gaucher disease ( gd ) is a rare autosomal recessive hereditary disorder of glycosphingolipid metabolism , characterized by the accumulation of glucosylceramide in cells of the reticulo - endothelial system , due to deficient activity of the lysosomal enzyme , beta - glucosidase ( glucocerebrosidase , ec 3.2.1.45 ) . there are various manifestations of gd but the disease is generally described by three main phenotypes : type 1 gd ( gd1 ) , which can manifest at any age and is distinguished by a lack of primary neurological manifestations , and type 2 ( acute ) and type 3 ( chronic ) neuronopathic phenotypes which manifest in early life , involve the central nervous system , and result in early death . usually in infancy ( type 2 ) and childhood or early adulthood ( type 3 ) . type 1 gd is , by far , the most common form of gd . the age of onset , rate of disease progression , severity of symptoms , and the organs affected can vary widely . some patients with gd1 may remain asymptomatic although the majority experience symptoms that commonly include fatigue , splenomegaly , anemia , thrombocytopenia , hepatomegaly , and skeletal disease . the skeletal manifestations of gd , which may lead to significant pain , morbidity , disability and a progressive reduction in quality of life , include abnormal bone remodeling , osteopenia , osteoporosis , lytic lesions , avascular necrosis , pathological fractures , osteonecrosis of humeral or femoral heads , and vertebral collapse . osteopenia is a common and often early manifestation of gaucher - related bone disease which may lead to reduced bone mineral density ( bmd ) , reduced peak bone mass , fragility fracture and poor bone healing . treatment for the systemic manifestations of gd has been available for over 20 years in the form of enzyme replacement therapy , firstly as alglucerase ( ceredase ; genzyme corporation , cambridge , ma , usa ) , and subsequently by the recombinant form imiglucerase ( cerezyme , genzyme corporation , cambridge , ma , usa ) . in more recent years , an alternative treatment , substrate reduction therapy , has also become available for adult patients who are unwilling or unable to be treated with enzyme therapy . another enzyme replacement therapy has also become available : velaglucerase - alfa ( shire human genetic therapies ) . before disease - specific treatment became available for gaucher disease , splenectomy was frequently carried out to control bleeding risk and the mechanical effects of an enlarged spleen in severely affected patients . it is now known that splenectomy may be associated with an exacerbation of gd - related bone disease . skeletal involvement is an important indication for treatment to ameliorate symptoms , prevent osteoporosis and reduce risk of pathological fractures . while some skeletal symptoms of gd , such as bone pain and bone crises , may be ameliorated within 12 years by treatment with imiglucerase enzyme replacement therapy , it is recognized that the response of some bone manifestations , such as bmd , to treatment can be slower , and some bone complications , such as avascular necrosis , are irreversible despite optimal treatment . this underlines the need for early initiation of therapy if the risk of irreversible complications is to be reduced . it is known that bmd is accrued mostly in the first 20 years of life , peaks in the third decade , and then declines , suggesting that there is a limited therapeutic window in which to gain maximal benefit from treatment . similarly , it has been shown that the risk of avascular necrosis ( avn ) appears to be reduced in patients who initiate enzyme replacement therapy within two years of diagnosis compared with those initiating therapy after two years , although the risk of avn may not be eliminated in patients receiving enzyme therapy . one of the challenges of monitoring bone involvement and treating bone manifestations is that the pathophysiology of bone disease is not completely understood . although the infiltration of gaucher cells into the bone marrow and bone is thought to be a central event in the pathophysiological process , it is unclear whether , and to what extent , complications are due to mechanical effects , such as compression , and/or the indirect effects of gaucher cells on bone ( through the activation of inflammatory mediators ) , or whether gd alters bone homeostasis independently of gaucher cells . the erlenmeyer flask deformity has been shown to be present in up to 59% of gd patients , suggesting that bone remodeling disorders are common in this disease and may have relevance to skeletal pathologies . bone turnover or remodeling in healthy individuals is a lifelong process by which mature bone is resorbed ( mediated by osteoclasts ) and new bone tissue is formed ( mediated by osteoblasts followed by calcification ) . osteoclasts are recruited from bone marrow precursors which attach to the surface of bone and break down the bone matrix through the release of proteolytic enzymes to leave surface pits . osteoblasts differentiate from connective tissue precursors and fill the bone pits with a protein matrix which subsequently becomes fully mineralized bone . the biochemical processes involved in bone turnover result in measurable changes in enzyme activities , the release of various bone components , and the expression of proteins involved in the mineralization of the bone matrix . table 1bone biomarkers used to assess bone turnover.markers