover included bone-specific alkaline phosphatase, C-telopeptides of type I collagen, and serum osteocalcin. Previous clinical fractures were registered and spine X-ray was obtained from all patients.

RESULTS

Three women were excluded for concomitant diseases associated to osteoporosis. Osteoporosis according with the WHO classification (T-score<-2.5) was found in 16 patients (20.0%) (7 females and 9 men). Mastocytosis-related low BMD (Z-score at either the spine or the hip<-2) was found in 3 women (9%) and 13 men (28%). The BMD was generally lower at the spine than at the hip. No significant correlation was observed between serum tryptase levels and T or Z-score BMD. One or more moderate or severe vertebral fractures were found in 17 patients (12 men); in 11 of them Z-score values were>-2 or not valuable at the spine. No significant difference was found in the prevalence of mastocytosis-related low BMD and/or vertebral fractures between patients with or without skin involvement. Two patients had radiographic and densitometric osteosclerosis-like characteristics. In osteoporotic patients higher, normal or lower serum BTM were found, without correlations with serum tryptase levels, while in patients with osteosclerosis both BTM and serum tryptase values were particularly increased.

CONCLUSIONS

Vertebral osteoporosis and fractures are frequent in patients with ISM. Spine X-ray and densitometric examination are warranted in all patients, also without skin involvement and particularly in males; Z-score other than T-score BMD must be evaluated. Patients with idiopathic osteoporosis should be evaluated for mast cell disease. Both high than low BTM can be observed in patients with osteoporosis while osteosclerosis is characterized by high bone turnover and serum tryptase levels.

A potential role for the mast cell in the pathogenesis of idiopathic osteoporosis in men

Osteoporosis is increasingly being recognized in men. Secondary causes are often implicated, but the mechanism of bone loss remains unclear in about a third of patients. The mast cell is a complex cell that stores a number of factors known to affect bone metabolism. Patients with systemic mastocytosis often demonstrate osteoporosis and bone marrow mast cells may be increased in osteoporotic postmenopausal women. We address the possible role of the mast cell in the pathophysiology of male osteoporosis by studying the relationship between bone marrow infiltration with mast cells and the 24 h urine excretion of N-methylhistamine, and the severity of osteoporosis in 48 consecutive men with idiopathic osteoporosis (bone mineral density Z score of <-1 and/or at least one prevalent vertebral fracture). Secondary causes for osteoporosis were excluded and none of the patients had systemic manifestations of enhanced mast cell activity. A widely variable number of morphologically normal mast cells were counted in toluidine blue-stained sections from 42 of 46 evaluable bone marrow biopsies. In 4 of the 42 biopsies (9%), clusters of abnormal mast cells were identified. These four patients were the only ones who also demonstrated increased 24 h urine excretion of N-methylhistamine. There was a significant positive relationship between mast cell number and the 24 h urine excretion of N-methylhistamine reflecting mast cell activity (p = 0.0001), and this latter measurement correlated negatively with bone mineral density (BMD) at the lumbar spine (p < 0.001). We identified clinically important bone marrow cell infiltration with pathologic mast cells in the absence of systemic manifestations of mast cell hyperactivity as an additional secondary cause for osteoporosis in some 9% of men with idiopathic osteoporosis, and found urinary excretion of N-methylhistamine to be above the upper limit of the normal laboratory reference range diagnostic for this cause of secondary osteoporosis. The more continuous spectrum in the relationship between mast cell activity and BMD supports a potential role for this cell in the pathogenesis of idiopathic male osteoporosis.

The osteoporosis of heparinotherapy and systemic mastocytosis

Heparinotherapy and systemic mastocytosis are two unusual aetiologies of diffuse osteopenia, possibly linked by common pathophysiological factors. Osteoporosis related to heparinotherapy has only been observed in patients treated with doses higher than 10,000 units per day and for more than 4 months. Even in these, it is a rare disorder which has only been reported approximately 15 times in the world literature. Bone histomorphometry has demonstrated the occurrence of marked hyperresorption. In vitro, heparin appears to have resorptive and collagenolytic effects which could play a pathophysiological part in the disorder. Diagnosis of the osteopenic form of systemic mastocytosis may be difficult. Urticaria pigmentosa is a very important clue but may be misdiagnosed or even missing. Hepato or splenomegaly are inconstant. X-rays may show the coexistence of osteosclerotic lesions. Standard biochemical tests are of little help. The urinary excretions of the histamine metabolites methyl histamine and methyl imidazolacetic acid have been found increased when measured. Finally, the diagnosis is made by bone histology which must be performed without decalcification and read by a pathologist informed of the potential diagnosis. Toluidine blue stain shows that mastocytes are numerous in the bone marrow where they are grouped in foci. Histomorphometry demonstrates a high bone turnover with excessive resorption, which could be mediated by heparin or PG E 2 contained in mastocyte granules. Treatment is difficult and may involve cytostatic drugs and/or inhibitors of bone resorption: Clodronate has recently been reported to be at least transiently effective.