Multiple myeloma pathophysiology

Multiple myeloma Microchapters
Home
Patient Information
Overview
Historical Perspective
Classification
Pathophysiology
Causes
Differentiating Multiple Myeloma from other Diseases
Epidemiology and Demographics
Risk Factors
Screening
Natural History, Complications and Prognosis
Diagnosis
Diagnostic Criteria
Staging
History and Symptoms
Physical Examination
Laboratory Findings
Electrocardiogram
X Ray
Echocardiograph and Ultrasound
CT
MRI
Other Imaging Findings
Other Diagnostic Studies
Treatment
Medical Therapy
Surgery
Primary Prevention
Secondary Prevention
Future or Investigational Therapies
Case Studies
Case #1
Multiple myeloma pathophysiology On the Web
Most recent articles
Most cited articles
Review articles
CME Programs
Powerpoint slides
Images
American Roentgen Ray Society Images of Multiple myeloma pathophysiology All Images X-rays Echo and Ultrasound CT Images MRI
Ongoing Trials at Clinical Trials.gov
US National Guidelines Clearinghouse
NICE Guidance
FDA on Multiple myeloma pathophysiology
CDC on Multiple myeloma pathophysiology
Multiple myeloma pathophysiology in the news
Blogs on Multiple myeloma pathophysiology
Directions to Hospitals Treating Multiple myeloma
Risk calculators and risk factors for Multiple myeloma pathophysiology

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Hannan Javed, M.D.[2]; Haytham Allaham, M.D. [3]; Shyam Patel [4]

Overview

Multiple myeloma, a disorder of clonal late B-cells, arises from post-germinal center plasma cells that are normally involved in production of human immunoglobulins. Although the exact pathogenesis and the stage at which myeloma cells arise from post-germinal B-cells remain unclear, a variety of factors have been implicated in pathogenesis of multiple myeloma. Of these, chromosomal abnormalities are thought to be the most important. It has been suggested that all cases of multiple myeloma pass through MGUS. Renal involvement by multiple myeloma is catergorized into three entities: light chain cast nephropathy, monoclonal immunoglobulin deposition disease, and amyloidosis. Osseous involvement by multiple myeloma is based on cytokine and cellular interactions that lead to bone breakdown. On microscopic histopathological analysis, abundant eosinophilic cytoplasm, eccentrically placed nucleus, and Russell bodies are characteristic findings of multiple myeloma.

Pathophysiology

Normal physiology and development of plasma cells

• Plasma cells are terminally differentiated B-cells which function to produce immunoglobulins. plasma cells arise from B lymphocytes through a series of events. These events take place in bone marrow and secondary lymphoid tissues.
• These events include immunoglobulin heavy-chain (IgH) VDJ gene rearrangement, migration into lymphoid tissues, somatic hypermutation (SMH) in the IgH and light-chain genes, antigen selection, switch in Ig class from IgM to IgG, IgA, IgD, or IgE, migration back to bone marrow and terminal differentiation into plasma cells.

Stem cells → Pre-B cells → Immature B-cells → Mature B-cells (naïve) → Activated B-cells → Memory B-cells and Plasmablasts → Plasma cells

• During these events, B cells express variety of surface markers which may be used to denote their developmental stage such as CD19, CD20, CD27, CD38, CD10, CD138. These markers and IgH chain gene sequences are important to define the nature of myeloma cells.
• Plasma cells secrete antibodies which function in humoral immunity.
• Plasma cells are typically polyclonal and can respond to a variety of antigens, which helps combat infections. Under normal circumstances, there is no monoclonality amongst the plasma cell population in a person. These plasma cells are functionally intact in their ability to contribute to humoral immunity.
• Transcription factors such as interferon regulatory factor 4 (IRF4), BCL6, B-lymphocyte-induced maturation protein 1 (BLIMP1, also known as PRDM1), paired box gene 5 (PAX5) and X box binding protein 1 (XBP1) play an important role in differentiation and survival of plasma cells.

Interferon regulatory factor 4 (IRF4) → Down-regulation of BCL6 → Up-regulation of B-lymphocyte-induced maturation protein 1 (BLIMP1) → Down-regulation of paired box gene 5 (PAX5) and Up-regulation of X box binding protein 1 (XBP1).

For more information on plasma cells, click here.

Normal physiology and development of Immunoglobulins

• Immunoglobulins (also known as Antibodies) are proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses.
• They are made of a few basic structural units called chains; each antibody has two large heavy chains H and two small light chains L. There are several different types of antibody heavy chains, and several different kinds of antibodies, which are grouped into different isotypes based on which heavy chain they possess.
• Five different antibody isotypes are known in mammals, which perform different roles, and help direct the appropriate immune response for each different type of foreign object they encounter.
• Class switch recombination (CSR), a region specific deletion recombination process, generates different immunoglobulin (Ig) isotypes by replacing one switch region with another. This process leads to enhanced functionality of immunoglobulins.
• Class switch recombination (CSR), just like somatic hypermutation (SMH), requires the expression of activation-induced deaminase (AID) and both are dependent on creation of double-stranded DNA breaks (DSB) in the Ig loci.

For more information on immunoglobulins, click here.

