Familial Transthyretin Amyloidosis

[Synonym: Familial TTR Amyloidosis. Includes: Familial Amyloid Cardiomyopathy, Familial Amyloid Polyneuropathy Type I (Portuguese-Swedish-Japanese Type), Familial Amyloid Polyneuropathy Type II (Indiana/Swiss or Maryland/German Type), Leptomeningeal Amyloidosis, Familial Oculoleptomeningeal Amyloidosis (FOLMA)]

Clinical Diagnosis

The diagnosis of familial transthyretin (TTR) amyloidosis is suspected in adults with the following:

• Slowly progressive sensorimotor and/or autonomic neuropathy that is frequently accompanied by:

  • Cardiac conduction blocks

  • Cardiomyopathy

  • Nephropathy
    
    and/or

  • Vitreous opacities

• Family history consistent with autosomal dominant inheritance supports the diagnosis; however, absence of other affected individuals in the family does not preclude the diagnosis of familial TTR amyloidosis especially in persons over age 50 years.

Testing

Tissue biopsy. To confirm amyloidosis, including familial TTR amyloidosis, the demonstration of amyloid deposition on biopsied tissues is essential. Deposition of amyloid in tissue can be demonstrated by Congo red staining of biopsy materials. With Congo red staining, amyloid deposits show a characteristic yellow-green birefringence under polarized light. Tissues suitable for biopsy include: subcutaneous fatty tissue of the abdominal wall, skin, gastric, or rectal mucosa, sural nerve, and peritendinous fat from specimens obtained at carpal tunnel surgery. Sensitivity of endoscopic biopsy of gastrointestinal mucosa is around 85%; biopsy of the sural nerve is less sensitive because amyloid deposition is often patchy [Hund et al 2001, Koike et al 2004, Vital et al 2004].

It is ideal to show that these amyloid deposits are specifically immunolabeled by anti-TTR antibodies.

Serum variant TTR protein. TTR protein normally circulates in serum or plasma as a soluble protein having a tetrameric structure [Kelly 1998, Rochet & Lansbury 2000]. Normal plasma TTR concentration is 20-40 mg/dL (0.20-0.40 mg/mL).

Pathogenic mutations in the TTR gene cause conformational change in the TTR protein molecule, disrupting the stability of the TTR tetramer, which is then more easily dissociated into pro-amyloidogenic monomers [Sekijima et al 2005]. Small amounts of TTR monomer (0.28-0.56 µg/mL) can be detected in the plasma of individuals with familial TTR amyloidosis and normal controls [Sekijima et al 2001].

After immunoprecipitation with anti-TTR antibody, serum variant TTR protein can be detected by mass spectrometry [Tachibana et al 1999]. Approximately 90% of TTR variants so far identified are confirmed by this method. Mass shift associated with each variant TTR protein is indicated [Connors et al 2003].

Molecular Genetic Testing

GeneReviews designates a molecular genetic test as clinically available only if the test is listed in the GeneTests Laboratory Directory by either a US CLIA-licensed laboratory or a non-US clinical laboratory. GeneTests does not verify laboratory-submitted information or warrant any aspect of a laboratory's licensure or performance. Clinicians must communicate directly with the laboratories to verify information.—ED.

Gene. TTR is the only gene known to be associated with familial TTR amyloidosis.

Clinical testing

• Targeted mutation analysis. The most frequent pathogenic mutation, Val30Met, has been identified in many individuals of different ethnic backgrounds.

• Sequence analysis of select exons and/or sequence analysis of all exons. Sequence analysis of the entire TTR gene can be performed efficiently because it consists of only four exons; and all the hitherto-identified mutations are present in exons 2, 3, or 4. Direct sequencing detects more than 99% of disease-causing (amyloidogenic) mutations.

• Deletion/duplication analysis. With the exception of del122Ile, no deletions or duplications involving TTR as causative of transthyretin amyloidosis have been reported. However, with newly available deletion/duplication testing methods, it is theoretically possible that such mutations may be identified in affected individuals in whom prior testing by sequence analysis of the entire coding region was negative.

Table 1 summarizes molecular genetic testing for this disorder.

