Chromatin Impacts in Human Genetics

Chromatin-mediated influences

• Gametic (parental) imprinting

• Regulation of gene expression

• Developmental programming

Gametic (parental) imprinting

• Gametic imprinting is the differential expression of gene that depends on which parent donated the gene

• Most genes are expressed from both maternally and paternally inherited chromosomes.

• However, some genes are expressed only from the maternally inherited allele, whereas others are expressed only from the paternally inherited allele.

Successful development requires both maternal and paternal genes

Diseases tracked to imprinting

Prader Willi Syndrome• Small hands and feet• Underactive gonads,

tiny external genitals• Short stature• Mentally retarded• Slow-moving• Compulsive overaters• Obese

Diseases tracked to imprinting

Angelman Syndrome• “Happy puppet”• Severe mental

retardation• Absence of speech• Happy disposition• Excessive laughing• Hyperactive, with jerky

repetitive motions• Red cheeks, large jaw

and mouth

Patterns of inheritance

Parental Origin

Mother Father Phenotype

Normal allele Normal allele Normal

Normal allele Deletion Prader-Willi

Deletion Normal Angelman

Both diseases are associated with deletions in the same region of chromosome 15

Genes in imprinted region of Chr 15

Blue: Prader Willi gene candidatesBlue: Prader Willi gene candidatesFailure to inherit a good allele from dad results in disease.Paternal allele is expressed. Maternal allele is silent.

Pink: Angelman gene candidatesPink: Angelman gene candidatesFailure to inherit a good allele from mom results in diseaseMaternal allele is expressed. Paternal allele is silent.

Gametic imprinting is epigenetic

• For imprinted genes, one allele is expressed and the other is silent.

• The silent alleles typically show high levels of DNA methylation and tightly packed chromatin, consistent with their transcriptional inactivity.

• The expressed alleles are unmethylated and associated with loosely packed chromatin.

What we don’t know about imprinting

• What targets a gene for imprinting?– Why are some genes expressed from both

alleles and other expressed from only one allele?

• How are the imprints imposed?– Do males and females have different

mechanisms for imprinting genes?

Rett Syndrome

• X-linked trait• Mainly girls affected• Normal at birth• At 6-18 months, begin

losing purposeful movement

• Persistent wringing of hands

• Loss of speech, gait• Mental retardation

ensues

Nature Genetics (1999) 23:127-128

Rett’s is due to defect in MeCP2

• Methyl-cytosine binding protein 2 (MeCP2) binds methylated DNA and recruits binding of a histone deacetylase

• Normal role is tightening chromatin packing, leading to gene silencing

Why is the phenotype neurological?

• The phenotype suggests that the targets are genes in the brain

• Normal neurological differentiation requires silencing of MeCP2 gene target(s)

• The target(s) of MeCP2 are not known

Mouse model for Rett’s

• Mice have a gene that is homologous to MeCP2

• Knocking out the gene in mouse gives a phenotype similar to human Rett’s

• This model offers good experimental system for studying the human disease

Male mice with MeCP2 knockout develop normally for a while (middle), but at 6 weeks of age, they begin to develop neurological symptoms, such as hindlimb clasping (right).

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Coffin Lowry Syndrome

• X-linked trait• Girls less affected than

boys• Mild to severe mental

retardation• Poor motor coordination• Defects in formation of

head and hands• Not all patients show all

symptoms

Coffin Lowry defect in Rsk2 gene

• Rsk2 codes for a protein kinase, which phosphorylates proteins that participate in stimulating cell division and cellular differentiation.

• The Rsk2 protein associates with a histone acetyltransferase. Together, these proteins phosphylate and acetylate histone H3.

• Modification of histone H3 is associated with activation of a suite of genes, whose identity is not yet known.

• When Rsk2 is not functional, expression of the target genes is repressed, thus leading to disease.

Chromatin and Cloning

• During differentiation of cells, many changes in chromatin occur.

• Cloning from adult cells requires re-programming to return the cell to an embryonic state.

• Re-programming requires erasing the differentiation-specific chromatin changes.

Cloning anomalies

• Initial successes in cloning are being re-evaluated.

• Observations include:– Dolly, the first cloned sheep, at age 5 years

is fat. – Cloned mice also tend to be overweight.– Some cow clones have been born with

abnormally large hearts and lungs.

The cloning process

www.bioteach.ubc.ca

What’s the problem with cloning?

• After nuclear transfer, the donor nucleus often fails to re-establish an embryonic pattern of gene expression.

• New studies from R. Jaenisch’s lab show that this is due to the differentiation and DNA methylation state of the donor nuclei.– Neural stems cells make better donors than

differentiated neural cells.– Cells from DNA methylation mutants make better

donors than normal cells.

• Conclusion: Successful cloning requires re-programming of chromatin modifications.

Summary

• Chromatin modifications play key roles in many important processes.– Parent-of-origin (imprinting) differences in

gene expression– Activation and repression of gene

expression during development– Differentiation of cells during development

• Disruption of normal chromatin modification can result in disease.