Gene expression studies in Prader-Willi and Angelman syndromes

Merlin G. Butler¹, Douglas C. Bittel¹, Nataliya Kibiryeva¹, Zohreh Talebizadeh¹, and Daniel J. Driscoll²

¹Section of Medical Genetics and Molecular Medicine, Children’s Mercy Hospitals and University of Missouri-Kansas City School of Medicine, Kansas City, MO; ²Department of Pediatrics, University of Florida College of Medicine, Gainesville, FL

 Prader-Willi syndrome (PWS) is a complex neurodevelopmental disorder which results from the loss of paternally expressed contiguous genes from the 15q11-q13 region. The 15q11-q13 region is known to contain imprinted sequences that are differentially expressed depending on the parent of origin.  The loss of maternally expressed gene(s) from 15q11-q13 results in Angelman syndrome (AS), a completely different disorder.  PWS and AS are generally caused by a paternal or maternal deletion of 15q11-q13, respectively.  In addition, maternal uniparental disomy (UPD) 15 resulting in the loss of the paternal chromosome 15 causes PWS and paternal UPD 15 resulting in the loss of the maternal 15 causes AS.  Furthermore, disruption of the imprinting center (IC) results in PWS or AS when the IC defect is paternal or maternal in origin, respectively.

The genetic subtypes of PWS and AS are known to have specific clinical phenotypes. It is unclear how subtle changes in gene expression due to the loss of both imprinted genes and reduced expression of nonimprinted genes lead to the clinical features associated with PWS and AS. In our continuing efforts to understand the molecular genetics of these disorders, we generated a custom cDNA microarray representing 77 genes/transcripts from the 15q11-q13 region.   We used our custom microarray to compare gene expression from actively growing lymphoblastoid cell lines established from male subjects. 

We compared the expression patterns obtained from PWS (paternal deletion and maternal UPD), AS (maternal deletion and paternal UPD) and control subjects.  We detected no difference in expression between the groups of genes with known biallelic expression located outside the 15q11-q13 region. We detected no expression from paternally expressed genes in PWS subjects (e.g., SNRPN, NDN) but the expression of these genes in AS subjects was similar to that of controls.  The known maternally imprinted genes/transcripts (paternally expressed) from the 15q11-q13 region were found to be over expressed as expected in the AS UPD subjects with paternal disomy 15.  In addition, we identified and confirmed several paternally biased genes/transcripts which had significantly greater expression from the paternal allele.  This group of genes/transcripts included the GABA receptor genes, GABRA5 and GABRB3. GABA is an inhibitory neurotransmitter that is widely distributed throughout the central nervous system.  GABAergic mechanisms have been implicated in a number of symptoms associated with PWS including hunger, compulsivity, metabolism, visual perception and visual memory. GABAergic mechanisms may have a direct effect on these characteristics or indirect effects due to feedback from other GABA-influenced systems.  Presumably, even a modest decrease in the synthesis of these receptor proteins could have a significant effect on brain development, with permanent consequences on central nervous system function.

Our findings suggest multiple layers of complex control of gene expression on chromosome 15.  Interconnected mechanisms can produce subtle and unexpected changes in gene expression when perturbed by the different mechanisms causing the genetic subtypes found in PWS and AS.  The resulting alterations in gene expression may help to explain, at least in part, the phenotypic differences observed among the genetic subtypes.

June 2004