Table 1. Molecular Genetic Testing Used in Familial Transthyretin Amyloidosis

Gene Symbol	Test Method	Mutations Detected	Mutation Detection Frequency by Test Method	Test Availability
TTR	Targeted mutation analysis	Val30Met	Unknown 1	Clinical
	Sequence analysis	Sequence variants	>99%
	Deletion/duplication analysis 2	Exonic or whole gene deletions	Unknown

1. Found in large clusters in Portugal, Sweden, and Japan

2. Testing that identifies deletions/duplications not readily detectable by sequence analysis of genomic DNA; a variety of methods including quantitative PCR, long-range PCR, multiplex ligation-dependent probe amplification (MLPA), or targeted array GH (gene/segment-specific) may be used. A full array GH analysis that detects deletions/duplications across the genome may also include this gene/segment.

Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.

Testing Strategy

Establishing the diagnosis in a proband. Diagnosis relies upon the following:

• Tissue biopsy to document the presence of amyloid using Congo red staining and immunohistohemical study with anti-TTR antibodies

• Molecular genetic testing of TTR

Note: Mass spectrometry to detect serum TTR protein variants is also useful for screening.

Predictive testing for at-risk asymptomatic adult family members requires prior identification of the disease-causing mutation in the family.

Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mutation in the family.

Genetically Related (Allelic) Disorders

Senile systemic amyloidosis (SSA; previously called senile cardiac amyloidosis) results from the pathologic deposition of wild-type TTR, predominantly in the heart. Pathologic deposits are also seen in the lungs, blood vessels, and the renal medulla of kidneys [Westermark et al 2003]. SSA affects mainly the elderly but is rarely diagnosed during life. Thus, the precise prevalence of SSA is still unknown, but the examination of autopsy samples revealed a prevalence of 22%-25% in the elderly (age >80 years).

SSA is typically asymptomatic or manifest by cardiac symptoms. Some individuals with SSA present with carpal tunnel syndrome. SSA should be distinguished from familial TTR amyloidosis with variant TTR or other forms of amyloidosis such as primary (AL) amyloidosis. In contrast to the rapid progression of heart failure in AL amyloidosis, SSA results in slowly progressive heart failure [Ng et al 2005]. Westermark et al [2003] have indicated that the lung may be a more reliable tissue for amyloid detection than the heart.

Natural History

Neuropathy. The cardinal feature of TTR-familial amyloid polyneuropathy type I is slowly progressive sensorimotor and autonomic neuropathy [Benson 2001, Hund et al 2001, Ando et al 2005]. Typically, sensory neuropathy starts in the lower extremities and is followed by motor neuropathy within a few years. The initial signs of this sensory neuropathy are paresthesias (sense of burning, shooting pain) and hypesthesias of the feet. Temperature and pain sensation are impaired earlier than vibration and position sensation. By the time sensory neuropathy progresses to the level of the knees, the hands have usually become affected. In the full-blown stage of the disease, sensory loss, muscle atrophy, and weakness of the extremities show a glove and stocking distribution. Foot drop, wrist drop, and disability of the hands and fingers are common symptoms of motor neuropathy.

The disease usually begins in the third, fourth or fifth decade in persons from Portugal and Japan, countries with large endemic foci; onset is later in persons from other areas. The following shows that the age at onset varies greatly even within ethnically identical populations with the same TTR mutation:

• For persons of Japanese ancestry with the Val30Met mutation who are related to two large endemic foci (Ogawa village and Arao city), the mean age at onset is 40.1±12.8 years (range 22-74 years) [Nakazato 1998].

• For persons of Japanese ancestry with Val30Met who are unrelated to two large endemic foci show, the mean age at onset is much later (62.7±6.6 years) (range 52-80 years) [Misu et al 1999, Ikeda et al 2002].

• For persons of Portuguese ancestry with the Val30Met mutation, the mean age at onset is 33.5 ±9.4 years (range 17-78 years).

• For persons of Swedish, French, or British ancestry, the mean age at onset is much later than that in individuals of Japanese or Portuguese ancestry [Planté-Bordeneuve et al 1998].

Sensorimotor and autonomic neuropathy progress over ten to 20 years. Various types of cardiac conduction block frequently appear. Cachexia is a common feature at the late stage of the disease. Affected individuals usually die of cardiac failure, renal failure, or infection.

Non-neuropathic amyloidosis. Individuals with familial TTR amyloidosis do not necessarily present with polyneuropathy. Cardiac amyloidosis and leptomeningeal amyloidosis are well-known non-neuropathic forms of familial TTR amyloidosis that are associated with specific TTR mutations. In these types of familial TTR amyloidosis, polyneuropathy is absent or, if present, less evident.

Approximately one-third of the TTR protein variants are accompanied by vitreous opacities.

