Scientists have discovered a recessive mutation in β amyloid precursor protein that causes early onset Alzheimer disease in two siblings from a small south Italian village. Paradoxically, the mutant Aβ peptide, when mixed with wild-type, prevents aggregation. That may protect heterozygotes from developing AD, said senior author Fabrizio Tagliavini of the Carlo Besta Neurological Institute in Milan. Tagliavini, first author Giuseppe di Fede, also at Carlo Besta, and colleagues published their results in today’s Science. The work may indicate a new direction for AD pharmacotherapy using small peptides to keep Aβ from clumping.

Tagliavini and colleagues were intrigued by the case of a 36-year-old man with cognitive decline who visited their clinic. They collected DNA samples and were surprised to find that their patient was homozygous for an alanine-to-valine substitution at position 673 of APP. This corresponds to position 2 of the Aβ peptide. The man’s condition has worsened over the eight years since his symptoms began and now he cannot care for himself. His younger sister, who now suffers from mild cognitive impairment as well, also had two mutant alleles. Family members with only one mutant allele provided additional test subjects. “These heterozygotes were healthy,” Tagliavini said. “Even the oldest one, an 88-year-old lady—she is perfect.”

The A673V mutation stands in contrast to the many other known APP mutations, which are inherited in a dominant fashion. Many of those mutations flank the cleavage sites and thus still yield a wild-type Aβ peptide, Tagliavini noted, and others reside in the core of the peptide. There are two known neighboring amino-terminal mutations, at the sixth and seventh positions of Aβ (Janssen et al., 2003; Wakutani et al., 2004), which are dominant. It is not known how common the A673V mutant allele is, Tagliavini said, although he suspects it is rare. The scientists did not find the mutation in 200 healthy people and 100 people with sporadic AD.

The man had decreased Aβ1-42 and increased tau levels in his cerebrospinal fluid, similar to other people with AD. Both he and his sister showed high levels of Aβ in blood plasma, while their heterozygous family members had intermediate levels, in comparison with healthy control subjects. Fibroblasts cultured from the initial patient also released high levels of Aβ1-40 and Aβ1-42, compared to control cells. Transfected Chinese hamster ovary and COS-7 (green monkey kidney fibroblast) cells also secreted more Aβ when expressing the mutant APP, with cleavage products including Aβ1-40 and Aβ1-42 as well as Aβ11-40, Aβ11-42, and pyroglutamate AβN3pE-42. The ratio of Aβ1-40 to Aβ1-42 was unchanged. These results suggested to the scientists that the mutation alters APP processing, boosting cleavage to form Aβ.

Di Fede and colleagues then turned to biochemistry to assay the mutation’s effects on aggregation. Using synthetic peptides and laser light scattering to measure clumping, they found that the mutant peptide aggregated faster than wild-type. “The effect of the mutation was dramatic,” Tagliavini said, but left him and his colleagues puzzled. “It was impossible to understand why the heterozygous carrier did not develop disease.”

To answer their questions, the scientists mixed mutant and wild-type peptides to simulate heterozygous conditions. In these experiments, the mixtures aggregated more slowly than either individual peptide. The hybrid clumps were also unstable; they dissolved within eight minutes when diluted in buffer, which neither single peptide will do. In cultured neuroblastoma cells, the mixture of mutant and wild-type Aβ1-42 was less toxic than either alone. Approximately 85 percent of cells survived 24 hours with the peptide cocktail, compared to 80 percent for wild-type and 70 percent for mutant. In heterozygotes, then, the single mutant allele may actually protect against Alzheimer disease by preventing stable Aβ aggregates from forming, Tagliavini said.

The results underscore the importance of the amino-terminus of Aβ. Sometimes thought to be a floppy, unimportant tail (Williams et al., 2004), these and other data suggest the region is involved in aggregation. Amino-terminal mutations promote Aβ fibril elongation (Hori et al., 2007) and antibodies to the amino terminus bind and clear plaques (see ARF related news story and Bard et al., 2003). To isolate the amino-terminal effect, the authors of the current study experimented with an Aβ1-6 fragment, with and without the A673V mutation. The mutant hexapeptide bound more tightly to wild-type Aβ than the normal hexapeptide did.