of bone formationmarkersourcespecificity for bonespecimencarboxyterminal propeptide of type 1 procollagen ( picp)pro - collagenweak ( also in soft tissue)serumamino ( n-)terminal propeptide of type 1 procollagen ( pinp)pro - collagenweak ( also in soft tissue)serumosteocalcin ( oc)protein produced by mature osteoblasts in bone matrixhigh . the major non collagen protein of bone secreted by osteoblasts . reflects both bone formation and bone resorption.serumtotal alkaline phosphatase ( ap)osteoblast enzymelow : present in plasma membrane of osteoblasts but also liver , kidney , intestinal , and spleen cellsserumbone specific alkaline phosphatase ( bsap)osteoblast enzymehighly specific for bone - but up to 20% cross reactivity between bone and liver enzymesserummarkers of bone resorptionpyridinolines ( free and total pyr)collagen breakdownspecific ( primarily bone and cartilage)urinedeoxy - pyridinolines ( d - pyr ) ( also known as d - pyrilinks , pyrilinks - d , or deoxypyd)collagen breakdownhighly specific ( primarily bone and cartilage)urinehydroxyproline ( hp)collagen breakdownweak ( found in bone , soft tissue , diet)urinecarboxyterminal telopeptide of type 1 collagen ( ictp)collagen breakdownhighly specific ( primarily bone)serumcross - linked type 1 collagen c - terminal telopeptide ( ctx ) amino ( n-)terminal telopeptide ( ntx)collagen breakdownhighly specific ( primarily from bone)serum and urinecross - linked type 1 collagen c - terminal telopeptide ( ctx ) n - terminal telopeptide ( ntx)collagen breakdownhighly specific ( primarily from bone)serum and urinetartrate - resistant acid phosphatase ( trap ) ( trap5b)osteoclast enzymetrap also found in other tissues specific for osteoclastsserumcathepsin kproteolytic enzymespecific for bone osteoclastsserumnf - kb ligand ( rankl ) and osteorotegerin ( opg)osteoclast recruitment and activityosteoclastsserum 
  the activity of osteoclasts and osteoblasts is normally tightly coupled , but in several pathological states and with increasing age the turnover process may be disrupted . signaling networks between osteoclasts and osteoblasts are central to the regulation of bone turnover . osteoclasts , for example , have a role in regulating the activity of other cells , including osteoblast precursors , by participating in immune responses and by secreting cytokines that may affect their own function and those of other cells . osteoclast formation , in turn , is dependent on the cytokine rankl through binding to its cognate receptor rank on the surface of osteoclast precursors . osteoprotegerin ( opg ) , a decoy receptor for rankl , inhibits osteoclast differentiation and osteoclast activity . the interaction between opg and rankl , therefore , has a role in regulating osteoclast formation and provides an additional means of estimating bone turnover . the opg / rankl system is of particular interest because it has been exploited as a therapeutic target in some bone diseases . bone biomarkers have been used to assess disease status and the response to therapy in a number of bone disorders , including post - menopausal osteoporosis , paget 's disease , rheumatoid arthritis , and cancer . here , we examine the literature for evidence of abnormalities in markers of bone turnover in patients with type 1 gd . while biomarkers of activated macrophages , such as chitotriosidase , are often used to assess the whole body burden of gaucher cells and the whole body response to therapy , there is currently no recommended biomarker that is specific to bone . skeletal disease in gd may develop silently , may not correspond with systemic disease in terms of onset , severity or rate of progression , and may not always be detectable by the commonly used imaging methods . some bone imaging methods , especially the more sensitive techniques such as quantitative chemical shift imaging ( qcsi ) , are expensive , complex and not widely available for routine monitoring . the identification of a sensitive biomarker specific for bone disease in gd would , therefore , be of significant benefit for detecting and monitoring bone involvement , and would help clinicians determine optimal timing of treatment for the prevention of bone complications . the available english language literature on bone markers in gd was reviewed using online search methods such as pubmed . an overview of biomarkers in gd patients who were untreated , treated with enzyme therapy , and treated with bisphosphonates is to be found in table 3 . table 2summary of findings on the use of bone biomarkers in gaucher disease.studypatientsstudymarker of bone formationmarkers of bone resorptioncomments / relationship with severity of bone diseaseuntreatedstowens et al . 198526 ( subset of 327 cases aged 1865 yrs ) all patients had symptoms of skeletal involvement spleen status?retrospective review of untreated gd patient data9 of 26 patient abnormally low urinary hydroxyproline excretion - none excreted high levels gd : 34mg/24hrs ( range : 964 ) ( normal 2577 mg/24hrs)concluded that while some patients had reduced bone turnover , the majority had enhanced bone cell activity ( measured by kinetic marker tetracycline)drugan et al . 200216 ( 5 m ; 11 f ) mean age : 27.44 yrs ( 954 yrs)bssi scores*stage 2 : n=9stage 3 : n=4stage 4 : n=3spleen status?untreated gd patients compared with 29 age matched controls)oc ( 3.5 times reduced vs controls : p=<0.003)ctx : 1.9x reduced in gd ( 3617.62 + /536.69 ng / ml ; vs 6808.57 + /865.66 ng / ml in controls : p<0.01)reduced bone turnover and imbalance favouring bone resorption.biomarkers decreased with age in controls ; not in gd.no correlation of bone markers with chitotriosidase . 