Pathogenesis

The pathogenesis of multiple myeloma is complex and probably is a result of multiple and multi-step oncogenic events such as hyperdiploidy and deregulation of cyclin D1, and interaction of myeloma cells with marrow environment. Recently it has been suggested that all cases of multiple myeloma pass through an MGUS phase. The events surrounding the progression of MGUS into multiple myeloma are not well-defined but environmental and genetic factors have been proposed to have an association. A brief description of events thought to play a role in pathogenesis of multiple myeloma is given here.

Biology of myeloma cells

• Myeloma cells are malignant plasma cells which exhibit the morphology of mature plasma cells or plasmablasts. Majority of these cells seem to be mature, differentiated and quiescent, appearing to be without long-term proliferative potential.
• Cells with similar morphology to mature B-cells but with immunoglobulin gene sequences and idiotype similar to myeloma cells have also been found in myeloma patients, both in bone marrow and the peripheral blood.
• Current hypothesis is the presence of functional heterogeneity in myeloma cells with only a minor group of specialized myeloma cells exhibiting clonogenic growth potential. Although studies have shown the presence of myeloma cells sub-populations with distinct phenotypes and functionality in multiple myeloma such as presence of CD138+ and CD138− sub-populations, the phenotype of these so-called clonogenic cells is yet to be determined.
• Myeloma cells express surface markers associated with plasma cells such as CD138, natural killer (NK) cells such as CD56/NCAM, T cells such as CD28, and sometimes the pan-B-cell marker CD20. Presence of CD19 and CD20 on sub-population of myeloma cells may suggest either early-lineage precursor for myeloma cells or possible de-differentiation of myeloma cells.
• Myeloma cells show complex chromosomal abnormalities and mutations. Studies have demonstrated the presence of translocations in up to 75% of the myeloma cases.

Environmental and hereditary factors

• The Key environmental and hereditary factors thought to confer a greater risk of developing multiple myeloma or play a part in pathogenesis are summarized in the table below.

Environmental and hereditary risk factors
Increasing age Gender (increased incidences in male) Familial history Past history of MGUS (Monoclonal gammopathy of undetermined significance) Obesity Immune dysfunction such as auto-immune disease, HIV (human immunodeficiency virus) and transplant recipients Exposure to chemicals, pesticides or radiation Greater prevalence of MGUS and multiple myeloma in African Americans* Inherited variation in genetic loci at 2p, 3p, 3q, 6p, 7p, 17p and 22q
* Likely influenced by environmental and behavioral confounding factors.

Chromosomal aberrations

• Two major sets of chromosomal abnormalities seen in multiple myeloma are translocations with extensive IgH rearrangements and numerical aberrations, either trisomy or monosomy.

Translocations

• Majority of translocations in multiple myeloma take place through class switch recombination (CSR). But few may occur through D_H–J_H recombination, possibly in early stages of B-cell development such as pre-B cell stage.
• Majority of these translocations put oncogenes such as cyclin D1 (CCND1), CCND3, fibroblast growth factor receptor 3 (FGFR3), multiple myeloma SET domain (MMSET; also known as WHSC1), MAF and MAFB under the influence of enhancers present at Ig loci. .
• These translocations cause over-expression of these oncogenes that in turn leads to dysregulation of cell-cycle such as increased cell survival, growth, progression and DNA repair. One of the key abnormality being the increased G1/S transition during the cell-cycle.
• Many other translocations may occur in later phases of multiple myeloma. One important example is translocation of MYC, an oncogene that encodes transcription factors.

Hyperdiploidy

• Gain of the odd numbered chromosomes including 3, 5, 7, 9, 11, 15, 19 and 21 is the other major abnormality observed in multiple myeloma.
• The mechanism of this gain phenomena is not well understood, one hypothesis being the gain of chromosome during single mitosis.

Chromosomal aberrations in multiple myeloma (MM)
Chromosomal Abnormality	Chromosome(s)/Protein(s) affected	Consequence
Trisomies	Odd-numbered chromosomes with the exception of chromosomes 1, 13, and 21
t(11;14)(q13;q32) t(6;14q)(p21;32) t(12;14)(p13;q32)	Cyclin D1 Cyclin D3 Cyclin D2	Over-expression; cell cycle dysregulation
t(4;14)(p16;q32)	FGFR3 or MMSET	Over-expression and activation; multiple myeloma cell proliferation/apoptosis prevention MMSET probably linked to crucial transforming event
t(14;16)(q32;q23) t(14;20)(q32;q11) t(8;14)(q24;q32)	c-MAF MAFB MAFA	Over-expression; involvement in IL-4 regulation
del 17p13	p53	Cell-cycle dysregulation/apoptosis
Monosomy 14	Chromosome 14
Chromosome 13 deletion and monosomy	Chromosome 13
Gain(1q21)	Chromosome 1
Abbreviations used: FGFR3:fibroblast growth factor receptor 3; MMSET:multiple myeloma SET domain; MAF:musculoaponeurotic fibrosarcoma oncogene homolog.

Mutations in myeloma

• Studies have demonstrated the presence of approximately 35 mutations per sample in multiple myeloma. These mutations cause loss of tumor suppressors and NFKB alterations.

Tumor suppressors

• These mutations result in cell-cycle dysregulation with an increase in G1/S transition. Some of these mutations are mentioned in the table below.
• The loss of tumor suppressors allows the cells to survive and grow without check points.