Cardiac amyloidosis is usually late onset. Most individuals develop cardiac symptoms after age 50 years; cardiac amyloidosis generally presents with restrictive cardiomyopathy. The typical electrocardiogram shows a pseudoinfarction pattern with prominent Q wave in leads II, III, _aV_F, and V₁-V₃, presumably resulting from dense amyloid deposition in the anterobasal or anteroseptal wall of the left ventricle. The echocardiogram reveals left ventricular hypertrophy with preserved systolic function. The thickened walls present "a granular sparkling appearance."

Among the mutations responsible for cardiac amyloidosis, Val122Ile is notable for its prevalence in African Americans. Approximately 3.0%-3.9% of African Americans are heterozygous for Val122Ile [Yamashita et al 2005]. The high frequency of Val122Ile partly explains the observation that, in individuals in the US older than age 60 years, cardiac amyloidosis is four times more common among blacks than whites [Jacobson et al 1997].

Individuals with leptomeningeal amyloidosis show CNS signs and symptoms including: dementia, psychosis, visual impairment, headache, seizures, motor paresis, ataxia, myelopathy, hydrocephalus, or intracranial hemorrhage.

When associated with vitreous amyloid deposits, leptomeningeal amyloidosis is known as familial oculoleptomeningeal amyloidosis (FOLMA) [Petersen et al 1997, Jin et al 2004].

In leptomeningeal amyloidosis protein concentration in the cerebrospinal fluid is usually high, and gadolinium-enhanced MRI typically shows extensive enhancement of the surface of the brain, ventricles, and spinal cord [Brett et al 1999].

Although meningeal biopsy is necessary to confirm amyloid deposition in the meninges, characteristic MRI findings and TTR gene mutations strongly suggest this pathology [Mitsuhashi et al 2004].

Other. Vitreous opacification has been reported in approximately 20% of families with various TTR mutations, including Val30Met [Benson 2001, Connors et al 2003, Kawaji et al 2004, Benson & Kincaid 2007]. Four out of 43 individuals with the Val30Met mutation developed vitreous amyloidosis as the first manifestation of familial TTR amyloidosis [Kawaji et al 2004]. In one case report, vitreous opacification was the only evidence of amyloid deposit caused by the Trp41Leu mutation [Yazaki et al 2002].

The kidney is consistently involved with marked deposition of amyloid demonstrated at postmortem examination. Mild to severe renal involvement is usually seen in the advanced stage [Haagsma et al 2004, Lobato et al 2004].

Amyloid deposition on the gastrointestinal tract wall, especially with involvement of the gastrointestinal autonomic nerves, is common [Ikeda et al 1982, Ikeda et al 1983].

Nodular cutaneous amyloidosis has been reported in an individual with the Thr114His mutation [Mochizuki et al 2001].

Shortness of breath induced by diffuse pulmonary amyloid deposition has been reported in two individuals with the Asp38Ala mutation [Yazak et al 2000].

Anemia with low erythropoietin has been reported in 25% of cases [Beirao et al 2004].

Genotype-Phenotype Correlations

Heterozygotes. With expanding lists of mutations in the TTR gene (see Table 7), genotype-phenotype correlations have been intensively investigated; however, they remain largely unknown.

In subsets of families with the Val30Met mutation, considerable variation in phenotypic manifestations and age of onset is observed. It is hypothesized that genetic modifiers and non-genetic factors contribute to the pathogenesis and progression of familial TTR amyloidosis [Holmgren et al 1997, Misu et al 1999, Munar-Qués et al 1999, Sobue et al 2003, Soares et al 2005].

It has been clinically and experimentally demonstrated that Thr119Met has a protective effect on amyloidogenesis in individuals who have the Val30Met mutation [Alves et al 1997, Hammarström et al 2001, Sebastião et al 2001].

Most of the more than 90 mutations in the TTR gene result in classic peripheral and autonomic neuropathy; but some mutations are considered to be associated with unique phenotypes of familial TTR amyloidosis, in which peripheral or autonomic neuropathy is clinically absent or less prominent:

• Cardiac amyloidosis is caused by Asp18Asn, Val20Ile, Pro24Ser, Ala45Thr, Ala45Ser, His56Arg, Gly57Arg, Ile68Leu, Ala81Thr, Ala81Val, His88Arg, Glu92Lys, Arg103Ser, Leu111Met, or Val122Ile (see Table 5) [Nakazato 1998, Benson 2001, Saraiva 2001, Connors et al 2003, Benson & Kincaid 2007]. Peripheral and autonomic neuropathy are absent or less evident in persons with these mutations.