200310 ( 5 m ; 5 f)mean age : 33.2 yrs ( range 2540 yrs)bssi scoresstage 12 : n=3stage 34 : n=4stage 5 : n= 2 ( 1 patient not assessed)6 patients osteopenic ( t score 2.5 to 1sd ) 3 patients normal bmd4 patients asplenicuntreated patients compared with 10 age and gender matched healthy controlsbsap ( normal ) gd : 28 u / l ; controls 32.06 u / l ( p=0.31 ) oc ( normal ) picp : reduced in gd ( 101.17 ng / ml vs 140.75 ng / ml ; p=0.0016)ictp : increased in gd ( 4.24 ng.ml vs 2.87 ng / ml in control ; p=0.012 ) normal parathyroid hormone , ca , p , hydroxyproline , free d - pyrimbalance in bone turnover favouring bone resorption . no statistically different bmd scores between bssi score groupsvan dussen et al . 201140 ( 17 m ; 23 f ) median age : 41.8 ( range 15.276.2 ) all patients : bone complications ( defined as history of avn , pathological fractures , vertebral collapse , osteomyelitis and bone crises / infarctions ) . 11 patients asplenic disease severity scored by zimran ssiin 30 patients , samples taken before or within 1 mth of starting treatment . 3 patients received pamidronate results compared with normal reference rangesoc : decreased in gd ( median 0.35 nmol / l ) pinp ( within normal range ) median 37 g / l ( ref range:1996 g / l)ctx ( within normal range)decreased bone formation . patients with bone complications had significantly lower oc vs patients without complications no significant difference in ctx or pinp between patients . oc : statistically significant negative correlation with chitotriosidase ( cr : 0.323 , p=0.042 ) , bmd , and positive correlation with qcsi scores ( cr : 0.423 ; p=0.025 ) negative correlation between oc and zimran ssi ( 0.531 ; p=<0.001)enzyme replacement therapy ( ert)gd patients treated with ert vs healthy controlsciana et al . 200512 ( 6 m ; 6 f)mean age : 32.3 yrs(range 2540 yrs)bssi score : stage 12 : n=7stage 34 : n=4stage 5 : n=12 patients asplenicall patients received ert ( 6.830 u / kg/2wks for 4.5 years ) + vitamin d ( interrupted in all during first 2yrs of treatment ) 12 age and gender matched healthy controlsat baseline : picp : reduced in gd ( 100.52 ng / ml vs 142.45 ng / ml in controls ; p=0.017 ) bsap , oc , parathyroid hormone , ca , p , hydroxyproline , free deoxypyridinoline normal although reduced urine ca / creatinine in gd group vs controls ( ns).ictp : at baseline increased in gd : ( 3.93 ng / ml vs 2.72 ng / ml in controls ; p=0.004after 4.5 yrs , no significant change in bone formation markers -except urine ca / creatinine ratio , ( from 0.065 mg / mg at baseline to 0.191 after 4.5 years ( p=0.0014)9 of 11 patients ( 1 not assessed for full period ) showed an increase in bmd during follow up ( p=0.039)no correlation between bone markers and bmdgd patients treated with ert vs gd patients treated with calcitriolschiffmann et al . asplenic all had bone involvement at baseline ( mri bone score mean=10 ( range 611 ) ( score = max number of sites of bone involvement out of 11 sites scored in pelvis and lower extremities)2 year study3 groups:1 : calcitriol ( control ) for 6 mths + ert after 6 mths2 . calcitriol and ert3 . ert alone(ert ; 60u / kg/2 wks , halved after 1 6 mths in all.in ert treated group : bsap : significant increase ( p=0.002 ) oc : significant increase ( p=0.008)in ert treated group : d - pyr : transient increase ; returned to baseline after 2 years.calcitriol - no effect on bone density . ert associated with progressive reduction in bmd - although marked improvement in lumbar spine fat fraction noted . authors suggest ert may have led to increased bone turnover , with sustained elevation of osteoblastic activity but only transient elevation of osteoclastic activity.gd patients treated with ert over timesims et al . 200833 ( 19m/14f ) 1270 yrs 5 patients asplenic at baseline : skeletal pain ( 73% ) ; lytic lesions ( 36% ) ; medullary infraction ( 36% ) osteoarticular necrosis ( 3% ) ; spine fracture ( 3% ) ; osteopenia ( lumbar spine ) ; ( 58% ) osteoporosis ( 19%)longitudinal study : all ert treated : most 60u / kg/2 wks ( 9 dose reductions ) bone markers measured every 6 months for 4 yearsat baseline : oc : normalbsap : normalmedian post treatment oc , bsap increased by 60% in first 12 months and remained above baseline ( p= 0.050.001)at baseline : ntx : normald - pyr : normal between 36 and 48 months median ntx and d - pyr decreased 2040% ( ns).shift towards new bone formation relative to bone resorption after starting treatment reflected in bmd increases over timeert followed consecutively by srtmikosch et al . 20087 ( 5m;2f ) mean age 42.6 yrs ( range 2484 yrs ) 1 patient asplenic 1 patient vit d deficient 5 with vitamin d insufficiencypatients treated consecutively with ert ( 1 mth - 7 yrs ) and srt ( minimum 6 mths ) . results compared at 6mths and 1218 mths after change to srtoc : decreased during follow up ( ns ) ap : decreased significantly during follow upctx : no significant change trap5b : no significant change - remained within normal rangeoc and ctx in lower reference range 1 patient showed intermittent increase in oc and ctx 1 patient showed increasing trap5b - also experienced increasing bone painbisphosphonatesciana et al . 