Tumor suppressor genes commonly affected in myeloma
FAM46C (family with sequence similarity 46, member C)
DIS3 (Exosome complex exonuclease RRP44)
CYLD (Cylindromatosis)
Baculoviral IAP repeat containing protein 2 (BIRC2; also known as cIAP1)
BIRC3 (Baculoviral IAP repeat containing protein 3)
tumor necrosis factor receptor associated factor 3 (TRAF3)
CDKN2C
CDKN2A
TP53

NFKB alterations

• NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) is a protein complex which regulates DNA transcription, production of cytokine and cell survival. NF-κB plays a key role in generating cellular responses to stimuli such as stress, free radicals, heavy metals, ultraviolet radiations, and foreign antigens. NF-κB is also crucial in response of the immune system towards infections.
• Deregulated NF-κB system leads to increased growth factors and cytokines such as IL-6 which promote growth and survival in myeloma cells.
• Mutations in multiple genes can cause activation of NF-κB system which in turn plays a role in pathogenesis of multiple myeloma. Some of these mutations, that have been documented in genes regulating NF-κB pathway, are mentioned below.

Genes mutated associated with canonical signaling	Genes mutated associated with non-canonical signaling
TLR4 TNFRSF1A IKBKB IKBIP CARD11 MAP3K1 RIPK4 CYLD BTRC	MAP3K14 TRAF3

Epigenetic changes in multiple myeloma

• Epigenetic events such as methylation of DNA and histones play an important role in pathogenesis of multiple myeloma.
• Of these events, the most notable is the global hypomethylation and gene-specific hypermethylation that takes place during progression of MGUS to myeloma.
• A subset of patients with t(4;14) translocation show prominent DNA methylation changes that lead to MMSET overexpression.
• Other dysregulations related to epigenetics in multiple myeloma may effect KDM6A (UTX), a histone demethylase, mixed lineage leukemia (MLL) protein, lysine demethylase 6B (KDM6B) and homeobox A9 (HOXA9).

Bone marrow microenvironment and multiple myeloma

• Bone marrow microenvironment comprises of bone marrow stromal cells (BMSCs), extracellular matrix (ECM) proteins such as collagen, fibronectin and laminin, and the extracellular fluid containing cytokines and growth factors. This microenvironment is conducive to normal hematopoiesis but this also helps myeloma cells to have increased replication activity and anti-apoptotic resistance.
• One of the key processes is the interaction and adherence of myeloma cells with bone marrow stromal cells (BMSCs) and extracellular matrix proteins through cellular adhesion molecules (CAMs) such as CD44 (H-CAM), CD56 (N-CAM), members of the CD49 integrin family (including very-late antigen VLA-4 and VLA-5), lymphocyte function-associated antigen-1, syndecan-1, and selectin.
• This interaction leads to activation of p42/44 mitogen-activated protein kinase, and nuclear factor κB (NF-κB) which cause increased adhesion molecules, both on myeloma cells and bone marrow stromal cells. These events ultimately lead to increased production of cytokines. Of these cytokines IL-6, TNFα, BAFF, IGF and HGF are of particular importance.
• These events cause localization of tumor cells in bone marrow, increased proliferation and survival, and resistance to apoptosis and chemotherapy. Bone marrow niche thus plays a crucial role in pathogenesis of multiple myeloma.
• Decreased cellular adhesion molecules such as CD56 and chemokine receptors such as CXCR4 on myeloma cells lead to decreased adhesion to bone marrow stromal cells and immune evasion, allowing the myeloma cells to spread outside the bone marrow.

Cytokines in multiple myeloma pathogenesis

• A variety of cytokines have been implicated in pathogenesis of multiple myeloma. Some of the relevant cytokines, their mechanisms and effects on tumor cells have been summarized in the table below.

Cytokines	Mechanism	Effects on tumor cells and pathogenesis
Interleukin 6 IL-6	Activates signal transduction pathways (JAK/STAT3 and PI3K/Akt)	Increased proliferation Increased production of vascular endothelial growth factor (VEGF) Decreased apoptosis Drug resistance Increased osteoclasts differentiation
Tumor necrosis factor α TNF-α	Activation of NF-κB Activation of the MAPK pathways	Increased survival Increased proliferation Increased cell migration Increased inflammatory mediators Increased osteoclast differentiation Increased bone resorption
B-cell activating factor (BAFF)	Activation of NF-κB	Increased survival Increased adhesion
Insulin-like growth factor-1 (IGF-1)	Activation of PI3K/Akt Activation of IKK/NF-κB	Increased growth and proliferation Decreased apoptosis and increased survival
Vascular endothelial growth factor (VEGF)	VEGF Receptor activation	Increased angiogenesis Increased tumor cell migration Increased growth, Increased survival Increased IL-6 production
Interleukin 17 (IL-17)	Interleukin 17 receptors activation	Increased survival Increased cytokines production Lytic bone lesions

Pathophysiology of renal involvement

Abnormal antibody fragments are produced in multiple myeloma and are deposited in various organs, such as the kidneys. There are three major forms of renal damage in patients with multiple myeloma.