• Leptomeningeal amyloidosis is associated with Leu12Pro, Asp18Gly, Ala25Thr, Val30Gly, Ala36Pro, Gly53Glu, Gly53Ala, Phe64Ser, Tyr69His, or Tyr114Cys (see Table 5) [Petersen et al 1997, Nakazato 1998, Brett et al 1999, Mascalchi et al 1999, Uemichi et al 1999, Connors et al 2000, Benson 2001, Ellie et al 2001, Saraiva 2001, Ikeda et al 2002, Blevins et al 2003, Connors et al 2003, Hammarström et al 2003, Sekijima et al 2003].
  It has been demonstrated that highly destabilized TTR variants induce leptomeningeal amyloidosis [Hammarström et al 2001, Sekijima et al 2003, Sekijima et al 2005].

Homozygotes. The vast majority of individuals with familial TTR amyloidosis are heterozygous for a TTR mutation; however, homozygotes have been reported:

• At least 19 homozygous for Val30Met from 14 families [Munar-Qués et al 2001, Tojo et al 2008]

• Seven homozygous for non-Val30Met (five Val122Ile, one Leu58His, and one Phe64Leu) (see Table 5) [Jacobson et al 1990, Nichols et al 1991, Ferlini et al 1996, Askanas et al 2000, Jacob et al 2007]

Homozygotes present with a slightly more severe clinical course (higher incidence rate and earlier onset) than heterozygotes within the same family [Tojo et al 2008]; amyloid deposition is more widespread in homozygotes than in heterozygotes [Yoshinaga et al 2004]. Most homozygotes are members of families characterized by incomplete penetrance of familial TTR amyloidosis.

Penetrance

Because the penetrance for familial TTR amyloidosis is not 100%, an individual with a TTR mutation may be symptom free until late adulthood. The penetrance may vary by mutation, geographical region, or ethnic group.

It is generally accepted that the penetrance is much higher in individuals in endemic foci than outside of endemic foci [Misu et al 1999]. In Portugal, 87% of individuals with the Val30Met mutation develop symptoms of familial TTR amyloidosis before age 40 years. However, in Sweden, disease penetrance is only 2%; and even some Val30Met homozygotes remain asymptomatic.

Anticipation

Genetic anticipation is observed in families with TTR amyloid polyneuropathy from endemic areas [Yamamoto et al 1998, Soares et al 1999, Misu et al 2000]. In Japanese families with the Val30Met mutation, which originated from one of two endemic foci, it was reported that affected children with maternal transmission showed more profound anticipation than those with paternal transmission, especially when the children were male [Yamamoto et al 1998].

Nomenclature

The neuropathy associated with TTR mutations, now called familial TTR amyloidosis, was formerly referred to as one of the following:

• Familial amyloid polyneuropathy type I (or the Portuguese-Swedish-Japanese type)

• Familial amyloid polyneuropathy type II (or the Indiana/Swiss or Maryland/German type)

Prevalence

The Val30Met mutation, found worldwide, is the most widely studied TTR variant and is responsible for the well-known large foci of individuals with TTR amyloid polyneuropathy in Portugal, Sweden, and Japan. Numerous families with various non-Val30Met mutations have also been identified worldwide (Table 7).

The frequency of familial TTR amyloidosis caused by the mutation Val30Met is estimated to be one in 538 in northern Portugal (Povoa do Varzim and Vila do Conde), the largest cluster worldwide of individuals with familial TTR amyloidosis.

In Caucasians in the US, the frequency of Val30Met familial TTR amyloidosis is estimated to be one in 100,000 [Benson 2001].

The frequency of Val122Ile in the African American population is 3.0%-3.9%; most heterozygous individuals develop late-onset cardiac amyloidosis. Over 5.0% of the population in some areas of West Africa is heterozygous for this mutation. In the US, the frequency of Val122Ile in the white and Hispanic populations is 0.44% and 0.0%, respectively [Jacobson et al 1997, Yamashita et al 2005].

