Animal models for human contiguous gene syndromes and other genomic disorders

The contribution of gene movement to the "two rules of speciation"

The two "rules of speciation"--the Large X-effect and Haldane's rule--hold throughout the animal kingdom, but the underlying genetic mechanisms that cause them are still unclear. Two predominant explanations--the "dominance theory" and faster male evolution--both have some empirical support, suggesting that the genetic basis of these rules is likely multifarious. We revisit one historical explanation for these rules, based on dysfunctional genetic interactions involving genes recently moved between chromosomes. We suggest that gene movement specifically off or onto the X chromosome is another mechanism that could contribute to the two rules, especially as X chromosome movements can be subject to unique sex-specific and sex chromosome specific consequences in hybrids. Our hypothesis is supported by patterns emerging from comparative genomic data, including a strong bias in interchromosomal gene movements involving the X and an overrepresentation of male reproductive functions among chromosomally relocated genes. In addition, our model indicates that the contribution of gene movement to the two rules in any specific group will depend upon key developmental and reproductive parameters that are taxon specific. We provide several testable predictions that can be used to assess the importance of gene movement as a contributor to these rules in the future.

Phenotypic consequences of genetic variation at hemizygous alleles: Sotos syndrome is a contiguous gene syndrome incorporating coagulation factor twelve (FXII) deficiency

PURPOSE

We tested the hypothesis that Sotos syndrome (SoS) due to the common deletion is a contiguous gene syndrome incorporating plasma coagulation factor twelve (FXII) deficiency. The relationship between FXII activity and the genotype at a functional polymorphism of the FXII gene was investigated.

METHODS

A total of 21 patients including those with the common deletion, smaller deletions, and point mutations, and four control individuals were analyzed. We examined FXII activity in patients and controls, and analyzed their FXII 46C/T genotype using direct DNA sequencing.

RESULTS

Among 10 common deletion patients, seven patients had lower FXII activity with the 46T allele of the FXII gene, whereas three patients had normal FXII activity with the 46C allele. Two patients with smaller deletions, whose FXII gene is not deleted had low FXII activity, but one patient with a smaller deletion had normal FXII. Four point mutation patients and controls all had FXII activities within the normal range.

CONCLUSION

FXII activity in SoS patients with the common deletion is predominantly determined by the functional polymorphism of the remaining hemizygous FXII allele. Thus, Sotos syndrome is a contiguous gene syndrome incorporating coagulation factor twelve (FXII) deficiency.

Rai1 deficiency in mice causes learning impairment and motor dysfunction, whereas Rai1 heterozygous mice display minimal behavioral phenotypes

Smith-Magenis syndrome (SMS) is associated with an approximately 3.7 Mb common deletion in 17p11.2 and characterized by its craniofacial and neurobehavioral abnormalities. The reciprocal duplication leads to dup(17)(p11.2p11.2) associated with the Potocki-Lupski syndrome (PLS), a neurological disorder whose features include autism. Retinoic acid induced 1 (RAI1) appears to be responsible for the majority of clinical features in both SMS and PLS. Mouse models of these syndromes harboring an approximately 2 Mb chromosome engineered deletion and duplication, respectively, displayed abnormal locomotor activity and/or learning deficits. To determine the contribution of RAI1 in the neurobehavioral traits in SMS, we performed a battery of behavioral tests on Rai1 mutant mice and the Df(11)17-1/+ mice that have a small deletion of approximately 590 kb. The mice with the small deletion were hypoactive like the large deletion mice and they also showed learning deficits. The Rai1+/- mice exhibited normal locomotor activity. However, they had an abnormal electroencephalogram with overt seizure observed in a subset of mice. The few surviving Rai1-/- mice displayed more severe neurobehavioral abnormalities including hind limb clasping, overt seizures, motor impairment and context- and tone-dependant learning deficits. X-gal staining of the Rai1+/- mice suggests that Rai1 is predominantly expressed in neurons of the hippocampus and the cerebellum. Our results suggest that Rai1 is a critical gene in the central nervous system functioning in a dosage sensitive manner and that the neurobehavioral phenotype is modified by regulator(s) in the approximately 590 kb genomic interval, wherein the major modifier affecting the craniofacial penetrance resides.

Rai1 duplication causes physical and behavioral phenotypes in a mouse model of dup(17)(p11.2p11.2)

Genomic disorders are conditions that result from DNA rearrangements, such as deletions or duplications. The identification of the dosage-sensitive gene(s) within the rearranged genomic interval is important for the elucidation of genes responsible for complex neurobehavioral phenotypes. Smith-Magenis syndrome is associated with a 3.7-Mb deletion in 17p11.2, and its clinical presentation is caused by retinoic acid inducible 1 (RAI1) haploinsufficiency. The reciprocal microduplication syndrome, dup(17)(p11.2p11.2), manifests several neurobehavioral abnormalities, but the responsible dosage-sensitive gene(s) remain undefined. We previously generated a mouse model for dup(17)(p11.2p11.2), Dp(11)17/+, that recapitulated most of the phenotypes observed in human patients. We have now analyzed compound heterozygous mice carrying a duplication [Dp(11)17] in one chromosome 11 along with a null allele of Rai1 in the other chromosome 11 homologue [Dp(11)17/Rai1(-) mice] in order to study the relationship between Rai1 gene copy number and the Dp(11)17/+ phenotypes. Normal disomic Rai1 gene dosage was sufficient to rescue the complex physical and behavioral phenotypes observed in Dp(11)17/+ mice, despite altered trisomic copy number of the other 18 genes present in the rearranged genomic interval. These data provide a model for variation in copy number of single genes that could influence common traits such as obesity and behavior.