• Cast nephropathy: End-organ damage to the kidneys is typically due to light chain cast nephropathy. The pathophysiology of this type of renal involvement is based on light chain deposition in the renal tubules, which results in obstruction. Free light chains are readily filtered at the glomerulus and are reabsorbed in the proximal tubule of the nephron. This reabsorption occurs via the megalin-cubulin transport system. In patients with multiple myeloma, there is excess production of free light chains, and the ability of the nephron to resorb light chains in the proximal tubule cannot meet the demands of the freely filtered light chains. This results in excess secretion of free light chains in the urine (known as Bence-Jones protein). Eosinophilic proteinaceous casts and crystalline structures can be seen. Cast formation occurs in the tubules due to excess abundance of free light chains that interact with Tamm-Horsfall proteins in the thick ascending loop of Henle. Tubular obstruction ensues, triggering local inflammation which results in interstitial nephritis and fibrosis. The onset of cast nephropathy can be very quick, requiring prompt treatment. Risk factors for development of cast nephropathy include monoclonal immunoglobulin secretion of >10 g/day, sepsis, and volume depletion. Patients can also develop Fanconi syndrome, resulting in dysfunctional reabsorption ability by the proximal tubule, and type II renal tubular acidosis.

• Monoclonal immunoglobulin deposition disease (MIDD): In this subtype of renal involvement by multiple myeloma, the initial pathophysiological process is filtration of monoclonal immunoglobulins and subsequent deposition of immunoglobulins along the tubular or glomerular basement membrane. Deposits of immunoglobulin can have a similar appearance as Kimmelstein-Wilson lesions (seen in diabetes). The immunoglobulins can appear fibroblast-like.

• Light chain amyloidosis: The pathophysiology of renal involvement by light chain amyloidosis begins with beta-pleated sheet formation in the tubules or glomeruli. Beta-pleated sheets form as a result of electrostatic interactions between heparan sulfate proteoglycan and amyloid proteins. Amyloid fibrils usually consist of immunoglobulin light chains (usually lambda light chain) and deposit in the basement membrane. The size of the fibrils vary from 7 to 10 nanometers. A diagnosis of this type of renal involvement is made by the visualization of apple green birefringence upon Congo red staining of the renal specimen. It is frequently associated with nephrotic range proteinuria, in which greater than 3 grams of protein is excreted daily.

Pathophysiology of osseous involvement

Bone disease characterized by progressive osteolytic bone lesions leading to bone resorption is hallmark of multiple myeloma. Abnormal bone remodeling is thought to be the cause. It has been reported that up to 80% of the patients have characteristic osteolytic bone lesions at presentation and 60% of the patients with multiple myeloma will develop at least one pathological fracture at some stage.The pathophysiology of bony involvement in multiple myeloma is complex and is briefly described here..

Increased osteoclastic activity

Osteoclasts are large, multinucleated cells of monocyte–macrophage lineage and play a crucial role in bone remodeling. Myeloma cells, in addition to their direct interaction with other cells, produce and release a number of factors which promote osteoclast differentiation and activation. Some of these factors and interactions have been described below in the tables.

Cell-cell interactions	Consequences
Myeloma cells to bone marrow stromal cells	Decreased production of OPG Increased RANKL expression Activation of the NF-kB system that has following effects on myeloma cells: ↑ growth ↑ survival ↑ drug resistance migration
Alpha4-beta1 integrin to vascular cell adhesion molecule 1 (VCAM-1) interaction	Increased osteoclastic activity independent of IL-6, TNF or PTHrP. Increased RANKL expression
Myeloma cells to osteoblasts	Decreased production of OPG
Myeloma cells to osteoclasts	Direct adherence of myeloma cells to osteoclasts may result in myeloma cell proliferation ↑ osteoclastic differentiation ↑ proangiogenic factors
Myeloma cells to immune cells	Increased production of cytokines, chemokines and factors associated with growth, survival and migration.

.

