Increased endothelial sclerostin caused by elevated DSCAM mediates multiple trisomy 21 phenotypes.

Trisomy 21 (T21), a recurrent aneuploidy occurring in 1:800 births, predisposes to congenital heart disease (CHD) and multiple extracardiac phenotypes. Despite a definitive genetic etiology, the mechanisms by which T21 perturbs development and homeostasis remain poorly understood. We compared the transcriptome of CHD tissues from 49 patients with T21 and 226 with euploid CHD (eCHD). We resolved cell lineages that misexpressed T21 transcripts by cardiac single-nucleus RNA sequencing and RNA in situ hybridization. Compared with eCHD samples, T21 samples had increased chr21 gene expression; 11-fold-greater levels (P = 1.2 × 10-8) of SOST (chr17), encoding the Wnt inhibitor sclerostin; and 1.4-fold-higher levels (P = 8.7 × 10-8) of the SOST transcriptional activator ZNF467 (chr7). Euploid and T21 cardiac endothelial cells coexpressed SOST and ZNF467; however, T21 endothelial cells expressed 6.9-fold more SOST than euploid endothelial cells (P = 2.7 × 10-27). Wnt pathway genes were downregulated in T21 endothelial cells. Expression of DSCAM, residing within the chr21 CHD critical region, correlated with SOST (P = 1.9 × 10-5) and ZNF467 (P = 2.9 × 10-4). Deletion of DSCAM from T21 endothelial cells derived from human induced pluripotent stem cells diminished sclerostin secretion. As Wnt signaling is critical for atrioventricular canal formation, bone health, and pulmonary vascular homeostasis, we concluded that T21-mediated increased sclerostin levels would inappropriately inhibit Wnt activities and promote Down syndrome phenotypes. These findings imply therapeutic potential for anti-sclerostin antibodies in T21.

Genetics and etiology of congenital heart disease.

Congenital heart disease (CHD) is the most common severe birth anomaly, affecting almost 1% of infants. Most CHD is genetic, but only 40% of patients have an identifiable genetic risk factor for CHD. Chromosomal variation contributes significantly to CHD but is not readily amenable to biological follow-up due to the number of affected genes and lack of evolutionary synteny. The first CHD genes were implicated in extended families with syndromic CHD based on the segregation of risk alleles in affected family members. These have been complemented by more CHD gene discoveries in large-scale cohort studies. However, fewer than half of the 440 estimated human CHD risk genes have been identified, and the molecular mechanisms underlying CHD genetics remains incompletely understood. Therefore, model organisms and cell-based models are essential tools for improving our understanding of cardiac development and CHD genetic risk. Recent advances in genome editing, cell-specific genetic manipulation of model organisms, and differentiation of human induced pluripotent stem cells have recently enabled the characterization of developmental stages. In this chapter, we will summarize the latest studies in CHD genetics and the strengths of various study methodologies. We identify opportunities for future work that will continue to further CHD knowledge and ultimately enable better diagnosis, prognosis, treatment, and prevention of CHD.

A review of non-invasive samples and tools in kala-azar diagnosis and test of cure.

The recurrence of visceral leishmaniasis (VL), also called kala-azar (KA), in endemic regions of tropical countries like India, is primarily attributed to asymptomatic VL, post-kala azar dermal leishmaniasis (PKDL), and human immunodeficiency virus (HIV) co-infection. To effectively manage VL cases and elimination targets, an early and rapid diagnosis as well as accurate field surveillance is highly essential. The traditional sampling methods like bone marrow (BM), spleen, and lymph node (LN) tissue aspirations are invasive, painful, tedious, and prone to nosocomial infections, require skilled persons and hospital facilities, and are not feasible in rural areas. Therefore, there is an urgent requirement for the adoption of a patient-friendly, non-invasive, non-hospitalized sampling procedure that ensures an effective VL diagnosis. This review aims to meticulously evaluate the most recent scientific research that focuses on the precision, feasibility, and applicability of non-invasive sampling (NIS) and techniques for the diagnosis and test of cure of VL, particularly in resource-limited settings. Apart from that, the non-invasive techniques (NIT) that have shown promising results while monitoring VL treatment response and relapse are also reviewed. The limitations associated with NIT and possible improvements in this regard are discussed as well to improve the diagnosis and management of VL.

Harnessing diversity to study Alzheimer's disease: A new iPSC resource from the NIH CARD and ADNI.

