BMSC3101 Inherited Disorders

Dr. Al Pickles

a.r.pickles@leeds.ac.uk

Sex of this individual?

Characteristic banding pattern on the chromosome is due Characteristic banding pattern on the chromosome is due to staining differences; light indicates areas of gene-rich to staining differences; light indicates areas of gene-rich areas as euchromatin stains lightly and contains actively areas as euchromatin stains lightly and contains actively expressed genesexpressed genes

Top = pTop = p

Lower = qLower = q

Central point of Central point of each chromosome each chromosome = centromere= centromere

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Autosomal dominant• Affects both males & females equally• Transmitted from one generation to the next• All forms of transmission are observed i.e. male-

male, male-female, female-male, female-female

Autosomal recessive• Disorder affects males & females equally• Usually only affects individuals in one generation

in a single sibship (brothers & sisters), not occurring in previous and subsequent generations

• Consanguinity in the parents provides further support for autosomal recessive inheritance

Paternal Age

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Achondroplasia

AchondroplasiaUSAAperts

Marfan

Case of sporadically occurring disorders – new autosomal dominant mutation

X-Linked recessive

• Affects males• No male-male transmission• Females may manifest the condition

X-Linked dominant

• Affects males & females, females more frequently

• Females less severely affected than males• Females transmit to son & daughter, males

only transmit to females = all affected

Mutations• Definition: A heritable alteration or change in the

genetic material• Genomes between any two individuals are

99.9% identical – any two people differ by 1 nucleotide in every 1000 = 3 million diff’

• Can occur following exposure to mutagens, but majority occur spontaneously following errors in DNA replication & repair

• Usually only cause problems when in coding areas

• Somatic cell mutation – can not be transmitted to offspring

• Gonadal tissue or gamete mutation transmitted to future generations

• Estimated that each person carries up to 6 lethal recessive mutant alleles – if homozygous then would be serious

• Harmful alleles constitute the genetic load of the population

Types of mutations

• Traditionally two types:- gross structural chromosomal changes- submicroscopic changes in one or more

nucleotides• Until recently mutations considered to be

transmitted unaltered termed fixed or stable mutations

• More recent are dynamic or unstable mutations – undergo further alteration as they are transmitted in families & explains unusual patterns of inheritance.

Fixed/stable point mutations: Classified according to DNA molecular changes

• Majority of these differences are called: Single Nucleotide Polymorphisms (SNPs – pronounced snips) (also called substitutions)

• Involves replacement of single nucleotide by another

Deletions – Involves loss of one or more nucleotides.

• If occurring in coding sequences it disrupts the reading frame

Insertions – Involves the addition of one or more nucleotides into a gene, same disruption can occur as above

Dynamic/unstable mutationsNew class, important cause of inherited disease in

humans• Consist of triplet repeat sequences which in

affected people occur more frequently• Increase in these, termed triplet amplification

identified as the mutational basis for single gene disorders such as Huntington’s

• Mechanism not clear, triplet repeats below a certain number are consistently transmitted during mitosis and meiosis

• However, above a certain number, they are transmitted unstably – usually with increase or decrease in repeat number = unstable mutations

• Triplet repeat sequences usually take place over a number of generations within a family= unusual inheritance patterns (anticipation), increasing severity

Mutation effects on proteins: STRUCTURAL

Mutations sub-divided into two main groups depending on the effect on the polypeptide sequence of the encoded protein:

- Synonymous

- Non-synonymous

Synonymous/silent mutations

• Mutation that doesn’t alter the polypeptide product of a gene

• SNP, due to degeneracy of the code will often result in another triplet which codes for the same amino acid

• Leads to no alteration in the resulting protein

Non-synonymous

• Term for mutation leading to alteration in the encoded polypeptide

• Occur less frequently than synonymous• Change in protein will lead to abnormal

function = disease and occur in three main ways:

- Missense- Nonsense- Frameshift

Missense • SNPs can lead to coding for a different amino

acid and subsequent altered protein• Can lead to gross reduction or complete loss of

biological activity = non-conservative substitution• Protein can retain normal biological activity (e.g.