Molecular pathways and factors associated with increased osteoclastic activity	Association with multiple myeloma
RANK/RANKL pathway Binding of RANKL to RANK → fusion of osteoclast precursors into multinucleated cells → mature osteoclasts → ↑ bone resorption Osteoprotegerin (OPG) is a soluble decoy receptor for RANKL → ↓ binding of RANKL to RANK → ↓ bone resorption RANK is a transmembrane receptor of TNF superfamily and is expressed by osteoclast precursors RANKL, a membrane-bound protein on BMSCs of osteoblastic lineage and activated T-lymphocytes, is a cytokine Osteoprotegerin (OPG), released by BMSCs and osteoblasts, is a member of the TNF superfamily ↑ RANKL and/or ↓ OPG → ↑ Bone resorption and destruction	Myeloma cells lead to increased RANKL and decreased OPG causing bone resorption and destruction. RANKL/OPG ratio is an idependent prognostic factor in multiple myeloma Myeloma cells produce and release soluble RANKL → ↑ Bone resorption Plasma cells secrete PTHrP that stimulates paracrine pathway → ↑ expression of RANKL by osteoblasts and BMSCs → ↑ Bone resorption Syndecan-1, an heparan sulfate proteoglycan expressed by myeloma cells, binds to OPG → endocytosis and degradation of OPG by myeloma cells → ↑ Bone resorption
Notch pathway Binding of Notch receptors to its ligands → ↑ RANKL production → ↑ bone resorption Notch family contains four transmembrane receptors (Notch 1–4) Their ligands include Jagged 1,2 and Delta-like 1,3,4	Myeloma cells express Notch 1,2,3 and their ligands Jagged 1,2 and Delta-like 1,3,4. This pathway, in addition to bone resorption, may also be involved in metastasis of myeloma cells by increasing expression of adhesion molecules, migratory chemokines, and angiogenetic factors, and disruption of the immune surveillance. Notch receptors on myelomacells can bind to Jagged 1,2 ligands on the same cell (homotypical interaction) → ↑ RANKL Notch receptors on myeloma cells can bind to Jagged or Delta-like ligands on adjacent BMSCs and myeloma cells (heterotypical interaction) → ↑ RANKL Notch receptors on BMSCs can bind to Jagged ligands on myeloma cells → ↑ RANKL
CCL-3 (MIP-1α)/CCL-20 Chemokine (C-C motif) ligand 3 (CCL-3), previously known as macrophage inflammatory protein-1α (MIP-1α), is a chemokine that binds to CCR1 and CCR5. Chemokine (C-C motif) ligand 20 (CCL-20) is a chemokine involved in Th17 pathway that binds to CCR6 and has been implicated in osteoclastogenesis and osteolytic lesions. CCL-3 may: attract osteoclast precursors and induce osteoclastogenesis potentiate RANKL and IL-6 effects on osteoclasts down-regulate RUNX2 and osterix → ↓ osteoblast activity ↓ bone mineralization promote survival of myeloma cells promote migration of myeloma cells	Multiple myeloma patients have high levels of CCL-3/CCL-20 detected in their bone marrow and serum. In myeloma cells with translocation t(4;14), up-regulation of fibroblast growth factor 3 (FGFR-3) leads to induction of CCL-3. High levels of CCL-3/CCL-20 in myeloma patients correlate positively with bone disease and negatively with survival.
Activin A Binds to type II transmembrane serine/threonine kinase receptor (ActRIIA/B) → recruitment and phosphorylation the type I receptor (ActRI, also called activin receptor-like kinase 4 (ALK4) receptor) → heterodimer formation → activation of the Smad signaling cascade → translocation of Smad2/3/4 complex in the nucleus → action as transcriptional factor → RANK expression and activation of NF-κB pathway → increased osteoclast differentiation It belongs to TGFβ superfamily. May also be involved in Akt/PI3K, MAPK/ERK, JNK, and WNT/β-catenin pathways.	Multiple myeloma patients have high levels of activin A detected in their bone marrow and serum. Increased activin A levels may be associated with advanced bone disease and worse prognosis in multiple myeloma. It has been suggested that interaction between BMSCs and myeloma cells may induce secretion of activin A. It is also involved in inhibition of BMP signaling in myeloma cells.
Osteopontin a non-collagenous bone matrix glycoprotein secreted by osteoclasts may cause ↑ Osteoclast activation ↑ angiogenesis	High levels may be associated with extensive osteolytic diseas advanced disease (myeloma) High levels may also play a protective role against bone resorption in myeloma patients with with maf gene translocations.
Interleukin 3 (IL-3) a bifunctional cytokine stimulates osteoclast formation, primarily through induction of activin A production inhibits osteoblast differentiation through participation of CD45 + hematopoietic cells.	Increased levels found in multiple myeloma Thought to be secreted by T-lymphocytes through a complex interaction with myeloma cells
Vascular endothelial growth factor (VEGF) important in angiogenesis and growth may promote osteoclast differentiation by substituting for M-CSF	Majority of myeloma cells secrete VEGF Increases survival and growth of myeloma cells
Interleukin 6 (IL-6) a multifunctional cytokine involved in bone metabolism stimulates osteoclast differentiation stimulates the PI3K/Akt/mTOR pathway. This pathway plays a role in regulation of expression of: IL-6 VEGF osteopontin	Increases survival of myeloma cells Stimulates myeloma cells to produce and release vascular endothelial growth factor (VEGF) that further stimulates osteoclasts correlates with bone turnover rate in multiple myeloma
Interleukin 17 (IL-17) a pro-inflammatory cytokine stimulates osteoclast activation	Primarily secreted by T-helper cells (Th17). Also secreted by T-lymphocytes and natural killer (NK) cells Associated with osteolytic lesions in myeloma models.
B cell-activating factor (BAFF) Binding to its receptor → activation of NF-κB → ↑ MM cell survival a member of TNF superfamily associated with increased osteoclast activation	Secreted by bone marrow stromal cells (BMSCs), osteoclasts, and myeloma cells Increased serum levels in multiple myeloma patients Increases survival of myeloma cells
Bruton’s tyrosine kinase (BTK) Osteoclast precursors with CXC chemokine receptor type 4 (CXCR4) and Bruton’s tyrosine kinase (BTK) expression → migration towards stromal cell-derived factor-1α (SDF-1α) → ↑ activation of Bruton’s tyrosine kinase (BTK) in myeloma cells. a nonreceptor tyrosine kinase involved in B cell receptor signaling pathway stimulates osteoclast differentiation	Expressed by myeloma cells Correlates with CXCR4 expression Associated with progression, invasion through the extracellular matrix and the blood vessels, and settling of myeloma cells
Stromal cell-derived factor-1α (SDF-1α) Osteoclast precursors with CXC chemokine receptor type 4 (CXCR4) and Bruton’s tyrosine kinase (BTK) expression → migration towards stromal cell-derived factor-1α (SDF-1α) → ↑ activation of Bruton’s tyrosine kinase (BTK) in myeloma cells. a chemokine stimulates osteoclasts by binding to CXC chemokine receptor type 4 (CXCR4)	Involved in migration of myeloma cells Plays a role in homing of tumor cells
Annexin II (Annexin A2) a calcium-dependent phospholipid-binding member of the annexin family normally expressed by: bone marrow stromal cells (BMSCs) endothelial cells mononuclear macrophages osteoblasts osteoclasts Promotes mineralization osteoclastogenesis	Secreted by myeloma cells, bone marrow stromal cells (BMSCs), osteoblasts, and osteoclasts Up-regulated in multiple myeloma Promotes myeloma cells' adhesion growth of myeloma cells angiogenesis
PU.1 a transcriptional activator binds to the PU-box, a lymphoid-specific enhancer may be associated with differentiation or activation of macrophages and B-cells involved in osteoclasts formations may modulate pre-mRNA splicing