19775 ( 2 m ; 3f ) 2460yrs severe skeletal involvement : osteopenia , osteosclerosis , osteonecrosis , severe chronic pain spleen status?pamidronate treatment for 35 mnths ( 45 mg/3 weeks+ 500 mg calcium daily ) in otherwise untreated gd patientsoc : decreased ( p=0.04 ) compared to untreatedhydroxyproline : decreased ( p=0.08 ) total d - pyr : ( p=0.08 ) urinary ca ( p=0.04 ) compared to untreatedbmd increased ( p=0.04 ) compared with baselinewenstrup et al . 200434 ( 10 m ; 24 f ) age range : 1850 years 1 year study : 2 arm double blind placebo controlled study for alendronate disodium spleen status?1 yr 2 arm double blind placebo controlled study all received ert ( 1560u / kg/2 weeks ) for at least 24 mths+vitamin d ( 400u / day)+calcium carbonate ( 1.5g / day ) 1 arm received alendronate ( 40 mg / day)oc : reduced in alendronate group bsap : reduced in alendronate group ( significantly reduced from 6 months  18 months ; differences not maintained at 24 months)ntx : reduced in alendronate group d - pyr : no changeat screening all markers in normal reference range ( even subjects with more severe osteopenia)significant improvement in bmd in lumber spine in alendronate group.findings do not support a simple osteoclast - mediated mechanism for gd bone manifestationsstudies involving mixed groups ( various treatment history / mixed gd type)fiore et al . 2002 ( untreated+variable treatment period)12 ( 8 m ; 4 f ) mean age : 33 + /13yrs ( range 2160yrs ) 9 patients asplenic bone involved scored by bssi ) : 3 patients with severe bone disease ( chronic pain , joint replacement / disability)gd patients with variable treatment history ( ert : 10 patients at 3060u / kg for 28 yrs ) results compared with age and gender matched controls ( database)bsap ( normal)oc ( normal)pyr ( increased in urine ; pyr=28.63+/ 7.6 nmol / mmol ( normal=12.8  25.6 nmol / mmol ) d - pyr ( increased in urine ; d - pyr=6.13+/2.33 ( normal 2.35.4 nmol / mmol)increased bone resorption in gd . 2006 ( untreated+treated for variable period + some treated with bisphosphonates)173 ( 74 m ; 99 f ) mean age : 36.9+/19.9 range : 1856 reported to have mild gd 20 patients asplenic 32 controls108 treated with ert . many patients ( unspecified ) also treated with bisphosphonatesopg : ( no difference between gd patients and controls)serum opg increased with age in patients and controls . no correlations between opg and gender , disease severity , need for ert , avn , bmd and spleen status . 3 opg polymorphisms detected , but no correlations between polymorphisms and bone density.yoshino et al . 2007 ( type 1 , and 3 patients , adults and children , variable treatment period)8 ( type 1 and type 3 gd ) ; 7 m ; 1f ; 4 children ( no manifest bone disease ) 4 adults ( evidence of bone disease ) 3 patients asplenicexamined role of cytokines in bone disease ( compared with published age / gender matched control data ) all received ert ( 1mth11.75yrs)adults : bsap : ( normal)oc : ( normal)children : bsap marginally elevated in alloc : lower than reference range.adults : ntx : slightly increased in 3 patients ( serum ) ; slightly increased in 2 patients ( urinary ) d - pyr : elevated in 3 patients . children : ntx elevated in 3 d - pyr lower than reference range in allil-18 ( in 7/8 patients ) and tgf-1 elevated ( all ) . concentration of il-18 correlated with bsap in adults , and oc in all 8 patients.gd , gaucher disease ; m , male ; f , female ; oc , osteocalcin ; ctx , c - terminal telopeptide ; bsap , bone specific alkaline phosphatate ; ictp , carboxyterminal telopeptide of type 1 collagen ; picp , carboxyterminal propeptide of type 1 collagen ; pinp , amino ( n-)terminal propeptide of type 1 procollagen ; ert , 
 enzyme replacement therapy ; srt , substrate reduction therapy.*herman bssi ( hermann et al . 1997 ) : as detected by x ray imaging . stage 1 : diffuse osteoporosis ; stage 2 : medullary expansion ; stage 3 : osteolysis ; stage 4 : necrosis / sclerosis ; stage 5 : destruction and collapse ( 12 mild involvement ; 34 
 moderate involvement ; 5 : severe involvement ) 
  gd , gaucher disease ; m , male ; f , female ; oc , osteocalcin ; ctx , c - terminal telopeptide ; bsap , bone specific alkaline phosphatate ; ictp , carboxyterminal telopeptide of type 1 collagen ; picp , carboxyterminal propeptide of type 1 collagen ; pinp , amino ( n-)terminal propeptide of type 1 procollagen ; ert , 
 enzyme replacement therapy ; srt , substrate reduction therapy . herman bssi ( hermann et al . 1997 ) : as detected by x ray imaging . stage 1 : diffuse osteoporosis ; stage 2 : medullary expansion ; stage 3 : osteolysis ; stage 4 : necrosis / sclerosis ; stage 5 : destruction and collapse ( 12 mild involvement ; 34 
 moderate involvement ; 5 : severe involvement ) 
 table 3overview of biomarker activity in untreated gaucher disease ( gd ) patients , gd patients treated with enzyme replacement therapy , and gd patients treated with bisphosphonates ( excluding studies involving both untreated and treated patients).untreatedmarkernormalincreaseddecreasedformationbsapocpicppinpresorptioncalcium / creatininectxhydroxyprolined - pyrictpntxin response to ertmarkerno changeincreaseddecreasedformationbsapocpicpresorptioncalcium / creatinehydroxyprolineictpin response to bisphosphonatesmarkerno changeincreaseddecreasedformationbsapocresorptiond - pyrhydroxyprolinentx*7 of 12 patients included in the study by ciana et al 200538 were also included in the study by ciana et al . 2003ert , enzyme replacement therapy ; bsap , bone specific alkaline phosphatase ; ctx , type 1 collagen c - terminal telopeptide ; d - pyr , deoxy - pyridinolines ; ictp , carboxyterminal telopeptide of type 1 collagen ; ntx , amino ( n-)terminal telopeptide of type 1 collagen ; picp , carboxyterminal propeptide of type 1 procollagen ; pinp , amino ( n-)terminal propeptide of type 1 procollagen . 