enzyme action) but has altered characteristics such as stability or pH

• SNPs that result in replacement of amino acid but without effect = conservative substitutions

Nonsense• Substitution that leads to generation of a

stop codon• Results in premature termination of

translation• Unlikely to retain biological function of the

protein, particularly if resulting in loss of functional domain

Frameshift• If mutation is insertion or deletion which

are not multiple of three = disrupts the reading frame

• Amino acid sequence will mean the protein’s function will be abnormal

• These type may also result in a stop codon immediately after the mutation

Mutation effects on proteins: FUNCTIONAL

Mutations exert phenotypic effect through either loss or gain of function

Loss of function• Result in reduced activity (hypomorph) or

decrease stability (null allele/amorph) or even loss of gene product

• Inherited enzyme function reduction usually X-linked recessive as normal allele would be enough to compensate……..

• Heterozygous state such as this where phenotypic effect is seen is termed haploinsufficiency ( e.g. familial hypercholesterolaemia)

• Number of autosomal dominant disorders have basis of this

• Homozygous mutations in this state result in more severe phenotypic manifestations of the reduction in protein function (surprise, surprise).

Gain of function• Result in increased levels of gene expression or

development of new functions of gene product- E.g. expanded triplet repeat mutations of Huntington gene cause aggregations in the CNS leading to the features of the disorder

Expression– is a statement about the features of the

condition, many conditions have variable manifestations.

Penetrance – is a measure of how many individuals who

carry the mutant gene will manifest features of the condition (heterozygotes).

Example: NeurofibromatosisStarting point: neurofibromin

Background• First described in the 18th Century• Most common genetic disorders in

humans (some thought was cause for original ‘elephant man’ – was actually Proteus syndrome

• Two main types NF1 & NF2; NF1 more common by far with incidence of 1:3000 births

Clinical features• Small pigmented skin lesions (café-au-lait

spots) & small soft fleshy growths• Café-au-lait spots first appear in childhood,

increasing in size & number until puberty• Diagnosis requires 6 spots>5 mm• Late childhood = neurofibromata; benign

tumours usually on the skin, increasing with age

• Neurofibromatosis

Clinical manifestation• Also may include:

- axillary freckling, macrocephaly, mild developmental delay & Lisch nodules (small raised pigmented hamartomata or iris)

• Mostly NF1 individuals enjoy a normal healthy life• Small number develop more major complications:

epilepsy, CNS tumour or scoliosis

Genetics• Autosomal dominant inheritance• Mutation in the neurofibromin (NF1) gene. • Located at 17q11.2

• Spans 350 kb containing 56 exons.

Gene product• Neurofibromin protein: very

large protein (350 kD); structural homology to GTPase activating protein (GAP) – important role in signal transduction; downregulates RAS activity(GTP-binding protein)

• The GAP activity of neurofibromin resides in a segment which represents only 10% of the protein and remains the only clearly defined biochemical function of the protein

Normal role: NF1 gene functions as a tumour suppressor

• GAP-related domain (GRP) interacts with RAS proto-oncogene product

• NF1 plays an important role in cell growth and differentiation

Mutations• >100 identified including deletions, insertions and

substitutions• Most lead to severe truncation or complete absence

of protein – loss of function!• Large deletions = severely affected with more

intellectual impairment + increased neurofibromata

• Almost complete penetrance by 5 yrs old• Expression variable amongst family

members (identical twins not so = modifying genes at other loci?)

• 50% of all NF1 cases are a result of new mutations (m.r. = 1:10,000 gametes)

Treatment• At present no cure for NF1• Drug therapy aimed at upregulating

neurofibromin GAP activity or downregulating RAS may be beneficial

• Difficulty in applying a systemic drug to diverse range of tissues and CNS

• Requires better understanding first – cloning studies

Module information

• Your seminars should be similar but contain more detail – YOU DO NOT NEED TO KNOW ABOUT NF1 FOR THE EXAM, IT IS PURELY INSTRUCTIONAL

• All you need to know will be explained in the session on Friday i.e. groups, topics, seminars/reports etc etc.