ANKK1-Related Disorders via the ANKK1 Gene

The ankyrin repeat and kinase domain containing 1 (ANKK1) gene maps to a cluster of four genes on chromosome 11 at 11q22–23 collectively known as the NTAD (NCAM1-TTC12-ANKK1-DRD2) cluster (Mota et al. 2012. PubMed ID: 23412349). Although the molecular details of these genes are not fully elucidated, these genes have roles in neurogenesis and brain function and have been associated with numerous disorders including alcoholism, attention deficit hyperactivity disorder, autism spectrum disorder, anti-social behaviors, obesity, and problems with memory and decision making (Comings and Blum. 2000. PubMed ID: 11105655; Mota et al. 2015. PubMed ID: 25989041; Gangi et al. 2016. PubMed ID: 26990357; Stice et al. 2008. PubMed ID: 18927395; Klein et al. 2007. PubMed ID: 18063800). In particular, the ANKK1 and DRD2 (dopamine receptor D2) genes are in linkage disequilibrium and polymorphisms within ANKK1-DRD2 are known to influence dopamine neurotransmission and have been well documented in psychiatric studies (Koeneke et al. 2020. PubMed ID: 32260442). Alteration of dopamine neurotransmission in the brain is considered one of the key factors involved in the pathophysiology of addiction and other psychiatric disorders (Nestler and Lüscher. 2019. PubMed ID: 30946825).

Addictions are classified as psychiatric disorders and defined as maladaptive behaviors that lead to distress, health issues, and impairment (Agrawal et al. 2012. PubMed ID: 22806211). Addictions have been compared to other types of complex disorders (i.e. diabetes or heart disease) that result from an interplay between genetic and environmental factors. It is estimated that ~20-40% of people who use substances of abuse will eventually develop an addiction (Koeneke et al. 2020. PubMed ID: 32260442), and genetic factors account for 30-80% of an individual’s addiction risk (Agrawal et al. 2012. PubMed ID: 22806211). Over 400 genomic loci and >560 variants have been associated with tobacco and alcohol use, indicating a polygenic mode of inheritance (Liu et al. 2019. PubMed ID: 30643251). Environmental factors that can increase addiction risk include early life trauma, abuse, situations of crisis, social pressures, and social isolation/deprivation (Koeneke et al. 2020. PubMed ID: 32260442). Having a mental health disorder also increases the overall risk of addiction (Drake et al. 1998. PubMed ID: 9853791).

Age of addiction onset varies from adolescents to elderly, and it is estimated that ~20 million Americans (ages 12 years and older) have a substance abuse disorder (2017, American Addiction Centers). Addictions, especially those relating to drug abuse, cost the United States ~$740 billion annually in lost productivity, healthcare expenses, and crime-related costs (National Institute on Drug Abuse).

Although specific polymorphisms within ANKK1 have been associated with various psychiatric disorders and addiction, the molecular role of the ANKK1 protein in the etiology in these disease remains largely unknown. The absence of functional data has made the interpretation of associations between ANKK1 genotypes and psychiatric disorders challenging (Koeneke et al. 2020. PubMed ID: 32260442). At this time the clinical advantages of ANKK1 germline testing are limited. Testing may be helpful in assessing an individual’s potential risk of addiction.

In addition, evolutionary studies examining the emergence of specific polymorphisms within the NTAD cluster suggest that the ordering of genes within this cluster (synteny) originated ~400 million years ago. The preservation of this cluster suggests that the NTAD genes share important co-regulatory functions during neurogenesis and neurotransmission (Mota et al. 2012. PubMed ID: 23412349; Yerushalmi and Teicher et al. 2007. PubMed ID: 17554587; Kikuta et al. 2007. PubMed ID: 17387144). Common polymorphisms in the NTAD, currently associated with behavioral and psychiatric phenotypes, could have possibly provided some adaptive advantage in the past. Because of this evolutionary relationship, it has been suggested that variation throughout the entire NTDA cluster should be considered as a whole when studying associations with behavioral and psychiatric disorders as opposed to looking at variation within each gene independently (Mota et al. 2012. PubMed ID: 23412349; Mota et al. 2015. PubMed ID: 25989041).

The ANKK1 gene, located on chromosome 11q23.2, is composed of 8 exons that encode a 765 amino acid (~84.6 kDa) protein (https://www.uniprot.org/uniprot/Q8NFD2). The ANKK1 protein belongs to the Receptor-Interacting Protein (RIP) serine/threonine kinase family which plays roles in cell proliferation, differentiation, and activation of transcription factors like NF-kappaB or AP-1 (Meylan and Tschopp. 2005. PubMed ID: 15752987). ANKK1 contains a single serine/threonine kinase domain and 12 ankyrin repeat domains. Ankyrin repeat domains are a common structural motif in proteins and are used to mediate protein-protein interactions (Mosavi et al. 2004. PubMed ID: 15152081).  

