Genetics: A Piece of the Puzzle

Genetics: A piece of the puzzle

Michael J. Ombrello, M.D.Senior Clinical Fellow, Inflammatory Disease SectionNational Human Genome Research InstituteNational Institutes of HealthMichael.Ombrello@nih.gov

Genetics: A piece of the puzzle

Michael J. Ombrello, M.D.Senior Clinical Fellow, Inflammatory Disease SectionNational Human Genome Research InstituteNational Institutes of HealthMichael.Ombrello@nih.gov

Himmelfarb J. et al. Kidney International (2002) 62: 1524–1538.

What is Juvenile Idiopathic Arthritis?

Juvenile arthritisJuvenile rheumatoid arthritisJuvenile chronic arthritis

• Juvenile idiopathic arthritis:– Onset of symptoms prior to 16th birthday– Arthritis present > 6 weeks without other cause

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Petty R.E. et al. J Rheumatology (2004) 31:390-392.

Juvenile Idiopathic Arthritis

Classification Criteria of JIA

International League of Associations for Rheumatology (ILAR) Classification Criteria:

1. Systemic arthritis2. Oligoarthritis3. Rheumatoid factor positive polyarthritis4. Rheumatoid factor negative polyarthritis5. Psoriatic arthritis6. Enthesitis related arthritis7. Undifferentiated arthritis

Petty R.E. et al. J Rheumatology (2004) 31:390-392.

REVIEW OF GENETICS PRINCIPLES

What is DNA?What is a chromosome?What is a gene?What is a protein?Why perform genetic research?What is a simple genetic trait?What is a complex genetic trait?

What is DNA?

Deoxyribonucleic Acid

What is a chromosome?

• Largest organizational unit of genetic material

• One cell has 6 feet of DNA• Linear DNA folds into

chromosomes

Normal Human FemaleImage courtesy of U.S. National Library of Medicine

What is a gene?

• The organizational unit of DNA that produces one specific protein

• Range of 300 to 4300 genes per human chromosome• Humans have 20,000 – 25,000 genes

DNA is the template for making proteins

Proteins are the things within cells that “do” thingsChanges in DNA code may change the structure and function of the protein it encodes

What can genetics research teach us?

1. We hope to identify:• Genetic variants that affect risk• Cellular pathways

2. Identification of these things may:• Reveal a novel or unrecognized therapeutic target• Allow prediction of therapeutic response• Uncover a novel biomarker• Advance our understanding of disease process

Mendelian (Simple) Genetics

• Simple Genetic Trait• Gregor Mendel first described in garden peas• A trait (phenotype) caused by variation in a single gene

– Dominant trait caused by 1 copy of genetic variant– Recessive trait caused by 2 copies of genetic variant

Mendelian (Simple) Genetics

Mendelian traits in humans• Freckles • Dimples• Cleft chin• Kinky hair• Nearsightedness• Astigmatism• Lip size• Eye size• Blood Rh factor • Earlobe type

Mendelian diseases in humans• Cystic fibrosis• Sickle cell anemia• Hemophilia• Duchenne Muscular Dystrophy• Huntington’s Disease

Complex Genetics

• Complex Genetic Trait• A trait (phenotype) caused by the interaction of one or more

genetic variants with environmental factors• Multiple genes may participate in a trait

» Common and rare genetic variants» They are not always intuitive» Not always related to one another

• Environmental factors may be involved» Infectious agents (viruses, bacteria, fungi, etc.)» Exogenous substances (diet, chemicals, etc.)» Endogenous substances (degradative or waste products)

• Chance is also involved

Complex Genetics

Complex genetic traits in humans• Eye color• Hair color• Blood type (ABO)• Tongue rolling

Complex genetic diseases in humans

• Cancer– Lung cancer

• Autoimmune Diseases– Rheumatoid arthritis– Inflammatory bowel disease– Type 1 diabetes mellitus

• Atherosclerosis• Parkinson’s disease• Autism• Juvenile Idiopathic Arthritis

Types of Genetic Studies

1. Candidate gene studies• Investigator chooses gene• Variants/markers in the region are typed• Compare frequency of markers

• Advantages of candidate gene studies» Low cost study design» Relatively easy to perform

• Limitations of candidate gene studies» Selection bias for genes studied» Differences in ethnic composition of cases and control groups» Sampling bias unless many individuals studied» Results are inconsistently replicated

Types of Genetic Studies

2. Genome-wide association studies• Performed using commercial arrays that type variants/markers across

the entire genome (usually over 1 million markers)• Compare frequency of many more markers

• Advantages of genome wide association studies» Genome wide data allows exclusion of ethnically dissimilar subjects» Unlimited ability to detect

• Limitations of genome wide association studies» Require very large numbers of cases and controls» Relatively expensive to perform» Arrays only examine common variants

Manolio T. N Engl J Med 2010; 363:166-176.

