Question

Background Information:

In the nervous system, the brain processes information by transmission of electrical and chemical signals through a network of neurons. These signals travel through synapses which connect two neurons. Neuroligins are cell-adhesion molecules that function to connect two neurons at synapses and mediate proper signalling across the synapse. Interestingly, neuroligin function has been linked to autism spectrum disorder (ASD) which is primarily manifested by delayed development and deficiency in social interaction and communication in patients. Mutations in genes encoding neuroligins have been identified in ASD patients. The molecular mechanism of how neuroligins function in ASD has been investigated using mouse models. The mouse gene NL3 encodes a neurolignin. Aberrant NL3 gene function in mice results in synaptic dysfunction and a diverse range of abnormal behavior including a subset that resembles ASD patients. A striking phenotype of NL3 mutant mice is increased repetitive behavior compared to wild type/normal mice.

There are two alleles of the mouse NL3 gene. One allele (referred as “+” or wild type) produces NL3 protein which results in the synthesis of neuroligins for synaptic function. In contrast, the other allele (referred as “-”) does not produce any NL3 mRNA (and hence no NL3 protein) resulting in an inability to produce neuroligins.

Question: Closer examination of the two alleles of the mouse NL3 gene reveals no changes in nucleotide sequence in the transcriptional unit. This observation indicates that the difference in nucleotide sequence between the two alleles must not occur within the transcriptional unit. Provide two possibilities where nucleotide changes could occur on the chromosome for the allele that does not produce any NL3 mRNA and explain why.

Answer 1

The NL3 ‘-‘ allele does not form the mRNA. Hence, there is should be some change in the DNA. The transcriptional unit consists of the coding sequence, the promoter and the terminator region. Eukaryotes may also have enhancer region. However, this allele did not show any change in nucleotide sequence of the transcriptional unit. The two possibilities of gene silencing could be:

1) The 3D structure of chromatin affects gene transcription. The genome is partitioned into megabase-size topological associated domains, or TADs. TADs are regulatory domains that prevent the interactions between enhancers and promoters. There may be mutation occurring in the TAD associated with NLE3 gene that could affect prevent interaction between enhancers and promoters. Such mutations are seen in Polydactyly disease.

2) There may be a mutation in the intergenic region that are present just before the transcription unit. These intergenic region codes for transcripts that could be involved in interaction between the core promoter and distal enhancers. There may be mutations in intergenic region that codes for a microRNA to NL3, resulting in increased transcription of this microRNA. MicroRNAs inhibits mRNA transcription by binding to the untranslated regions.