chr16-176718-T-A

Position:

• chr16-176718-T-A

Variant summary

Our verdict is Pathogenic. Variant got 14 ACMG points: 14P and 0B. PVS1 PS1_Moderate PM2 PP5_Moderate

The NM_000558.5(HBA1):​c.2T>A​(p.Met1?) variant causes a start lost change involving the alteration of a non-conserved nucleotide. The variant was absent in control chromosomes in GnomAD project. In-silico tool predicts a pathogenic outcome for this variant. Variant has been reported in ClinVar as Likely pathogenic (★).

• Frequency:
  • Genomes: not found (cov: 28)
  • Exomes 𝑓: 6.9e-7 (0 hom.)
  • Failed GnomAD Quality Control
• Consequence: HBA1 NM_000558.5 start_lost
• Clinical Significance: Likely pathogenic criteria provided, single submitter P:1
• Conservation: PhyloP100: 2.48

Genes affected

HBA1 (HGNC:4823): (hemoglobin subunit alpha 1) The human alpha globin gene cluster located on chromosome 16 spans about 30 kb and includes seven loci: 5'- zeta - pseudozeta - mu - pseudoalpha-1 - alpha-2 - alpha-1 - theta - 3'. The alpha-2 (HBA2) and alpha-1 (HBA1) coding sequences are identical. These genes differ slightly over the 5' untranslated regions and the introns, but they differ significantly over the 3' untranslated regions. Two alpha chains plus two beta chains constitute HbA, which in normal adult life comprises about 97% of the total hemoglobin; alpha chains combine with delta chains to constitute HbA-2, which with HbF (fetal hemoglobin) makes up the remaining 3% of adult hemoglobin. Alpha thalassemias result from deletions of each of the alpha genes as well as deletions of both HBA2 and HBA1; some nondeletion alpha thalassemias have also been reported. [provided by RefSeq, Jul 2008]

ACMG classification

Verdict is Pathogenic. Variant got 14 ACMG points.

• PVS1: Start lost variant, no new inframe start found.
• PS1: Another start lost variant in NM_000558.5 (HBA1) was described as [Pathogenic] in ClinVar as 915293
• PM2: Very rare variant in population databases, with high coverage
• PP5: Variant 16-176718-T-A is Pathogenic according to our data. Variant chr16-176718-T-A is described in ClinVar as [Likely_pathogenic]. Clinvar id is 2573442.Status of the report is criteria_provided_single_submitter, 1 stars.

Transcripts

RefSeq

Gene	Transcript	HGVSc	HGVSp	Effect	#exon/exons	MANE	UniProt
HBA1	NM_000558.5	c.2T>A	p.Met1?	start_lost	1/3	ENST00000320868.9

Ensembl

Gene	Transcript	HGVSc	HGVSp	Effect	#exon/exons	TSL	MANE	Appris	UniProt
HBA1	ENST00000320868.9	c.2T>A	p.Met1?	start_lost	1/3	1	NM_000558.5	P1
HBA1	ENST00000472694.1	n.21T>A		non_coding_transcript_exon_variant	1/2	1
HBA1	ENST00000397797.1	c.-46T>A		5_prime_UTR_variant	1/3	2
HBA1	ENST00000487791.1			upstream_gene_variant		1

Frequencies

GnomAD3 genomes Cov.: 28

GnomAD4 exome Data not reliable, filtered out with message: AS_VQSR AF: 6.88e-7 AC: 1 AN: 1453242 Hom.: 0 Cov.: 31 AF XY: 0.00 AC XY: 0 AN XY: 722450

Gnomad4 AFR exome AF: 0.00

Gnomad4 AMR exome AF: 0.00

Gnomad4 ASJ exome AF: 0.00

Gnomad4 EAS exome AF: 0.00

Gnomad4 SAS exome AF: 0.00

Gnomad4 FIN exome AF: 0.00

Gnomad4 NFE exome AF: 9.02e-7

Gnomad4 OTH exome AF: 0.00

GnomAD4 genome Cov.: 28

ClinVar

Significance: Likely pathogenic

Submissions summary: Pathogenic:1

Revision: criteria provided, single submitter

Submissions by phenotype

alpha Thalassemia Pathogenic:1

Likely pathogenic, criteria provided, single submitter

clinical testing

Women's Health and Genetics/Laboratory Corporation of America, LabCorp

Jun 27, 2023

Variant summary: HBA1 c.2T>A (p.Met1Lys) alters the initiation codon and is predicted to result either in absence of the protein or truncation of the encoded protein due to translation initiation at a downstream codon. An alternative downstream in-frame start codon (Met33) is located in the encoded protein. An activation of potential downstream translation at this initiation site would result in a shortened protein missing the first 32 amino acids from the protein sequence. At least one pathogenic missense variant has been reported upstream of this alternate codon in ClinVar. Four of four in-silico tools predict a damaging effect of the variant on protein function. The variant was absent in 230058 control chromosomes (gnomAD). c.2T>A has been reported in the literature as a homozygous and as a heterozygous genotype in individuals suspected of alpha thalassemia and in the heterozygous state in alpha thalassemia trait carriers presenting with mild microcytosis and hypochromia (e.g. Waye_2016, Keikhaei_2018). These reports do not provide unequivocal conclusions about association of the variant with Alpha Thalassemia. To our knowledge, no experimental evidence demonstrating an impact on protein function has been reported, however another variant affecting the initiation codon (c.2T>G, p.Met1Arg) has been classified as pathogenic in ClinVar. The following publications have been ascertained in the context of this evaluation (PMID: 29627922, 28865746, 27821014). No submitters have cited clinical-significance assessments for this variant to ClinVar after 2014. Based on the evidence outlined above, the variant was classified as likely pathogenic. -

Computational scores

Source: dbNSFP v4.3

Name	Calibrated prediction	Score	Prediction
BayesDel_addAF	Pathogenic	0.30	D
BayesDel_noAF	Pathogenic	0.20
CADD	Uncertain	24
DANN	Uncertain	0.98
DEOGEN2	Uncertain	0.63	D
Eigen	Benign	0.069
Eigen_PC	Benign	0.065
FATHMM_MKL	Uncertain	0.87	D
LIST_S2	Uncertain	0.90	D
M_CAP	Pathogenic	0.92	D
MetaRNN	Pathogenic	0.99	D
MetaSVM	Uncertain	0.74	D
MutationTaster	Benign	1.0	D;D
PROVEAN	Uncertain	-3.6	D
REVEL	Pathogenic	0.90
Sift	Pathogenic	0.0	D
Sift4G	Pathogenic	0.0	D
Vest4		0.91
MutPred		0.98		Loss of stability (P = 0.0065);
MVP		0.99
ClinPred		0.99	D
GERP RS		3.3
Varity_R		0.96
gMVP		0.84

Splicing

Name	Calibrated prediction	Score	Prediction
SpliceAI score (max)		0.010		Details are displayed if max score is > 0.2

Find out detailed SpliceAI scores and Pangolin per-transcript scores at spliceailookup.broadinstitute.org

Publications

LitVar

Other links and lift over

dbSNP: rs1316527998; hg19: chr16-226717;