The iDA Project (iPSCs to Study Diversity in Alzheimer's and Alzheimer's Disease-related Dementias) is generating 200 induced pluripotent stem cell lines from Alzheimer's Disease Neuroimaging Initiative participants. These lines are sex balanced, include common APOE genotypes, span disease stages, and are ancestrally diverse. Cell lines and characterization data will be shared openly.

A fully automated FAIMS-DIA mass spectrometry-based proteomic pipeline.

Here, we present a standardized, "off-the-shelf" proteomics pipeline working in a single 96-well plate to achieve deep coverage of cellular proteomes with high throughput and scalability. This integrated pipeline streamlines a fully automated sample preparation platform, a data-independent acquisition (DIA) coupled with high-field asymmetric waveform ion mobility spectrometer (FAIMS) interface, and an optimized library-free DIA database search strategy. Our systematic evaluation of FAIMS-DIA showing single compensation voltage (CV) at -35 V not only yields the deepest proteome coverage but also best correlates with DIA without FAIMS. Our in-depth comparison of direct-DIA database search engines shows that Spectronaut outperforms others, providing the highest quantifiable proteins. Next, we apply three common DIA strategies in characterizing human induced pluripotent stem cell (iPSC)-derived neurons and show single-shot mass spectrometry (MS) using single-CV (-35 V)-FAIMS-DIA results in >9,000 quantifiable proteins with <10% missing values, as well as superior reproducibility and accuracy compared with other existing DIA methods.

Partner and localizer of BRCA2 (PALB2) pathogenic variants and ovarian cancer: A systematic review and meta-analysis.

OBJECTIVE: Approximately 20% of ovarian cancers are due to an underlying germline pathogenic variant. While pathogenic variants in several genes have been well-established in the development of hereditary ovarian cancer (e.g. BRCA1/2, RAD51C, RAD51D, BRIP1, mismatch repair genes), the role of partner and localizer of BRCA2 (PALB2) remains uncertain. We sought to utilize meta-analysis to evaluate the association between PALB2 germline pathogenic variants and ovarian cancer. METHODS: We conducted a systematic review and meta-analysis. We searched key electronic databases to identify studies evaluating multigene panel testing in people with ovarian cancer. Eligible trials were subjected to meta-analysis. RESULTS: Fifty-five studies met inclusion criteria, including 48,194 people with ovarian cancer and information available on germline PALB2 pathogenic variant status. Among people with ovarian cancer and available PALB2 sequencing data, 0.4% [95% CI 0.3-0.4] harbored a germline pathogenic variant in the PALB2 gene. The pooled odds ratio (OR) for carrying a PALB2 pathogenic variant among the ovarian cancer population of 20,474 individuals who underwent germline testing was 2.48 [95% CI 1.57-3.90] relative to 123,883 controls. CONCLUSIONS: Our meta-analysis demonstrates that the pooled OR for harboring a PALB2 germline pathogenic variant among people with ovarian cancer compared to the general population is 2.48 [95% CI 1.57-3.90]. Prospective studies evaluating the role of germline PALB2 pathogenic variants in the development of ovarian cancer are warranted.

Apolipoprotein E in lipid metabolism and neurodegenerative disease.

Dysregulation of lipid metabolism has emerged as a central component of many neurodegenerative diseases. Variants of the lipid transport protein, apolipoprotein E (APOE), modulate risk and resilience in several neurodegenerative diseases including late-onset Alzheimer's disease (LOAD). Allelic variants of the gene, APOE, alter the lipid metabolism of cells and tissues and have been broadly associated with several other cellular and systemic phenotypes. Targeting APOE-associated metabolic pathways may offer opportunities to alter disease-related phenotypes and consequently, attenuate disease risk and impart resilience to multiple neurodegenerative diseases. We review the molecular, cellular, and tissue-level alterations to lipid metabolism that arise from different APOE isoforms. These changes in lipid metabolism could help to elucidate disease mechanisms and tune neurodegenerative disease risk and resilience.

Do people with hereditary cancer syndromes inform their at-risk relatives? A systematic review and meta-analysis.

