Health and Community 17: Genetics of Aging

Genetics of Aging

Introduction

  • Counterintuitive view of aging: As you age, the more likely you are to die, but the more resilient you become.
  • Diseases, wars, and strife act as active selective pressures.
  • The genetics governing aging will be discussed because it is not just wear and tear.
  • Question: Do active phenotypes in older people represent their genotype?
  • Goal: Identify mutations that decrease or increase lifespan.

Progeria: Early Aging Syndromes

  • Mutations that decrease lifespan will be looked at via the genetics of Progeria: early aging syndromes.
  • Three types:
    • Hutchinson-Guilford Progeria Syndrome (HGPS): Very rare.
    • Werner Syndrome.
    • Down Syndrome: Most common.
Hutchinson-Guilford Progeria Syndrome (HGPS)
  • Recognized since the 1880s.
  • No mental retardation is associated with HGPS.
  • Shockingly rare: one in four million to one in eight million people.
  • Rate of aging is so great that people die before reproducing.
  • Mutations are de novo.
Features of HGPS
  • Senile appearance with a pinched face.
  • Early loss of hair (complete body alopecia).
  • Disproportionately large head due to small body.
  • Scleroderma: tight, thin, and hard skin due to lipodystrophy (problem with body fat distribution).
  • Incomplete extension at knees and elbows (stiff joints).
  • Coronary heart disease is common, leading to death in teens.
  • Slow growth, dwarfism, very fragile bodies.
  • Average age of death is about 13 years old, occasionally up to 30.
Discovery of the Genetic Basis
  • Sam Berns, a famous person with HGPS, died at 17 and gave a TED talk.
  • His mother, Leslie Gordon, a physician, and his father, a pediatrician, established a scientific foundation to study HGPS.
  • In 2003, the mutations causing HGPS were identified in the LMNA gene.
  • The gene was mapped to chromosome 1q (long arm of chromosome 1).
  • Sequencing was done on 20 classic HGPS cases and compared with the Human Genome Project consensus sequence.
Mutations in LMNA Gene
  • 18 of the HGPS cases had the same nucleotide change: a C to T transition.
  • This transition created an alternative splice site, resulting in a lamin A protein with 50 amino acids missing at its C terminus.
  • One case had a different substitution in the same codon, causing a splicing error.
  • One case had a substitution in exon two, resulting in a mutant protein.
  • The end result in all cases was the expression of a mutant protein called progerin (a defective form of lamin A).
Function of Lamin A
  • Lamin A is part of the lamin network that lies under the inner nuclear membrane.
  • Lamin A is an intermediate filament.
  • Passage: is a measure of the age of cells that you grow in vitro; every time a flask with cells is divided into another flask, that's one passage.
Effect of Mutant Lamin A (Progerin) on the Nucleus
  • Progressive change in the shape of the cell nucleus.
  • Normal lamin A and lamin C accumulate in normal cells.
  • In HGPS cells, mutant lamin A (progerin) accumulates.
  • Lamin A (red) and lamin B tend to co-localize in the nuclear lamina in normal cells, but in late passage HGPS cells, mutant progerin is mislocalized into blebs, while lamin B is at the nuclear membrane.
Effect of Progerin
  • Wild-type lamin A promotes genetic stability.
  • It maintains the level and placement of proteins involved in DNA repair, particularly double-stranded DNA breaks.
  • Mutant progerin reduces the efficiency of DNA repair.
  • HGPS is effectively a defect in DNA repair.
  • Mouse cells deficient for maturation of Prelamin A accumulate DNA damage and chromosomal aberrations and are more sensitive to DNA-damaging agents.
HGPS as a Model for Aging
  • HGPS is not the best model for aging.
  • Males do not develop prostate cancers, there's no increased risk of cancers or cataracts, high blood pressure is rare, diabetes is rare, and they don't get Alzheimer's, probably because they die too early.
  • It gives a clue about genetic stability.
Werner Syndrome
  • Adult-onset progeria.
  • Advanced accelerated aging phenotype at puberty.
Symptoms
  • Grey hair, loss of voice, bird-like face.
  • Thin, hardened skin.
  • Thin arms and legs with fat deposition around the middle.
Genetics
  • Can be inherited (autosomal recessive).