The results point to potential therapies for AD, Tagliavini said. Although a hexapeptide would probably be degraded too quickly to have a protective effect, the scientists are working on ways to modify the peptide so that it lasts long enough to interfere with Aβ aggregation. The scientists have also engineered a transgenic mouse carrying the new mutation, although they have yet to discover a disease phenotype. In addition, Tagliavini hopes to discern the protein structure of the mutant Aβ in order to discover what makes it different from wild-type peptide.—Amber Dance

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Comments on News and Primary Papers

  1. There may be a reversible formation of dimers in the APP molecule, before being cleaved by γ-secretase and the subsequent formation of Aβ dimers, thus preserving the molecule for cleavage by α-secretase.

    Data provided by this extraordinary article based on a mutation of the amino-terminal region of Aβ in its precursor molecule APP (changes in its primary sequence trigger peptide assembly and fibril formation) point to this possibility, since the relationship between the carboxy-terminal fragments generated by β- and α-secretase (1.9 ± 0.2 increase in C99: C83 ratio in patient's fibroblasts) implies double the rate of β product versus α in the mutant APP.

    View all comments by Miguel Rodríguez-Manotas
  2. The identification of two families with homozygous APP variants (Tomiyama et al., 2008 and now Di Fede et al., 2009) is interesting, but the linkage analysis in neither pedigree is sufficient for us to be sure that the mutations are pathogenic, let alone sufficient for us to tell whether they act in a co-dominant or recessive fashion. The homozygosity in both families is almost certainly caused by unrecognized consanguinity and this is common even in ostensibly outbred populations (Nalls et al., 2009). Thus, in both cases it is still possible that the variants are simply harmless polymorphisms or that they additively increase risk of disease. In both families, it might be informative to carry out whole genome genotyping to see which other loci are homozygous. Interpretation of these variants as recessive is premature.

    References:

    . A new amyloid beta variant favoring oligomerization in Alzheimer's-type dementia. Ann Neurol. 2008 Mar;63(3):377-87. PubMed.

    . A recessive mutation in the APP gene with dominant-negative effect on amyloidogenesis. Science. 2009 Mar 13;323(5920):1473-7. PubMed.

    . Measures of autozygosity in decline: globalization, urbanization, and its implications for medical genetics. PLoS Genet. 2009 Mar;5(3):e1000415. PubMed.

  3. I recommend the primary paper.

References

News Citations

  1. Plaque Clearance, Antibody Isotype Are Key for Passive Aβ Immunization

Paper Citations

  1. . Early onset familial Alzheimer's disease: Mutation frequency in 31 families. Neurology. 2003 Jan 28;60(2):235-9. PubMed.
  2. . Novel amyloid precursor protein gene missense mutation (D678N) in probable familial Alzheimer's disease. J Neurol Neurosurg Psychiatry. 2004 Jul;75(7):1039-42. PubMed.
  3. . Mapping abeta amyloid fibril secondary structure using scanning proline mutagenesis. J Mol Biol. 2004 Jan 16;335(3):833-42. PubMed.
  4. . The Tottori (D7N) and English (H6R) familial Alzheimer disease mutations accelerate Abeta fibril formation without increasing protofibril formation. J Biol Chem. 2007 Feb 16;282(7):4916-23. PubMed.
  5. . Epitope and isotype specificities of antibodies to beta -amyloid peptide for protection against Alzheimer's disease-like neuropathology. Proc Natl Acad Sci U S A. 2003 Feb 18;100(4):2023-8. PubMed.

Further Reading

Papers

  1. . The E693Delta mutation in amyloid precursor protein increases intracellular accumulation of amyloid beta oligomers and causes endoplasmic reticulum stress-induced apoptosis in cultured cells. Am J Pathol. 2009 Mar;174(3):957-69. PubMed.
  2. . Aggregation and catabolism of disease-associated intra-Abeta mutations: reduced proteolysis of AbetaA21G by neprilysin. Neurobiol Dis. 2008 Sep;31(3):442-50. PubMed.
  3. . Swedish amyloid precursor protein mutation increases cell cycle-related proteins in vitro and in vivo. J Neurosci Res. 2008 Aug 15;86(11):2476-87. PubMed.
  4. . Identification of PSEN1 and APP gene mutations in Korean patients with early-onset Alzheimer's disease. J Korean Med Sci. 2008 Apr;23(2):213-7. PubMed.
  5. . Atypical early-onset Alzheimer's disease caused by the Iranian APP mutation. J Neurol Sci. 2008 May 15;268(1-2):124-30. PubMed.

Primary Papers

  1. . A recessive mutation in the APP gene with dominant-negative effect on amyloidogenesis. Science. 2009 Mar 13;323(5920):1473-7. PubMed.