Purpose: To evaluate rates of familial disclosure of hereditary cancer syndrome information. Methods: A systematic review and meta-analysis was conducted in accordance with PRISMA guidelines (PROSPERO no.: CRD42020134276). Key electronic databases were searched to identify studies evaluating hereditary cancer syndrome cascade relative disclosure. Eligible studies were subjected to meta-analysis. Results: Thirty-four studies met inclusion criteria. Among 11,711 included relatives, 70% (95% CI 60 - 78%) were informed of their risk of carrying a cancer-associated pathogenic variant; of 2,875 relatives informed of their risk who were evaluated for uptake of cascade testing, 43% (95% CI 27 - 61%) completed testing. Rates of disclosure were higher among female vs male relatives (79% [95% CI 73% - 84%] vs 67% [95% CI 57% - 75%]) and first-degree vs second-degree relatives (83% [95% CI 77% - 88%] vs 58% [95% CI 45 - 69%]). Conclusion: Nearly one-third of at-risk relatives remain uninformed of their risk of carrying a cancer-associated pathogenic variant. Even among those informed, fewer than half subsequently complete genetic testing, representing a critical missed opportunity for precision cancer prevention. Innovation: Five studies evaluating interventions to improve disclosure rates were generally ineffective. Urgent work is needed to elucidate barriers to relative disclosure by probands to develop targeted interventions that can optimize proband-mediated cascade genetic testing rates.

Metaplastic carcinoma of the breast: matched cohort analysis of recurrence and survival.

PURPOSE: Metaplastic breast carcinoma (MBC) is a rare subtype of breast cancer, defined as mammary carcinoma with squamous or mesenchymal differentiation, that may include spindle cell, chondroid, osseous, or rhabdomyoid differentiation patterns. The implications of MBC recurrence and survival outcomes remains unclear. METHODS: Cases were ascertained from a prospectively maintained institutional database of patients treated from 1998 to 2015. Patients with MBC were matched 1:1 to non-MBC cases. Cox proportional-hazards models and Kaplan-Meier estimates were used to evaluate outcome differences between cohorts. RESULTS: 111 patients with MBC were matched 1:1 with non-MBC patients from an initial set of 2400 patients. Median follow-up time was 8 years. Most patients with MBC received chemotherapy (88%) and radiotherapy (71%). On univariate competing risk regression, MBC was not associated with locoregional recurrence (HR = 1.08; p = 0.8), distant recurrence (HR = 1.65; p = 0.092); disease-free survival (HR = 1.52; p = 0.065), or overall survival (HR = 1.56; p = 0.1). Absolute differences were noted in 8-year disease-free survival (49.6% MBC vs 66.4% non-MBC) and overall survival (61.3% MBC vs 74.4% non-MBC), though neither of these reached statistical significance (p = 0.07 and 0.11, respectively). CONCLUSION: Appropriately-treated MBC may exhibit recurrence and survival outcomes that are difficult to distinguish from those of non-MBC. While prior studies suggest that MBC has a worse natural history than non-MBC triple-negative breast cancer, prudent use of chemotherapy and radiotherapy may narrow these differences, although studies with more power will be required to inform clinical management. Longer follow-up among larger populations may further elucidate the clinical and therapeutic implications of MBC.

A reference human induced pluripotent stem cell line for large-scale collaborative studies.

Human induced pluripotent stem cell (iPSC) lines are a powerful tool for studying development and disease, but the considerable phenotypic variation between lines makes it challenging to replicate key findings and integrate data across research groups. To address this issue, we sub-cloned candidate human iPSC lines and deeply characterized their genetic properties using whole genome sequencing, their genomic stability upon CRISPR-Cas9-based gene editing, and their phenotypic properties including differentiation to commonly used cell types. These studies identified KOLF2.1J as an all-around well-performing iPSC line. We then shared KOLF2.1J with groups around the world who tested its performance in head-to-head comparisons with their own preferred iPSC lines across a diverse range of differentiation protocols and functional assays. On the strength of these findings, we have made KOLF2.1J and its gene-edited derivative clones readily accessible to promote the standardization required for large-scale collaborative science in the stem cell field.

What happens in the long term: Uptake of cancer surveillance and prevention strategies among at-risk relatives with pathogenic variants detected via cascade testing.