Evaluations Following Initial Diagnosis

To establish the extent of disease in an individual diagnosed with familial transthyretin (TTR) amyloidosis, the following evaluations are recommended:

• Complete neurologic assessment including baseline nerve conduction studies

• Evaluation of the heart:

  • Echocardiogram, the most useful noninvasive test for cardiac amyloidosis, for visualization of ventricular wall thickness, ventricular septal thickness, and hyperrefractile myocardial echoes (so called "granular sparkling appearance")

  • Electrocardiogram (ECG) to show characteristic findings of cardiac amyloidosis including low voltage in the standard limb leads and QS pattern in the right precordial leads with or without conduction blocks

  • Myocardial technetium-99m-pyrophosphate scintigraphy to visualize amyloid deposition in heart [Ikeda 2004]

• Gadolinium-enhanced MRI of the brain and spinal cord to evaluate CNS amyloidosis [Mitsuhashi 2004]

• Ophthalmologic evaluation

• Evaluation of renal function

Treatment of Manifestations

The following are appropriate:

• Carpal tunnel release surgery for carpal tunnel syndrome

• Vitrectomy for vitreous involvement

Prevention of Primary Manifestations

Orthotopic liver transplantation (OLTX). The only effective therapy for the neuropathy of familial TTR amyloidosis is orthotopic liver transplantation (OLTX), which removes the main production site of the amyloidogenic protein. Successful OLTX results in rapid disappearance of variant TTR protein from the serum and thus halts the progression of peripheral and/or autonomic neuropathy. It has been shown by pre- and postoperative sural nerve biopsy that myelinated nerve fibers regenerate after OLTX [Ikeda et al 1997].

Recommended clinical criteria for OLTX in individuals with TTR amyloid polyneuropathy [Takei et al 1999, Adams et al 2000] include the following:

• Age younger than 60 years

• Disease duration less than five years

• Either polyneuropathy that is restricted to the lower extremities or autonomic neuropathy alone

• No significant cardiac or renal dysfunction

As of the end of June 2008, 1336 individuals with familial TTR amyloidosis, approximately 90% of whom were heterozygous for the Val30Met mutation, had undergone liver transplantation (www.fapwtr.org/ram_fap.htm) [Ericzon et al 2000, Ikeda et al 2003, Herlenius et al 2004, Stangou & Hawkins 2004]. The five-year survival rate was significantly higher in individuals with the Val30Met mutation than in those with other mutations (80% vs 57%, p=0.001) [Ericzon et al 2000, Ikeda et al 2003]. The most common causes of postoperative death were cardiovascular events (29%) and septicemia (26%) [Ikeda et al 2003].

Poor outcomes of transplanted individuals based on ten years' experience [Ikeda et al 2003] include:

• Poor nutritional condition (mean body mass index <600)

• Severe polyneuropathy (Norris score <55/81)

• Permanent urinary incontinence

• Marked postural hypotension

• A fixed pulse rate

OLTX is not effective in the non-neuropathic forms of familial TTR amyloidosis (i.e., cardiac amyloidosis, leptomeningeal amyloidosis, and familial oculoleptomeningeal amyloidosis [FOLMA]).

Individuals with leptomeningeal involvement may not be candidates for liver transplantation because amyloidogenic TTR variants that cause intracranial amyloid deposits are considered to derive from the choroid plexus.

Vitreous opacities may also progress after OLTX, possibly as the result of de novo production of variant TTR in the retinal epithelium.

Note: Because liver involvement in familial TTR amyloidosis is minimal, the liver of an individual with familial TTR amyloidosis can be grafted into an individual with liver cancer or end-stage liver disease (so-called "domino" liver transplantation). Since 1995, more than 330 domino liver transplantations have been performed. Stangou et al [2005] recently reported that an individual who had received a liver graft from a Val30Met heterozygote with familial TTR amyloidosis developed systemic amyloidosis eight years after the domino liver transplantation. Also, subclinical cutaneous TTR deposits were reported in five other recipients of domino liver grafts [Sousa et al 2004].

Prevention of Secondary Complications

Cardiac pacing for persons with familial TTR amyloidosis with conduction block helps prevent sudden death.

Surveillance

Serial nerve conduction studies can be used to objectively monitor the course of the polyneuropathy.

Serial electrocardiogram and echocardiography can be used to monitor the course of cardiomyopathy and conduction block.

Testing of Relatives at Risk

It is appropriate to offer molecular genetic testing to at-risk relatives if the disease-causing mutation is identified in an affected family member so that morbidity and mortality can be reduced by early diagnosis and treatment.

If the disease-causing mutation in the family is not known, it is appropriate to offer clinical diagnostic evaluations to identify those family members who will benefit from early treatment.