GENETICS OF JUVENILE ARTHRITIS

“Lumping” studies of Juvenile Arthritis

ARGUMENT AGAINST “LUMPING” STUDIES• The seven JIA subtypes are different diseases• We should not expect the same genetic causes

ARGUMENT FOR “LUMPING” STUDIES• JIA is rare, and lumping increases statistical power• Though different, all subtypes involve arthritis developing at a

young age• All JIA subtypes appear to be complex diseases• Some genes are bound to overlap among types

“Juvenile Idiopathic Arthritis”

• Autoimmunity susceptibility genes

• TNF-a: Inflammatory cytokine with broad effects» Numerous therapeutics are widely used in JIA

• CTLA-4: Signaling molecule that protects “self” from T cells» Therapeutic in use as second line agent in JIA

• PTPN22: Signaling molecule that affects adaptive immune system» Variants can influence autoimmunity

• STAT4: Transcription factor for inflammatory genes

Systemic Arthritis

• Systemic juvenile idiopathic arthritis (sJIA) is characterized by:

• Arthritis• Fever• One or more of the following:

– Rash– Generalized lymphadenopathy– Organomegaly– Serositis

• Causes are poorly understood• Innate immune involvement• Unclear role of adaptive immunity• Autoimmune vs. autoinflammatory

Sir George Frederick Still

Adaptive vs. Innate Immune Systems

Innate Immunity• Innate immune cells

– Macrophage/monocyte– Neutrophil– Mast cell

• Receptors (sensors)– Detect products of pathogens– Detect host danger signals– NOT variable

• Inflammatory cytokines

Adaptive Immunity• Adaptive immune cells

– B lymphocyte (antibodies)– T lymphocyte

• Receptors (sensors)– Detect pieces of protein– VARIABLE (via recombination)

• Inflammatory cytokines

Genetic Studies of Systemic Arthritis

Inflammatory cytokines

Inflammatory cytokine

Anti-inflammatory cytokine

Antigen recognition

Inflammatory cytokine

Receptors for inflammatory cytokines

Receptor for inflammatory cytokines

Ion channel

Genetic Studies of Systemic Arthritis

Major Histocompatibility Complex• Display peptides for T cells• Small, specific “binding cleft”• TCRs recognize antigen + MHC

Systemic Arthritis

• Translation of genetic findings to therapeutics?1. Anti-interleukin 1 therapies

» Anakinra» Canakinumab» Rilonacept

2. Anti-interleukin 6 therapy» Tocilizumab

Genome Wide Association Study of Systemic Arthritis

Oligoarthritis

• Oligoarticular JIA is characterized by:• Arthritis present in 4 or less joints at presentation

» Persistent oligoarthritis: Arthritis “persists” in 4 or less joints » Extended oligoarthritis: Arthritis “extends” to involve >4 joints

• High rate of inflammatory eye disease (uveitis)

• Very strong association with specific MHC types• Functional importance is not clear

Polyarthritis (RF negative)

• RF negative polyarticular JIA is characterized by: • Arthritis present in 5 or more joints at presentation

» Typically more large joint involvement» Often asymmetrical involvement

• Absent rheumatoid factor• Rate of eye disease comparable to oligoarthritis

Polyarthritis (RF positive)

• RF positive polyarticular JIA is characterized by:• Arthritis present in 5 or more joints at presentation

» Typically involves large and small joints» Clinical course is comparable to rheumatoid arthritis

Psoriatic Arthritis

• Psoriatic JIA is characterized by:• Arthritis and psoriasis• Arthritis PLUS 2 of the following:

» Dactylitis (sausage-like swelling of a digit)» Characteristic fingernail changes» Psoriasis in a first degree relative

• Much genetic work in adult form of the disease• Interleukin 23 receptor plays a central role

» Therapeutic against this already FDA approved for psoriasis» Still under consideration for psoriatic arthritis

Enthesitis Related Arthritis

• Enthesitis-related JIA (ERA) is characterized by:• Arthritis and enthesitis, OR• Arthritis OR enthesitis, AND 2 or more of the following

» Sacro-iliac joint tenderness or inflammatory back pain» HLA-B*27 antigen positive» Arthritis in male over 6 years of age» Acute anterior uveitis» History of similar disease in first degree relative

• HLA-B*27 is strongest association (MHC gene)• Other genes have been identified in adult form of disease

Summary

• State of the art genetic studies are coming to JIA• In the past candidate gene studies• Today genome wide association studies• Tomorrow whole genome sequencing studies

• The scale and scope of the studies are expanding• Larger numbers of patients being enrolled (collaboration)• Larger numbers of markers being checked (technology)• Separate investigations of different JIA subtypes

If you are interested in more information about our ongoing genome wide association study of systemic juvenile idiopathic arthritis, please ask me or send an email to:

Michael.Ombrello@mail.nih.gov

Questions or Comments?