Speech and language are uniquely human-specific traits that have contributed to humans becoming the predominant species on earth from an evolutionary perspective. Disruptions in human speech and language function may result in diverse disorders, including stuttering, aphasia, articulation disorder, spasmodic dysphonia, verbal dyspraxia, dyslexia, and specific language impairment (SLI). These disorders often cluster within a family, and this clustering strongly supports the hypothesis that genes are involved in human speech and language functions. For several decades, multiple genetic studies, including linkage analysis and genome-wide association studies, were performed in an effort to link a causative gene to each of these disorders, and several genetic studies revealed associations between mutations in specific genes and disorders such as stuttering, verbal dyspraxia, and SLI. One notable genetic discovery came from studies on stuttering in consanguineous Pakistani families; these studies suggested that mutations in lysosomal enzyme-targeting pathway genes (GNPTAB, GNPTG, and NAPGA) are associated with non-syndromic persistent stuttering. Another successful study identified FOXP2 in a Caucasian family affected by verbal dyspraxia. Furthermore, an abnormal ultrasonic vocalization pattern (USV) was observed in knock-in (KI) and humanized mouse models carrying mutations in the FOXP2 gene. Although studies have increased our understanding of the genetic causes of speech and language disorders, these genes can only explain a small fraction of all disorders in patients. In this paper, we summarize recent advances and future challenges in an effort to reveal the genetic causes of speech and language disorders in animal models.

Uncovering enzyme (UCE), encoded by the human NAGPA, is a trans-Golgi enzyme that adds the mannose-6- phosphate recognition tag on lysosomal enzymes destined for the lysosome. Mutations in NAGPA are known to cause stuttering, a common speech disorder with unknown etiology. The human NAGPA gene is transcribed into two different forms, probably due to alternative splicing. One of them, known as a brain isoform, is lacking exon 8 (102-bp). We performed quantitative real-time PCR for the NAGPA brain and non-brain isoforms in a cDNA panel originating from 16 human tissues and 24 sub-brain regions. According to our findings, the relative quantity of the NAGPA brain isoform in the brain was 4.7 times more than that in the control cDNA, a pooled mixture of equal amounts of cDNAs from the 16 different tissues. Further analysis using the cDNA panel originating from 24 different sub-brain regions revealed that the cerebral cortex contained the largest amount of NAGPA brain isoform. Relative quantity in the cerebral cortex was 8.6 times more than that in the control cDNA (P=0.00004). The lowest quantity of this isoform was detected in cDNA from the pituitary gland. In conclusion, findings of the current study suggest that the cerebral cortex, expressing the highest quantity of the NAGPA brain isoform, might be the region associated with speech function.