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

Strategies of potential molecular therapies for familial TTR amyloidosis include the following [Saraiva 2002, Ando 2003, Sekijima et al 2008]:

• Inhibition of synthesis of variant TTR

• Stabilization of variant TTR

• Inhibition of aggregation of amyloidogenic intermediates

• Disruption of insoluble amyloid fibrils

Drugs that stabilize TTR tetramer and prevent dissociation into monomers and drugs that disrupt TTR amyloid fibrils into amorphous materials have been designed [Saraiva 2002, Miller et al 2004, Almeida et al 2005].

Phase I and II clinical trials of diflunisal showed that diflunisal increased serum TTR stability in persons with familial TTR amyloidosis beyond the level of normal controls without adverse effects. A randomized double-blind placebo-controlled multicenter/multinational clinical trial is currently underway to determine whether diflunisal will alter the progression of familial TTR amyloidosis [Sekijima et al 2006, Tojo et al 2006].

Recently, Fx-1006A, a small molecule that binds TTR tetramer selectively and potently, has been developed as a TTR kinetic stabilizer to ameliorate familial TTR amyloidosis. In a dose escalation Phase I study in healthy volunteers, Fx-1006A was found to be safe and well tolerated. In addition, Fx-1006A showed strong TTR stabilization effects in plasma of study participants. A Phase II/III study of Fx-1006A is currently fully enrolled; results are anticipated in the summer of 2009. Fx-1006A has orphan drug designation in both the US and European Union and Fast Track designation in the US for the treatment of familial TTR amyloidosis.

Inhibition of variant TTR mRNA expression by small interfering RNAs (siRNA) is also under investigation.

Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions.

Other

Plasma exchange, affinity column binding with a monoclonal antibody, and use of a special column with affinity for TTR were considered as possible methods for elimination of amyloidogenic TTR from the blood circulation. Serum TTR levels decreased significantly immediately after treatment, but then returned to the same levels as before treatment because of the rapid turnover of TTR. Therefore, these methods were concluded not to be effective for familial TTR amyloidosis.

4’-iodo-4’-deoxydoxorubicin (IDOX) has been reported to bind to several types of amyloid and lead to the catabolism of amyloid in deposits. In a multicenter clinical trial, IDOX was administered to persons with AL amyloidosis; however, no obvious benefit could be detected.

Genetics clinics, staffed by genetics professionals, provide information for individuals and families regarding the natural history, treatment, mode of inheritance, and genetic risks to other family members as well as information about available consumer-oriented resources. See the GeneTests Clinic Directory.

See Consumer Resources for disease-specific and/or umbrella support organizations for this disorder. These organizations have been established for individuals and families to provide information, support, and contact with other affected individuals.

Mode of Inheritance

Familial transthyretin (TTR) amyloidosis is inherited in an autosomal dominant manner.

Risk to Family Members

Parents of a proband

• Some individuals diagnosed with familial TTR amyloidosis have an affected parent.

• A proband with familial TTR amyloidosis may have the disorder as the result of a new gene mutation. The proportion of cases caused by de novo mutations is unknown.

• Recommendations for the evaluation of parents of a proband with an apparent de novo mutation include molecular genetic testing if the disease-causing TTR gene mutation has been identified in the proband.
  
  Note: The family history may appear to be negative because of failure to recognize the disorder in family members, early death of the parent before the onset of symptoms, or late onset of the disease in the affected parent.

Sibs of a proband

• The risk to sibs depends on the genetic status of the parents.

• Sibs of an affected individual have a 50% chance of inheriting a mutation in the TTR gene if one parent has a mutation.

• At least 26 homozygous persons with familial TTR amyloidosis have been reported. In this circumstance, sibs of the proband have a 50% chance of inheriting one TTR mutation and a 25% chance of inheriting two TTR mutations.

• If the disease-causing mutation found in the proband cannot be detected in the DNA of either parent, the risk to sibs is low but greater than that of the general population because (although no instances have been reported to date) germline mosaicism remains a possibility.

Offspring of a proband

• Every child of an affected individual has a 50% risk of inheriting a mutation in the TTR gene.

• If the proband is homozygous for a TTR mutation, all offspring will inherit the mutation.

Other family members of a proband. The risk to other family members depends on the genetic status of the proband's parents. If a parent is affected or has a disease-causing mutation, his or her family members are at risk.

Related Genetic Counseling Issues

See Management, Testing of Relatives at Risk for information on testing at-risk relatives for the purpose of early diagnosis and treatment.