  7 of 12 patients included in the study by ciana et al 200538 were also included in the study by ciana et al . 2003 ert , enzyme replacement therapy ; bsap , bone specific alkaline phosphatase ; ctx , type 1 collagen c - terminal telopeptide ; d - pyr , deoxy - pyridinolines ; ictp , carboxyterminal telopeptide of type 1 collagen ; ntx , amino ( n-)terminal telopeptide of type 1 collagen ; picp , carboxyterminal propeptide of type 1 procollagen ; pinp , amino ( n-)terminal propeptide of type 1 procollagen . three studies were carried out in untreated gaucher patients in which results were compared with healthy control data . one study examined bone biomarkers in patients from whom samples were taken before or within one month of starting enzyme therapy , considered here as a study of untreated patients . of the studies examining bone markers in treatment - nave patients , stowens et al . found decreased hydroxyproline excretion in 9 of 26 patients ( 35% ) compared with controls , suggesting reduced bone turnover ( resorption ) in these patients . reduced bone turnover was also found in a study involving 16 untreated gaucher patients , in whom there was a 3.5 times decrease in osteocalcin levels and a 1.9 times decrease in type 1 collagen c terminal telopeptide ( ctx ) , suggesting reduced osteoblastic and osteoclastic activity but with a net imbalance in bone turnover favoring bone resorption . no correlation was found between bone markers and genotype , chitotriosidase activity , or the zimran severity score index ( ssi ) for gd . a significant difference was , however , noted in serum osteocalcin levels and ctx according to the stage of skeletal involvement , as assessed by the hermann bone severity score index , with both markers exhibiting the highest levels in stage 3 disease . a reduction in bone biomarker levels in the more advanced stage 4 disease was suggested to be related to an extended loss of functional bone tissue due to osteonecrosis , with a marked decline in both osteoblast and osteoclast activity . a further study involving 10 gd patients also concluded that there was increased bone resorption in gd compared with healthy controls based on a decrease in carboxyterminal propeptide of type i procollagen ( picp ) and an increase in carboxyterminal telopeptide of type 1 collagen ( ictp ) while osteocalcin was unchanged . no differences were found in hydroxyproline or free deoxypyridinoline ( d - pyr ; a more sensitive marker unaffected by collagen synthesis or dietary collagen ) or the urinary calcium / creatinine ratio between gaucher patients and controls . there was no correlation between bone marker levels and patient demographics , splenectomy status , spleen and liver volume , or bone mineral density . found that osteocalcin was decreased in 50% of gd1 patients ( n=40 ) with no significant change in type 1 collagen ctx , a marker of bone resorption , and concluded that imbalances in bone turnover result primarily from a decrease in bone formation . osteocalcin levels showed a statistically significant negative correlation with chitotriosidase activity , a positive correlation with qcsi scores ( 0.323 , p=0.042 ; and 0.423 , p=0.025 , respectively ) and other bone turnover markers pinp and ctx ( 0.483 , p=0.002 ; and 0.466 , p=0.002 , respectively ) . osteocalcin also showed a negative correlation with the zimran ssi score ( 0.531 , p<0.001 ) . the correlations between ssi values and qcsi scores remained significant when the possible confounding contribution of patients receiving bisphosphonates ( 3 of 40 patients , 7.5% ) or who had a recent bone crisis ( 4 of 40 patients , 10% ) were excluded . there was also a negative correlation between osteocalcin and bmd in a subset of patients relatively mildly affected ( n=12 ; r=0.639 ; p=0.885 ; p<0.001 ) but no statistically significant correlation between osteocalcin and the bone marrow burden score ( - 0.382 ; p=0.08 ) . in an analysis of pre - treatment data cited by studies investigating bone marker levels during treatment , sims et al . found normal biomarker levels [ osteocalcin , bone specific alkaline phosphatase ( bsap ) , type 1 collagen n - terminal telopeptide ( ntx ) and d - pyr ] at baseline while ciana et al . one study examined bone biomarkers over time in enzymetreated patients in whom bone markers were measured every six months for four years . another looked at markers in enzyme - treated patients ( who also received interrupted vitamin d supplementation ) treated for 4.5 years compared with healthy controls . compared bone markers in 29 splenectomized patients treated with either enzyme therapy alone or enzyme therapy and calcitriol ( 1,25-dihydroxyvitamin d3 ) , and mikosch et al . examined biomarkers in patients treated with enzyme therapy ( one month to seven years ) followed consecutively by substrate reduction therapy with follow up for up to 18 months . ciana et al . found no significant change in bone markers over 4.5 years after the initiation of enzyme replacement therapy except for a statistically significant increase in the calcium / creatinine urinary ratio . patients showed a gradual improvement in bmd