Expression studies have shown that ANKK1 is constitutively expressed in astrocytes of the central nervous system and is found in both the developing and adult brain (Hoenicka et al. 2010. PubMed ID: 19853839). ANKK1 expression has also been observed in muscle fibers and myogenic precursor cells (Rubio-Solsona et a. 2018. PubMed ID: 29758057). Hoenicka et al. showed evidence for the transcription of three distinct proteins from the ANKK1 gene including a full length (Ser/Thr kinase domain + ankyrin repeats), and two shorter versions of ~40 kDa (Ser/Thr kinase domain only and ankyrin repeats only). They suggested that each protein had a distinct biological function; however, no functional data was provided (Hoenicka et al. 2010. PubMed ID: 19853839).

According to the gnomAD public population database, ~1030 variants including synonymous, missense and loss of function variants have been documented throughout the entire genomic region of ANKK1 with the vast majority of these variants (~980) occurring at an allele frequency of <1%. The allele frequency for the ANKK1 c.2137G>A (p.Glu713Lys) variant, also known as the TaqIA allele and the most well-studied ANKK1 polymorphism, is about 26.3% overall with the highest frequency in the Latino population (~45%) (gnomAD). De novo single nucleotide variants and copy number variants within ANKK1 have not been documented. The mode of inheritance for described variants in ANKK1 appears to be polygenic.

The TaqIA allele of ANKK1 (c.2137G>A, p.Glu713Lys) is one of the most commonly studied polymorphisms in psychiatry and has been considered as a genetic marker for psychiatric disorders and addiction risk (Koeneke et al. 2020. PubMed ID: 32260442). The TaqIA allele is found in exon 8 and occurs within ankyrin repeat 11 (Neville et al. 2004. PubMed ID: 15146457). This allele has been found to be in linkage disequilibrium with other polymorphisms in the NTAD cluster including an interaction with the c.957C>T (p.Pro319=) variant within DRD2. ANKK1 and DRD2 are close genomic neighbors, and studies have shown that the two genes have overlapping 3’ ends, share haplotype blocks, and have identical cis elements for transcriptional regulation in their promotors (Hoenicka et al. 2010. PubMed ID: 19853839). The close relationship between these two genes leads to epistatic effect between the TaqIA allele and the DRD2 c.957C>T variant. This effect has been studied in antisocial traits in alcoholic patients (Ponce et al. 2008. PubMed ID: 18669994; Koeneke et al. 2020. PubMed ID: 32260442).   

Although the exact mechanism of how ANKK1 impacts dopamine signaling is not known, it has been speculated that ANNK1 is able to indirectly regulate DRD2 expression through activation of NF-kappaB (Huang et al. 2009. PubMed ID: 18354387). Specific variants within ANKK1 alter its ability to activate NF-kappaB, which in turn leads to a reduction in DRD2 expression within neurons and ultimately impacts dopamine neurotransmission (Huang et al. 2009. PubMed ID: 18354387). Individuals that have variants within ANKK1, DRD2, or other genes involved in the dopamine pathway generally have a lower level of DRD2 expression and are thought to have an increased addiction risk when compared to those without variants (Valli et al. 2019. PubMed ID: 30847741; Trifilieff and Martinez. 2014. PubMed ID: 23851257). However, due to the high levels of linkage disequilibrium within this genomic region as well as the absence of functional studies for these genes, the results of association studies have been conflicting and challenged to pinpoint the variants within ANKK1 and DRD2 that have the greatest impact on addition risk (Koeneke et al. 2020. PubMed ID: 32260442; Deng et al. 2015. PubMed ID: 25500252; Klaus et al. 2019. PubMed ID: 30836122). Development of cellular and animal models would provide further insight into the biological role of ANKK1 variants in addiction (Koeneke et al. 2020. PubMed ID: 32260442). At this point in time, the clinical significance of rare variants within ANKK1 has not been studied. This gene is relatively tolerant to both missense and loss-of-function variants (gnomAD).

It is difficult to estimate the clinical sensitivity for ANKK1 testing. Analytical sensitivity should be high as reported variants are detectable by sequencing.

This test is performed using Next-Gen sequencing with additional Sanger sequencing as necessary.

This test provides 100% coverage of all coding exons of the ANKK1 gene plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define coverage as ≥20X NGS reads or 2x Sanger sequencing.

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).