  • One in a million people for a de novo mutation, but one in thirty thousand in Japan and Sicily.
  • Most common cause of death: cancer and atherosclerosis.
  • People with Werner's get relatively early onset cataracts, skin ulcers, type two diabetes, they do show diminished fertility with atherosclerosis, relatively early onset osteoporosis, and they usually develop multiple and very rare cancers in their lifetime.
Cellular Features
  • Werner's fibroblasts grow very slowly in tissue culture (about 20 doublings before senescence, compared to 60 in normal cells).
  • Gene isolated in 1996 called WRN, encoding a DNA helicase.
  • Mutations are normally loss of function.
  • Some patients also have LMNA (lamin A) mutations.
Mechanism
  • DNA helicases are used for DNA repair.
  • Wild-type Verna protein is involved in DNA repair, especially at telomeres.
  • Lagging strand replication has problems in Verna cells.
  • Overexpression of telomerase in furnace cells gets a partial correction.
  • Wild-type Werner protein interacts with the nuclear lamina.
  • Focuses attention on telomeres, which are an inbuilt mechanism of cell death.
Werner Syndrome as a Model for Aging
  • Reinforces the idea that poor repair of genetic lesions is associated with aging.
  • Tells us something now. It reinforces the idea that, the idea that this repair of genetic lesions is associated, or poor repair of genetic lesions is associated with ageing.
  • No prostate problems, generally raising of blood pressure, strokes not recognized in Burton's patients, they don't get Alzheimer's, they don't suffer mental degeneration.
Down Syndrome
  • Most common progeria: one in seven hundred live births.
  • Treatments and support have improved dramatically (life expectancy increased from 10 years in 1960 to 60 years in 2017).
Symptoms
  • Premature graying and hair loss.
  • Very early vascular disease.
  • Almost universal early-onset Alzheimer's disease (typically by age 35-40).
  • Brains have significant levels of tangles and plaques associated with Alzheimer's disease.
Down Syndrome as a Model for Aging
  • It's not the best model for ageing. There's no prostate or breast cancer recognised as a common feature, no osteoporosis, no cataracts, there's no obvious high blood pressure, and generally there's not too many wrinkles.
Causes of Down Syndrome
  • Trisomy 21 (classic): caused by nondisjunction during meiosis.
  • Translocation Down's (4% of cases): extra chromosome 21 attached to a different chromosome.
  • Mosaic Down's (1% of cases): some cells are Down's, some cells are non-Down's in the same person.
Genetics of Down Syndrome
  • Approximately 230 genes have been identified on chromosome 21.
  • About 300 RNA genes are protein-coding.
  • About 500 or more genes on this chromosome.
  • Six million Down's people worldwide, of these, there's about two hundred only with an extremely rare partial trisomy of chromosome twenty one.
  • Downs Critical Region of 30 genes that cause the effects of Down's when replicated.
  • TS65DN mouse model of Down syndrome: partially trisomic for 132 genes equivalent to genes on human chromosome 21.
    • Mice have behavioral and cognitive changes that mimic those of Down's.
  • A key protein target is USP16 (ubiquitin-specific protease 16).
    • USP 16 is a in that Downs critical region. Normally, cells have two copies of it. People with Down's would have three, so that gene is overexpressed.
    • Overexpression of USP16 is the critical driver of Down's syndrome. (If you down regulate USP 16, you should increase growth rate.)
  • High expression of USP16 decreases growth rate.
  • Reducing the expression of USP16 increases cellular growth rate.
  • Overexpressing USP16 in normal cells reduces growth rate.
Function of USP16
  • Ubiquitin-specific protease; removes ubiquitin from histones.
  • Histones can also be ubiquitinated by the attachment of a small protein, 76 amino acids, called ubiquitin.
  • Ubiquitination targets histones stranded breaks and acts as a recruitment site for DNA repair enzymes.
  • USB 16 removes ubiquitin, and it's therefore a critical regulator of this DNA repair response. If you've got overexpressed USB 16, you're removing the flags that allow the cell to repair DNA too quickly.
  • Critical regulator of DNA repair response.
  • Overexpression removes ubiquitin flags, impairing DNA repair.