Considerations in families with an apparent de novo mutation. When neither parent of a proband with an autosomal dominant condition has the disease-causing mutation or clinical evidence of the disorder, it is likely that the proband has a de novo mutation. However, possible non-medical explanations including alternate paternity or maternity (e.g., with assisted reproduction) or undisclosed adoption could also be explored.

Testing of at-risk asymptomatic adults. Testing of at-risk asymptomatic adults for familial TTR amyloidosis is available using the same techniques described in Molecular Genetic Testing. Such testing is not useful in predicting age of onset, severity, type of symptoms, or rate of progression in asymptomatic individuals. When testing at-risk individuals for familial TTR amyloidosis, an affected family member should be tested first to confirm the molecular diagnosis in the family.

Testing for the disease-causing mutation in the absence of definite symptoms of the disease is predictive testing. At-risk symptomatic adult family members may seek testing in order to make personal decisions regarding reproduction, financial matters, and career planning. Others may have different motivations including simply the "need to know." Testing of asymptomatic at-risk adult family members usually involves pretest interviews in which the motives for requesting the test, the individual's knowledge of familial TTR amyloidosis, the possible impact of positive and negative test results, and neurologic status are assessed. Those seeking testing should be counseled regarding possible problems that they may encounter with regard to health, life, and disability insurance coverage, employment and educational discrimination, and changes in social and family interaction. Other issues to consider are implications for the at-risk status of other family members. Informed consent should be procured and records kept confidential. Individuals with a positive test result need arrangements for long-term follow-up and evaluation.

Related liver transplantation donors. In Japan, where liver transplantation from living, related donors is the generally accepted therapy of familial TTR amyloidosis, molecular genetic testing of asymptomatic adult relatives is always performed on family members volunteering to be donors.

Molecular genetic testing of asymptomatic individuals younger than age 18 years who are at risk for adult-onset disorders is not considered appropriate, primarily because it negates the autonomy of the child with no compelling benefit. Further, concern exists regarding the potential unhealthy adverse effects that such information may have on family dynamics, the risk of discrimination and stigmatization in the future, and the anxiety that such information may cause.

Children who are symptomatic usually benefit from having a specific diagnosis established. See also the National Society of Genetic Counselors resolution on genetic testing of children and the American Society of Human Genetics and American College of Medical Genetics points to consider: ethical, legal, and psychosocial implications of genetic testing in children and adolescents (pdf; Genetic Testing).

Family planning

• The optimal time for determination of genetic risk and discussion of the availability of prenatal testing is before pregnancy. Similarly, decisions about testing to determine the genetic status of at-risk asymptomatic family members are best made before pregnancy.

• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected or at risk.

DNA banking. Because it is likely that testing methodology and our understanding of genes, mutations, and diseases will improve in the future, consideration should be given to banking DNA (typically extracted from white blood cells) of affected individuals for possible future use. DNA banking is particularly relevant when molecular genetic testing is available on a research basis only. See for a list of laboratories offering DNA banking.

Prenatal Testing

Prenatal diagnosis for pregnancies at increased risk is possible by analysis of DNA extracted from fetal cells obtained by amniocentesis usually performed at approximately 15 to 18 weeks' gestation or chorionic villus sampling (CVS) at approximately ten to 12 weeks' gestation. The disease-causing allele of an affected family member must be identified before prenatal testing can be performed.

Note: Gestational age is expressed as menstrual weeks calculated either from the first day of the last normal menstrual period or by ultrasound measurements.

Requests for prenatal testing for adult-onset conditions which (like familial TTR amyloidosis) do not affect intellect and have some treatment available are not common. Differences in perspective may exist among medical professionals and in families regarding the use of prenatal testing, particularly if the testing is being considered for the purpose of pregnancy termination rather than early diagnosis. Although most centers would consider decisions about prenatal testing to be the choice of the parents, discussion of these issues is appropriate.

Preimplantation genetic diagnosis (PGD) has been reported [Almeida et al 2005] and may be available for families in which the disease-causing mutation has been identified. For laboratories offering PGD, see .