Decreased osteoblast activity

Inhibition of osteoblasts leading to decreased bone formation is now considered to be a critical event in bone disease in multiple myeloma. This inhibition leads to bone loss as well as inability to repair the osteoytic lesions caused by increased osteoclastic activity. Several pathways and factors have been associated with suppressed osteoblastic activity. Some of them are mentioned below in the table.

Molecular pathways and factors associated with osteoblastic activity	Association with multiple myeloma
Wingless and integration-1 (Wnt) signaling Canonical pathway Activation of Wnt pathway → binding of Wnt ligands to Wnt co-receptors LRP5/6 and one trans-membrane receptor of the FDZ family → formation of DVL–Axin–FRAT1–GSK-3β complex → translocation of β-catenin from cytoplasm to nucleus → activation of T cell factor/lymphoid enhancer factor (TCF/LEF) transcription factors → ↑ bone formation and ↓ bone resorption Non-canonical WNT–planar cell polarity (WNT–PCP) pathway Formation of WNT ligand–receptor tyrosine kinase-like orphan receptor 2 (ROR2) or the receptor-like tyrosine kinase (RYK)–FZD–DVL complex → activation of one of these 3 pathways: 1) Disheveled-associated activator of morphogenesis 1 (DAAM1)–RHO–RHO-associated kinase (ROCK) pathway; 2) RAC–Jun kinase (JNK)–RUNX2 pathway; WNT–Ca2+ pathway Binding of PTH to PTH1 receptor may result in activation of Wnt pathway This pathway is also involved in cell-cycle promotion and cell growth	Dickkopf 1 (Dkk-1) an extracellular inhibitor of this pathway expressed by osteoblasts and bone marrow stromal cells, is thought to play the crucial role in osteoblastic inhibition in multiple myeloma. The serum levels and the expression of Dickkopf 1 (Dkk-1) by bone marrow cells were found to increased in patients with multiple myeloma. Sclerostin secreted by osteocytes, sclerostin is a cysteine knot-containing protein activates the caspase pathway, resulting in apoptosis of mature osteoblasts binds to LRP5/6 transmembrane receptors preventing the Wnt ligand- LRP5/6 transmembrane receptors binding, that leads to inhibition of Wnt signaling pathway promotes osteoclastogenesis by increasing RANKL/OPG ratio elevated levels have been found in all phases of multiple myeloma and is considered to be a worse prognostic factor. Furthermore, myeloma cells have been shown to secrete sclerostin Secreted frizzled related protein-2 (sFRP-2) prevents binding of Wnt ligands to frizzled, a membrane bound receptor some studies suggest that myeloma cells express sFRP-2 that antagonizes bone formation Periostin produced by bone marrow stromal cells, periostin is an adhesion protein of fasciclin family thought to activate the integrin–AKT–FAK–β-catenin pathway although the role is still unclear, elevated levels have been found in almost all the patients presenting with multiple myeloma Runt-related transcription factor 2 (Runx2)/corebinding factor runt domain alpha subunit 1 (CBFA1) a transcription factor that is a part of the non-canonical WNT-signaling pathway myeloma cells decrease osteoblast differentiation by inhibiting RUNX2 activity in BMSCs and osteoblast precursor cells
EphrinB2/EphB4 signaling pathway Binding of ligands called ephrins (Eph receptor-interacting proteins) → activation of two cascades: 1) the forward signaling → ↑ osteoblast differentiation by downregulating RhoA; 2) the reverse signaling → ↓ osteoclast differentiation by ↓ Fos and Nfatc1 transcription EphrinB2 is found in osteoclasts EphB4 is found in osteoblasts and bone marrow stromal cells	Multiple myeloma patients have decreased expression of both EphrinB2 and EphB4 in bone marrow stromal cells.
Transforming growth factor-β (TGF-β) a ubiquitous, multifunctional growth factor produced by osteocytes and osteoblasts and is activated by osteoclasts primary effect in bone marrow is the inhibition of terminal differentiation of osteoblasts	Inhibition of Transforming Growth Factor-β have been shown to prevent myeloma cells to block osteoblastic differentiation.
Bone morphogenetic proteins (BMPs) belong to TGFβ superfamily promote osteoblastogenesis through Smad-dependent and Smad-independent pathways	Myeloma cells express negative regulators of bone morphogenetic proteins that result in decreased activity of these proteins which in turn causes decreased osteoblastogenesis.
Hepatocyte growth factor (HGF) inhibits BMP-induced osteoblastogenesis, thought to be mediated by inhibition of nuclear translocation of receptor-activated SMADs	Produced by myeloma cells Increased levels have been shown to correlate with worse prognosis
Interleukin 3 (IL-3) inhibits BMP-induced osteoblastogenesis, thought to be mediated indirectly by increasing CD45+ hematopoietic cells	Elevated levels had been demonstrated in bone marrow of myeloma patients.
Interleukin 7 (IL-7) Causes increased osteoclastic activity decreased osteoblasts stimulation and maturation decreased Runx2/CBFA1 activity
Growth factor independence-1 (GFI1) a transcriptional repressor that causes decreased expression of RUNX2	Bone marrow stromal cells (BMSCs) in myeloma patients have been shown to over-express growth factor independence-1 (GFI1).
Tissue necrosis factor-α (TNF-α) inhibits osteoblast differentiation through inhibits osteoblast precursor recruitment suppresses RUNX2 and its transcriptional co-activator, TAZ	Tissue necrosis factor-α (TNF-α) levels are increased in multiple myeloma and it is thought to play a complex role in pathogenesis of multiple myeloma.
Adiponectin adipocyte-derived hormone thought to act on osteoblasts and osteoclasts	Increased adiponectin secretion through pharmacologic interventions have shown to decrease osteolytic lesions in myeloma.