over the 4.5 years which was not reflected in bone biomarker levels . sims et al . found that the markers of bone formation ( osteocalcin and bsap ) increased by approximately 60% in the first 12 months after the initiation of enzyme replacement therapy ( without any other medications ) and remained above baseline for the remainder of the study ( four years ) . patients reported a reduction in bone pain and bone crises . at baseline , 13 of 33 patients had a history of bone crises ; 2 patients reported bone crises in the first year , one in the second year , and none thereafter . in a study involving splenectomized patients , bsap and osteocalcin increased significantly after the initiation of enzyme therapy while there was a transient increase in d - pyr ( marker of bone resorption ) . clinically , enzyme therapy was associated with a progressive reduction in bmd in these splenectomized patients , although there was an improvement in lumbar spine fat fraction , an indication of the burden of gaucher cells in the bone marrow . in patients treated consecutively with enzyme replacement therapy and substrate reduction therapy , osteocalcin and alkaline phosphatase were within the normal range ( lower part of reference ranges ) at the time of the change from enzyme replacement therapy to start of reduction therapy , and decreased during follow up of 1228 months ( statistically significant for alkaline phosphatase ) . there was no statistically significant change in markers of bone resorption ( ctx , trap5b ) . imaging parameters of bone disease ( mri , dexa ) generally remained stable throughout follow up , although one patient experienced worsening bone parameters observed by mri . in untreated patients with severe skeletal involvement , pamidronate therapy ( plus calcium ) resulted in a decrease in osteocalcin , hydroxyproline , total d - pry , and urinary calcium after 35 months of therapy compared to baseline . the second study involved gd patients who had all received enzyme replacement therapy for at least 24 months ( plus vitamin d and calcium ) . biomarkers were within normal reference range at baseline even in patients with severe osteopenia ( t<2 ) . one group of patients received bisphosphonate therapy ( alendronate ) for 24 months . there was a significant reduction in osteocalcin and bsap for a period of 18 months after the initiation of bisphosphonates but markers of bone resorption were inconsistent with reduced ntxs , but not d - pyr links . there was a statistically significant increase in bmd in the alendronate treated group ( p=0.001 ) . three studies looked at different mixed groups of patients : i ) patients who had and those who had not been treated with enzyme therapy ( and for varying periods of time ) ; ii ) patients who had been treated with other medications such as bisphosphonates ; iii ) patients who were diagnosed with different gd types ( types 1 and 3 ) and who had been treated with enzyme replacement therapy for varying periods ( 1 month to over 11 years ) . examined bone biomarkers in 12 patients , 10 of whom had received enzyme treatment for 28 years . markers of bone formation were within normal ranges while the markers of bone resorption , pyr and d - pyr , were significantly increased over control values , leading the authors to conclude that bone resorption was up - regulated in gd . were the only authors to examine the role of the rankl / opg system in gd - related bone disease . they found no difference in serum opg between gd patients and controls , and did not identify any correlation between opg and disease severity . the authors concluded that opg - related osteoclast activity was probably not a major underlying mechanism for bone disease in their cohort . the patients ( n=173 ) were described as having non - severe involvement and were heterogeneous in terms of age , spleen status and treatment ( untreated , enzyme - treated for variable periods , or treated with bisphosphonates ) . a third study , which mainly focused on examining the role of cytokines in bone remodeling , involved both pediatric and adult patients : 4 with type 1 and 4 with type 3 disease . osteocalcin and bsap were found to be within normal ranges in adults , while bsap was marginally elevated and osteocalcin was below the normal reference range in children . studies involving bone markers in gd cover a wide range of different study designs : patient groups in terms of the severity of bone disease , demographics , treatment regimens , and bone biomarkers . such diversity means it is difficult to make a direct comparison between one study and another . . however , the available evidence suggests variability in whether , and which , biomarkers are elevated or decreased in both untreated gd and treated gd patients ( tables 2 and 3 ) . no one bone biomarker appears to reflect the presence of bone disease consistently across multiple studies of gd patients or the response to enzyme replacement therapy . patients may have differed in the severity of their disease and also in their bone manifestations , both within and between studies . three pathological presentations may be associated with gd and often all three are presented together : i ) focal lytic and/or sclerotic lesions as a result of infarction , thrombosis and inflammation ; ii ) local disease such as remodeling deformities and cortical bone thinning ; and iii ) generalized osteopenia and osteoporosis . it is not known whether , and to what