Immunoglobulin-Related Amyloidosis 

Immunoglobulin-related amyloidosis is a monoclonal plasma cell disorder in which the secreted monoclonal immunoglobulin protein forms insoluble fibrillar deposits in 1 or more organs. In nearly all cases, the deposits contain immunoglobulin light (L) chains or L-chain fragments, termed L chain–type amyloidosis (AL). In a few reported patients, the amyloid deposits have contained immunoglobulin heavy (H) chains; these are termed H chain–type amyloidosis AH). Before the discovery that the major fibril component in these patients was an immunoglobulin fragment, patients with light chain–type amyloidosis were described as having primary (in the sense of idiopathic) amyloidosis or, when the burden of monoclonal plasma cells was large, myeloma-associated amyloidosis.

Immunoglobulin L and H chains are 2 of 20 different fibril proteins that have been described in human amyloidosis. For a general discussion of the human amyloidoses, the types of human amyloidosis, and an approach to the diagnosis of amyloidosis, see Amyloidosis, Overview.

L chain–type amyloidosis (AL) is related to both multiple myeloma and monoclonal gammopathy of undetermined significance (MGUS). These monoclonal plasma cell disorders can be categorized according to the total body burden of monoclonal plasma cells. When this burden is large, the diagnostic criteria for multiple myeloma are fulfilled; when this burden is lower, MGUS is diagnosed. Multiple myeloma and MGUS fall on a continuum, with 20% of patients with MGUS progressing to multiple myeloma within 10 years.

In most patients with a monoclonal plasma cell disorder, whether multiple myeloma or MGUS, the monoclonal L chain secreted by the clone remains soluble in the bloodstream. However, in some patients, the physicochemical characteristics of the immunoglobulin L chain or L-chain fragment lead to its deposition as amyloid. Thus, some patients with light chain–type amyloidosis meet the diagnostic criteria of multiple myeloma, whereas other patients can be considered as having MGUS in which the clonal immunoglobulin product is amyloidogenic.

In addition to cases of monoclonal gammopathy in which the secreted clonal immunoglobulin remains in solution and those in which secreted clonal immunoglobulin forms amyloid deposits, a third group consists of cases in which the monoclonal proteins accumulate in various organs, but the deposits do not form fibrils. Patients with this form are described as having nonamyloid monoclonal immunoglobulin deposition disease (MIDD). The relationship among the plasma cell dyscrasias and the amyloidoses is depicted in the image below.

The most common light chain–type amyloidosis precursor proteins are L chains of the lambda (l) class. The lambda light chain–type amyloidosis is approximately twice as prevalent as the kappa (k) light chain–type amyloidosis, and L chains of the V_l 6 class are the most amyloidogenic. Clonal plasma cell proliferative diseases in which the V_l 6 gene is expressed are always associated with amyloid deposition. Among V_k genes, the V_k 1 subgroup is overrepresented among amyloid-forming L chains.

Within the V region families, certain amino acid residues occurring at particular positions in the L-chain sequence render those chains are more amyloidogenic, with a combination of such residues increasing the chances of a particular L-chain protein being associated with tissue amyloid deposition. Another structural feature that appears to predispose to L chain – type amyloid deposition is enzymatic glycosylation of the L chain. Although 15% of human L chains bear sugar residues, almost one third of amyloidogenic L chains are glycosylated. Why certain amino acid and glycosylation characteristics in L chains predispose to amyloid formation remains unknown.

L chain – type amyloid deposits contain intact L chains, L-chain fragments, or both (most patients). The fragments always include the amino terminus of the chain and range in mass from 5000 to 16,000 d. In 90% of patients, the deposited peptides include at least some constant region sequence; therefore, the peptides react with commercially available anti–L chain sera, which are specific for constant region determinants. These observations explain why 10% of deposits do not bind either commercial anti-k or anti-l antisera.

L chain – type amyloid deposits can develop in any organ system. The most common organs involved are the kidneys, the heart, the gastrointestinal (GI) tract, the peripheral nerves, and the liver. In most cases, the deposits affect multiple organ systems. Factors leading to the specific pattern of organ involvement in a particular patient are not understood.

In a minority of cases, localized amyloid deposits, including amyloid masses (amyloidomas), may be found in various sites, even in the absence of systemic disease. The pathogenesis of localized light chain–type amyloidosis is not well understood, but a small, localized clone of plasma cells apparently produces immunoglobulin, which forms deposits near the site of synthesis. In some patients, plasma cells have been demonstrated histologically, accompanying the localized amyloid deposits. In one patient, DNA sequencing revealed that local plasma cells were producing the locally deposited L chains.

Researchers from the University of Arkansas for Medical Sciences (UAMS) studied the gene profile of more than 500 patients treated for multiple myeloma at UAMS.^[1] Of the about 25,000 genes in the body, the UAMS team found the expression of just 17 genes revealed which form of the disease the patients had.^[1] A gene is expressed when its DNA is transcribed into RNA, which is later transcribed into protein. The expression level of those 17 genes becomes an overpowering and overriding predictor of outcome in therapy. It is questionable whether a drug that targets those genes will be developed.

Frequency

United States

Annually, 1-5 cases of immunoglobulin-related amyloidosis per 100,000 people occur.

The best available direct data on L chain–type amyloidosis prevalence in the United States come from Olmstead County, Minn, where the annual prevalence of L chain–type amyloidosis was calculated to be approximately 1 case per 100,000 people. The population in this location is primarily of northern European ancestry. Whether this prevalence applies to different populations is not known.

Based on indirect calculations, the prevalence may be higher. The annual incidence of multiple myeloma is approximately 5 cases per 100,000 people, and the prevalence of L chain–type amyloidosis in patients with myeloma is approximately 20-35%, producing an overall incidence of combined L chain–type amyloidosis and myeloma of 1-1.5 cases per 100,000 people. Only 1 in 5 patients with L chain–type amyloidosis has frank myeloma; therefore, the total number of patients with L chain–type amyloidosis type is 5 times the number of patients with L chain–type amyloidosis and myeloma, or at least 5 cases per 100,000 people.

International

The prevalence of L chain–type amyloidosis appears to be the same in all populations. The only population-based direct measurement comes from the United States.

Mortality/Morbidity

Symptoms of immunoglobulin-related amyloidosis reflect the organs containing amyloid deposits. Factors that cause deposits in different organs in different patients are unknown. Cardiac deposition is the most severe consequence of systemic L chain–type amyloidosis, eventually occurring in most patients. Cardiac L chain–type amyloidosis is the cause of death in most patients with systemic L chain–type amyloidosis.

Race

L chain–type amyloidosis affects people of all racial and ethnic groups. No data are available comparing the incidence of disease in different groups.

Sex

The male-to-female incidence ratio of L chain–type amyloidosis is 2:1.

Age

The median age at diagnosis of immunoglobulin-related amyloidosis in the largest published series (from the Mayo Clinic) was 64 years.^[2]