Novel and reported variants in Parkinson's disease genes confer high disease burden among Indians

BACKGROUND

Genetic heterogeneity in Parkinson's disease (PD) has been unambiguously reported across different populations. Assuming a higher genetic load, we tested variant burden in PD genes to an early onset PD cohort from India.

METHODS

Whole exome sequencing was performed in 250 PD patients recruited following MDS-UPDRS criteria. The number of rare variants in the 20 known PD genes per exome were used to calculate average rare variant burden with the 616 non-PD exomes available in-house as a comparison group. SKAT-O test was used for gene level analysis.

RESULTS

80 patients harboured rare variants in 20 PD genes, of which six had known pathogenic variants accounting for 2.4% of the cohort. Of 80 patients, 12 had homozygous and nine had likely compound heterozygous variants in recessive PD genes and 59 had heterozygous variants in only dominant PD genes. Of the 16 novel variants of as yet unknown significance identified, four homozygous across ATP13A2, PRKN, SYNJ1 and PARK7; and 12 heterozygous among LRRK2, VPS35, EIF4G1 and CHCHD2 were observed. SKAT-O test suggested a higher burden in GBA (p_unadjusted = 0.002). Aggregate rare variant analysis including 75 more individuals with only heterozygous variants in recessive PD genes (excluding GBA), with an average of 0.85 protein-altering rare variants per PD patient exome versus 0.51 in the non-PD group, revealed a significant enrichment (p < 0.0001).

CONCLUSION

This first study in an early onset PD cohort among Indians identified 16 novel variants in known genes and also provides evidence for a high genetic burden in this ethnically distinct population.

Contribution of Rare Copy Number Variants to Bipolar Disorder Risk Is Limited to Schizoaffective Cases

BACKGROUND

Genetic risk for bipolar disorder (BD) is conferred through many common alleles, while a role for rare copy number variants (CNVs) is less clear. Subtypes of BD including schizoaffective disorder bipolar type (SAB), bipolar I disorder (BD I), and bipolar II disorder (BD II) differ according to the prominence and timing of psychosis, mania, and depression. The genetic factors contributing to the combination of symptoms among these subtypes are poorly understood.

METHODS

Rare large CNVs were analyzed in 6353 BD cases (3833 BD I [2676 with psychosis, 850 without psychosis, and 307 with unknown psychosis history], 1436 BD II, 579 SAB, and 505 BD not otherwise specified) and 8656 controls. CNV burden and a polygenic risk score (PRS) for schizophrenia were used to evaluate the relative contributions of rare and common variants to risk of BD, BD subtypes, and psychosis.

RESULTS

CNV burden did not differ between BD and controls when treated as a single diagnostic entity. However, burden in SAB was increased relative to controls (p = .001), BD I (p = .0003), and BD II (p = .0007). Burden and schizophrenia PRSs were increased in SAB compared with BD I with psychosis (CNV p = .0007, PRS p = .004), and BD I without psychosis (CNV p = .0004, PRS p = 3.9 × 10^-5). Within BD I, psychosis was associated with increased schizophrenia PRSs (p = .005) but not CNV burden.

CONCLUSIONS

CNV burden in BD is limited to SAB. Rare and common genetic variants may contribute differently to risk for psychosis and perhaps other classes of psychiatric symptoms.