extent , different pathological processes in gd - related bone disease might influence bone biomarker profiles , or how recent bone events might have affected overall results . recent bone crises , for example , may result in increased levels of bone markers since bone crisis is associated with enhanced bone resorption . similarly , bone markers may increase by 2060% for periods of up to six months following a fracture . most of the bone markers studied in gd have been used to assess the response to treatment in other disease states , notably osteoporosis . in these other conditions , a decline in bone biomarkers within weeks of starting bisphosphonate therapy indicates that anti - resorptive therapy is having an effect , even in the absence of noticeable changes in bmd , which may take several years . studies in gd patients that measure biomarker response after the initiation of bisphosphonate therapies showed a decrease in bone biomarkers . a reduction in osteocalcin and hydroxyproline , and osteocalcin , bsap and ntx , together with statistically significant increases in bmd suggest that bone markers may be responsive to bisphosphonate intervention in gd , and that the use of biomarkers may be relevant to the study of bisphosphonate treatment response . the observation that biomarker levels changed significantly in response to bisphosphonates in these studies also underlines the possible confounding effects of attempting to compare biomarkers in bisphosphonate - treated gd patients with controls and correlating biomarkers with clinical parameters in populations that include both bisphosphonate and non - bisphosphonate - treated patients . for example , magel et al . found no differences in opg levels between gd1 patients , many of whom were treated with bisphosphonates , and healthy controls . in our own studies of the opg / rankl system in 5 enzyme - treated gd patients who had not been exposed to bisphophonates , we found a statistically significant decrease in opg , and a consequent statistically significant decrease in the rankl / opg ratio ( p=0.0002 ) compared with age and gender matched controls . no statistically significant differences in the levels of picp , osteocalcin or trap5b were found ( giuffrida et al . , 2010 , data not cited ) . this review does not clarify whether biomarkers could be applied reliably to the measurement of the bone response to enzyme replacement therapy or srt . markers of bone formation , osteocalcin and bsap increased by 60% in the first 12 months of enzyme treatment in one study with no significant change in markers of bone resorption , suggesting a shift towards new bone formation . increased bmd has been demonstrated in another long - term study of enzyme - treated gd patients ( who also received interrupted vitamin d supplementation ) . another study examining biomarker levels in a group of gd patients receiving enzyme therapy , all of whom had undergone splenectomy , demonstrated statistically significant increases in osteocalcin and bsap ( p=0.002 and 0.008 , respectively ) with only a transient increase in d - pyr . however , bmd decreased in enzyme - treated patients despite an apparent shift towards enhanced bone formation , as demonstrated by bone biomarker levels . splenectomy in gd patients is associated with exacerbated bone disease thought to be related to an increased burden of gaucher cells in bone . however , baseline data in splenectomized patients did not indicate that bsap , osteocalcin and d - pyr were outside the normal reference range . several other studies also included splenectomized patients in their cohorts . including the study by sims et al . these data tend to suggest that bone markers may not reflect skeletal changes that might occur as a consequence of splenectomy . unlike bisphosphonate therapy , where treatment is targeted directly at reducing bone resorption by a process believed to be related to increased osteoclast apoptosis , any effects of enzyme therapy ( and srt ) on bone biomarkers are likely to be indirect and may occur through different mechanisms . the limited evidence of changes in bone markers in gd appears to suggest that bone markers tend to decrease in response to bisphosphonate therapy ( consistent with the response in non - gd populations ) but tend to increase in response to enzyme therapy ( table 3 ) . enzyme replacement therapy aims to reduce the burden of glucocerebroside in macrophages and thereby reduce gaucher cell infiltration into organs . glucocerebroside laden macrophages in bone tissue could have an impact on bone turnover by displacing bone cells , causing vascular occlusion , increasing intra - osseous pressure , and/or by influencing osteoblast and osteoclast activity through inflammatory mechanisms . several studies have reported disturbances in the levels of serum cytokines in gd , including il-1 , il-6 and tnf , which may affect osteoclasts , their function , and bone turnover . indeed , yoshino et al . found correlations between il-18 and bsap and osteocalcin , and m - csf and urinary ntx , although in a limited number of enzyme - treated type 1 ( n=4 ) and type 3 gd patients ( n=4 ) . a reduction in glucocerebroside burden , therefore , might be expected to impact bone turnover , but the temporal relationship between initiation of therapy and changes in different bone turnover markers has not been studied in detail . the rate of glucocerebroside clearance is likely to depend on multiple factors , such