BACKGROUND: Cascade genetic testing for hereditary cancer syndromes offers affected relatives the opportunity to pursue cancer screening and risk-reducing surgery and thus reduces morbidity and mortality. The purpose of this study was to measure the long-term utilization of targeted cancer prevention and quality of life among at-risk relatives offered clinician-facilitated cascade genetic testing. METHODS: In a pilot study, at-risk relatives of patients with a hereditary cancer syndrome were contacted directly by the clinical team and offered telephone genetic counseling and genetic testing via an at-home, mailed saliva kit. Two-year follow-up results evaluating the use of targeted cancer prevention strategies and the quality of life for enrolled relatives were reported. Quality-of-life was measured with validated surveys, and scores were compared to the time of initial contact by the Wilcoxon signed-rank test. RESULTS: Ninety-five at-risk relatives were enrolled in the initial pilot study, and 72 (76%) participated in the 2-year follow-up; 57 of these (79%) had completed genetic testing. Twenty-five of those 57 relatives (44%) were found to harbor an inherited pathogenic variant. Guideline-based cancer surveillance was recommended to 18 relatives; 13 (72%) completed at least one recommended screening, and six (33%) completed all recommended screenings. Risk-reducing surgery was recommended to 10 relatives; four (40%) completed a total of eight procedures. Quality-of-life surveys demonstrated low levels of anxiety, depression, distress, and uncertainty. CONCLUSIONS: The 2-year follow-up of the original pilot study revealed that clinician-facilitated cascade testing resulted in genetically targeted cancer screening and prevention with preserved quality of life. These results, to be confirmed by larger randomized controlled trials, suggest that medical systems should consider supporting clinician-facilitated cascade testing programs.

Cascade Testing for Hereditary Cancer Syndromes: Should We Move Toward Direct Relative Contact? A Systematic Review and Meta-Analysis.

PURPOSE: Evidence-based guidelines recommend cascade genetic counseling and testing for hereditary cancer syndromes, providing relatives the opportunity for early detection and prevention of cancer. The current standard is for patients to contact and encourage relatives (patient-mediated contact) to undergo counseling and testing. Direct relative contact by the medical team or testing laboratory has shown promise but is complicated by privacy laws and lack of infrastructure. We sought to compare outcomes associated with patient-mediated and direct relative contact for hereditary cancer cascade genetic counseling and testing in the first meta-analysis on this topic. MATERIALS AND METHODS: We conducted a systematic review and meta-analysis in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines (PROSPERO No.: CRD42020134276). We searched key electronic databases to identify studies evaluating hereditary cancer cascade testing. Eligible trials were subjected to meta-analysis. RESULTS: Eighty-seven studies met inclusion criteria. Among relatives included in the meta-analysis, 48% (95% CI, 38 to 58) underwent cascade genetic counseling and 41% (95% CI, 34 to 48) cascade genetic testing. Compared with the patient-mediated approach, direct relative contact resulted in significantly higher uptake of genetic counseling for all relatives (63% [95% CI, 49 to 75] v 35% [95% CI, 24 to 48]) and genetic testing for first-degree relatives (62% [95% CI, 49 to 73] v 40% [95% CI, 32 to 48]). Methods of direct contact included telephone calls, letters, and e-mails; respective rates of genetic testing completion were 61% (95% CI, 51 to 70), 48% (95% CI, 37 to 59), and 48% (95% CI, 45 to 50). CONCLUSION: Most relatives at risk for hereditary cancer do not undergo cascade genetic counseling and testing, forgoing potentially life-saving medical interventions. Compared with patient-mediated contact, direct relative contact increased rates of cascade genetic counseling and testing, arguing for a shift in the care delivery paradigm, to be confirmed by randomized controlled trials.

A regulatory network of microRNAs confers lineage commitment during early developmental trajectories of B and T lymphocytes.

The commitment of hematopoietic multipotent progenitors (MPPs) toward a particular lineage involves activation of cell type-specific genes and silencing of genes that promote alternate cell fates. Although the gene expression programs of early-B and early-T lymphocyte development are mutually exclusive, we show that these cell types exhibit significantly correlated microRNA (miRNA) profiles. However, their corresponding miRNA targetomes are distinct and predominated by transcripts associated with natural killer, dendritic cell, and myeloid lineages, suggesting that miRNAs function in a cell-autonomous manner. The combinatorial expression of miRNAs miR-186-5p, miR-128-3p, and miR-330-5p in MPPs significantly attenuates their myeloid differentiation potential due to repression of myeloid-associated transcripts. Depletion of these miRNAs caused a pronounced de-repression of myeloid lineage targets in differentiating early-B and early-T cells, resulting in a mixed-lineage gene expression pattern. De novo motif analysis combined with an assay of promoter activities indicates that B as well as T lineage determinants drive the expression of these miRNAs in lymphoid lineages. Collectively, we present a paradigm that miRNAs are conserved between developing B and T lymphocytes, yet they target distinct sets of promiscuously expressed lineage-inappropriate genes to suppress the alternate cell-fate options. Thus, our studies provide a comprehensive compendium of miRNAs with functional implications for B and T lymphocyte development.

