By searching online databases, Cen et al. (2018) found that 8 transcripts of the SAMD12 gene had been described. The largest transcript was the most dominantly expressed transcript in the brain, particularly in the frontal cortex, followed by the cerebellum. The expression increased with age and remained stable after adolescence. In mouse cerebellum, Samd12 expression was mainly found in the Purkinje cell and granule cell layer.

In affected members of 49 Japanese families with familial adult myoclonic epilepsy-1 (FAME1; 601068), Ishiura et al. (2018) identified a heterozygous 5-bp expanded TTTCA(n) insertion in a noncoding region within intron 4 of the SAMD12 gene (618073.0001). The mutation was found by a combination of linkage analysis of 6 families and whole-genome sequencing of 1 family. Subsequent families were identified by targeted analysis of the SAMD12 gene. The TTTCA(n) repeat expansion was associated with an upstream 5-bp TTTTA(n) repeat expansion in all families, although in 1 family (F6115), the TTTCA(n) repeat expansion was found between 2 TTTTA(n) expansions. The TTTCA(n) expansion was not observed in the reference sequence or in 1,000 control individuals, whereas the TTTTA(n) repeat was found in the reference sequence and an expanded TTTTA(n) repeat was found in 5.9% of controls, suggesting that the TTTCA(n) expansion is responsible for the phenotype. Four patients from 3 families had homozygous repeat expansions, which was associated with a more severe phenotype. Neuropathologic examination of brain tissue from 6 patients using a probe targeting the UUUCA repeat showed accumulation of abnormal RNA foci in cortical neurons and, to a lesser extent, in Purkinje cells. Western blot analysis showed decreased levels of SAMD12 protein in patient brains. One patient with homozygous repeat expansions had mild and diffuse loss of Purkinje cells or Purkinje cells with halo-like amorphous materials around the cytoplasm, referred to as 'somatic sprouts,' suggesting possible degeneration. These histologic abnormalities were not detected in the brains of patients with heterozygous repeat expansions or in controls. RNA-seq analysis of patient brains showed transcriptional dysregulation and accumulation of altered repeat motifs. There was an inverse correlation between repeat length and age at onset of epilepsy and myoclonic tremor. Affected members from 2 additional families (F9283 and F8241) were found to have similar heterozygous abnormal expansions of TTTCA and TTTTA repeats in introns of the TNRC6A (610739.0001) and RAPGEF2 (609530.0001) genes, respectively. The finding of identical expanded-repeat motifs in separate genes strongly suggested that the expression of RNA molecules containing expansions of UUUCA and UUUUA repeats per se is involved in the pathogenesis of the disorder, rather than altered physiologic function of each individual gene.

In affected members of 18 Han Chinese families with FAME1, Cen et al. (2018) identified a heterozygous 5-bp expanded TTTCA(n) repeat in intron 4 of the SAMD12 gene. The mutation was found by a combination of linkage analysis in 11 informative families, followed by detailed analysis of repetitive motifs within the candidate region. The TTTCA(n) insertion was located between an Alu element and polymorphic TTTTA(n) repeats. Repeat-primed PCR demonstrated cosegregation of the expanded TTTCA(n) insertion with the disorder in all 18 families. The size of the insertion in patients was estimated to be at least 105 repeats; the insertion was not found in 119 Chinese control individuals. Haplotype analysis of 5 Chinese pedigrees and 3 Japanese FAME1 pedigrees (Ishiura et al., 2018) showed a shared core haplotype containing the pathogenic TTTCA(n) insertion, consistent with a founder effect between the 2 countries. The haplotype had a frequency of 0.226 in the 1000 Genomes Project database for the Chinese population. Functional studies of the SAMD12 mutation were not performed, but Cen et al. (2018) postulated that the expanded repeat would form RNA foci resulting in neurotoxicity, similar to that observed in SCA37 (615945), which results from a heterozygous 5-bp ATTTC(n) expanded repeat insertion in the DAB1 gene (603448.0001).

By whole-genome sequencing in 2 families with FAME1, one of Sri Lankan origin and the other of Indian origin, Bennett et al. (2020) identified heterozygosity for the TTTCA repeat insertion in SAMD12 in affected members of both families. Haplotype analysis showed that the families shared the same core ancestral haplotype that had been found in Japanese and Chinese families with the disorder. This finding expands the TTTCA repeat expansion beyond the Japanese and Chinese populations to a broad region of southern Asia. The age of this ancestral haplotype is estimated to be 17,000 years old.

Mizuguchi et al. (2021) used a CRISPR/Cas9-mediated method to cut and enrich for DNA fragments capturing the SAMD12 repeat region and single-molecule nanopore sequencing to measure the length of expansions in 25 patients from 22 families with benign adult familial myoclonic epilepsy (BAFME). The long-read sequencing was able to identify differences within the repeats and how they were configured. The authors noted that insertions of TTTCA motifs within canonical TTTTA repeats were associated with development of BAMFE. The study implied that minor differences in sequence composition, as well as overall repeat length, can influence phenotype.

The studies of Ishiura et al. (2018) and Cen et al. (2018) indicated that FAME1 is particularly frequent in the Chinese and Japanese populations, consistent with a founder effect. Bennett et al. (2020) broadened the core ancestral haplotype of the TTTCA repeat insertion to a broad region of southern Asia.