as vascularization , degree of osteonecrosis , fibrosis , and sanctuary sites in the bone in individual patients . the difficulties of determining whether and how biomarkers may be affected in gd , and their response to therapy , are compounded by the knowledge that bone biomarkers fluctuate according to age , diet , exercise , hormonal status , stage of the menstrual cycle , season , the use some medications ( e.g. anticonvulsants , estrogen - based therapies , corticosteroids , insulin , fluoride , vitamin d supplements , calcitonin ) , immune factors , and inflammatory status , and that different biomarkers show different degrees of spontaneous and intra - individual variability . the response of markers of bone formation and bone resorption may also vary ; bone resorption markers are reported to respond faster ( 212 weeks ) than markers of formation ( 36 months ) . urinary biomarkers may also show diurnal variation ; pyr and d - pyr are reported to show 2-fold differences over a period of 24 h , and biomarker measurements may show inter - laboratory variation , even when identical assays and methods are used . , statistically significant correlations between biomarker levels and several measures of clinical bone status have been demonstrated in gd in one study . one of the limitations of the studies reviewed here is that bone marker levels are often not the study end point , and data on bone biomarkers were of secondary interest . there have been no detailed studies of changes in biomarker levels over time in untreated and treated gd and their relationship with bone manifestations , leaving clinicians with very little guidance on which biomarkers might be useful in clinical practice . the ideal biomarker in gd should : i ) accurately reflect the presence and activity of disease ; ii ) predict clinically meaningful outcomes ; iii ) change rapidly with the response to therapy ; iv ) be easily measured and stable in accessible clinical samples ; v ) be specific and sensitive ; vi ) be subject to little or no genetic variation ; vii ) be reproducible according to agreed standards of measurement ; and viii ) be inexpensive to measure . evidence from currently available studies does not suggest that any one bone biomarker , or group of biomarkers , reliably reflects the presence and/or activity of disease or changes in a predictable manner to enzyme replacement therapy . bone biomarkers may also be subject to considerable variation as a result of patient status ( hormonal , medications , etc . ) or as a result of inter - laboratory variability . bone markers reflect activity in the whole skeleton and may not be specific or sensitive enough to detect subtle localized changes over a general background of activity . in addition , most studies tend to report biomarker levels as being indicative of either bone formation or bone resorption , although some markers may be released as a consequence of both processes . osteocalcin , for example , is produced by mature osteoblasts in the bone matrix , but is also released from the bone matrix during resorption . there is no evidence as to whether bone markers might have any predictive value in terms of the progression of bone disease or the need for treatment . a greater understanding of the temporal relationship between bone biomarkers and clinical consequence is required . the difficulties of measuring biomarkers are compounded in rare diseases , such as gd , where few patients are available for study and where bone disease may reflect multiple pathological mechanisms . future advances in identifying meaningful biomarkers of bone disease are , therefore , likely to depend on large internationally coordinated studies . such studies might be able to compare carefully controlled groups of patients in sufficient numbers for meaningful statistical analyses to be made . key questions to address include the temporal relationship between fluctuations in bone biomarkers and clinical disease . do biomarkers measured today reflect current bone abnormalities or do they reflect an ongoing pathological process , the consequences of which are not yet measurable by dexa , mri or other commonly used monitoring methods ? studies involving bone biomarkers in gd show variable results which do not currently support their routine use for the clinical assessment of bone status , as an indication for therapy initiation , or for monitoring the response to therapy . a greater understanding of bone markers and their relation to the bone manifestations of gd is required .
bone complications occur frequently in gaucher disease ( gd ) and reduce the quality of life of these patients . 
 skeletal involvement is an important indication for treatment to ameliorate symptoms and reduce the risk of irreversible and debilitating disease . 
 bone biomarkers have been used to assess disease status and the response to therapy in a number of bone disorders . here 
 , we examine the literature for evidence of abnormalities in bone turnover markers in patients with type 1 gd to assess whether they might be useful for the assessment of bone involvement in gd . 
 we have found that bone biomarkers in gd show highly variable results which do not currently support their routine use for clinical assessment of bone status , as an indication for therapy initiation , or for monitoring the response to therapy . 
 a greater understanding of bone markers and their relation to the bone manifestations of gd is required .