Renal involvement in multiple myeloma

Renal involvement is common in multiple myeloma as up to 50% of the patients go on to develop kidney disease at some point during the disease course. Half of these patients recover kidney function while the rest of them develop chronic kidney dysfunction. The causes of renal involvement in multiple myeloma are multiple and diverse. They can broadly be classified into Ig dependent and Ig independent mechanisms. Some of them have been discussed below in the table and then described briefly.

Ig-dependent renal injury		Ig-independent renal injury
Cause	Notes	Cause	Notes
Cast nephropathy (myeloma kidney)	Risk factors light chain myeloma with >10 g/d monoclonal Ig excretion volume depletion sepsis medications Ig deposition is primarily in the renal tubules	Volume depletion	May cause pre-renal azotemia acute tubular necrosis and/or contribute to cast nephropathy
Monoclonal Ig deposition disease	May be associated with systemic syndrome Ig deposition may be demonstrated in either tubules or glomeruli but typically not in both	Sepsis	May cause pre-renal azotemia acute tubular necrosis and/or contribute to cast nephropathy
Light chain amyloidosis (AL)	May be associated with systemic syndrome often with nephrotic-range albuminuria and λ-light chains amyloid deposition is primarily in the glomeruli	Hypercalcemia	May result in acute kidney injury directly or contribute to cast nephropathy
Glomerulonephritis	Following types have been demonstrated membranoproliferative diffuse proliferative crescentic cryoglobulinemic	Tumor lysis syndrome	Caused by uric acid or phosphate nephropathy
Tubulointerstitial nephritis	May be caused be either Ig-dependent or Ig-independent mechanisms	Direct parenchymal invasion by plasma cells	Rare cause. Association with advanced or aggressive multiple myeloma
Minimal change disease	Often with albuminuria and light chain proteinuria	Pyelonephritis	Rare cause. Pathogenesis is typically multifactorial and may include: immunodeficiency deficient Ig chemotherapy
Hyperviscosity syndrome	Seen primarily in cases of IgA, IgG3, or IgM myeloma	Medication toxicity	Following drugs may cause kidney injury: Zoledronate: May cause acute renal failure in rare cases Pamidronate: May be associated with collapsing focal and segmental glomerulosclerosis in rare cases Nonsteriodal anti-inflammatory drugs Angiotensin converting enzyme inhibitor Angiotensin receptor blocker Loop diuretics Iodinated contrast
Henoch–Scholein purpura/IgA nephropathy	Associated with IgA myeloma
Immunotactoid glomerulopathy (and possibly fibrillary GN)	Rare conditions. The association between fibrillary disease and paraproteins is is not understood at this point.
Thrombotic microangiopathy (TMA)	Endothelial injury caused by paraprotein leads to thrombotic microangiopathy.
Membranous glomerulopathy

 

Cast nephropathy (myeloma kidney)