Perineural invasion as a risk factor for locoregional recurrence of invasive breast cancer.

Perineural invasion (PNI) is a pathologic finding observed across a spectrum of solid tumors, typically with adverse prognostic implications. Little is known about how the presence of PNI influences locoregional recurrence (LRR) among breast cancers. We evaluated the association between PNI and LRR among an unselected, broadly representative cohort of breast cancer patients, and among a propensity-score matched cohort. We ascertained breast cancer patients seen at our institution from 2008 to 2019 for whom PNI status and salient clinicopathologic features were available. Fine-Gray regression models were constructed to evaluate the association between PNI and LRR, accounting for age, tumor size, nodal involvement, estrogen receptor (ER), progesterone receptor (PR), HER2 status, histologic tumor grade, presence of lymphovascular invasion (LVI), and receipt of chemotherapy and/or radiation. Analyses were then refined by comparing PNI-positive patients to a PNI-negative cohort defined by propensity score matching. Among 8864 invasive breast cancers, 1384 (15.6%) were noted to harbor PNI. At a median follow-up of 6.3 years, 428 locoregional recurrence events were observed yielding a 7-year LRR of 7.1% (95% CI 5.5-9.1) for those with PNI and 4.7% (95% CI 4.2-5.3; p = 0.01) for those without. On univariate analysis throughout the entire cohort, presence of PNI was significantly associated with an increased risk of LRR (HR 1.39, 95% CI 1.08-1.78, p < 0.01). Accounting for differences in salient clinicopathologic and treatment parameters by multivariable Fine-Gray regression modeling, the association between PNI and LRR was potentiated (HR 1.57, 95% CI 1.2-2.07, p = 0.001). We further conducted propensity score matching to balance clinicopathologic parameters and treatments between the two groups (PNI vs not), again showing a similar significant association between PNI and LRR (HR 1.46, 95% CI 1.03-2.08, p = 0.034). PNI is significantly associated with LRR following the definitive treatment of invasive breast cancer. The excess risk conferred by PNI is similar in magnitude to that observed with LVI, or by ER/PR negativity. Breast cancer prognostication and therapeutic decision-making should consider the presence of PNI among other salient risk factors. Larger studies among more uniform breast cancer presentations may elucidate the extent to which these findings apply across breast cancer subtypes and stages.

Bevacizumab in advanced endometrial cancer.

OBJECTIVE: Prospective data have demonstrated the efficacy of bevacizumab monotherapy in the treatment of advanced endometrial cancer. Bevacizumab is used off-label, and real-world data regarding the role of bevacizumab in endometrial cancer treatment are scant. In this largest single-institution retrospective study of its kind, we report our experience with bevacizumab monotherapy in the treatment of advanced/recurrent endometrial cancer. METHODS: All eligible patients (n = 101) had histologically confirmed endometrial cancer and were treated with bevacizumab at our institution from 2004 to 2017. Demographic data and tumor characteristics were obtained through chart review. Primary objective was response to therapy determined by Response Evaluation Criteria in Solid Tumors (RECIST v1.1). RESULTS: Analysis included 13 grade 1/2 endometrioid, 15 grade 3 endometrioid, 44 serous, 8 carcinosarcoma, and 21 other/mixed histologies. No patients achieved complete (CR) or partial (PR) responses; 19 achieved stable disease (SD). The clinical benefit rate (CBR; CR + PR + SD) was 19% (95% CI: 12-28%). The CBRs were 7%, 17%, 21%, and 23% for patients with 1, 2, 3, and ≥ 4 prior treatment lines. Median PFS ranged from 2.6 months (2 lines) to 4.9 months (≥4 lines). The 3-year OS rate was 58% (95% CI: 47-67%). The median OS was 3.4 years (95% CI: 2.9-4.2), ranging from 2.5 years (2 lines) to 4.5 years (≥4 lines). The most common treatment-related adverse event was hypertension; 35 (78%) of 45 were grade 1 or 2. CONCLUSIONS: In heavily pretreated advanced endometrial cancer, bevacizumab was associated with modest clinical efficacy and remains a viable palliative option in this setting.

APOE4 disrupts intracellular lipid homeostasis in human iPSC-derived glia.