Molecular Genetic Pathogenesis

The main component of amyloid is protein fibrils. In familial transthyretin (TTR) amyloidosis, the fibrils are mainly composed of self-aggregated TTR protein. TTR protein is potentially amyloidogenic because of its extensive beta-sheet structure. The key factor in amyloidogenesis in familial TTR amyloidosis is the stability of the TTR protein [Kelly 1998, Rochet & Lansbury 2000, Sekijima et al 2005]. The TTR protein normally circulates in plasma as a soluble protein having a tetrameric structure. The amyloidogenic process is understood to comprise two steps: soluble TTR tetramers dissociate into pro-amyloidogenic monomers that, in turn, polymerize into amyloid fibrils in certain tissues [Kelly 1998, Rochet & Lansbury 2000]. Pathogenic mutations in the TTR gene cause significant conformational change in TTR protein molecules, in turn disrupting the stability of the TTR tetramer. In other words, a tetramer containing variant TTR monomers is more easily dissociated into pro-amyloidogenic monomers than is a normal TTR tetramer [Sekijima et al 2005].

It has been demonstrated that all disease-associated TTR variants are energetically (thermodynamically and kinetically) less stable than wild-type TTR. On the other hand, suppressor mutations (Thr119Met and Arg104His) are more stable than wild-type TTR. In vitro amyloidogenicity correlates very well with protein stability. However, extremely destabilized (highly amyloidogenic in vitro) TTR variants do not induce severe systemic amyloidosis because serum concentrations of these TTR variants are very low. The low serum concentration of highly destabilized TTR variants is a result of degradation by endoplasmic reticulum (ER) quality control system (ERAD) of the hepatic cells. The most pathogenic TTR variant (Leu55Pro) exhibiting the earliest disease onset is the most destabilized variant that can be secreted at levels comparable to the wild-type, barely avoiding ERAD. TTR variants that predominantly induce CNS amyloidosis are the least stable variants. The choroid plexus secretes highly destabilized TTR variants more efficiently than hepatic cells, thus, it is thought, accounting for CNS selective amyloid deposition (leptomeningeal amyloidosis) [Hammarström et al 2003, Sekijima et al 2003, Mitsuhashi et al 2005, Sekijima et al 2005].

Pathologic allelic variants. To date, more than 90 point mutations and one in-frame microdeletion have been identified in exons 2-4 in the TTR gene in individuals with familial TTR amyloidosis [Connors et al 2000, Benson 2001, Saraiva 2001, Connors et al 2003, Benson & Kincaid 2007]. No mutation has been described in exon 1, which encodes amino acids 1 through 3.

The Val50Met mutation, found worldwide, is the most widely studied TTR variant and is responsible for the well-known large foci of individuals with TTR amyloid polyneuropathy in Portugal, Sweden, and Japan. Several haplotypes are associated with Val50Met in different ethnic groups, suggesting that multiple founders spontaneously occurred in each group.

Val122Ile, present in 3.0%-3.9% of African Americans and more than 5.0% of the population in some areas of West Africa, is the most common amyloid-associated TTR variant worldwide [Jacobson et al 1997, Yamashita et al 2005].

Normal gene product. The human TTR cDNA encodes a 20-amino acid signal peptide plus a 127-amino acid mature protein with molecular mass 14 kd. TTR is a normal plasma protein synthesized predominantly by the liver. TTR is secreted into plasma as a tetrameric form (Mw = 55 kd) composed of four identical monomers; its plasma half-life is approximately one to two days. TTR concentration in plasma normally ranges from 20 to 40 mg/dL.

TTR is considered to transport thyroxine and retinol-binding protein (RBP) coupled to vitamin A. TTR binds virtually all of serum RBP and approximately 15% of serum thyroxine. In the cerebrospinal fluid, TTR is required for transport of serum thyroxine across the blood-brain barrier.

The choroid plexus is the source of the cerebrospinal fluid TTR. The TTR concentration in cerebrospinal fluid ranges from 10 µg/mL to 40 µg/mL.

The other site of synthesis is the retina.

Abnormal gene product. It is speculated that amyloidogenic TTR variants reduce the stability of the physiologic TTR tetramer, and consequently produce a pro-amyloidogenic monomer more easily than normal TTR (see Molecular Genetic Pathogenesis) [Kelly 1998, Rochet & Lansbury 2000, Saraiva 2002, Sekijima et al 2005].

Literature Cited

Published Statements and Policies Regarding Genetic Testing

Suggested Reading

Revision History

• 15 September 2009 (me) Comprehensive update posted live

• 15 March 2006 (me) Comprehensive update posted to live Web site

• 2 March 2005 (cd) Revision: mutation scanning and sequencing of select exons no longer clinically available

• 5 March 2004 (ky) Revision: molecular genetic testing

• 9 January 2004 (me) Comprehensive update posted to live Web site

• 5 November 2001 (me) Review posted to live Web site

• 25 June 2001 (ky) Original submission