• Normally circulating monoclonal free light chains are relatively freely filtered through the glomerulus and a receptor-mediated process leads to their endocytosis in proximal tubule cells. They bind to the tandem scavenger receptor system cubilin/megalin, followed by endocytosis through the clathrin-dependent endosomal/lysosomal pathway. They are then degraded within lysosomes.
• In multiple myeloma, myeloma cells produce excess of monoclonal free light chains that overwhelms the capacity of the proximal tubule cells to absorb and catabolize them. This leads to large amounts of monoclonal free light chains reaching distal renal tubules.
• In distal renal tubules, they interact with Tamm-Horsfall protein (uromodulin), a glycoprotein produced in the medullary thick ascending limb of the loop of Henle in kidneys. This interaction leads to formation of myeloma casts.
• These casts block the glomerular flow, compromising renal function and may lead to proximal tubular atrophy. This may also contribute to interstitial fibrosis.
• This blockage leads to increased luminal pressure, resulting in decreased blood flow and causing further renal injury.
• The excessive amounts of monoclonal free light chains being absorbed in proximal renal tubules leads to induction of apoptosis and DNA damage in proximal renal tubules.
• Free light chains also leads to increased pro-inflammatory cytokines such as IL-6, macrophage chemoattractant protein-1 (MCP-1), and tumor necrosis factor α (TNFα) through transcription factors nuclear factor kappa B (NFκB) and mitogen-activated protein kinases.
• All these events leads to kidney injury and inability of renal cells to repair the injury. On histopathological studies, chronic tubulointerstitial nephropathy with marked tubular atrophy, laminated casts in the tubules, and interstitial fibrosis may be observed.

Factors associated with increased risk of AKI or CKD in multiple myeloma	Mechanism of injury
Volume depletion	Increased FLC concentration Decreased GFR Pre-renal azotemia
Hypercalcemia	Renal vasoconstriction Decreased GFR Pre-renal azotemia
Iodinated contrast media	Nephrotoxicity
Nonsteroidal anti-inflammatory drugs	Renal vasoconstriction Decreased GFR Medullary hypoxia
Diuretics	Increased sodium chloride concentration Increased cast formation
Aminoglycosides	Renal vasoconstriction Decreased GFR
Hyperuricemia
Comorbidities such as chronic kidney disease, diabetes, aging, hypertension, and cardiovascular disease.

Hypercalcemia

• Hypercalcemia in multiple myeloma primarily results from increased osteoclastic bone resorption. It is the second most common cause of renal injury in myeloma patients.
• Hypercalcemia leads to intra-tubular calcium deposition and vasoconstriction in renal vasculature. The resultant decrease in GFR not only causes renal injury by itself but also favors cast formation.
• By impairing renal concentrating ability through resistance to anti-diuretic hormone, it may also cause nephrogenic diabetes insipidus. Polyuria may develop, leading to hypovolemia. Net effect is decreased GFR, concentrated urine, reduced urine flow, and pre-renal azotemia.

Light-chain glomerulopathy

• Deposition of immunoglobulins either in the form of amyloid or non-amyloid, typically resulting in non-selective proteinuria.
• Amyloid deposits are fibrillar structures, consist of variable region fragments of the light chain, and are seen primarily within the glomeruli.
• Some patients may have predominant vascular amyloid deposits rather than glomerular deposits and present with renal failure instead of nephrotic syndrome.

Light chain deposition disease (LCDD)

• Non-fibrillar, granular deposits are topically observed in mesangial area along with thickening of basement membrane which resembles type II membranoproliferative glomerulonephritis or diabetic glomerulopathy. The light chain deposits may also be observed in renal vasculature in addition to mesangial area.
• Typical clinical picture is that of nephrotic syndrome but renal impairment is more severe with almost all patients developing renal failure.

Immune dysfunction in multiple myeloma

• Immunodeficiency is an important characteristic of multiple myeloma and is thought to associated with increased risk for infections, and affects disease progression and response to treatment.
• The typical clinical picture is that of global immunosuppression. The interaction between myeloma cells and bone marrow stromal cells and cytokines produced as a result of this interaction are thought to play a key role.
• Following abnormalities have been demonstrated in myeloma patients:
  • Decreased numbers of natural killer cells, B-cells, and memory T cells
  • Decreased non-myeloma immunoglobulins
  • Decreased expression of cell-surface markers on CD4+ and CD8+ T cells such as CD28 and CD152. These markers are associated with cell-signaling.
  • Abnormal signal transduction in T-cells
  • Abnormalities in cytokine production
  • Abnormalities in antibodies response and production
  • Aberrations in B-cells maturation
  • Defects in natural killer cells and antigen presenting cells

Gross Pathology

• 

  Vertebrae in multiple myeloma
  (Image courtesy of Melih Aktan M.D.)

• 

  Calvarium in multiple myeloma.
  (Image courtesy of Melih Aktan M.D.)

Microscopic Pathology

On microscopic histopathological analysis, multiple myeloma is characterized by the following:

• Abundant eosinophilic cytoplasm
• Eccentrically placed nucleus
• Clock face morphology of the nucleus due to chromatin clumps around the edges
• Russell bodies which are eosinophilic, large (10-15 micrometres), homogenous immunoglobulin-containing inclusions
• Dutcher bodies which are PAS stain +ve intranuclear crystalline rods

• Shown below is a series of microscopic images seen in multiple myeloma:

• 

  Bone marrow aspiration in multiple myeloma.
  (Image courtesy of Melih Aktan M.D.)

• 

  Bone marrow biopsy in multiple myeloma.
  (Image courtesy of Melih Aktan M.D.)

• 

  Bone marrow in multiple myeloma.
  (Image courtesy of Melih Aktan M.D.)

• 

  Bone marrow in multiple myeloma.
  (Image courtesy of Melih Aktan M.D.)

• 

  Multiple myeloma slide with intermediate magnification

• 

  Multiple myeloma slide with high magnification

• 

  Multiple myeloma slide with russell bodies