The E4 allele of the apolipoprotein E gene (APOE) has been established as a genetic risk factor for many diseases including cardiovascular diseases and Alzheimer's disease (AD), yet its mechanism of action remains poorly understood. APOE is a lipid transport protein, and the dysregulation of lipids has recently emerged as a key feature of several neurodegenerative diseases including AD. However, it is unclear how APOE4 perturbs the intracellular lipid state. Here, we report that APOE4, but not APOE3, disrupted the cellular lipidomes of human induced pluripotent stem cell (iPSC)-derived astrocytes generated from fibroblasts of APOE4 or APOE3 carriers, and of yeast expressing human APOE isoforms. We combined lipidomics and unbiased genome-wide screens in yeast with functional and genetic characterization to demonstrate that human APOE4 induced altered lipid homeostasis. These changes resulted in increased unsaturation of fatty acids and accumulation of intracellular lipid droplets both in yeast and in APOE4-expressing human iPSC-derived astrocytes. We then identified genetic and chemical modulators of this lipid disruption. We showed that supplementation of the culture medium with choline (a soluble phospholipid precursor) restored the cellular lipidome to its basal state in APOE4-expressing human iPSC-derived astrocytes and in yeast expressing human APOE4 Our study illuminates key molecular disruptions in lipid metabolism that may contribute to the disease risk linked to the APOE4 genotype. Our study suggests that manipulating lipid metabolism could be a therapeutic approach to help alleviate the consequences of carrying the APOE4 allele.

PICALM Rescues Endocytic Defects Caused by the Alzheimer's Disease Risk Factor APOE4.

The ε4 allele of apolipoprotein E (APOE4) is a genetic risk factor for many diseases, including late-onset Alzheimer's disease (AD). We investigate the cellular consequences of APOE4 in human iPSC-derived astrocytes, observing an endocytic defect in APOE4 astrocytes compared with their isogenic APOE3 counterparts. Given the evolutionarily conserved nature of endocytosis, we built a yeast model to identify genetic modifiers of the endocytic defect associated with APOE4. In yeast, only the expression of APOE4 results in dose-dependent defects in both endocytosis and growth. We discover that increasing expression of the early endocytic adaptor protein Yap1802p, a homolog of the human AD risk factor PICALM, rescues the APOE4-induced endocytic defect. In iPSC-derived human astrocytes, increasing expression of PICALM similarly reverses endocytic disruptions. Our work identifies a functional interaction between two AD genetic risk factors-APOE4 and PICALM-centered on the conserved biological process of endocytosis.

Translocon Declogger Ste24 Protects against IAPP Oligomer-Induced Proteotoxicity.

Aggregates of human islet amyloid polypeptide (IAPP) in the pancreas of patients with type 2 diabetes (T2D) are thought to contribute to ß cell dysfunction and death. To understand how IAPP harms cells and how this might be overcome, we created a yeast model of IAPP toxicity. Ste24, an evolutionarily conserved protease that was recently reported to degrade peptides stuck within the translocon between the cytoplasm and the endoplasmic reticulum, was the strongest suppressor of IAPP toxicity. By testing variants of the human homolog, ZMPSTE24, with varying activity levels, the rescue of IAPP toxicity proved to be directly proportional to the declogging efficiency. Clinically relevant ZMPSTE24 variants identified in the largest database of exomes sequences derived from T2D patients were characterized using the yeast model, revealing 14 partial loss-of-function variants, which were enriched among diabetes patients over 2-fold. Thus, clogging of the translocon by IAPP oligomers may contribute to ß cell failure.

Quantitative analysis of co-oligomer formation by amyloid-beta peptide isoforms.

Multiple isoforms of aggregation-prone proteins are present under physiological conditions and have the propensity to assemble into co-oligomers with different properties from self-oligomers, but this process has not been quantitatively studied to date. We have investigated the amyloid-ß (Aß) peptide, associated with Alzheimer's disease, and the aggregation of its two major isoforms, Aß40 and Aß42, using a statistical mechanical modelling approach in combination with in vitro single-molecule fluorescence measurements. We find that at low concentrations of Aß, corresponding to its physiological abundance, there is little free energy penalty in forming co-oligomers, suggesting that the formation of both self-oligomers and co-oligomers is possible under these conditions. Our model is used to predict the oligomer concentration and size at physiological concentrations of Aß and suggests the mechanisms by which the ratio of Aß42 to Aß40 can affect cell toxicity. An increased ratio of Aß42 to Aß40 raises the fraction of oligomers containing Aß42, which can increase the hydrophobicity of the oligomers and thus promote deleterious binding to the cell membrane and increase neuronal damage. Our results suggest that co-oligomers are a common form of aggregate when Aß isoforms are present in solution and may potentially play a significant role in Alzheimer's disease.