Genetics Governing Normal Aging

  • Three progerias provide clues: nuclear lamina controls DNA replication and repair (double-stranded breaks).
  • Accumulation of DNA damage and loss of telomeres leads to genetic instability.
  • Studies have not yet led to any treatment of any progeria.
  • Question: Are there mutations that increase lifespan?
  • Human Aging Genomic Resources: database of genes related to human aging.
  • Gendr: collection of genes associated with the effects of dietary restriction.
  • CellAge database: genes associated with senescence and old age.
Genes Influencing Lifespan
  • More than 2,000 genes influence lifespan.
    • Less than 1,000 in Baker's yeast.
    • About 900 in nematode (C. elegans).
    • About 200 in fruit fly.
    • A bit less than 150 to be identified in a mammal (mouse).
  • Mutations in key genes in nematodes can increase lifespan 10 times.
  • Lifespan in fruit flies has been doubled by mutation.
  • Some mutations in mice give them an extra 50% life.
Age-1 Gene in Nematodes
  • First mutation in Age-1 gene increased average lifespan by almost seventy percent, and the maximum lifespan increased by over one hundred percent.
  • Null mutation of Age-1 increases lifespan 10 times.
  • Age-1 encodes PI3 kinase.
PI3 Kinase and Insulin Pathways
  • Removal of PI3 kinase activity extends lifespan in worms.
  • PI3 kinase is involved in glucose utilization and insulin pathways.
  • In the absence of glucose, you only get the tenfold increase in lifespan from removing PI3 kinase activity.
  • High blood glucose levels are toxic.
  • Sugars make you older.
DAF-2 Gene
  • DAF stands for dower formation.
  • DAF2 encodes a receptor tyrosine kinase, that's the C. Elegans insulin, or insulin like growth factor receptor ortholog.
  • During starvation starvation or crowding, then between L1 and L2 you get formation of a dower form, which is non feeding, not particularly active, but it's non ageing. And then when the conditions get better, anytime up to four months, it can now develop into L four.
  • So indications then, would inhibition of PI three kinase in humans also extend lifespan?
  • One way to inhibit PI three kinase activity is to reduce the amount of glucose in your diet, Eat fewer sugars.
  • Will that extend human lifespan? There is some evidence that if you reduce sugar consumption, you live longer.

Human Lifespan

  • Great increase in people over 90 years old from the 1990s to modern day.
    • There is this little blip somewhere about twenty ten, when there were fewer than expected, and we need to address that before we go any further, and this goes straight back to World War one, when they had a reduced birth rate, really quite dramatically
  • Females live longer than males, with an increasing exaggeration at older ages.
Centenarians
  • 3% of people who make it to 90 years old reach 100 years old.
  • Estimated 600,000 centenarians worldwide.
  • Expectation of a 40-fold increase in centenarians.
  • Approximately one in thousand centenarians makes it to 110 years old or more.
Characteristics of Centenarians
  • Low body mass index and body fat levels.
  • Low plasma triglyceride levels.
  • Highly sensitive to insulin.
  • High levels of active insulin-like growth factor one.
  • Slow accumulation of age-related decline and onset of chronic diseases.
Genetic Lottery
  • How much of lifespan is determined by genetics versus lifestyle choices?
  • GWAS (genome-wide association studies) and SNPs (single nucleotide polymorphisms) are used to address this.
SNPs
  • A single nucleotide polymorphism is a change in one nucleotide that's in some part of the population, and not in others. In other words, it's just a mutation.
  • They don't alter any functions, but they're easily mapped because you can just sequence them.
  • SNPs are inherited just like normal genes.
  • We can map thousands of them at a time.
GWAS
  • Genome wide study of mutations.
  • Associates mutations with different phenotypes.
  • Researchers screen for associations between SNPs and diseases like Alzheimer's.
APOE Gene and Alzheimer's
  • GWAS identified two SNPs associated with Alzheimer's.
  • Strongly associated with a mutation in the APOE gene.
  • APOE4 allele is strongly associated with Alzheimer's: fifteen-fold increased risk.
Function of APOE
  • Regulates lipid homeostasis (mediates lipid transport).
  • In the central nervous system, cholesterol is synthesized in astrocytes and transported to neurons via APOE.
  • APOE4 binds amyloid beta, leading to deposition of amyloid protein as tangles and plaques in the brain.
APOE Alleles and Lifespan
  • French study found that the APOE4 allele is less frequent in older people (causes early death).
  • The frequency of the APOE2 allele was increased (APOE2 allele increases lifespan).
  • Swedish study confirmed that APOE4 allele causes earlier death, and the E2 allele increases lifespan.
FOXO3 Gene
  • In hydra, FOXO makes organisms immortal.
  • Humans have four FOXOs (FOXO1, FOXO3, FOXO4, FOXO6): transcription factors that regulate energy metabolism, oxidative stress, proteostasis, apoptosis, cell cycle regulation, and inflammation.
  • FOXO3AGG SNP is associated with people who live a long time in Japanese, German, and Danish populations.
  • FOXO03 is regulated by AKT phosphorylation (insulin pathway).
  • They all encode transcription factors that encode genes that regulate energy metabolism, oxidative stress, proteostasis apoptosis, cell cycle regulation, and so on, immune seed inflammation.

Summary

  • Key finding indicate anAccumulated DNA repair defects strongly associated with aging phenotype.
    • DNA repair defects at the nuclear lamina and telomeres.
  • Insulin signaling pathway and lipid metabolism: three key drivers of aging phenotype.
  • AKT insulin pathway and FOXO3 (induces autophagy for protein homeostasis).